RRT vette

Hey guys, I havent been around in awhile (few years). Got into different hobbies and such. My 86 has been running good and just replace the waterpump, opti, plugs and wire on my 95 that I haven't driven in a few years (still working on it).

The problem on the 86E: the car has been running great the last 5 year's or so after I got the kinks worked out. The battery died and I let it sit for a few months. I replace the battery and the problem shows up. It starts up good and idles around 500 to 600rpm. Once it goes closed loop, the idle goes to about 1,000rpm and sits at idle. It drops down when placed in drive but its ready to go.

Nothing else wrong (like missing, surging, ect). I've checked the TPS voltage, which is order, reset the IAC, and checked all the vac lines. I haven't used propane or starting fluid on the lines yet, tho. It feels like a vac leak, but why after the battery is replaced and only in closed loop? I got rid of all my scanning equipment so I don't have access to check the O2 sensor, but the SES light flashes accordingly in field sercive mode.

I did find a few dirt dobber nest around the engine bay.

Any help appreciated or just a chat with you C4 guys I haven't seen in awhile.

Cliff Harris

I recommend pulling the error codes.

The main difference between open loop and closed loop mode is that closed loop mode uses the O2 sensor (it's not hot enough to work in open loop mode). That would seem to point at your problem.

You might be running very rich and the ECM is compensating by adding more air. Is your fuel pressure regulator diaphragm intact? If it ruptures it will feed fuel into the plenum through the vacuum line. Remove the vacuum line to check.

RRT vette

I forgot to mention there are no codes showing. I haven't check the fuel pressure lately, but that will be next when I get time. I have an adjustable FPR that is not all that old, but stranger things happen. Thanks Cliff.

RRT vette

Hah, it's only been 2.5 years since I started this thread. That's about how long since I drove it last. I start it up regularly and shift thru the gears. I finally got the C4 bug again.

Now my 86 will start up but hunts for idle when cold. It will die if I don't feather the throttle to keep it going. Once it hits closed loop it idles fine and will run strong. I reset the IAC lately without help. Still wondering about vac leaks. I don't have a lot of time in my hands but plan on checking fuel pressure (I have an AFPR), checking timing, TPS voltage, vac leaks.

Without any codes to go by, it's a game.

I have a custom prom for my set up. EGR, cold start Injector, VATS have been deleted. I'm sure some other things I can't think of right now.

Any other ideas will be appreciated. Thanks guys.

billschroeder5842

Sounds like a CTS ---Coolant Temperature Sensor to me, You are displaying all the classic symptoms.

RRT vette

I will check that as well...thanks Bill. I replaced it it about 10 years or 500 miles ago as it were. I can't remember the ohms but i remember it being a code 15?? I will refer to my FSM when I get on it.

I acquired more knowledge then most people by being a part of this forum, but haven't used it in years. Trying to remember the tech stuff.

RRT vette

My timing was about 10 degrees off. I have it lined up and the hot idle did drop. The fuel pressure is steady at 39 psi. Going to wait til morning to see if the cold idle straighten out. Still no codes. I checked for vac leaks with starting fluid but didn't find any. Didn't hear any hissing.

aklim

If you set the idle speed for timing 10 degrees off, IOW base timing 16, I would think it would somehow affect the cold idle as well. Best to reset everything using IAC counts, ASSUMING you got the timing right, there are no air leaks and your tach (might be wrong too) is spot on which I highly doubt.

ex-x-fire

Check the grounds, especially the ones at the back of the heads. Loosen & retightening them is all that's needed sometimes.

RRT vette

I believe resetting the timing might have fixed the problem. It started up cold and hunted for a few seconds and then ran steady. I did reset the IAC and set the RPM at 450ish. I'm going to take it for a drive to see how it acts....in a little while.

aklim

How did you determine 450? The tach isn't the greatest. Until I had it recalibrated, it was a couple hundred RPM off and you wouldn't notice it.

RRT vette

The tach and audible exhaust noise was my main way. Anyway, I drove it around the block and it seemed fine other than the rust on the calipers. I cold started it about an hour ago and it stumbled once, picked up and idles fine. I think the timing was the biggest problem as it was off a bit. I will update if anything else occurs. Thanks everyone. Tune in to my next project. 95 that cranks but won't start.

aklim

I'm beginning to understand why people still cling on to the carburetor. It is more aligned to their nature to guess an answer than to really see what is going on.

All that means is the ECM is compensating. It can also shrink the available window of compensation when really needed. Think of a wallet with $50. You buy $20 of Twinkies and Ho Hos and you are left with $30. Should you need more than $30 to buy gas, well.....

RRT vette

The cold start was also after the battery was disconnected for 6 hours. In my learning the ecm takes time to establish it's parameters after battery reset. Sorry, I'm not a novice, but I've been out of mechanical knowledge for years (especially corvettes...since I haven't drove them in years). I use to tell people what their problem was on here when I worked one them. Is CFI-EFI still here with his rude precise diagnosis??

I have never owned a carburated vehicle.

aklim

I have pulled the fuse to the ECM but got no change in cold start. I have tried several times. I really don't know if it can learn much more than closed loop.