I have an '85 TPI Trans-am which is having some problems which seems to be getting worse and no-one seems to know why it occurs:
The engine idles and cruises reasonably well, but it stumbles within the first 20% throttle open.

The check engine light comes on under normal driving conditions and I've connected an X-Ray diagnostic onto the computer:
The results are that there is an error in the "speed sensor" and the "oxygen sensor", but when taking "snap shots" of the sensors, all sensors appear to be functioning perfectly.

I've checked for vacuum leaks, I've checked for spark, fuel pressure is good, but seems to drop when the engine is stumbling...

In order to solve this problem I've also tried swapping the O2 sensor, the throttle position sensor, the MAF sensor all to no avail !

Any 5 minute "try this" suggestions are welome, as I am not happy with giving wads of cash to mechanics, as I have done in the past (never solving any of these problems in the past)

 

Have you checked the fuel filter lately and possibly the coil could be breakingdown. Quite possible the fuel pump maybe going on you since the fuel pressure drops when it stumbles

 

What were the actual error numbers the computer read. Also is it stumbling on acceleration, steady, hot or cold or all the above?

 

Can't remember both error codes, but one of them was 44...
The problem originally occurred on cold start and also if the motor was left off for about 20-30 minutes and then re-started...

but now the problem seems to happen more often (cold, warm, whatever) and also now seems to happen when doing things like "U" turns (engine under load at low revs) but it seems to be only the first initial couple of revs from start (1000-2500 rpm) ie accelleration from idle.

I suppose the filter would be a good thing to check first, as for the fuel pump, I don't even want to think about swapping that !! Yikes !
It idles quite well and one it gets on the freeways, it's smooth, so I would have throught that if the fuel is not coming through, it would affect all driving conditions??

 

Hi The good news is it definately sounds like fuell starvation, code 44 is lean mixture, which means not enough gas in the air.

Always start with the cheapest easiest fix, unless you are getting paid, then start at the most expensive.

From what you describe, you might want to change the in line filter before you do anything else.

If that does not fix it, you need to recheck the fuel pressure - you mention you have a guage. What you have to check is the back pressure control valve is working OK. I don't know if there is a section of rubber hose on your model, at the fuel return, but try sqeezing it flat, and seeing if the fuel pressure comes up or not. It should then let it go and see how low the pressure goes. It only gives a rough clue, but its better than nothing at first.

Run the engine from cold and see what the pressure is, and how steady it remains. If it drops much when you start the engine, you need to check the back pressure valve is working. You might have to take it off.

The problem with doing a check on the pump, is needing to do a flow check, and that is just too dangerous, because flowing gas can create static electricity and ignite the gas.

So, if that valve is OK, then you need to think about the pump and the pump suction filter.

The conditions you describe the problem occuring are at the highest fuel demand in your driving. Cruising requires almost no gas by comparison to any acceleration, slow speed turning or cold. You can see that with new cars which show instantaneous fuel consumpstion. If you drove flat out, the problem would likely reoccur.

You have to fix this soon, or you could burn out a valve or piston.

 

I go along with Andy on this one remember if it was electrical it would probably come and go for a while and then just quit. It sounds like a fuel delivery issue since it is occuring all the time or near most of the time when fuel demands are greatest excelerating coming out of a turn when the engine is cold etc. check your filter pump and pressure relief valve like Andy said. I personaly thing it is an obstructed fuel line or a pump that seems to be going on you. But, thats only my 2 cents.

 

Thanks my fellow 3rd genners, I will be taking it to someone soon to try those things you've suggested...

fingers crossed !

 

Good luck

 

And if the filter is more than two years old, it's time to change it anyway.