85 Idle Search problems - no codes

[OSU-VETTE]

1985 corvette - idle problems. Hi, I am out of ideas, I have changed the TPS (voltage perfect at .054 idle, 4.0 at WOT), the IAC, new throttle body last year, new Bosch MAF this year, and cannot get it resolved. The car will not idle when cold, and will search when warm in Drive and drop low enough to die. It will throttle up to 900+ down to 350 and die and all in between. Performance and mileage are good at speed, any thoughts to what I am missing? It is an auto with a/c and cruise, all work well. This just came up last summer and I did replace the burn off relay 2 years ago, hoping and praying not it. I changed the TPS over the weekend since a code tripped saying the low side was faulty, no codes now after I disconnected the battery and drove it ~50 miles. Just the idle problems. Thanks

 

[sstearns2]

.....

Have you changed the PCV valve? It's cheap and easy. Have you checked the resistance on the fuel injectors?

Cheers,
Scott

 

[OSU-VETTE]

PCV valve is good, it was new 2 years ago and shakes well, may give it a shot though, like you said it is a cheap fix if that is the problem.

I have not checked the FI, not sure I would know what to check, is it individual checks or a common feed? Any help is appreciated, thanks.

I also should add I have not changed the temperature sending unit, that was a suggestion in another thread, the IAC is only a year old, although it is possible that went bad, but again, no codes are thrown, at least none with my cheapo scanning tool.

 

[Hib Halverson]

When the idle is unstable, it means the engine controls are incapable of controlling the idle. When it won't idle cold that means the open loop cal probably can't supply the right amount of fuel.

When the engine is warmed up and it's not stalling but the idle is unstable, what are the IAC counts and fuel trims doing?

Also, have you checked for a vacuum leak?

If you rebuilt or replaced the throttle body did you perform the "minimum air" adjustment?

Have you cleaned the IAC pintle and seat?

 

[aklim]

voltage perfect at .054 idle, 4.0 at WOT

Isn't that a little low for WOT?

 

[aklim]

Have you changed the PCV valve? It's cheap and easy. Have you checked the resistance on the fuel injectors?

Cheers,
Scott

While ohming the fuel injectors is a first step, I would certainly suggest having them sent out for testing. DVOM will NOT tell you if it is spraying right.

 

[TedC]

OSU-Vette, sure hope you mean .54V at idle. I agree with aklim that 4.0V at WOT is on the low side. ECM doesn't engage the enrichment mode based on some preset voltage, but I like to set WOT at 4.3V+ using the gas pedal. One L98 I worked on was set at 3.9V. We trashed that TPS and the new one came in at 4.5V. This vette with 2.59 auto could now burn rubber. There is some play in the TPS position, and the trick is to increase the arc to reach 4.5V+. Mine is set at 4.6V. Reference voltage is 5.0V. I would measure each injector for resistance. They all should be close in ohms. Again, I agree with aklim and that this is only a static measurement and doesn't indicate proper fuel flow. I've worked on enough L98s to know that owners mess around with the min air speed screw thinking this is an idle speed adjustment. Then the L98 runs like crap. You can search this site for my instructions how to set it. Best to adjust using a scanner so everything is set properly based on what the ECM is seeing. TPS voltages and a properly set min air speed are key in having a good running L98. I would check fuel pressure at the rail. You should check the ECM temp sensor to make sure the resistances are correct. If you don't have a factory service manual. I would start looking on ebay.

 

[aklim]

Reference voltage is 5.0V.

I was just thinking of that. Could it be that his reference voltage is FUBAR and he isn't getting the whole 5.0V? Maybe 4.5V and that might explain how he is able to get .54 at idle and 4.0 at WOT?

 

[aklim]

I like to set the TPS last.

How is the timing? Is it more than 6 degrees BTDC until you remove the advance wire? After that, it should drop to 6 degrees. If so, your timing is correct. Reconnect the wire.

After that, I like to set the idle using IAC counts. I shut off all the draws to the motor like the AC, radio, etc, etc. Only engine runs. Read the IAC counts. Target count is about 15-20. More counts means the IAC has to open up more to compensate. That means you have to turn your idle adjustment screw in. Less counts means it needs to be closed so turn your idle adjustment screw in. Each time you adjust, you rev the motor up and let it settle down.

After that, I like to adjust the TPS to 0.54V

 

[OSU-VETTE]

Wow, CAC comes through as always. I have a lot to work on this weekend now, another thread reminded me I never set the IAC after I installed it, just slammed it in with the new throttle body and went to town...

I will be back with updates, thanks to all for the insight

Jeff

 

[TedC]

More counts means the IAC has to open up more to compensate. That means you have to turn your idle adjustment screw in. Less counts means it needs to be closed so turn your idle adjustment screw in.

I hope aklim will accept my correction in bold...
More counts means the IAC has to open up more to compensate. That means you have to turn your idle adjustment screw in. Less counts means it needs to be closed so turn your idle adjustment screw OUT.

 

[OSU-VETTE]

Does anyone have an image of the idle adjust screw? I have searched and the one on my throttle body has a screw that has seizing compound on it and I don't think is correct (I do have an aftermarket, cannot recall the brand). Will take a picture this evening when I get home.

Thanks everyone, looking forward to attacking this problem over the weekend.

Jeff

 

[aklim]

I hope aklim will accept my correction in bold...
More counts means the IAC has to open up more to compensate. That means you have to turn your idle adjustment screw in. Less counts means it needs to be closed so turn your idle adjustment screw OUT.

NP. Sometimes I type faster than the brain can go.

 

[OSU-VETTE]

Many, many thanks to everyone again for all their help, I (finally) got around to adjusting the idle today. After I adjusted the IAC trick, then I did get the TPS to go above 4 volts, holy crap what a difference at WOT. Never been able to turn the back tires, now I have to let off to go, huge diff.

Thanks again, she is running like a top!