I have exactly the same problem with my car... I've replaced about every sensor, plugs, wires, and optispark without any noticeable improvement. The only new theory I've heard recently involves the catalytic converter. If it becomes plugged, the higher back pressure exhaust will mess with the O2 and cause the engine to run rich once it's warm and in closed loop mode. My only hestitation with this theory is that it doesn't explain why the car suddenly went south unless my unorthadox driving habits pushed the cat to its untimely death.
I did find one work-around.. try unplugging the IAT sensor in the front elbow of the air intake. This forces the car to behave normally since it comes out of closed loop back to open (like when it first starts in the morning - cold).

 

I have the same problem, the hesitation in the 94 z28. I have done spark plugs, wires, water pump, opti, and cat converter, also have done fuel filter, nothing has seemed to change the hesitation. I might try the TCC that Pete was talking about, havent looked to that area yet.

akohnz

 

Just an update for folks........Have now run to and from work for a couple of days with the scanner attached and monitoring the TCC Solenoid status.

Now bearing in mind that it is only around 20 mins each way - the engine & transmission doesn't get too hot but we do get into closed loop.

On these journeys I haven't seen the problem and I find that the scanner shows the TCC solenoid acting as designed....above 50mph and in top gear it locks; pressing gas to accelerate it releases and allows the transmission to kick down; applying brake to slow below 45mph again it releases.

Over next few days I intend to take a longer journey and see if I can get the problem to repeat - more typical of the times I have experienced it in truth.

 

WOW i cant believe every1 is have'n the same problem on their 94's Like i said, i too have had the opti replaced and the fuel filter/pump. the water pump was replaced not to long ago when it leaked all over the electronic distributer.(same thing as opti?) thats prolly a dumb question, but i need to learn somehow. i checked the spark plugs and wiring.
My friend (kinda) works for Chevy and said it could be the MAF sensor. He said if u hit it with like a wrench or something (while your car is running) and it revs or sputters, anything like that, then its broken. the MAF sensor is right before the elbow intake. i'll check that tomorrow.

 

You can pull the wires off the MAF and the car will run in open loop mode. It will set a code, but don't worry about it for now. If the car runs better in this condition, it means that one or more sensors are bad, but it does not necessaraly mean that the MAF is bad. Pulling the MAF wires is an easy way to get the car into open loop without causing it to go into the limp mode home. Good luck.

 

Anyone know where i can find the SES light?

 

i know thats a dumb question

 

It is in your instrument cluster,and it lites up momentarily when you turn the key on. Could be labeled check engine, service engine soon, MIL (official name, Malfunction Indicator Light). Anyways, if you have a trouble code it will be on at all time, unless it brned out from being on too long.

 

oo ok.
i took your advice pete and unpluged the MAF sensor. it did set off a code and the fans went on high, but the car ran great. it didn't bog out at all.
so can auto zone tell me which sensor it is? when i have the MAF in, the Sevice Engine Soon light goes away.
thanks for all the help.
Nick

 

Thet may be able to read some retained codes and that could help to pin down the bad sensor. Another possibility is to get a sensor tester and check all the sensors, don't bother with thw MAP sensor because it controlling your engine when the MAF is unplugged. Stil lanother is to get a professional grade scanner and read the data from the sensors and see which one is screwing up. If you have a buddy with a similiar car you can try swapping the MAF sensors & see if clears the trouble. Other sensors to test: ECT (engine coolant temp), IAT (intake air temp sensor), and the two O2 sensors.

Now at least you know that the fuel system and ingnition are OK.