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E30 Throttle Randomly Cuts Off

Discussion in 'E30 (1984-1993)' started by az3579, Apr 7, 2008.

    • Member

    az3579

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    I've searched high and low for a solution to this problem. I have visited several mechanics, one of them a certified Tech-Inspector for the Driver's School at Lime Rock. So, here I am, $2100 later, and still have this rediculous problem.


    My E30 has developed a nasty habit of cutting throttle response while driving. What happens is to the same effect as accelerating and lifting your foot off the throttle suddenly and completely. This is what happens, except that I am still accelerating. I could be driving along and suddenly, the throttle would cut off completely for a few seconds, and naturally, the car would jerk because my foot is still on the throttle when it finally resumes responding.

    When the throttle cuts off, the engine doesn't stall immediately. It would slowly rev down as though your foot wasn't on the accelerator at all until it reached below idle speed, afterward it would stall. This is a more extreme case. Most of the time, the throttle would cut off for a good second or two, sometimes as much as 5 seconds, before finally responding to throttle input again.

    When the engine revs down, to prevent stalling, I would downshift (letting out the clutch slowly to rev the engine back up) successfully, so as long as I have gears to downshift to, it wouldn't stall. When it stops responding, I usually floor the accelerator to try to get some response, but usually to no avail.



    Things that have been looked at/replaced by the shop I took it to include fuel pressure tests (they have ruled out the fuel system), some leak was fixed at the transmission (I think this is unrelated), and they have replaced the spark plugs as the auto parts store gave me the incorrect ones the last time I did a tune-up and the ones from the parts store were prematurely fouled.




    I've reached the bottom of my patience barrel and now I'm starting to really worry. I'm going to the Driver's School at Lime Rock on Friday, April 11, and I would hate to lose out on the Driver's School for the second time in a row due to some kind of vehicle-related issue at the last second. If somebody has SOME kind of information for me by tomorrow, that would probably help because I'm taking it to another shop tomorrow. If I miss out again, I'm gonna shoot myself; $2100 was dumped into this car already and that is way over my budget. I just want this problem GONE.



    Thanks in advance.

    -Botond Pal


    EDIT: Some possibly useful information:
    1987 BMW 325e, aftermarket air filter, chip tuning (~140hp), 5-sp manual, custom exhaust
    otherwise unmodified
    • Member

    Brian A

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    I am not an automobile technician. I am not sure if I am just advocating voodoo with this suggestion. Take it skeptically …

    I had the similar problem in my 87 325i. It would lose all throttle then a few seconds later would go full on (if I had my foot on the gas) then cycle again to full off …etc.

    My car wasn't holding fault codes so I decided to change my Service Indicator batteries in hopes that I could capture a fault code to give some clues what was going on. Magically, after changing the batteries, the problem has not returned!

    This may have just been coincidence! The back of the instrument cluster has an output labeled "pulse", so maybe there is a relationship to the two somehow. I don't know.

    Since you are about to shoot yourself, I thought I'd let you know. Hate to see a nice car lose a good driver.
    • Member

    az3579

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    Give me your name and address and I'll have the car shipped to you before I go. ;)


    Strangely enough, I have just replaced my instrument cluster with a used one and have also replaced my SI board with a brand new one that has soldered-in new longer-life batteries. This was back in February; I have gone about 2-3,000 miles since then. I don't know if this problem existed then, but it sure seems like it hasn't happened until after I replaced all of the above-said parts. I would try to swap back my old parts, but I have a hard time letting go of my current functioning odometer and fuel gauge. Besides, the problem occurs so randomly, I don't want to be going for such a long time without a working odometer or fuel gauge in the mean time.

    I've tried a method for obtaining fault codes via the Check Engine light. The post says that it's for 1988 and on models, but I thought I'd try it anyway. It didn't work; my check engine light just keeps flashing constantly (because of my brake lights; one of them isn't illuminated at night but works fine when braking -- any fix on this, by the way?).
    Here is the link to where I saw this:
    http://www.bmwpower.ca/forum/viewtopic.php?p=51959&sid=012be03f507f0fe45f5305610e380784
    • Member

    ctexter

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    I have an '88 325is that had what may be a similar problem. The engine would essentially shutdown (tach to 0 revs). Sometimes it would come back while still rolling, sometimes I would end up pulling over and have to restart. I replaced the main power relay and haven't had this problem since. Some say these relays deteriorate over time and will become intermittent. It might be worth a shot and shouldn't be too expensive to at least try.

    Chris
    • Member

    az3579

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    The difference here is that the engine revs don't go down to 0 immediately; the engine slowly revs down and then stalls when it finally doesn't have enough revs to stay "awake". This is a rarer occasion; most of the time it just revs down while it doesn't respond and then comes back to responding within a few seconds.
    • Member

    MGarrison

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    you might try an ecu swap, if you can find a spare from someone or a shop to use. If you shop for one (ebay, perhaps), make sure it's the same as what you have.

    I'm not quite sure about the setup in the later 'large tail-light' 89'-on E30's, but I think the earlier tail-lights are all the same, presumably yours are identical to my '87 325is. If so, the brake lights and running lights are separate bulbs. Pop out the light panel (from the inside of the trunk) and check the bulbs and connectors; if you're getting water leaking into the taillights, there might be some corrosion evident.
    • Member

    az3579

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    That's really unfortunate that you think it's the ECU; a while back, a friend of mine had a spare ECU for a 325e that he offered but I refused. This was over a year ago and the problems just started not more than 2 months ago. Darn.

    I put in a performance chip that I bought off of eBay that added a nice bit of oomph (~20+ hp). Do you think this might be a cause? If so, what would just cause that particular chip to malfunction after half a year of use?

    What makes this particularly hard to figure out is that it hasn't acted up in almost a week, making it impossible to replicate for someone that would fix the car. This is frustrating to no end. I really hope that it just works on Friday and that I don't have to take the chip out of the ECU box; I really do enjoy the extra juice it gets, even though it's still not nearly as much as the 325i engine...


    As for the brake lights, there are only bulbs for the reverse lights, brake lights, and turn signals. The same lights that are illuminated are the ones to light up when the brake pedal is pressed, therefore I would assume that I only have one brake light bulb per assembly. I know it's not the bulb because it lights up just fine when braking, it just doesn't illuminate when the lights are on but not braking. I checked the connector for the assembly and didn't see anything unusual about it (dry as the desert). The bulbs didn't have significant corrosion nor did their sockets. The lights do have some moisture in them though as I see from the outside, but this always has been the case and this hasn't been an issue 'till now. Strangely enough, this issue arose after I have new mounts tapped to hold the battery in place for the Driver's School. It's not worth me bringing the car back to the shop that did that just to spend another $100+ dollars for them to fix something stupid, yet I feel that may have had something to do with the problem because of the work near the battery (which is in the trunk), which just happens to be next to the particular light in question.
    • Member

    MGarrison

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    The ecu is just a guess, something to try. I wish I had enough insight to tell you the most likely causes. As you well know, unfortunately problems like this are difficult to diagnose because it could be attributed to any variety of the car's systems. Swapping an ecu is just an ez thing to try, hardly takes a minute, if you have a known good one in hand.

    The idle circuit in mine was bad, changed most of the stuff that was idle related before trying a different ecu - woulda saved myself all that had I tried that first.

    Chip? it's a possibility, but I wouldn't suspect it as a very likely one - and I doubt you'd have to give it up. If there was an issue, if you got it from a Roundel advertiser, they'd probably exchange it for you or work with you somehow (presumably).

    Considering you've had work done around the battery, I'd say doublecheck what they've done - maybe pull out the trunk trim around the battery, make sure your ground wire is solidly connected to the battery and body, and check that no wires were pulled or nicked in the course of the work. See if you can identify which wire in the tailight cluster connector is for the bulb you're trying to trace, might want to see if you're getting the right voltage w/ the lights and brakes on, if you can determine which wire.

    Here's a realoem pic of a taillight cluster from a 325e production date 4/87 (pick the correct date through the realoem site)

    http://www.realoem.com/bmw/showparts.do?model=1254&mospid=47308&btnr=63_0193&hg=63&fg=20

    It shows 5 bulb sockets - if my memory is correct, the far left is the turn signal, the far right is the backup light, the center one that lines up w/ the clear plastic section in the middle is your running light, one of the remaining ones is wired, which is the brake light, and the other socket isn't wired. On Euro cars, that's for the required-in-Europe rear 'fog' lights, but it's not wired on U.S. cars.

    So, I think there's a few possibilities -
    1. You're missing the running light bulb, as the brake bulb is not supposed to light except when the brake pedal's pressed (try sticking a bulb in the other sockets, see what ya get).

    2. If you are finding your brake light bulb's on when you have the lights on, and going brighter when the brake pedal's pressed, you have a dual filament brake light bulb, and the taillight is wired to work that way (either originally, or modified by someone at some point in the car's history to work that way).

    3. You have a dual filament bulb installed that's supposed to be a single filament bulb, and for some reason is operating as is typical for American cars.

    Oh ya - if you only have sockets for the brake lights, turn signals, and reverse lights, then it would seem that surely it's supposed to be that way - would seem wholly illogical for someone to actually remove a couple of light sockets, their wiring, and rewire the taillight cluster for dual filament bulbs. Possible, but seems unlikely, I don't know why anyone would bother to do that!

    Well anyway, best of luck w/ resolving these issues, particularly the intermittent running one!
    • Member

    az3579

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    I appreciate you trying to help. Only thing at this point I can do is hope that the shop finds the problem, as I don't have a spare working ECU on hand.

    I don't know if this has any relevance, but my idle control valve doesn't work, so I have it disconnected. In its place, I use a plumbing valve mounted inline with the ICV and have it set to control the amount of air manually (as a result, I have to keep the revs up for a minute so it doesn't stall while first starting). When my ICV is unplugged, the car doesn't seem to stall but instead just do its regular 'hesitation' thing that I described before stalling. When the ICV is plugged in, it usually gets to the point of stalling. Perhaps this narrows it down a bit?

    MissingParts guest

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    Try checking the connector for the fuel injector harness it's on the left side of the motor under the intake manifold.Start the engine and wiggle the connector if the engine starts to cut out thats your problem . The other possibillity is that your return line is partaily blocked .Your fuel tank may have rust inside it that is blocking the line(it would not normally be an issue at idle but could become one at speed).
    GOOD LUCK

    Bob Balbirnie guest

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    I see that you mentioned the fuel system has been ruled out but I have heard of a few cases now where the fuel pump intermittenly stops working and then after it cools down (random times from seconds to minutes) starts working again. Similar explaination of symptoms to what you have described. If you can get the problem to appear when you are stopped try a light tap on the pump to see if it restarts. Barring that you may want to change it out with a known to be good pump and see if the problem goes away (I know these are expensive little devils but may be worth a shot).

    Cheers, Bob
    • Member

    az3579

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    This thread is over two months old. The problem has long been resolved. I guess I forgot to post it in here but I have mentioned it in other threads.

    So, for the record, the problem is resolved. Overview:
    I have a leak into the car (still have it) and it seems that water gets in behind the rear seat. The wires leading to the fuel pump have corroded and as a result the car wouldn't even start after a while. The wires have been repaired and these problems have never resurfaced. Now my problem is finding the leak, but that's another story for another day.

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