First a few specs:

1993 Z28 LT1 A4

155K Miles, Around 30K on plugs and wires..
155K Miles on OPTI, coil, IAC, ICM

Car runs GREAT when its cold, and even when it gets up to operating temperature. It will run fine on short trips on the highway, or stop and go traffic. However, after a longer trip on the highway (1 hour long) and then preceding into stop and go with lights...the car will stumble, sometimes even stall (but then come right back on). This typically happens under normal acceleration and higher. Between 1800-2400 RPMS seems to be when it does it the most.

Also sitting at a light with the car in DRIVE...my RPMS will pick up and go back down, and pick up and go back down. IAC???

Anyway, it seems obvious to me that some electrical culprit is getting to hot on longer drives and malfunctions. I know many would be quick to say the OPTI, but I would assume the car would run pretty bad all the time if in fact something was seriously wrong with the OPTI. Lemme here what some of ya'll got to say. Thanks.

 

Look at the O2's.

Wait, it's a 93. That one have 1 wire or 4 wire O2's?

 

The opti all beit a weird beast, still uses conventional type rotor and cap. Any rotor and cap should be replaced within 2 years. Therefore your lack of proper tune up procedures is not helping matters. In other words, although it may not be the direct cause of the problem, it certainly will exaserbate it.

If you were to go to a mechanic, the first thing they would do is properly tune it up. That would include cap and rotor (if they don't insist on replacing the whole opti), new coil since they are considered tune up items. While they are there, they will notice the condition of the wires that connect to the opti as well as the wires at the coil and ICM and those in between.

The O2s should be replaced regularly since they become lazy. The TB, EGR and IAC should at least be cleaned. New fuel filter should be put on and the old one torn apart to see what's in it. (good indication of what's in the tank).

The plugs should at least be looked at to see their condition. Don't forget to look at the air filter. Then after all that is done, let's see how it runs.

 

O2's are fine. Throttle Body has been cleaned religiously. EGR is gone and blocked (did this a long time ago). IAC was cleaned about 8K miles ago. I think I will just go buy a new one and see if that helps at all.

I did look at all the plugs and wires, plugs look great...wires are fine. The air filter is K&N cold air setup, just recently cleaned out the filter.

Fuel filter has about 10-15K miles on it. I stay up with all the maintenence on this car. I have not changed the cap and rotor. I would not consider the cap and rotor a "tune up" regular maintenence thing that needs to be done every 20-30K miles. However, it could totally be time for a new opti. I just would like to weigh my options before sinking 300 bucks into an opti when it could be the coil or something else.

Cory

 

I had somewhat similar problems with mine. I thought for sure it was the opti on its way out or something electrical such as the wires being burned through.

Turns out it was the #8 fuel injector that was bad.

I would go get it scanned by a good mechanic.

 

I havent ruled out something fuel related....but it really seems to me that its ignition related. Thanks for all the replies guys.

Cory

 

did you oil your k&n? if so you may have over oiled it. might want to clean your maf and see if that helps. on another note I am having the exact same problem and cleaning the maf didn't help me.

 

Think about the components that are vulnerable to heat soak... IC Module, Opti optical sensor, coil, injectors, and most wire insulation, particularly when the wires are old.

I've never heard anyone say the Opti cap and rotor should be changed every 2 years.

 

Injuneer,

Im definitely taking all of those into consideration here. I think I'm going to start with the IAC module as maybe that will fix my idle problem when sitting in traffic. From there, maybe I will try a coil or something. Does anyone have Jason's contact information to buy a new delco opti? Is he still the best guy to purchase an opti from. Mine would need to be non-vented. I have ruled out anything fuel related as my fuel pressure seemed fine.

Caminiti,

Thanks for the reply, but I would never forget to oil up the K&N. That isn't the problem. Also, my car is speed density...does not have a MAF.

Cory