Hey Everyone,

What I have is a 1985 Corvette. L98 Engine with no mods.

I've been beating my head over this issue for quite a while now and the best way I can describe it is as a loop. Basically at idle, when it is most pronounced, the RPM's will loop where is jumps anywhere from 50 to 100 RPM's in a shot. 550 to 600, back to 550, up to 650, back to 520, you get the idea.

The worst part is that it's intermittently consistant. Basically I can reproduce the issue under certain conditions. For instance, on a cold startup it may do it initially before it's warmed up. After I get it up to temp it will be fine and when I park it will idle normally. Now if I shut the car off and walk away for a little bit, come back, it will start to loop shortly after starting it up again. Then it usually takes a little bit to get back to normal. Sometimes it does not get back to normal.

One thing I have noticed is that around 1100 to 1200 RPM I feel as if something is either missing or dragging behind a little bit as things fire. Again, a little bit of a loop that's consistent in pattern, but not as pronounced as it is when idling.

So far I have spent a considerable amount of money in replacing various parts that could possibly be causing the issue without much success.

The parts I've replaced are as follows:

• PCV Valve
• EGR Valve
• Idle Air Control Valve
• Throttle Positioning Sensor
• Spark Plugs
• Spark Plug Wires
• Distributor Cap
• Rotor

I've tried to clean the MAF but I have not replaced that as of yet. When I pulled out the spark plugs, two of them were slightly questionable. One of them, the insulator was kind of a burnt yellow color, and another one had a fair amount of oil around the threads of the spark plug. However there is no visible leak on the outside leaking down onto the plug. I'll upload pictures of each plug later tonight so you all can see each plug. Plugs have roughly 1000 miles on them.

Given this information so far, any thoughts as to what may be causing this loop?

Regards,

Vision

 

I think the correct term for your condition is a "hunting" idle. 550 to 600 rpm seems somewhat low. Maybe your throttle position opening is not open enough and the IAC cannot compensate for the lack of air entering the throttle body. Try opening that up a half turn or one turn. Is the throttle position sensor adjusted correctly?

 

Check for a vacuum leak.

 

seems a bit low check the TPS voltage value first.
Next, I'd remove the IAC clean it and the TB housing....then do the IAC reset proceedure....if that fails at that point I'd mess with the throttle stop screw.
Loose/worn throttle body bushing can also this issue by not allowing it return to the correct position allowing excessive air through once your off the throttle.

 

383vett, I'll have to check on the TPS adjustment. When I replaced it I tried to line it up according to where the old one was placed. Granted, that's not to say that the old one wasn't adjusted properly. I'll also try as you suggested regarding the opening not being open enough.

Paul, regarding the vacuum leak. Are there any particular spots that are common locations where the leak may occur that I can check first, before going through the entire system?

Regards,

Vision

 

The rubber hose where the pcv tube connects to the intake often gets hard and leaks. The plastic vacuum lines can crack aswell.

 

What would be the optimal value this should be set for? I've seen some say that when in gear, at a dead stop, it should be between 520-550 RPM. Thoughts?

Thanks for the input Paul. I'll check those places as well first.

 

Vision, you definitely cannot just eyeball the tps sensor position. You will need to use a voltmeter or a scanner to get the .5 volts that is specified. As far as a vacuum leak, that would normally result in an increased idle speed; similar to cracking the throttle open. Good luck.

 

At a minimum you'll need some paper clips and meter for this one. Voltage value should be between .52 -.63 volts with TPS in closed position....most set the value @ .54
A good "cold" engine target RPM idle # will be ~ 800-900 RPM on a stocker.

 

with engle1147. You can't eyeball, but have to use a voltmeter to set the TPS. It should be at .54 volts with the key on and about 4.49 volts when you push the gas peddle to the floor.

You replaced the IAC valve, but did you clean the passages under the TB that this screws into? To do this you have to take the TB off and carefully remove the screws on the underside of this piece holding it to the TB. Once you have it off, clean it with some TB cleaner or carb cleaner. You will need a new gasket. Did you also adjust the IAC valve when you installed the new one?

I think I would concentrate on these two areas first before hunting for a vacuum leak. Leak usually results in the idle going UP, not hunting like your problem description.

 

Thanks for the responses everyone. This helps out a lot. Should've come here sooner. I'm planning on attacking it this afternoon. I'll let you know how I make out.

Regards,

Vision

 

John,

Forgot to answer your question. I have not made any adjustments to the IAC since installation.

Regards,

Vision

 

Hey everyone,

Just wanted to say thanks for the advice. Once I got those two pieces tuned into place properly I was able to get the hunting to stop. Runs much better now. On to the next piece.

Thanks again!

Vision

 

nice; a success story!