Have a 93 c4 starts ok run's ok but when it warms up for about 15 minutes it does not want to idle, motor starts skipping. ran the idle up and down with scanner and worked ok no codes are showing up in computer scan and i checked fuel pressure it was 42 lbs car has 27,000 miles on it anyone has a guess on what it might be

 

bad o2 sensors? Need new plugs and wires?

 

Two items to check. IAC and Opti. Iac won't throw a code, and a scaner will only show what the ecm is telling the iac to do, Not what the Iac is actually doing. The Opti, whis common, may just be getting old and needs to be replaced. I would do plugs and wires first. See if there is any change, then change opti. It may have few miles, but it is a crappy design and old. The iac, take it out and test it with a ohm meter, look up shbox iac test and there I a drawing showing what to test, ad whbile it is out clean it with sensor safe cleaner. Do not try to turn it etc, it is very fragile.

 

opti is that near the water pump? pump has to be removed to replace correct?

 

So it idles ok cold?
You say you got a scan tool, no codes right?

Ok, hook up the tool and obverse the Open Loop/Closed Loop status on the data monitor. If it idles ok in open loop cold, but after it warms up into closed loop then idles bad, this will be a major window into what is going on. Please report back your findings.

Also, Cheese and Crackers for the love of Pete, don't just throw parts at it, any monkey can do that. Diagnostics, saves you money. Just take it one step at a time and keep good notes.

 

Yes

 

93 Rubie, I am telling him to diagnose. You have to start somewhere. The open closed loop routine isnt going to tell him anything. He has no codes.
If the O2's were bad, he would get a code, as the O2's arent used during warm up, which in 93's is 90 seconds to when it will be in closed loop.
I have owned my 93 since 96.
IACS(no codes, none, never will, but they drie up the plastic worm gear and/or electrically crap out) and Optis are very very common. Rarely get codes from an Opti, but they do die, and they die often. Washing a motor, not running very much.....

 

i changed the water pump once what a ( p i t a) my back still hurts, anyone have a weightless room i can use?

 

My statement was a general one not directed at anyone in particular, I just see to much of that and not enough diag. around here.

Also, I disagree with the OL versus CL thing. If the car is ok, in OL, then we can think that fuel, spark, air, compression related things are ok, Opti-spark included, obviously heat is causing something to run afoul. The question is what...

Also, the operation parameters from OL to CL are much different. Yes, primarily 02 sensor feedback, however, just because you do not have codes, does not mean a problem does not exist. These OBD1 cars do not throw codes unless something is very, very wrong. They are not as code happy as Post-96 OBD2 stuff.

This just came to mind, if heat is causing the issue to arise, I have heard of lots of ignition module failures. Might want to check that.

Also, ridewot, does the car run ok hot, just not idle right? or not run ok either? The more we know the more we can help.

 

car runs good for about 15 minutes then acts up skipping and wants to stall in gear i did hear a pop out of the lift exhaust pipe once. i have a gm tech that's helping me with the problem he thinks maybe a coil or an injector coil breaking down when it heats up. thanks for the help i'll keep you posted.

 

Hae you checked the ohm readings on the fuel injectors they should all be very close to each other mine was 12.5 ohms.

 

i'll check good point

 

Ridewot have you figured out what your problem is yet?
My 94 has been doing the same exact thing for a few weeks.
Changed the MAF sensor and O2 sensors, car drives a lot better and smoother but still after about 15 minutes the car starts to erratically idle, almost like its missing. Even on the freeway it roughly accelerates and almost tops at 80, if i try to go any faster it misses.
Car runs perfect up to this point.
Any Luck on yours? I think it might be the same?
My next bet is to check the IAC & EGR valve?

 

Hi after many test a gm teck says its the distributor there is a service bulletin on it. the motor runs good for about 15 min, then starts to skip and a little pop thu the exhaust. but its a real pain to change it you have to remove the water to get to it.

 

The Optispark? damn I was hoping it wouldn't be that.... Would you by chance happen to have the link for the bulletin?
My car does almost the same as you are describing...
thanks again

 

check the ignition control modual (above the coil) when these warm up they can do weard things that when benched tested (when there cold) test fine. oh, 93's OBD1 don't throw codes unless its an emission problem or your car is going to blow up, OBD1 blows.

Oh yea, get some cold water and spary it or hold an ice cube on it when it starts acting up (don't get your opti wet), ICM's are not cheap.

 

Thats a great idea, thanks Drew1Down.
I have recently noticed that the ignition coil and cooling fins are covered in oil and gunk, Used carb cleaner to clean the fins but not sure if that made a difference.

 

np, its a little bit of work to get it off (1 nut and 3 bolts if i remember correctly) but the icm and coil come off together and i cleaned them up when i did my water pump and opti (made it ez to put the water pump on with it removed too)

i read on another post that someone used washers to space the ICM off the block to allow better air flow and so the motor doesn't transfer the heat directly like it does being in contact with the head.

 

Thanks for all the help...
Im gonna do that tomorrow instead of changing the TSP and IAC valves ill go ahead and remove the ICM and give it a good cleaning.
Do you think the problem in the ignition coil is because of a broken or loose wire or just whn it gets too hot it starts degrade?

 

don't change (it costs to much) test, most parts stores will test the part your replacing.

You can back probe the TPS by sticking a needle in the back of the wire thru the rubber insulation (mind you, this is with it still installed on the throttle body and connected) bottom wire ground, middle wire signel (correct me if im wrong on this people) and with a volt meter with out touching the throttle you should be inbetween 0.3 to 0.6 volts and when you move the throttle SLOWLY the voltage should have a steady climb to when you reach wide open throttle you get 5.0v. ANY drop in voltage at any point when your moving the throttle indicates a broken connection (it works like a light dimmer switch, potentiometer).

The test for the IAC is a different story, you need a scanner to read the "counts" or "steps" as this is a little motor that allows mettered air into the intake. our cars read them from 0 steps to 132-136 steps, 0 steps is closed off (no air passing thru the IAC) 132 steps is wide open allowing air to enter, thou it may not be this high, if these "counts" change when at idle, you can be surre its working correctly. The Tech2 can do an IAC test where you can manualy control the IAC counts and raise/lower the engine RPM's.

Testing the ICM is hard, cooling it off is the only methed i would use if i were you as your probems happen when your at normal operating temps. so to me something is getting hot and failing or wanting to fail, or sending bad information to the ECM (computer). you can take the ICM in to a local parts store and they can test it. But a cold failing ICM would cause hard/no starts and not telling the coil to spark.

the order of ignition in our systems is the opti tells the computer where the engine is and it tells the icm when to spark and the icm tells the coil to spark that sends the spark to the opti distributor to the wires to the plugs to the plug gap ground.