My '89 Iroc Z is a TPI 305 5 speed car with about 203,000 miles. The car is completely stock except for a Hypertech chip which was installed in 1990. The plugs, wires, cap, and rotor were all recently replaced. The car runs and accelerates perfectly, hot or cold, with one exception. When the car is fully warm, like after a 25 minute drive on the freeeway, at idle, there is a slight intermittant misfire. If I turn on the A/C, the misfire will go away. Also, if I disconnect the connector with the tan wire that you disconnect when you are setting the timing, the misfire also goes away. Does this sound like the Electronic Spark Timing (EST) module is not doing its job and needs to be replaced? Is there any other dianostic procedure that should be performed? Any other ideas on how to cure the rough idle when fully warmd up?

 

Anyone???????

 

there's some type of vacuum from the A/C I believe. I know I looked into this before but I just CANNOT remember exactly what it is. GAH I'll do some searching and see if I can find it.

 

It sound like timing. Try pulling a couple of degrees of timing, and see if that helps. If you prefer, you can take your ignition module in the distributor to your local NAPA and have them check it for you.

 

Interesting comment on the timing.....The manufacturer's specification for this engine is 6º BTDC, but I normally set it at 2º BTDC to avoid pinging under load using 87 octane fuel. However, I did reset it to 6º BTDC just to see if it changed the misfire situation and it didn't. However, I noticed when the connector with the tan wire was disconnected while the timing was being set, the misfire disappeared. However, after the connector was reconnected and the engne was restarted, the misfire returned no matter if the timing was set at 2º or 6º BTDC.

Also, concerning the ignition module, this is the original module with 203,000 miles on it, but I discounted that as a possible faulty component since I can make the misfire disappear by disconnecting the tan wire used when setting the timing. Does this sound logical, or am I missing something? I guess it wouldn't hurt to have it checked out at NAPA.

 

It sound like the ignition module could be at fault. That is a very common symptom of a bad ignition module. Fine when cold, not when warm. Something else to look into would be the coolant temperature sensor, the two wire sensor in the front of the intake manifold.

 

By the way. If you do take the ignition module to NAPA, have them test it multiple times. You can't really get a good reading until the module gets nice and warm.

 

OK, I just got back from NAPA. The ignition module was tested as described above and it tested OK. The coolant temp. sensor tested OK per the resistance values at given temperatures as indicated in the service manual. Any other ideas or suggestions are welcome.

 

Test all these and then you can figure out what's wrong hopefully

 

Just replaced the module in my son's 85 Fiero GT last night. The diagrams/pictures above look identical to the set up on the HO V-6 in those cars. Anyway, the car was missing (when good and hot) when the rpm passed 2500. While running up and down the road in front of the house trying to trouble shoot it the car up and died. Checked wiring, tried old coil, then replaced 2 week old high performance module with old stock piece and it fired up and ran. The hi-po job was okay till the engine compartment got real hot (those fieros do with that cat. convertor sitting beside the engine.) Anyway, hope you get it figured out.