1991_Z28

Early January I did some work to my Z28. I swapped the stock cam with a ZZ4, had the stock iron heads and intake manifold ported, I put in 1.6:1 roller rockers, and I modified my fuel pressure regulator and turned it into a homemade adjustable one. Anyway, after getting it put back together and everything, the idle isn't very good and it has stalled at low RPM's. It doesn't feel as powerful as I thought is should have, either...it actually doesn't seem much different than stock, so I hope that's related. It idles around 500 rpm now instead of 750, and like I said, it's also stalled at low RPM's (below 1250). Sometimes when it gets hot it will start to fluctuate between like 400 rpm and 600 at idle, just kinda bouncing back and forth. When it's cold, it's almost right. Additionally, the air doesn't work at all. Vent, defrost, heater, A/C, none of them are blowing anything at all through the vents, with the exception of when I started it up this morning it was back to normal for about 15 minutes and then it went away again. I've checked all the vacuum lines according to diagrams and pictures I took before the work, and it all seems correct; still, I can't help thinking it has something to do with engine vacuum. Maybe it has something to do with my homemade AFPR? Could it be leaking around the bolt I put in? I almost forgot to mention, when it's been running and I turn it off, sometimes it won't start for like 5 minutes or something. Acts just like it's flooded.

Okay, enough of my rambling. I'm sorry I made such a long post, but I haven't been able to figure out what's wrong for about a month. Thank you in advance.

Admin: I posted this in TPI, too, so if this is in the wrong spot, I apologize.

Vader

1991_Z,

I would tend to agree that at least part of your problem is a lack of vacuum. What is the vacuum at idle, anyway? The lack of any heat or A/C controls is a good indication of a vacuum problem.

I seriously doubt that the FPR vacuum hose would pass enough air to drop the vacuum that much, however. It's going to take a lot larger (or more) leak to do that. But disconnect it and cap the connection just to be sure. The extra fuel pressure isn't going to be a big issue at idle for right now, so there's no harm in trying it.

Check all the other vacuum lines by disconnecting and capping them as well. Don't forget to temporarily try the brake booster and cap the PCV valve as well. The EGR is always another suspect when vacuum is low, as well as all intake and plenum gaskets. Don't overlook the throttle body, and make sure the MAP sensor is getting a good source of vacuum.

It sounds like the IAC is trying to control the idle speed at least, or the idle wouldn't change with the temperature increase. This is a good sign, since it means the ECM is at least alive and trying to do its job.

If you are having trouble starting it after five minutes of hot soak, try flooring the accelerator while cranking, and get ready to release it as soon as the engine fires. This sets the ECM into a "clear flood" mode and prevents any fuel injector pulses. If you are flooded, this can help. Another thing to do is to check the fuel pressure at idle, then watch the pressure after you shut off the engine. Pressure should remain steady for at least 15 minutes before any changes. Any faster drop in pressure could indicate a leaky injector(s).