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Engine stumble - real puzzler

Discussion in 'E28 (1982-1988)' started by JPuglisi1954, Jul 27, 2008.

    JPuglisi1954 guest

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    My 1982 BMW 528e has almost 300,000 miles on it. Mileage has begin to really decline. It was running way too rich and had a serious hesitation. Moreover, after running hard (highway speeds) it would not start again until it sat for a while. The engine will crank and fire up and then stall immediately. When it does manage to catch and run it sounds like a diesel, chugging and chugging until the idle finally gets up to normal. One final clue - at speed on the highway when I hit a bump the engine stumbles.

    Now, I have replaced:

    • Idle control unit
    • Idle control valve
    • Cold start valve (eliminated the run too rich problem)
    • Fuel pump relay
    • air flow meter
    • air filter

    Have checked readouts on ICU with meter. All check out.

    Note: when I remove the vacuum hose from the fuel pressure valve, the engine idle increases significantly. If I plug the hose with my thumb, the engine idle comes down low again.

    I am at a loss to know what to do next.

    Amateur Joe P :confused:
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    jimoreno

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    Your stumble when you hit a bump sounds like one of two things: either a bad electrical connection in the engine compartment (ground straps, battery cables) or bad ECU solder joints. I would go over each electrical connection (ground straps too!) and remove, clean and re-attach them including the connections on the starter and alternator. After you are done with the electrical connections, the ECU is the next stop so check this link for info.
    If you haven't done it, replace the main and fuel pump relays and clean and re-tension the fuse box connections.
    Good Luck!

    JPuglisi1954 guest

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    Thank you ...

    .... for such a quick and complete response. I will make an effort to track down electrical connections. I have replaced the Fuel Pump relay. In fact, I notice a green wire there which does not connect to anything. It is a ground wire (green) that comes from the small collection of wires in the receptacle for the Fuel Pump relay.

    Would this electrical or ECU issue result in the hesitation too or do you think I am fighting on two fronts?

    Smooth depression of the gas peddle and engine revs climb nicely. Quick tap makes the engine choke and then rev up. Would this be electrical?

    Again, thanks for the help. After 26 years of owning this car this is my first real foray under the hood. I am having fun --- or was until I got stumped. Hopefully the cure won't be beyond my beginner skill set.

    jP
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    mooseheadm5

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    I suggest you check fuel pressure. One clue is that the mileage is getting worse. Does it ever blow black smoke? That would tell me the fuel pressure regulator may be bad. Was there any fuel in the vacuum hose you pulled off the regulator? If so it is definitely bad. If not, I would try a different ECU (actually I would try this first because it is easy to do if you can borrow one.) My E30 325e did this sort of thing when I first got it and it turned out that on bank of injectors was getting hung wide open. Previous owner replaced $1400 worth of stuff trying to figure it out. Replacement ECU cost $50.

    JPuglisi1954 guest

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    Yes and No

    Yes, it did blow black smoke. In fact, when the problem of stalling / restarting was severe, it would really chug and blow black smoke.

    No, there was no fuel (that I noticed) in the vacuum hose when removed from the fuel pressure unit.

    You referred to the "ECU" which I assume is the "brain" located in the glove box. Pulling that out and replacing it seems like a big job to me. Perhaps I should replace the fuel pressure regulator first?

    Thanks for your suggestions.
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    jimoreno

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    Do you have a Bentley maintenance manual? If you intend to keep and work on this car yourself do yourself a favor and get one. It is THE book for troubleshooting everything in our cars.
    As for your problem, I would say that yes you are fighting on two different fronts unless the electrical problem is screwing up all signals to the ECU. The ECU is held in place by four 10-mm screws and one huge big plug has all of the connections.

    Good luck!

    JPuglisi1954 guest

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    Bentley Maintenance Manual

    ... has been a godsend. I agree it is terrific at showing how to test and repair or replace. It was among the first things I purchased when I started to take an interest in maintaining the car myself. (After that I started spending money on tools and parts)

    If you have been tracking along, you see the other posts here. I am going to order and replace the fuel pressure unit. As soon as that is done I will report on any change.

    This weekend is when I inserted the new fuel pump relay. The ride in this morning included a few bumps in the road but there was no stumble.

    The hesitation and poor gas mileage continue.
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    jimoreno

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    Does it stumble when cold? Check the Thermo Time Switch for proper resistance and O2 sensor per Bentley. Other possibilities are a leaking Cold Start Valve (CSV) or injectors.
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    az3579

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    Just for reference it isn't a big job at all. It can be accessed in all of 5 minutes. All you have to do is remove the glove box portion and unscrew a few screws on the ECU. The Bentley outlines it in a very small section (that's all it needs).

    Open the glove box. It is held on by two straps on either side (left and right) and little pins that hold the box to the straps. Those can be pushed out by pinching the little arms on the inside portion and pushing the pins out through the hole. Be sure to support the glove box so it doesn't crash to the ground when you unpin the second strap.

    JPuglisi1954 guest

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    Got it !!

    Yes, I already replaced the ICU which is in the glove box above the ECU. I just found it difficult to work in that confined space ... not that working under the hood is easy for me.

    I won't take it out unless I have a replacement in hand so this may also take a few more days before I will get to it.

    Thank you and all the others for providing suggestions and guidance. :)

    JPuglisi1954 guest

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    Just to be clear, I use the term "stumble" to mean a severe interuption in power, a momentary pause in the engine which happens when I hit a bump. It is infrequent and did not happen this morning.

    The hesitation is a chronic problem where hitting the gas results in a pause, almost a choke like response, before the engine begins to take off. If I depress the pedal slowly, the revs will increase smoothly. This problem does seem to be related to engine temperature since it does not happen on cold start but after warning the hesitation becomes more prominent. It is usually worst on exit from the highway.

    The CSV has already been replaced (see my original post)
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    mooseheadm5

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    You should also test the resistance of the ECU temperature sensor. It is located at the front of the head. Let me know if you don't know how to do this. Basically, there are 4 things that commonly cause a too-rich condition on these cars:
    1-Bad engine coolant temp sensor for the computer (not the gauge.)
    2-bad fuel pressure regulator
    3-bad pressure compensation device. This is located on the side of the airbox near the air flow meter. It has a 3 pin plug and is a little black box about 1x1x3". Just unplug it and see if that helps.
    4-Bad ECU

    ECU takes 5 minutes to swap out. It is held in by 4 screws. If you replace the fuel pressure reg, DO NOT install it without some kind of lube on the O-ring, DO push it all the way in by hand, DO NOT use the bolts to force it in if it won't go. No time for the whole story now, but doing this wrong can cause a fuel leak, conveniently into the alternator and catch your car on fire!!!!!

    JPuglisi1954 guest

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    UPDATE - August 1

    Good news. I replaced the fuel pump relay and the bump/stumble has gone away. I drove the car about 300 miles over three days, hitting all the usual bumps in the road without one stumble.

    Bad news. An animal got up in the engine compartment and ate some wires making quite a mess. It wasn't until last night that I had the chance to replace the ignition wires and repair all the broken sensor wires. I also replaced the vacuum hose leading to the fuel pressure valve.

    Good news. The car runs. I didn't foul up the wire job or lose any ground. Still no stumble on bumps and gas mileage is better, but not nearly where it should be. Idle is still low and start is rough, particularly after the car warms or has run for a while. I have the new fuel pressure valve in hand and plan to install it tomorrow morning. I will post again next week after I have a chance to determine the status (assuming I can get the old valve out, the new one in and I don't break anything else.)

    Again, thanks to all of you who posted with pointers.

    jP

    PS: I found the pressure compensation component and the wire / connector leading to it. That wire is covered with grease. If the FPV does not cure my ills I know where to look next.
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    mooseheadm5

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    Just unplug the pressure compensation box. Unless you live in Colorado, you probably won't need it. The wires that got chewed are important. If one was the wire for the ECU coolant temp sensor it could have grounded causing an overly rich condition. The fuel pressure regulator may help. It certainly can't hurt unless you install it wrong.

    JPuglisi1954 guest

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    All chewed wires were repaired

    Once again, thank you for the instant reply!

    All of the wires including the sensors at the front of the block (temp sender, coolant temp, etc) were reconnected before driving. The control wire to the Idle Control Valve was damaged right at the connector so I cannot do much about that. Anyone know where I might get a new connector?

    I am pretty confident now that the relay was the culprit causing momentary loss of power to the fuel pump, hence the stumble. At least I hope that problem does not come back.

    The remaining symptoms are consistent with a fuel pressure problem as described by some of the respondents to this thread.

    How does the pressure comp device unplug? Do I just pull it out or is there a clip, screw or other fastener?
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    mooseheadm5

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    I think you can actually open that connector and resolder the wire since it is the early style. Check into it, you may be in luck.
    Fuel pressure regulator is held down by 2 bolts but seals with a fat O ring. Once you undo the screws, the thing just pulls out, BUT there will be pressurized fuel behind it, so watch out. Then to reinstall it, lube the new O ring and press all the way in by hand. Only then should you install the bolts.

    JPuglisi1954 guest

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    Okay, so after two hours of wrestling with it I managed to get the old fuel pressure regulator out. It screws on to the injector fuel channel and is held in place by one screw (bolt) and another large nut with o-ring.

    I installed the new one. I drove the car. It still has the hesitation. And warm starts have a very low, near stall level idle which works itself up to a normal steady low idle. :mad:

    I have not unplugged the pressure compensation unit. I am not sure I understand how this would cause hesitation? :confused:

    Perhaps the sensor is the issue? How do I test it? :)
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    mooseheadm5

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    Unplug the sensor. Just do it. It may not help but it rules it out.

    Your car will not idle correctly until you fix the wiring to the idle valve. Even after you do, it may still idle too low unless you adjust it properly by the Bently manual (or by guess.) Even if you do the idle control system may be bad. This is very common and not cheap to fix.

    If you do not have a Bently manual, go buy one. I can't possibly tell you everything in that manual.

    I would not hang any more parts on it unless you go through a proper diagnostic procedure. As a mechanic, this is the only way I can work on a car- go through step by step to find the problem. I would not have even put the FPR on it without testing fuel pressure first. Since you have, at least you have ruled that part out. Now you need to test the coolant temp sensor. Get a repair manual. It tells you the specs for your sensor and how to test at the sensor and at the ECU. You need to test both because the chewed wiring may be an issue for you.

    First fix the wiring to the idle valve. Then, to bump your idle, locate the idle control valve and mess with the screw to see if changes your idle speed to an acceptable level. Yours may have an externally adjustable screw or you may have to unhook it and trial and error adjust it from the inlet hole.
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    az3579

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    Or, to fix your idle (if you have a valve problem) would be to do what I did. I stuck a plumbing valve in to do the job of the idle control valve. I have attached two pictures so you have an idea: (the valve is located next to the master cylinder in the picture)

    [IMG]

    [IMG]


    Clearly it works because my idle control valve is pictured as disconnected. Upside of this is that when warmed up, it will idle at a fine-tuned rpm that you select. Downside is that if you set the idle too low, it will stall out when the engine is cold, so it makes tuning the idle to normal (around 750rpm) almost impossible unless you keep the revs up while the engine warms up.

    I have my idle set to 1000rpm warm and it's working fine.
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    mooseheadm5

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    I was actually going to suggest that if his idle control unit turned out to be bad. Also, you can take a 3/4" copper plumbing cap and drill a hole in it (1/8", I think) and stuff it in the hose. That only works if your car idles too high, though.

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