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Strange Engine, ABS, and braking behavior on start.

Discussion in 'E30 (1984-1993)' started by Tyjosimo43, Jun 9, 2019.

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    Tyjosimo43

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    So it's been a long time since my last post and a lot has happened since. Now I'm working on a weird issue that I can't quite put my finger on. Here is my best try at explaining it in detail.

    1. Start engine. Everything is okay and normal. Good idle, no roughness.
    2. Immediately start driving, again no issue.
    3. Then I hit the brakes for the first time. This is when the weird stuff happens. The engine starts revving up like crazy to 5k and the brake pedal is super stiff the car sort of stops but does not have the same brake pressure as it does 99% of the time. Suddenly, you feel the pedal give a bit and it settles into normal feel and the brakes work just fine and the engine goes back to normal, you feel the ABS kick on at the same time and flutter. This usually happens once or twice each time you start the car. But sometimes not at all If I start it and let it sit for 5 minutes and I press the brake pedal in the parking lot. When I start driving for about 5 minutes and its done the weird thing, then it completely goes away for the rest of the drive. If I'm driving and turn the car off and then on again when its warm it seems less likely to do it.

    Here are my initial guesses. I've heard of issues similar to this being caused by a vacuum leak or a bad master cylinder (or both?) and I also suspect the ABS pressure regulator because the issue seems connected to the ABS. The only thing that throws a bit of a wrench into that theory is the fact it only happens on start.

    So there you go! Please send help! Weird brake things give me the creeps! Thanks everybody!

    - Tyler

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    MGarrison

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    This is an off-the-cuff oddball guess, but confirm your motor mounts & transmission mounts are solid & intact & not just a bunch of oil-saturated, deteriorated goo, or broken. If they did have major problems and the motor was shifting, presumably the fan would be into the radiator, but, the motor mounts are at least aren't too bad to check visually.

    But, yes, the brake pedal suddenly getting hard would be consistent with a vacuum leak, and there's numerous things related to the lines running to the booster that could cause vacuum leaks. Important to make sure all junctions/hoses/connections are tight, & in good condition. Speculatively, the ABS could be related if, in having to press the brakes hard and suddenly they're boosted when before they're not, in effect might be the same as if you were braking as hard as possible, having the tires trying to lock up, & then triggering the abs. It's pretty simple to disable the ABS system to check that part of it, if memory serves all you need to do is unplug one of the ABS sensor wires, I think at least one of the front ones is right at hand under the hood along the sides ahead of the shock tower. Going from memory here, although I could double-check the Bentley manual here, just don't have it at hand. Generally, a bad master sounds plausible - google up symptoms of a bad master brake cylinder.
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    • Technical Service Advisor

    charlson89

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    Yes, check for vacuum leaks first. Makes sense that on start and driving off the vehicle isn't able to to have all sensors up and running to compensate for the air leak. But if you sit and let the vehicle warm up the sensors will. You spray some brake clean around the intake and vacuum lines on a cold engine and when it revs up that's where your leak is. ABS could be activating from the issues with brake pressure changing. I would fix the first issue and see if the ABS problems stop.

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