I have a '91 stock Corvette, automatic trans, approx 74,000 miles. All of a sudden I have a problem with acceleration. The car starts fine and idles perfectly. When accelerating (slowly) there is hesitation at normal slow speed driving and delayed shifting. Once at speed there is also some lag in acceleration. Going up a moderate grade the transmission will shift down (under load) No codes are being set Local mechanic has replaced the EGR, knock sensor and O2 sensor with minor improvement. He substitued an ECM with no change in results. This is driving me crazy!

The mechanic thinks my car is a 49 state emissions car since I bought it used in Ohio. Would this change a diagnosis?

I'm new to the forum and apprecite any help offered.

 

have you checked ign timing? your tps is not adjustable but you can check it. fsm lists many things, MAP, injectors, plugs.

my 91 was set at 0 base timing when I firt checked it. it ran fine, but when I set at 7, the accelleration was noticeable. be sure you follow the instructions on the "emissions control info" label on radiator shroud.
I belive it would be easy to have the ICM advancing timing, and you would set it back, effectively retarding base timing.

no idea what your mechanic is talking about.

joe * forgot: also lists an open circuit in 453, "ign reference low" wire from 4 pin connector at distributor, the red/blk wire. ck for open of connection. also plug wires leaking to ground. it's a stretch, but be sure teh tan bypass ecm wire from dist is shielded away from any plug wires or the high voltage can upset the ecm.

 

Here kitty kitty........

 

Hi Ellis,

I think you have a plug wire/ignition problem.

Using scan software will give you a MUCH better picture of what's going on. I can help you with that.

See ya.

 

Retarded is no longer politically correct, what you have is mentally challenged timing....
dws

 

There is nothing different about your Vette - they all came with same stuff (as have all cars have for the last 30 years or so - will this folklore ever die?)

Scanning could help, but I think you start with the old fashioned stuff first - like checking compression and doing a leakdown test paying particular attention to cylinder 7 which is where this Year gives it up first, though the '91 was a hell of a lot better than some of the earlier 113 headed motors.

Ok - what happens when you accelerate? In simplistic terms, the TPS signals the ECM that it's getting more air and the computer opens the injectors a little longer to meter in the precise amount of fuel. The fuel system (pump, filter and lines) is designed to provide more than it needs, but it can''d do that if the filter is plugged up and if the filter has never been changed, I'd swap it out.

If that doesn't fix it, TPS's rarely if ever go bad, but injectors do, so to find out if they're doing their thing, you need to test them. They need a certain amount of resistance across the coils to have enough oomph to open and close and that's usually in the range of 15 to 17 ohms so get out your DVM and measure the resistance across each of the injectors terminals hot and cold. They need to be the same under all conditions.

If that checks out, there's a possibility one or more are plugged up. I've never had any luck with over the counter remedys including 3M's port injector cleaner which is fed directly into the fuel rail. Nor have I had much success energizing each injector (NAPA sells a tool for about 70 Bucks that lets you do this) and measuring the fuel pressure drop that happens when an injector is opened for milisecond. It's not that the tool doesn't work; it's the fact that the numbers on fuel pressure gages are so small you can't really see what the drop is! In the distant past, some enterprising members removed the rail with the injectors attached and with a graduated beaker under an injector, energized it and measured how much it squirted - that's a lot of work. I have had luck with Dealers using whatever they use to clean these things and I've had success by just starting over with new set of cheapo Accels, but you might as well at least check out the electrical part before you drop a few bills on new ones.