btr85vette

I've been battling a surge problem in my 85 for a while. If it idles for about 45 seconds or more, it gets a wicked surge that goes to about 900, then almost stalls, and repeats.

Thinking vacuum leak, I've spent a lot of time with my MityVac checking hoses and replacing them. I just got a set of injectors from FIC, and that didn't fix it. Off idle the car runs great, its just the idle that is bad.

Here's the fun part: If I pull the PCV line, or the crank vent line, the idle immediately evens out. Not great, but the surge is immediately gone. WTF? This should cause a monster vacuum leak, but it does the opposite. When I plug the line back in, the surge is back.

If I pull the FPR vacuum line and plug it, the fuel pressure is rock steady. I replaced the IAC a couple of months ago with a new acdelco unit, so that is new. The surge is there with the IAC disconnected also. I tried to set the idle, but because of the surge the car won't idle low enough to set the RPM correctly. Of course there are no error codes.

Any ideas?

leesvet

If it were me, I'd pull the throttle body apart and clean and inspect. Its very possibly an air leak. You prove that by opening the pcv and letting it gulp unmetered air.
Loping on the other hand is caused by irregular fuel injection. That can come from the TPS as well as the MAF where the ECM is searching for data. The o2 sensor completes that "loop" when its hot enough to generate a signal somewhere above fast idle.
These wire harnesses are TRASH and at 25+ yrs old they are almost useless. They have always had issues with insultion cracking and exposing the wire to cross signaling, and direct shorting. Carefully inspect the fuel injection harness and sensor cables for signs of wire rot...

Go thru the grounds AND the power source connections at the jumper post behind the battery. The ECM does not get its power nor any of the sensors or control system from the battery cable. Power to the electronics comes thru fusable links that do not all go to the fuse box and these all originate on the jumper post. Take that assy apart and clean each connection. I assembled mine dirty once and had a terrible misfire and stumble...cleaned those connections and it all went away...
These engines are controlled with sensitive resistance and control or reference voltage so dirty connections have a direct effect on what the engine does and what the ECM tells it to do.When your o2 sensor is trying to send a voltage of .02V and a value of less means a change in the injector performance, a dirty connection can mean the difference in whether it idles or not. Same for the TPS...the idle setting is .54V IIRC.
Thats pretty exact. Again, dirty or corroded connections make a difference.

take a good look at the pcv valve and all the lines, both rubber and metal. Throttle body gasket, and even the runner gaskets. You can use the propane torch method...carefully spray the propane around suspect gaskets and take note of any sudden engine surge. Don;t get TOO carried away...since a propane cloud might become problematic around a hot engine.

Very often its electrical. Once in a million yrs its something like a worn cam lobe or some other obscure mechanical issue. With these cars, its almost a sure thing that its a control system.

Yours sounds to me like its air leaking or NOT. The IAC might be new, but the passages are old and full of carbon. It has to seat to seal and it has to allow air to pass to maintain an idle. Pulling the pcv apart is loetting more air in, and it stabilizes...so that is saying that it may not be getting air when its all assembled. Pull the TB apart and clean and look at the IAC pintle for damage.

btr85vette

A question. I have long tube headers with the O2 sensor in the collector. Its unheated. Could that be a problem? I live in Tucson, so I don't imagine it would be a problem.

Cliff Harris

The O2 sensor will never get hot enough for the car to go into closed loop mode. You need a heated O2 sensor. That said...

Check for error codes:

http://corvettephotographs.com/c4vettes/codes.htm

Find out what they mean:

http://forums.corvetteforum.com/1576116036-post35.html

gunnerjuju

that is what I was advised to do when I had a similar problem and that not only fixed my idle issues but also mem/cal comm issue. @ least for the last 70 miles.

btr85vette

Believe me, I'm familiar with the error codes. The problem is none are thrown when it surges.

If I ground the ALDL, and the SES light flashes quickly, that means its in closed loop correct? Would that mean the its reading the O2 sensor output?

And I'm not forgetting everyone with the electrical suggestions. That is tomorrow night's job.

AGENT 86

Maybe the throttle blades are too closed.
Try turning the minimum idle screw in a bit, then reset the TPS idle voltage.

kimmer

A bad egr can cause this.

leesvet

I agree with kimmer....

Get your EGR vac hose and operate it by applying a vac and see how that effects the idle. Simple test. leaking EGR are air leaks.

Cliff is also making a good point. Your ambient in Tucson although uncomfortable for you is ice cold to the o2 sensor...its got to be heated IIRC a min of 600* before it even wakes up. When you use long tube headers that places the sensor so far away from the flame of exhaust that it only gets luke warm ex temps...so its always sending bad data to the ECM. That may not be all of your problem but its part of it, I can assure you. There are many write ups on wiring in a heated sensor to end this problem since relocating is not practicle. At the collector is the first place to set a sensor and thats too far south on long tubes.

btr85vette

I no longer have an EGR, but the car has run fine since it was eliminated.

I'll check on the heated O2 sensor. Would anyone know offhand a part number?

btr85vette

I checked for a jumper post behind my battery, and there isn't one. Does an 85 have this terminal? I know the 85's have some one-off things being the first year of the TPI.