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Anonymous Poster

Idle Surging on Old BMW 85 325e

08/24/2008 1:11 PM

I have an old BMW 325e 1985. When at idle the engine RPM will cycle from normal 800 RPM up to 1220 or 1500 RPM and then back to norma. This cycling will repeat for extended periods of time. Then at times the RPM will run normally.and then the process repeats.... Any suggestions...

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#1

Re: Idle Surging on Old BMW 85 325e

08/24/2008 6:38 PM

Early seventies quadrajet carburetors were plagued with this after about a few months. Turned out the carburetor bolts were not tight enough. The return spring would pull the carb off it's base ever so slightly. the now created vacuum leak would cause the engine to rev higher. the increased engine speed would cause the vacuum to rise. Additional vacuum would suck the carb back onto it's base. At that time the vacuum leak would be sealed and the speed would drop to normal. Repeat again.

Check the bolts holding the carburetor. Check for vacuum leaks in general. Spraying short bursts of carb cleaner at the vacuum leak will help find it. Another tool is a length of hose held to one ear. The other end moved to follow a systematic search. Good luck.

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#2
In reply to #1

Re: Idle Surging on Old BMW 85 325e

08/24/2008 8:16 PM

Bob's advice is excelent. It does sound like a vacuum leak and the methods he mentioned for finding it are right on. Be careful with the carb cleaner however. If you have any leakage from your spark plug wires you could get a flash-over. I've come away with singed hair more than once.

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#3
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Re: Idle Surging on Old BMW 85 325e

08/24/2008 9:20 PM

Very true. Did you ever check for spark with starting fluid?

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#4
In reply to #3

Re: Idle Surging on Old BMW 85 325e

08/25/2008 12:36 AM

No Bob, I never had to. I always pull a wire and hold it while cranking. That usually tells me right away. When they came out with HEI it made it easier.

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#6
In reply to #1

Re: Idle Surging on Old BMW 85 325e

08/25/2008 3:36 AM

so Bob, exactly what kind of carburetor does a 1985 BMW 325e have?

I'm *real* curious about that!

-RF_G

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#12
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Re: Idle Surging on Old BMW 85 325e

08/25/2008 7:30 AM

I don't know what the fuel system is on this year BMW. My answer was meant to be more general in nature. Whatever is connected to the intake side of the cylinder head could cause a surging, if allowed to move. Those long runners on the 6 cyl BMW would never come loose would they?

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#5

Re: Idle Surging on Old BMW 85 325e

08/25/2008 2:14 AM

I can tell you exactly what's wrong with your car. You, my friend, have a head gasket that's getting ready to go.

I know it doesn't make sense, and don't believe me, but I'll bet $5.00 that when the end comes it will be a head gasket.

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#7
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Re: Idle Surging on Old BMW 85 325e

08/25/2008 3:40 AM

Clueless....

This is nothing but typical 'hunting' by the fuel injection system!

Typically caused by the mixture being a bit lean...BUT that said, it can

be that the cause is the lambda sensor (located in the exhaust manifold just downstream of the exhaust ports - it gets HOT and thus ages) going bad, which can cost a couple of bills to change, but worth it if the everything else is ok with the car..

RF_G

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#8

Re: Idle Surging on Old BMW 85 325e

08/25/2008 5:05 AM

Without looking anything up, I'd say that the car has fuel injection, and in 1985, it wasn't as well worked out as the present day cars.

A fuel injection system needs to have a continuous flow of fuel, at pressure.

It also needs to open and close the fuel injector valve many times per second, for varying lengths of time, depending upon the rpm of the engine, the load, etc.

So . . . one system that was used in some cars was a "continuous flow" system. The fuel was not just pressurized, it was in continuous circulation. When the system signaled an injector to open, the fuel just ran through that opening. When it closed the fuel just kept running around in the feedback loop.

This created a problem - sudden rise in pressure at injector closing. So they added a chamber with a diaphragm at the end. A spring worked against the diaphragm, keeping pressure up, and absorbing the shock of a sudden rise in pressure at injector shut off.

Some of the diaphragms were adjustable, so perhaps you have one, and perhaps it's adjustable. Or the diaphragm materiel may be deteriorated, and would need to be replaced.

If you can adjust the diaphragm, it is very, very sensitive. So be sure to back it off a tiny bit, then begin to screw it in, very, very slowly. Make your adjustment attempts in very small steps, or you won't even be able to start the car.

Good Luck,

Tom

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#9
In reply to #8

Re: Idle Surging on Old BMW 85 325e

08/25/2008 6:18 AM

Again: probably not.

The BMW 325e was equipped with Motronic...

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

Take it to any competent BMW garage/'European car' garage, they'll put a probe in the exhaust and try to adjust out the hunting. If they can't then they'll either plug an analyzer into the control unit (if they have one) or measure the voltage coming from the lambda sensor & compare it with the CO reading coming off the exhaust analyser (which will say if the lambda sensor is fried)

*tons* of stuff about this on the web.............................

Contact your local chapter for more info:

http://www.bmwcca.org/

rf_g

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#10

Re: Idle Surging on Old BMW 85 325e

08/25/2008 6:30 AM

We had an '88 316 with the same problem which we cured by taking out the solenoid actuator which was doing the hunting and replaced the carb. with twin webers manual choke.

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#11
In reply to #10

Re: Idle Surging on Old BMW 85 325e

08/25/2008 6:47 AM

but with Motronic, you can do *anything*....if that's the goal.

200hp without hardly thinking, more if you want it

Very sweet, very powerful little engine (with the appropriate re-working)

http://www.kormanautoworks.com/

etc etc etc

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#13

Re: Idle Surging on Old BMW 85 325e

08/25/2008 9:04 AM

an even better place(s) to ask these questions would be either www.roadfly.com or www.bimmerboard.com . Both have model specific boards, that range from old 2002's through the all current models They are the web equivilant of hanging out with about 100 reasonably knowledgeable guys all working on the same car... On most of the boards you will find a good protion of the heads of service from almost all the major BMW dealers, a very large amount of BMW master techs and people from most of BMW's suppliers like ZF transmissions. And it has been my experience (I'm on my 4th BMW) that almost every single thing that you can possibly do to our cars from hood badge replacement to complete V12 rebuilds is one there and 90% of the time someone took the time to not only write up a procedure but photo document as well. Both also have a small mountain of links to virtually anything you could possibly need or desire for our cars. You can also try www.e38.org which has links to other model specific sights which has even more info!!! Go there search and enjoy!!!

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#14

Re: Idle Surging on Old BMW 85 325e

08/25/2008 10:51 AM

I can't help but put my oar into this. I have an 84 325e with more miles than I like to admit on the odo. My experience with BMW's both with carbs (2002 and Bavaria) and fuel injecion (325e, 325i, 328i and 330xi) is that if there is a vaccum leak, they do not start or at least are very hard to start. All it takes is something as simple as a loose oil filler and it will not go. If they do start then the fuel injected ones often surge (vary in idle rpm). Vaccum leak is possibility number one and it is the easiest to check and has been covered here. My test solution to find a vaccum leak has been to spray starter fluid around various suspect connections and if there is a surge then I found the problem. You do need to be careful since this can blow up in your face (quite litterally).

Possibility number two (and the one that contunually plagues our car) is the idle control valve itself. Over the years sludge and gunk tend to build up inside causing it to stick and become slow to react to the signals sent by the computer. In addition it often doesn't shut off completely (sticks) paticularly when warm and the computer cannot compensate on the mixture side thus the surging at idle or low speeds when not under much load. It does not affect the car at driving speeds because the throttle opeing is such that the computer can compensate to keep the mixture within programmed limits.

Easiest fix is to take the idle control valve out (undo two hose clamps, one retaining clamp and undo one wiring connection), which takes about three minutes and run anything from WD-40 to brake cleaner to gunk remover through it, let it soak a few minutes and repeat. Shake the ICV and if you can hear it rattle or see the slide move, it will work properly for a while. If this doesn't work, take lots of money to your BMW dealer and have them run a diagnostic on the car and then be prepared to leave most of your cash on the the counter!

As for the possibility of a head gasket going. It is rare but it is possible. Easiest check on that is to look in the oil to see if there is any water and in the rad to see if there is any oil. If there isn't then a compression test is a good idea. If you have the tols available, a leak down test is also a good idea as it will let you know how well your engine holds the compression it has.

Along with remote possibilities, when these cars were new, there was a problem with some of them surging and the problem was electrical not mechanical. Some wiring harnesses were not assembled correctly. In theory it is possible for age to corrode some of the electrical connections in the harness or ruin a ground connection. The last one I saw this happen to was about 20 years ago ( I live in a very dry climate), so a I said, it is a "remote" possibility.

By checking out the first two issues, I have managed to solve my idle problems over the years for about the cost of two cans of brake cleaner and a couple hours of work. Much better than contributing heavily to my dealer's annual holiday in the Bahamas.

Good luck in fixing your car, Two boys learned to drive on this car and currently use it as their daily driver now that we have moved on to newer versions.

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#15
In reply to #14

Re: Idle Surging on Old BMW 85 325e

08/25/2008 12:28 PM

go easy with the starting fluid. One spark from anywhere and your engine bay will light up. then the panic attack happens.

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#19
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Re: Idle Surging on Old BMW 85 325e

08/26/2008 4:52 AM

Really, man, that can get real pyrotechnical real quick....

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#16
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Re: Idle Surging on Old BMW 85 325e

08/25/2008 12:43 PM

Had this car come into my shop I would have known by looking that it was an injected model and hit the books. It is also old enough that most cars of that vintage would be carburated. That said, I guess both Bob and I were shooting from the hip since a vacuum leak is the most common cause of the problem.

But hey, ya get what you pay for - right? Excuse me, I've got to go wipe this egg off my face.

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#18
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Re: Idle Surging on Old BMW 85 325e

08/25/2008 10:38 PM

Pass the rag please.

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#22
In reply to #16

Re: Idle Surging on Old BMW 85 325e

08/26/2008 4:36 PM

Most? Maybe...before about 1980...or around '84 at the latest. But...not a BMW officianado...but would expect FI to have been in them early on.

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#25
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Re: Idle Surging on Old BMW 85 325e

08/27/2008 1:23 AM

sorry, I haven't had any coffee yet, a bit early here (7:15), a little cranky...but that said,

WHAT IS GOING ON HERE?! THAT CAR IS NOT I REPEAT NOT CARBURATED!

NO BMW WAS CARBURATED IN THE '80'S (esp in the US, ESPECIALLY a 325e).

(unless some Bimmerhead installed 3 fat side-draft Webers on it..)

Sorry, but really...

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#26
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Re: Idle Surging on Old BMW 85 325e

08/27/2008 2:09 AM

I'd recomend you have some more coffee and read my post again (for the first time).

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#20
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Re: Idle Surging on Old BMW 85 325e

08/26/2008 11:43 AM

Great answer, GA to ya'. My experience with Herr Bosch's fuel injection systems of that vintage (I have an '87 Jetta with CIS-E with 240K+ miles) is that you can get vacuum leaks around the injectors that result in the idle hunting you describe. There is a really simple way to find if you have a small vacuum leak. Gently flow (not with the DeWalt pressure washer please) water in and around the injectors while the engine is idling. If the idle settles down when you flow water around the injectors, chances are the O-rings have lost their seal because the water will temporarily seal the leak if it is small. For about $10, you can get four new O-rings and the installation only takes about 30 min depending on how much you have to remove to get at the injectors.

Good luck with your quest.

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#24
In reply to #14

Re: Idle Surging on Old BMW 85 325e

08/27/2008 1:00 AM

I'll chime in once again... If your're also having some odd symptoms with your cooling system, you're looking at a bad head gasket.

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#17

Re: Idle Surging on Old BMW 85 325e

08/25/2008 12:57 PM

No expert here but ...

TPS (Throttle Position Sensor). I've had a similar problem on one of my cars and the 25$ piece was all that cause it. I think it has something to do with regulating the air/fuel ratio.... worth looking at.

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#21

Re: Idle Surging on Old BMW 85 325e

08/26/2008 4:21 PM

Throttle position sensor.

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#23
In reply to #21

Re: Idle Surging on Old BMW 85 325e

08/26/2008 4:58 PM

The transitions from curb to fast idle would be indications of transitions from closed loop to open loop engine control, respectively. Based on signals from throttle position sensor, open-loop engine control (rich fuel mix & EGR-closed) is set at curb idle and WOT (wide-open throttle) operating conditions. At partial throttle (PTCRuise), the engine is under closed loop control with monitoring of fuel mixture and open EGR.

These open-loop/closed-loop engine control transitions are also caused by AC cycling...so that open loop momentarily provides sufficent rich fuel mixure when AC compressor cycles on in order to prevent engine stumbling.

Sluggish or misaligned TPS will cause just the kind of inexlicable "surging" you speak about. It is be expected that the TPS will eventually fail altogether, leading to a no-start condition...as I recall, failure of throttle bypass is the reason. Sorry, but TPS replacement is not inexpensive; nor can it be diagnosed under the shade tree; nor can TPS be replaced and aligned as DIY amateur effort.

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#27

Re: Idle Surging on Old BMW 85 325e

08/15/2014 3:42 PM

It's the IDLE AIR SOLENOID controller located behind the glove box. Most likely it's a

failure of the solder joints.

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