Odd Idle Under Braking Issue

[weitrhino]

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?

[JonB]

Sounds like there's some vacuum leaks going on there.

[rednucleus]

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?

[weitrhino]

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.

[kelso840]

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.   

[Thatmint200]

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.

[weitrhino]

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.

I'm abandoning the weight-shift theory and chalking it up to a crappy idle since I replaced the head last winter. I had to replace the oil pan seal so while I had stuff off the car I removed the crank pulley, sanded it down, and painted it flat black. Then I painted the timing marks white so I could actually see them with a timing light. Nissan really could have done a better job with this initially. Anyway, the timing was waaaay retarded and now sits about 5 degrees BTDC. There's some improvement but idling is still inconsistent. It can settle in at 750-850 rpm then either still want to stall out at a red light or run over 1200.

One day I hope to have the time and gumption to take a deep dive through learning the procedure for diagnosing this. Aside from this stupid issue I'm pretty happy with the car.

[weitrhino]

So there IS a vacuum leak. Something got boogered with the gasket between the intake manifold and the head and it's sucking in unmetered air. I don't know exactly why but stepping hard on the brake
causes the idle to either go high when the engine is warm or low when the engine is cold. No doubt that's all part of the vacuum system.

I can't fix this now. I've just moved and don't have space to work until the garage is sufficiently cleared out, and it's a smaller garage now.

[kelso840]

Glad you got it figured out but it sounds like you accidentally moved to the wrong house with a smaller garage...

[weitrhino]

Glad you got it figured out but it sounds like you accidentally moved to the wrong house with a smaller garage...

You win some, you lose some. The new place has mountain views and an in-ground pool. I'll accept that trade for the space of one car.

[rednucleus]

The pool will be great after a hard day of wrenching in a small garage - sounds ok to me!!

[kelso840]

You win some, you lose some. The new place has mountain views and an in-ground pool. I'll accept that trade for the space of one car.

Definitely sounds like a win. Congrats on the new spot.