Paul G

My problem is if I crack open the throttle the engine does not rev up right away. There is a slight hesitation and sometimes a backfire from the intake. This is warm or cold. What I have done so far is Check base timing, @6 degrees. Set TPS to .57 closed, 4.76 WOT, TPS gives an even voltage rise / drop with throttle movement. Cleaned the throttle body and IAC pintle and seat. No engine codes are present. Car seems to drive okay down the road. I hate the hesitation, and the backfire means something is wrong.
Idle is around 700 to 800 RPM. I am open for suggestions. Thanks

emoshn

It is probably something more sophisticated than a bad ECM, but it sounds like your advance is not advancing and after the intial idling RPM, you are running slightly retarded.

Also, I was having an idling problem and occassionaly running bad throughout the RPM range. I had the engine "Auto Vac'd". This cleans all things related to fuel: fuel rails. injectors, IAM, TPS,valves, pistons, etc. It made a TREMENDOUS difference in my vette (92 LT1 roadster), car runs like new, if not better. Smooth idle, smoooooth acceleration throughout the entire range...and it really JUMPS! I strongly suggest this even if it proves not to correct your particular problem. Up here it cost $99, probably cheaper where you are...money well spent.

Bat Man

I too get the hesitation and backpressure (gun shot noise) at start up.

I think I might have found a small solution. Today I had my car on, and reved it to 1.5k for about a minute.

After that all the hesitation was gone.

Wonder what is wrong? some times, I will floor it when it is like this, and car falls on its face. Not good when you make a turn with cars coming..

Paul G

Checked a few more things tonight. Pulled the plugs, AC 5's, all look new, were gapped over .040. Set the gap to .035. Checked codes again. Had a code 42. Haynes manual says EST circuit. Check connector between ignition module and ECM. The connector at the ECM looks fine. Where is the ignition module? Then it says replace ignition module, replace ECM. What a big help that was. Took the car out for a short drive and rechecked codes. No codes.
Sounds like that "auto vac" might be something to try before replacing parts that might not be bad. Looking for more ideas if anyone has any.
Thanks, Paul G

Steve85

Paul G,
Your TPS sensor can read O.K. when you move it slowly but may be missing the signal change when moved quickly. For example if you just touch the test leads on your DVOM (set to ohms) together for a split second it won't react, not enough time in that position to get a reading. Same thing in TPS sensor it may just be "worn" enough to read different levels, just not as quickly as before. Hence good signal at idle, none as it sweeps briskly from idle to anything off-idle, and signal again after staying at any given throttle position (or moving slower).

Your problem sounds as though the rest of the engine has no idea you just opened the wings on the butterfly . This is the job of the TPS.

I have been out of this line of work for several years now so you might want to double check my info before throwing parts at it. Hope this helps and best of luck.
Steve
85 6spd
90 BMW 325ix AWD

wetzel

Hi Paul,

I have an 88 that I bought 1 year ago that has the same problem. I have replaced all 8 injectors, new fuel pump and filter, new ECM, new wires and plugs, removed and cleaned the throttle body, and replaced the TPS. Nothing has corrected the hesitation when I floor it. Sometimes it is not all that noticeable and others the car just sits there doing nothing and I release the gas for a second, then give it gas again and the car goes. I have not replaced the MAF yet as it costs a fortune and if it does not fix the problem I will be even more unhappy that I am now. There must be someone out there that knows what is causing this problem as I am not the only one it is happening to. I am so unhappy with this Corvette that I am selling it at a huge loss considering all the money I have sunk into it to try to fix this hesitation issue. Can someone PLEASE give input as to what the problem is and NO it is NOT a dirty throttle body and NOT the TPS.

PeteL

Could be lots of things.

Best advice is to get a scanner on it and find out what's wrong - otherwise you could spend a lot on parts guessing.

Fuel pressure test and vacuum test would be a good idea as well (tape gauges to windshield and note reading while driving).

Some items to check (all on the road): TPS, MAF, spark advance, engine load, fuel pressure, engine vacuum.

Most likely culprits include MAF (incl false air?), TPS, Fuel filter (fuel pressure in general), vacuum leaks, ignition (advance) problems.

The code 42 was from when you checked the timing (the brown wire).

Good Luck,

Pete

Mace

You may not want to hear this but my 85 did the exact same thing. At WOT the thing would backfire like crazy,but if I went steady it ran fine. The cam was bad.
Good luck
Mace

Togo

may sound stupid, but i am gonna replace the oxygen sensor(s) on my '85, if that don't do the trick, then i will replace the MAF with one of those hi-po granatelli's, and if that don't fix it, then i am gonna put a new motor in mine!

wetzel

I actually forgot some items that I have replaced/checked already. I have replaced the Oxygen sensor, EGR valve, checked for Vacuum leaks, timing good, never any stored codes. My CAT is also cleaned out. I actually do not have the backfire thing happening, just bog or hesitation that can be anything from not happening to my foot on the floor and the car not going anywhere. My 88 Trans AM ran to 220,000KM without any problems and when I sold it last year to buy this Vette of the same year I never thought Chevy would do such a poor job of putting the car together compared to Pontiac. Once I sell this expensive experiment I will be going back to a Trans Am.