This is gonna be long winded but I will try to go into as much detail as possible so you guys can give me an informed answer. I still am having problems with my idle and I cannot figure out what is causing it. If its not entirely obvious I have a 91 Z28 with the L98 engine. With the IAC plugged in it runs a little rough but the RPMs usually stay within 50 RPM of 600. With the IAC unplugged it goes between 500 and 800 RPM at pretty steady intervals of about 1 second.

Alright, now for what I have already done or checked:

Fuel system- I have already replaced the pump, filter, and FPR and I got some used injectors. The guy I got them from said they were 19 lb/hr out of a GTA. I thought all GTAs were 350s with the 22 lb/hr injectors, but who knows. Fuel pressure was good last time I checked, which was a month ago and the same problem still existed.

Sensors- The O2 sensor is less than a year old but I had some severe fuel system issues (hence replacing the pump, filter and injectors) so its possible that it is fouled. I don't think this is the source of the problem though because the problem exists in open loop as well. The TPS is set correctly and does not stick, jump, or anything else abnormal so I assume it is good. I cleaned out the MAP but I have no idea how to check to see if it is good or not. I have no fuel related SES lights.

Emissions systems- The EGR and charcoal canister are both blocked off. I do get an EGR related SES light when cruising on the highway for like 10 minutes or more. The AIR system I have not checked or disabled in any way.

Other info- All vacuum hoses are in good shape but I don't know for sure if there is a vacuum leak at any of the gaskets. I have reused the plenum and runner gaskets twice in the past year so maybe thats an issue. I did a check for misfire by removing each plug wire and checking for a reduced RPM and all cylinders checked out good in that respect. The plugs, wires, cap, rotor, and coil are all less than a year old. Timing is set to 6 degrees above top dead center and is stable. There is no unusual color or smell to the coolant or oil so I believe the head gasket is good. The IAC seems to be good since when it is disconnected the idle gets far worse. My car has no mods except for the throttle body coolant bypass. I have the dual cat exhaust... could a single clogged cat cause my problem?

I plan on doing a compression check soon on all cylinders and while I'm at it I will replace the spark plugs again. If anyone has any other suggestions on what I should examine or replace please speak up. I am just stumped at this point.

 

Alright, I know that its a lot to process so I'll start a little more simple. I think that I have an incorrect fuel mixture caused by either a vacuum leak below the plenum or an incorrect MAP sensor reading. How can I tell if the MAP is reading incorrectly?

 

Preliminary results on the compression test (cylinders 1-5) showed all cylinders between 190-200. I'll check the other 3 when I get a chance but I don't think this is the problem. The new spark plugs on these cylinders showed no change as well.

Also I forgot to mention earlier that I have cleaned the IAC, intake, and throttle body. That may make a difference.

 

I'm starting to think that my problem lies in a crankcase air leak. I think this because I get no idle change when I remove the PCV valve from the valve cover but I do get a change when I plug the PCV valve with my finger. Also, I am less than confident that I adequately sealed the intake manifold to the block when I replaced the intake manifold gasket. I'm guessing this added air is causing me to run lean (The spark plugs were all a little white also.) Is there another way to test this before I reseal the intake manifold to the block?

 

Wow just wanted to follow up seeing if you fixed your problem

 

WARNING! Do this test at YOUR OWN RISK. Don't ask me how I know...

With the engine COLD, start it up. Spray some carb cleaner, or brake cleaner around the intake manifold base, listening for a change in idle. If there is a change, it IS leaking from the area you sprayed. BE SURE ALL YOUR PLUG WIRES ARE SECURE AND IN GOOD CONDITION ALSO. A FIRE CAN RESULT IF THE ENGINE IS HOT AND PLUG WIRES ARE ARCING. PLEASE BE CAREFUL. HAVE A FIRE EXTINGUISHER AT YOUR SIDE.

P.S. I know cause a arcing plug wire casued a small fire in my engine bay, that I almost had a coronary over, but luckily escaped with only a melted vacuum hose.

 

if it stays at like 550 rpm that is the cause of your rough idle and in my opinion perfectly normal. on my 91 rs(i know different engine) at a stop fully warmed up it idled at like 450 rpms and cause a slight vibration. i asked about this and was given the following answer. the vibration is caused by a imbalnce in the engine caused by a low idle. in other words at higher rpms the engine is "more balanced" and at lower \rpms the engine is "less balanced". id just consider it a design flaw and not worry about it. my 89 iroc idles at like 750 rpms and still causes a slight vibration. if its really bothering you you could alwas increae the idle. again imo ist perfectly normal.

 

I refer you to this post for any updates on my status: http://web.camaross.com/forums/showthread.php?t=448952

 

Have you checked the IAC ajustment? Just because it's working doesn't mean it's working right. It may just need ajustment to bring it into specs.

 

And actually its more like 500-950. If the IAC was my problem then my idle without it would be more stable. As it is its the only thing keeping my car idling at all.

 

to see if you have a intake leak in the lifter vally, pull the PCV valve from the hole to the crankcase, and pull the breather tube off the valve cover, plug one hole, and put a vacuum gauge in the other, start the engine, let it run, if the pressure starts going up, pop the gauge out quick, you don't want the pressure over about 2 psi, but if it starts showing vacuum, time to pull the intake