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Author Topic: Odd Idle Under Braking Issue  (Read 79 times)

Offline weitrhino

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Odd Idle Under Braking Issue
« on: 06:04:55 PM / 04-May-19 »
Last summer I replaced the head on my blacktop CA20 with a reconditioned head from a pro headshop.  Since then I've had an unusual issue with the engine dying, at cold engine temp, when I brake hard enough to really shift the weight forward.

Scenario:

Start up in the garage and roll down the street toward stop sign. If I drive up like normal and step on the brakes, RMPs drop to zero and kill the engine. If I disengage the clutch and roll up to the stop and brake lightly, stopping slowly, then there's no problem. Usually by the time the engine has warmed to temp the issue goes away, usually.

Additional factors:

Many times if I turn on the blower the RPMs increase just enough to sidestep this issue, usually.

Idle is often irregular regardless of engine temp, even from one stop to the next. It may vary as much as 500 RMP.

What really puzzles me is the braking momentum. It's clear that when weight is shifted forward the engine has a much higher propensity to stall.

I simply haven't been able to puzzle my way through this one. Has anyone had a similar issue? Does anyone have insight to the cause?

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Online JonB

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Re: Odd Idle Under Braking Issue
« Reply #1 on: 06:45:32 PM / 04-May-19 »
Sounds like there's some vacuum leaks going on there.

Offline rednucleus

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Re: Odd Idle Under Braking Issue
« Reply #2 on: 08:53:47 PM / 04-May-19 »
Vacuum leak in the brake booster??
After you start it cold, if you are sitting still in driveway and hit brakes hard, will that kill it as well?

Offline weitrhino

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Re: Odd Idle Under Braking Issue
« Reply #3 on: 08:31:50 AM / 05-May-19 »
Thanks guys. I’ll go over all the vacuum lines today. As far as braking goes, it’s really been my observation that the forward weight shift has been required to cause this problem. Simply sitting still with my foot on the brake hasn’t been an issue.

Offline kelso840

Re: Odd Idle Under Braking Issue
« Reply #4 on: 08:55:22 AM / 05-May-19 »
Yeah, I'm going vacuum leak or sticky idle air control valve. You could also manually raise the idle of your car 100 or so RPMs which is essentially what's happening when you turn the AC on.

Be careful conflating the momentum shift of your car with the reality of what you're actually doing to create that sensation. You're loading the cold-ish engine when you approach a stop sign in a stall scenario, and then slam on the brakes (>50% pressure)-- big vacuum draw as RPMs are already dropping. It's likely unrelated to weigh shifting, more the brake pressure being applied.   
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Offline Thatmint200

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Re: Odd Idle Under Braking Issue
« Reply #5 on: 07:26:02 PM / 10-May-19 »
sit with the car idle and warm then quickly and with a lot of force pump the brakes. If you can pump it till the car dies or you notice a SIGNIFICANT change in idle, you have a vacuum leak. If this doesnt change your idle, then something about the momentum is causing it to die. To me it sounds electrical as if something is making contact, then not making contact once you are pushing momentum forward. Check your ECU is securely mounted to the chassis as the CA and KA ECU's get ground signal through the ECU body itself, not just through the ground wires.
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