I have a 89 Corvette that has developed a rough idle once the engine warms up. Vehicle has 43000 miles. I have replaced fuel pump, fuel filter, plugs , wires, pick up coil, cap, rotor , TPS , IAC , O2 sensor, ESC module and ECM. Problem remains the same. Took it to the dealership and they had their "best guy" spend hours trying to find the problem. They could find nothing. Ruled out injectors , vacuum leaks and all the easy stuff. When you unplug the Set timing connector the miss goes away and the vehicle idles fine. No codes .

Only thing I know left to replace is the PROM and the MAF sensor.

Anyone had similar problems or any ideas.

Any help would be greatly appreciated.

 

You need to hook up a scanner and see whats happening when it goes into closed loop. Did the shop do this?

 

What did they do to rule out the injectors? Sounds like you replaced everything else but the MAF and EPROM. Have you scanned it for what the IAC counts are like on a warm idle? I'd let you use my ECM but it wouldn't work and neither will the EPROM but someone should have those and a known good MAF to test.

Before I throw those things you did at it, I'd dump the injectors and get rebuilt ones that are cleaned and tested. You never know if the pintles are dirty unless you bench test them

 

..... Keep throwing new parts at it , you will eventually stumble on the problem and maybe fix it .....

..... FYI ... There is a diagnostic flow chart in the Factory Service Manual that might prove to be quite helpful ... I'll bet a 12 pack of your favorite brew that the dealer's "best guy" did not have access to the correct FSM .....

 

Same exact problem on my 89, started at 43000 and got worse as time went on. Checked and replaced everything, lots of money, numerous times I thought I had fixed it. Even bought scanner (mt-2500), checked out test tools from auto zone, could not find problem. Finally found it, when it got worse by pulling plugs. Found number seven to be wet with water. Blown head gasket.

 

Thanks everyone for the posts !

The GM dealer assured me it was not the injectors and that the IAC was doing what it should. I don't think that they had the exact diagnostic scanner needed due to the age of the car. I think the car is older than the guy who worked on it ! But I told them that I had checked the Ohms(hot and cold) on the injectors and they were within specs. They said they had done some other kind of test on them and they were fine. They are not the original injectors.

Again the car runs fine other than the rough idle and that goes away when you disconnect the Set timing connector. Would a bad MAF sensor cause this ?

I am new to this forum and any and all posts are appreciated.

 

Age of the technician is irrelevant. Ohm test is just about as irrelevant. Ohm test only tells you the coil is in our out of spec. How clogged the pintle is and what your spray pattern and volume is a guess until you pull it out and put onto a bench. What sort of test have they done? is a proper test. Without that test, you have no clue if the injectors are spraying as far as pattern. is another part of the test where it tells you if the volume is correct IOW, do you have 24pph injectors or do you THINK you have 24pph injectors?

It could be the MAF is bad however, before I go that route, I would try borrow one from someone in your area to test. Swap them and run them both. If the problem goes to his car, there is your answer.

If you send them to FIC and they say they are Multec, they can be rehabbed but I'd toss them. Ethanol eats away at the coils so dump them for the Bosch injectors. Get a reman set. My testing shows they get dirty within 3 years of use so that is about how long before I send them in for cleaning and testing.

Get yourself a Factory Shop Manual and a scanner. FSM can be had on ebay for your year. Get a scanner and NOT a code reader. You want to be able to see the data your ECM sees. With a scanner we can guide you through the testing. Without, we can guess.

 

You like many keep your location secret. Look for help from local corvette, f body, or impala ss, fleetwood club for help finding someone who knows what there doing. Without a scanner, FSM, and most likely a few more things, your unlikely to DIY. it sounds like all the easy fixes other a MAF have been taken. Disconnecting set timing connector fixes timing, ecm can raise idle to set value. Were any repairs done or did happen prior to rough idle?