G'day from Australia, Guys
I've just joined and am new at this forum so please forgive any mistakes, etc. I was hoping someone could put an end to my suffering! My 86 C4 Auto L98 is running beautifully, except when idling at operating temperature. I've had it datalogged to reveal an unstable MAF sensor and oxygen sensor that runs <500mv (about 450mv). This only occurs in closed loop. Above around 1000RPM in open loop everything runs normal. At idle the MAF spikes down randomly. Any thoughts? Your assistance will be very much appreciated!

 

First off, welcome to the forum. The reason that your car runs better in open loop is because most of the sensors don't start feeding info to the ecm until the engine reaches 145 degrees and then goes into closed loop.Up until closed loop the car runs off a set of perameters in the ecm..This tells me one or more of the sensors is not working correctly. As far as the O2 sensor goes I would just replace it because its only a $35.00 item and should be changed periodically anyway. The maf sensor is a high dollar item and tests should be run before sinking that much money into something you possible don't need. What you need is a (Factory service manual), this has the test procedures that you need to run to check out the maf sensor. Alot of times its not the maf sensor itself, but can be the maf power relay or the wiring. .Also unplug and clean the connections on the maf sensor and the relay, sometimes these get corrosion on them from sitting for 20 years without moving..You should be able to pick up a used FSM on ebay or on one of the Corvette sites, or possible on here in the for sale section fairly cheap. You will need this book if your going to work on the car...WW

 

And as mentioned many times in here (searching fixed most of my idle issues) always go over the basics that cause this on any motor, the L98's are as prone as any motor to simple tuning issues.

Timing must be correct.
Cap/rotor, wires and plugs must be good.
Vacuum leaks including bad PVC valves, leaking brake boosters and malfunctioning EVAP systems.
Gunk and carbon on the IAC and its passages.
EGR valves that don't fully close due to carbon.

Those items seem to cover 95% of all the fixes I searched on this board. No point at all buying expensive stuff until all the basics are cleaned up and covered off first.

If it hasn't been done in a while, new plugs, wires, PVC and O2, all cheap and all part of a regular tune up. Then, using injector cleaner, spray into IAC inlet (center/low of air inlet at throttle body) and if possible, take the IAC off and spray the pintle and into the seat on the throttle body. At idle, the EGR is supposed to be closed, check the pipe going to it, if it's hot, it is sucking exhaust gas when it shouldn't, not so easy a fix though if it is. Set or verify your base timing, instructions are found here, search it out. Don't go for the 8d advance trick until everything is working right..

Try all that first, it may not fix it but I bet it settles it down a lot. May also be all that is needed. On mine (90 MAP) 5 different things were contributing, nothing but regular tune up parts were all I had to buy.

 

Thanks Guys for your comprehensive analysis. Very much appreciated. I've checked some of those items but not all so that'll give me some work to do this weekend! Cheers.

 

Hi
I have an 1985 but had erratic idle. I know the ECM is not the same, but never the less the reason for erratic idle may be the same.
I agree with the advice you have got, but there is another that might help you.
Take the MAF out. Buy theCRC Air sensor rinse or MAF rinse. This is in an aerosol bottle and is quite effective.. If the burnout realays are bad the hot wire in the MAF may be contaminated as it would be by using the K&N air filter with a little too much oil on.
Hold the filter upside/down ie with the electronic part up so that residual rinsing liquid cannot run into the electronics. Spray from the front end of the MAF on to the wire becomin hot during burn off when shutting down the engine. Do this thoroughly.
When I did it the BLM values went from bad to normal in the ECM and the idle became better. I had ensured that all other things were ok.
Normally I don't believe in what I call snakeoil, but the values in the ECM proved that the rinse was effective.
Just a little recommendation that may help a bit.

 

Thanks Jan-Erik. That's all good advice. I just did a full spray with MAF sensor aerosol cleaner. All the metal components in the MAF cleaned up a treat. I also checked the wiring connector only to find a broken clamp that didn't securely hold the wiring connection. Sprayed the individual contacts as well. Seems to have made some improvement. I'll check the rest out on the weekend. Thanks again guys you've all been very helpful!