'91 running oddities
 

I have a '91 coupe that runs fine until it is shut down warm, though there seems to be a more or less constant very minor miss, and some surge at idle. When restarted (warm) --sometimes-- it runs very rough. Replaced plugs, cap/rotor and wires, and cleaned throttle body, before whioch I had a TOO RICH code; that code is now gone. All checks out well except that #2 cylinder is putting out low power (but injector is working and compression is fine)...nor, incidentally, does the check engine light work (maybe owing simply to a bad bulb). I'd much appreciate any advice about working towards a possible fix?

John Sutherland
Midlothian, VA

RE: '91 running oddities
 

Sounds like a leaking injector. Pull your plugs and see if one is more gas fouled than any other. You need to have someone run an injector balance test. Tha will reveal a bad injector if you have one. Runs rough on warm start, rich code set, all sounds like injector. Is your Vette black smoking @ idle? [8D]

RE: '91 running oddities
 

Thanks much for the reply. The car runs great under acceleration and at speed, so the problem now seems to be confined mainly to idle. In any case, I'll check the plugs and see if any stands out. But I don't know how to do an injector balance test, or even what equipment is required. But we did determine that when the injector is fired manually, the fuel pressure drops from 40 to 20. The "service engine soon" light is in fact lit at start-up, so it works. As of now, it seems like maybe the best thing is to go ahead and get a new injector (and seals and manifold gaskets) and hope that will be a fix. I'll let you know if that solves the problem.

RE: '91 running oddities
 

Well, doing an injector balance test require a injector pulsator and fuel pressure gauge. The tester must be able to fire the injector around 50 times, you have to turn the key off between each injector then turn it back on to pressurize the fuel rail so you can get consistant readings. [8D]

RE: '91 running oddities
 

We’ll try the injector balance test you suggested next time we get a chance to work on the car. In the meantime, we did use on-board fuel injector cleaner, to no real effect. The car still runs fine cold, but goes to pieces –very bad miss and power loss-- once it gets warm/hot. This afternoon it quit altogether once it got hot and would not restart for some time. Eventually got it to limp back to the garage, but not until it had cut off another couple of times. Fuel pressure gauge does not show consistent pressurization when ignition is turned on (sometime it goes to 45, other times it does not move at all). Fuel pressure now seems to be leaking down, whereas before it held steady at 40. We'll try isolating a bad injector(s) again, though all were working previously, and when we fired the one we thought was bad we got a fuel pressure drop from 40 to 20, so it was probably delivering fuel.Is it usual for an injector problem to appear only on a warm engine? John Sutherland [Midlothian, VA.]

RE: '91 running oddities
 

How long did you pulse the injector and how many times? Injector pulse is measured in milliseconds, around 3 - 4 milliseconds per injector @ idle. If you have no way to accurately pulse injectors, then it will be hard to tell. About an 8 pound drop is about average for about 50 pulses, but the important thing is all should be within a half of a pound of each other. [8D]

RE: '91 running oddities
 

I'll see if I can come up with an injector pulsator this coming weekend. In the meantime, do you have any suggestions about the possible sources of the "warm" running problems...again, the car idles and runs very well when cool.