Bleeding a Diesel Engine Question

Because if there is air in the injection pump this is compressed on the delivery stroke, but expands again when the piston returns. The air is not forced out via the injectors. Releasing air at the bleed nipple on the injection pump allows diesel into the pump cylinder. Usual to work the lift pump by hand until fuel flows freely from the bleed nipple. You could do it by cranking the engine but might get a flat battery before the engine starts. Maybe some modern engines have electric lift pump so you only need to switch on. Unusual to have to bleed on the high pressure side. As you say some engines are 'self bleeding'. I'm not familiar with the details but it doesn't sound difficult, just an automatic air release on the injection pump.

I don't see how bleeding at the lift pump would work, but if the layout was appropriate, it might.

Cheers.........Codey

If the valves in the lift pump are allowing fuel to drain back to the tank it could take quite a lot out of the starting system before it primed up properly. As for cracking the high pressure lines which we do whilst the engine is running on load it is usually to detect the performance of the injectors, a bad injector will not effect performance as much as a good injector.

All of the diesel cars I have driven since about 1987 (7), were able to eject the air happily by just stepping on the accelerator, down to the floor, and then turning the engine over on the starter. Generally 20-30 seconds was required.

I always run a new car till the tank is empty, to see where the tank needle is exactly when that happens. Naturally I have a can of fuel in the boot.......

I have heard of people having problems and when I asked them what they did, they all forgot/didn't know to step hard on the gas pedal!!

(By the way, does a diesel have a "gas" pedal??? Sounds strange....)

I call it the "throtle".

Thats a new word !!! Or did you mean "throttle"?

Furthermore it cannot be right for a diesel engine as they are as good as never "throttled", which means "to restrict" which is used in a petrol engine to control the amount of air being able to reach the engine, and therefore the speed of the engine.

A diesel engine is never (as far as I am aware) ever "throttled", as the way the speed is controlled is by increasing or decreasing the amount of fuel that gets pumped into the motor by the injectors.

The main aim is always to have more air rather than less (black smoke, incomplete burning due to a lack of air) in a diesel engine. Often today (and yesterday too!) Turbos are used to get some power from the heat and pressure in the exhaust, to blow extra air into the cylinders, which results in generally more complete burning and a cleaner exhaust......

On a petrol engine, the throttle is one of the main reasons that these engines are far less efficient than diesel engines and there have been several clever methods to do away with the throttle and improve the efficiency, none of which has made it into a car as far as I am aware!!! Yet anyway.....

Have a great Sunday and don't throttle your imagination!! Think FULL TIME!

I'm with you on modern diesels having no throttle, speed control just by varying fuel injected. But I remember from somewhere round 1955 we had a Ferguson TE20 tractor with a diesel, and that had a throttle. There were 2 impulse pipes from either side of the throttle, connected to a diaphragm on the injection pump to control the position of the fuel rack. It was an in-line Simms pump. Not unlike the vacuum advance on a distributor. Not sure why they thought 2 impulse pipes were needed, I'd have thought just 1, with atmosphere the other side, would be OK. Or none, as in later engines.

Of course diesels had been around for decades by the mid-fifties, and I don't know offhand whether earlier designs had a throttle, till somebody realised it was unnecessary. Anybody know?

Cheers.......Codey

Generally speaking, no mass produced diesel has a throttle, its entirely unnecessary and just reduces efficiency on any engine (= uses more fuel for the same power!)

I expect some "clever clogs" have tried it as you say.......but they also probably found that it brings as good as no benefits.....

You can read here:-

http://en.wikipedia.org/wiki/Throttle

This comment:-

The power output of a diesel engine is controlled by regulating the quantity of fuel that is injected into the cylinder. Because the engines do not need to control air volumes, they lack a butterfly valve in the intake tract, and therefore do not have throttles.

and this:-

In some instances, a "throttle" is known as a "thrust lever" (as in most Airbus and Boeing aircraft). This is chiefly due to the fact that "throttle" is associated with traditional gasoline engines.

I hope this helps further.

Commonly is a very hard work for the starter motor.

If its a well made car, the starter motor can handle that with no problems whatsoever.......quality tells!

it is easier to fully bleed the hydraulic system of the fuel circuit by cracking the cyl lines and pumping the lift pump handle then cranking the main to get air out of the system before starting, it can be hard on starter and also engine to just crank the hell out of it hoping it will eventually start up, and it is hard on a large diesel to start up with the system not fully working, wrong injected amounts and times are why the clattering and knocking take place when you do this, just take the time to purge the low pressure side with lift pump then the high pressure precision side with cranking with the hp lines cracked before starting, it would make me happier knowing you were doing this,,

Sincerely
Mitch retired Peugeot mech and large boat engine wrencher

For modern and relatively modern diesel cars completely no need to do that, the engine will start within 30 seconds. As far as I am aware, the manufacturers intended it this way, so its not wrong to this. Its detailed in the manuals....

I expect you are right with very large diesel engines.

May be Andy you are right with small engines and is easy and fast in your way but..i would say almost one starter motor rebuild by each.BOSCH both types common and KB type, same with Delco Remy.

It probably varies with the quality of the car driven.

I have not replaced a started in probably over 30 years (and only one alternator after 360,000 Kms.).

Since 1981 I have had only Japanese and German cars. Perhaps thats the reason for so few problems.....

The last starter I replaced was in a British Sunbeam (sh*t) Chrysler......about 1976 or so, the car was a 1970 model ......and it wasn't a diesel either......

My personal opinion only is that starters for Diesel motors are probably generally built to a higher standard, as turning over a diesel requires a lot of torque, but I expect some manufacturers just use what is to hand......and cheap!!

one boat design I am constantly dealing with is the volve kad43p-a, it has supercharger on ac type clutch and a turbo, anyhoo the starter it came with was a bosch reduction drive design and when I replaced it it waswith a paris rhone direct drive, neither of them would I want to sit and crank for 30 seconds to purge injection lines, at 3-4kw+ they are pulling a good deal of amps but it would tell you if there was a problem in your starting circuit, Volvo in thier wisdom decided to electronically control the super charger and put the control module on the engine, salt enviroment made this short lived,

anyhoo when I was working on the peugeots they pulled 5kw on starter and the manuals specifically said to crack the lines to purge before starting, if you ran out of fuel and did not try to restart you could get by with about ten second crank to start but with the rough clatter for a few until the air was purged, I still would properly purge after teardown and rebuild so that minimum crank and run was needed to have engine able to idle down and lube all those new parts before run in, the problem with new designs is that you cannot reach the lines to fit a wrench on them anyway, like why new cars have platinum iriduim plugs designed for 100,000 mile life, you need 3-4 hours to replace them, good luck with your decision.
Mitch retired Peugeot mech

Hi Mitch,

I likes my old Peugeot Diesels - 505 XD3T and XD2S - simple, reliable, mechanical, repairable .... good stuff

Bill