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Checking Engine Oil and Tansmission Fluid

12/10/2020 4:47 PM

Tinkering in the garage today, I was checking fluids and wondered why we check oil with the engine off, but trans fluid with the engine running. Both situations seem similar: there's a pan full of fluid, a dipstick to check the level, and an internal pump sends fluid to the moving parts. Now the oil part is logical; I imagine with the engine running there would be lots of splashing in the pan, so we check oil with the engine off. But why check the trans fluid with the engine running?

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#1

Re: Checking engine oil and transmission fluid

12/10/2020 5:20 PM

I agree with your assessment of measuring the engine oil. You couldn't read the dipstick. Transmission oil should be read warm when it has been expanded to the maximum extent to prevent overfilling.

"Transmission fluid however is best checked when your vehicle is running. The reason for this is that transmission fluid, like any fluid expands when it's warm and contracts when it's cold. You'll get a better reading of how your transmission fluid levels are in your vehicle if you check it when it's running warm. If you check it when it's cold, the levels may seem low and you could end up over filling the reservoir as a result."

https://www.cashcarsbuyer.com/how-to-check-transmission-oil/#:~:text=Transmission%20fluid%20however%20is%20best,it%20when%20it's%20running%20warm.

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#2
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Re: Checking engine oil and transmission fluid

12/10/2020 6:37 PM

I had heard at one time, and I am having a hard time confirming it, that the reason to have the car idling when checking the automatic transmission is because it makes sure the torque converter is full of oil. (Maybe when the engine is off, some of the oil in the torque converter drains back into the sump?)

Otherwise, we could just warm the car up, shut it down and check the tranny.

As for engine oil, sometimes you get air pressure pulsations in the crankcase that can blow around oily air and you may get some "puffs" that may make the viewing process unpleasant or skew the reading on the dipstick. Windage trays in the lower end should limit the slewing of oil from the crank and big ends.

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#7
In reply to #2

Re: Checking engine oil and transmission fluid

12/11/2020 7:38 AM

You are right.There is drain back from the torque converter when the engine stops.Keeping the torque converter moving when checking fluid ensures that the running fluid level is correct.It is also good to cycle between the gears before checking the fluid level in park.This ensures that all fluid passages are full.

The color of the fluid is a good indicator of the transmission health.

If it is dark,it is very oxidized,either from age or excessive heat.

The automatic transmission actually produces more heat at low speed,or towing than the engine.For frequent towing,an auxiliary radiator is recommended .This heat is produced by the fluid coupling churning in the torque converter and the wet clutches.

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#3

Re: Checking engine oil and transmission fluid

12/10/2020 6:37 PM

Turning the engine off allows the oil above the pan to return. It's best to give it 5-10 minutes to drain down.

Otherwise, see Rixter's comment.

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#4

Re: Checking engine oil and transmission fluid

12/10/2020 10:42 PM

Most/all of the cars I've owned, with automatic transmissions, specified to apply the brake, shift from park to drive, then back to park prior to checking the tranny fluid. Now I have a 2006 Ford Explorer with no transmission fluid dip stick. The good news is at 150K miles I had it test driven with full diagnostics by my favorite transmission shop and they said it is functioning extremely well for the years and miles.

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#6
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Re: Checking engine oil and transmission fluid

12/11/2020 7:26 AM

You must be a Harley rider."Live to ride"

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#10
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Re: Checking engine oil and transmission fluid

12/11/2020 11:25 AM

Never been on a harley in my life!

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#19
In reply to #10

Re: Checking engine oil and transmission fluid

12/11/2020 10:43 PM

Yes, not much in touring more agile and comfortable than my Wing. Rent Harleys to get around in Hawaii. Always good to get home to the wing.

And Yes, engine must be idling to be sure the toruqe converter is full to give you an accurate fluid level.

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#5

Re: Checking engine oil and transmission fluid

12/11/2020 12:41 AM

Most of the time when checking oil in the engine the supply in the oil pan with the engine off will be enough to keep all the bearings and wear points lubricated during normal operating conditions.

However all us screwball car nuts have to modify stuff to go fast or corner harder or climb steeper or just hit the rev limiter for fun. No matter what you are doing along those lines you need to be sure that there is oil at the suction side of the oil pump.

Changes to the engine to make more power and make it turn more RPM usually require thing that allow more oil to flow through the engine during high RPM operation. Roller rocker arms, roller tappet cams, looser bearing clearances, and many other modifications increase the amount of oil that the engine requires per revolution.

Without getting into a bunch or gallons per minute mathematics a little test that is a real eye opener is to get the engine oil up to operating temperature by driving it a couple of miles at high speed to get the oil temperature up to what it would be in the race. The stop the car, raise the hood and remove the dip stick. Wipe it clean.Then bring the engine RPM up to 3 do 4 thousand RPM. Hold it there for 5 or 6 seconds. Then rapidly insert the dip stick and pull it out just as quickly. You may be shocked at what you see......Or don’t see.....

The next thing you need to do is guesstimate how far down into the pan the dip stick goes into the pan. Then determine what the vertices distance the oil level is above the oil pump pickup. Anything less than 2 inches of vertical depth is asking for instant engine explosion. Add cornering force and acceleration forces your oil pump pickup is exposed to air. Air is a poor lubricant.

I have found that less than 2 inches vertical level at all times above the pump will vortex air into the oil pump.

Add oil to keep that 2inch distance and you engine will love you.....

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#8

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 7:46 AM

Why do We park in a driveway and drive in a parkway? Somethings make no sense at all. However all the reasons given below for checking fluids do make sense.

oilcan13

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#9

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 11:21 AM

I'm thinking the transmission needed a standardized level when operating and there are many different transmission types that require different amounts of transmission fluid(9 - 16 quarts), but when operating they all require a minimum level in the pan (4 -5 quarts) for the pump to function properly and maintain minimum hydraulic pressure...so most of the transmission fluid is held in the upper part of the transmission and torque converter...The engine however holds most of it's oil in the pan...

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#11

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 1:01 PM

OP here. I've been cogitating on this and think I may have an answer. Given that we know how much trans fluid expands when hot (here's some info: hot stuff) and knowing the volume of fluid contained in the trans works and the torque converter when the engine is running, we should be able to determine the correct pan level with the engine off and the fluid cold (I assume that all the fluid in the trans works and 1/2 the fluid in the converter drains back into the pan). BUT...since we can mate any number of different converters (with different fluid capacities) to a given transmission, we would have to know the fluid capacity of the converter and do some math and then mark the dipstick accordingly. Not a big deal for gearheads, but for the average user it's simple to check the fluid hot and running, which automatically accounts for the fluid capacity of the trans works and the converter.

As an aside, I use an aftermarket deep pan (Derale) that takes an extra 3 qts of fluid and that's what got me to wondering about levels. Also my dipstick is real close to the headers and it's easy to get burned!

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#12
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Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 2:13 PM

That variation in converters can happen at the manufacturer level. I have a 1998 Lincoln Town Car Signature Edition. The SE was made from 1998 to 2002, and was the closest Lincoln to the Crown Victoria Police Interceptor. One of the features on this model was a 1" larger diameter converter, compared to the other LTC models of the same model years.

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#13

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 5:33 PM

This is basically what I learned in High School Automotive Shop Class... now I just found out, things changed, which I is explained later at the end of the post... btw,... this is easily searchable.

There are two main things about tranny fluid:

  1. Transmission fluid is checked when it's hot (up to running temperature). As the fluid gets up to temperature it expands. If the fluid is cold, it won't give the proper level reading, thus overfilling will ensue.
  2. One of the main fluid compartments of the transmission is the torque converter. When full, the fluid level of the torque converter is much higher than is the level of the pan (almost entirely above the pan in most cases). Due to this, when the engine is stopped, a significant amount of the fluid will drain down into the pan, showing the wrong amount of fluid on the dipstick. With the engine running, not only is the transmission paths filled with fluid, but the torque converter is as well. Then a check of the fluid will give you a proper level.

Until recently, depends it on the type of vehicle you have, last Wednesday, I had my MB in for a Schedule B service... and on the transmission, to check the fluid in the transmission, it’s a little more intensive because it’s sealed? And would take an additional 2 hours... and an additional $600.00.
I was going to have it done right away, when they told me, it would be an additional 2 hours, I didn’t plan on that with my schedule that day so, I’ll have to return.

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#14
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Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 5:53 PM

$600 to check the oil? I don't think I would bother. Ever.

Unless your car is leaving oil spots on the ground or your tranny is getting that oily, grimy film, the oil isn't going anywhere.

Since I assume you are not drag racing your car, or running down the Autobahn 8 hours a day, chances are the tranny will last the life of the car without a hitch.

Germans build good stuff.

B.T.W. I've seen on a number of my vehicles over the years, one line on the dipstick for hot and one for cold (running in both cases of course).

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#15
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Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 6:10 PM

Yes I agree, schedule B was 625.00, and as they were going through it... the service manager came up to me that he forget to mention, about the tranny oil inspection...

And additional to that, to replace the spark plugs, that would be an additional 500 and some change.

when the Warranty expires on my car, so will my patronage.

it was the same with my 911, my patronage ends when the warranty ends... changing the plug on this,... it was somewhat earned... to change plugs,... you need to pull the engine... . That’s why I always used the longer lasting premium plugs..

my vehicles that were domestic, I normally put on 250,000 miles... and that only due to living inWisconsin, where winter salts eats them away... otherwise I’m sure they’d last longer.

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#16
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Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 6:24 PM

Yep, I've heard that about 911's. But then again, with the long-life plugs platinum tipped and so on, I've let my plugs go 150K+ and when I went to replace them, the gap was still within spec. I probably could have put them back in and run another 100K miles.

In the old days of leaded gasoline, I was replacing plugs every 5,000 or 10,000 miles along with the points, condenser and sometimes the cap and rotor if they looked really nasty.

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#21
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Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 4:53 AM

I replace all my plugs with Iridium plugs.

They last longer than even platinum.

250+ miles,and still no change in gap or signs of erosion.

All interstate miles,very few short trips.

Iridium is basically a set it and forget it plug.

Oil+filter changes every 3000 miles,even though manual recommends 7500 miles.

Just a habit from the old days.

Timing belt,water pump,pump belt,idler pulley,idler tension adjuster at 100k, DIY.

Takes about 1 1/2 hours.

Dealer price over $1400.

Bought the car right as it rolled off the delivery truck.

Everything was still shrink wrapped.

They did not have time to install the dealer name tag on the car.

They had it ready to go in about 1 hour.

Never been back to dealer except for an air bag recall.

Tranny fluid? I change filter and fluid every 100 k when doing the timing belt.

So far,so good.Just getting the car broken in now.

'Bout time to pass it on down to one of the grand kids so they can pass it on down to their kids eventually,after I "Exit stage right"...

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#27
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Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 11:30 PM

My son wants to pass the '87 Jetta (he came home from the hospital in it) to his son in another 15 years. It's at 300K+ at this point. None of the engine internals have ever been touched. Just the usual stuff like alternator, water pump, timing belt etc.

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#29
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Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:05 AM

I had a Oldsmobile Intrique...

everything mechanical on it was it great shape. It the body that could hold up in to the Wisconsin winter.

The interesting thing,... the paint was relatively great, it was behind the paint...

I knew I had problems... because after a rain,... there would be a slight rust hue on my driveway... i think that had about 200,000 and some change miles on it,. I went to rotate my tires, put the Jack under it, started to lift the car, rust started dropping out, and the uniframe started bending as I jacked it up and crumbling, and the tire stayed on the ground.

I was already looking for a replacement because the Olds Intrique was known to rust from the inside out and I already was looking for it’s replacement for a while,... I was just surprised how bad it was.

had a replacement car within the week... Buick Lucerne... with 70,000 miles,... that lasted until 225,000 until my wife totaled it... very disappointed,... it was pristine,... that car was going to be my record mileage car...

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#35
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Re: Checking Engine Oil and Tansmission Fluid

12/14/2020 4:11 PM

Automobiles have become so good in general, that with reasonable care, they will last a very long time except for the rust issue. (Or premature body damage. )

My 2000 Ford F-150 7700 still has less than 200K miles but the body and bed are rotting away with surprising speed. It doesn't get a lot of use and generally isn't run in the snow and ice because it's 2WD. Other than the body rot, it's a great truck with a payload capacity of 3,000 lb. and the running gear is still tight.

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#36
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Re: Checking Engine Oil and Tansmission Fluid

12/14/2020 9:04 PM

I have to say along with the design of the cars, the roads are better than back in the 50’s.

talking about old trucks,...

in high school, one of my industrial arts teachers, (had him in junior high also), he was our graphic design teacher (damn hippie )

he had a like a ‘49 chev or maybe Ford pick-up... the kind where the passenger window was spider webbed cracked... rust,... inside door handles are off, to open the door, you hand to crank down the window (with a vise grip handle) and use the outside latch...

anyways, in phy Ed, we had to walk along side it to go to the baseball field, because it was parked next to the gym exit... I looked inside, and busted out laughing... it had no seat... conventional seat that is...

what he did have for a seat behind the steering wheel was this... ya,... the truck musta had a billion miles on it...

our last class reunion we still laugh about it...

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#37
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Re: Checking Engine Oil and Tansmission Fluid

12/14/2020 9:36 PM

That's funny. An original environmentalist. Why throw away something that is still useful (to you)? Nowadays, you have to throw stuff away so you can get the newest environmentally friendly product.

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#38
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Re: Checking Engine Oil and Tansmission Fluid

12/14/2020 10:38 PM

Yep,.. one other thing... He chewed tobacco also.

well a few things... we had some characters as teachers... him, our art teacher and high school principal would go poach deer.

anyways, they finally got caught... how?,... well there was a petting zoo in our area,... it had bears, where you’d buy a grape soda and feed the bear, raccoons that would pick mini marshmallows out from you hand,... which you also bought and small handful for a quarter,... you can feed the trout with the marshmallows too,... goats where you’d bucket corn on the suspension bridge... a few other animals...

it also quite a few deer, where you walk in a pasture with a 8’ high fence,... put a quarter in the vending machine that had corn in,... and the deer would run up to you... anyways,... they had an albino deer,.., it was a sight...

i guess having a trophy like that was too much... and this albino buck disappeared... I’ll give you one guess where they found it?...

the high school principal had to turn in his resignation... he was actually a pretty cool guy... well except for the poaching...

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#39
In reply to #38

Re: Checking Engine Oil and Tansmission Fluid

12/15/2020 3:56 PM

One place I worked many years ago had a herd of albino deer.

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#42
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Re: Checking Engine Oil and Tansmission Fluid

12/15/2020 6:03 PM

Well, when there’s one in the area,... and it turns up missing... pretty tough to hide it, especially getting it mounted...

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#40
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Re: Checking Engine Oil and Tansmission Fluid

12/15/2020 5:07 PM

Sounds like you grew up in a colorful town.

These days no one can be 'colorful' anymore in this cancel culture of thin-skinned wussies. Spines have become an endangered species and appear to be disappearing from homo sapiens. Won't be long before the species devolves into gelatinous blobs plopped down in front of a screen.

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#41
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Re: Checking Engine Oil and Tansmission Fluid

12/15/2020 6:00 PM

Maybe... the sheriff was a pretty decent guy like by the kids... he’d take them deer hunt and give them the confiscated M80’s.

also was part of a big gambling ring...

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#17

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 8:02 PM

Yeah, my Jag XK8 has a sealed tranny. No dipstick.

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#18

Re: Checking Engine Oil and Tansmission Fluid

12/11/2020 10:20 PM

I'm going to bring Scotty Kilmer's opinion of the subject.

Don't use high mileage oil and don't charge your transmission fluid.

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#20
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Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 12:21 AM

"...don't charge your transmission fluid." Does that mean you should pay cash for it insted?

I gotta confess that it's been at least 20 years since I knew whether my cars even had/have a dipstick for the transmission! ... and the last one when I did know was a stick shift.

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#23
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Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 12:13 PM

Just saying it's not considered wise to try any change a tranny like they used too.

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#25
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Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 1:44 PM

Oil has came a long ways since the 70’s... other then once being plant life...

The ICE are a lot more efficient and the oil is a lot more robust and refined where it doesn’t break down as it once did... in what ways or how... I can’t say.

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#30
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Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:18 AM

The best lubricant ever,was whale oil.

It has a particular molecular structure that until recently (20th Century) was not found anywhere else.

It was used in extreme pressure applications,like thread cutting oil.

A plant was discovered whose oil was identical to whale oil...The Jojoba bean.

Chemists have used the molecular structure as a basis for creating new advanced lubricants,converting regular vegetable oil into a synthetic version.

Superior lubricants and closer tolerances have created longer lasting engines that we all use today.In the 1970's if a car ran 100k,you had bragging rights.

Now,they are just getting broken in.

The Mars rovers were only expected to run for 90 days,but look at how long they have lasted.Failures were not related to lubricants.

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#31
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Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:28 AM

Whale oil?... that was quite a commodity in its day. (Kinda the duct tape of its day)

As a layman, I understand proteins can be pretty tough and robust...

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#32
In reply to #31

Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:51 AM

You are right about the proteins.

Some are stronger than steel,like spider web.

The adhesives of barnacles are superior,esp underwater.

A scientist used soybean protein and modified it to be like the barnacle adhesive,and created a water proof,super strong adhesive that will hold up even in boiling water.

This has found applications in plywood manufacturing,and dentistry.

I tried to purchase some when it first came out,but they were not selling it retail.

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#22

Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 12:02 PM

Pragmatism and reality:

Automotive engines, at least the ones that are not in race cars, are designed to drain to the crankcase sump. Yes, the oil will expand at operating temperature and there is some residual in the rocker boxes and oil filter, but that can be easily compensated for by graduating the dipstick. Ease of checking takes priority.

Transmissions on the other hand are designed to hold oil in the valve bodies, ports and tubings. Older transmissions are no so good at holding oil in those critical places when the car is shut off. So graduating the dipstick becomes extra special problematic. Accuracy of checking takes priority.

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#24

Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 1:17 PM

Even if it was OK to check oil level with the engine running, it would not help much with the engine in my narrowboat because it is down under the rear deck, and almost impossible to get the far end of the long shaking dipstick to line up with the tiny hole at the bottom of the engine when it is wobbling all over the place.

...and to be honest, it is still difficult when the engine is stationary.

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#28
In reply to #24

Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:04 AM

I have noticed that when I am trying a delicate task,such as threading a needle or such as you describe,that the thread,needle,or dipstick will go everywhere except where I want it.So the work around is to pretend that I want it to go somewhere else and eventually it goes where I want it.Kinda like out guessing Murphy.Actually,I think the oscillations are a sine wave,and it is a matter of timing to get in phase with the oscillations.There is a delay in what you see and the muscle movement,which creates a sustained error.The more frustrated you become the more the error is amplified.

Watch the point where the oscillation stops and aim for that spot.Works for me.

(I learned this from a man who used to work at a sewing machine factory,and his job was to thread the sewing machines as they came down the line..and they were running...)

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#26

Re: Checking Engine Oil and Tansmission Fluid

12/12/2020 5:00 PM

I have had a couple of cars that did have a tranny dipstick, but the instructions were to shut the engine off when it was warm to check the fluid level. Reason: The tranny dipstick was in such an awkward place that they didn't want you there when things were turning.

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#33

Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 4:04 PM

OP here again. I've had an epiphany I think. We check the trans fluid hot and engine running. Ensure it is OK on the dipstick, then we turn off and cool down and let all the fluid run back into the pan. Now we check the dipstick, which should read much higher. Mark the dipstick at this point and we can then check the level cold and engine off at any time.

Sounds too easy, n'est-ce pas? What have I missed?

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#34

Re: Checking Engine Oil and Tansmission Fluid

12/13/2020 5:02 PM

Then there is this...

https://mechanics.stackexchange.com/questions/35616/why-is-the-atf-level-checked-with-the-engine-running

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