Hello everyone. This is my first post here. I recently bough a 89 Firebird Formula with a LO3 in it. She runs pretty strong for the first 5 minutes or so after starting up in the morning, but by the time she is warmed up it feels like i have a v6! The engine is slower to rev and surges when accelerating at part throttle. Is this normal? It feels like it is running lean or not getting enough timing. Where should I start looking for the problem?

Thanks

 

Sounds like a bad 02 sensor. If you cannot remember the last time it was changged chances are it is about due. Your ECM only uses the 02 data when in closed loop and would explain why your car runs fine when cold. It may be time for a general tune up anyways.

By the way, Welcome aboard!

 

I'll try a new oxyegn sensor this weekend. It's likely the original one. Thanks for the quick reply!

 

it could also be that your ecm is running the car rich when its cold and then when it sense the engine has warmed up it leans out the fuel trim for emmisions reasons. my car feels the same way but since i took out my A/C all i do is turn on my A/C on low so it runs the fan and cools down the coolant making the ecm run the car more rich.

 

That is the switch from open loop to closed loop that you are seeing. Amongst other things a rich condition at start up helps the engine heat up quicker. Once it heats up and goes into closed loop it recieves info from all of your sensors to maintain a consistant AF ratio.

 

i'm not sure about our cars but obdII cars open loop is basically 3 conditions:

1. 5-15 seconds after start-up
2. 40* raise in temp
3. at least 160* in temp


this is what we were taught in school for obdII, but our third gens are not obdII so i'm not sure how they do it. but aside from open/closed loop the engine temp being high or low will also affect fuel trim, when my car liked to not heat up above 120* i used so much more gas then when i fixed it and now it heats up to a good 180-200.

 

I swapped out my oxygen sensor with a new Bosch sensor. My car runs much smoother now but still feels flat above 3K RPMs. I'm going to check my timing next.

 

If the car still has problems when warm, another possibility is a bad cts sensor. Thats a pretty critical component as far as the fueling is concerned. You can check it with a DVM or a scan tool. Its pretty easy to change out.

 

Here's a stupid newbie question. What's a cts sensor?
(forgive me, I'm a recoviring Fordaholic)

 

coolant temp sensor