91-Z28-L98
01-02-2006, 05:40 PM
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, 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.