1989 Firebird Formula rough start
 

I have a 1989 Firebird Formula 305 that starts and drives great on the first start, but after driving it and letting it sit for about half an hour to two hours it has a really rough time starting again. It starts up on the first crank but then immediately shoots down to low roms and dies. Sometimes I can keep it alive by pumping the gas pedal but most of the time I have to start it several times and hold it at about 2000 rpm until the rpm steadies out and then it will run fine after that. I've replaced the ECM coolant temperature sensor and the fuel pump was replaced 8000 miles ago, rebuilt the throttle body and put new injectors in. Any other ideas what it could be

Re: 1989 Firebird Formula rough start
 

Check the simple things. When it happens does it have good spark? What's the fuel pressure? The coolant temp sensor may be new but that doesn't make it good. Check it. DON'T replace it just because you can.. check it first. Also worth testing the MAF. Disconnect it when it runs terribly...does it run better?

Re: 1989 Firebird Formula rough start
 

Your injectors are shot. The Multec's do that when they get heat soaked. When it's having trouble Ohm them and you'll find that a bunch are out of spec when hot.

GD

Re: 1989 Firebird Formula rough start
 

I tried the MAF it didn't make a difference, didn't get a chance to try anything else. I think the coolant temperature sensor works because it helped the car run allot better but didn't fix the start issue.

Re: 1989 Firebird Formula rough start
 

I replaced the injectors last month with these

Re: 1989 Firebird Formula rough start
 

Oh it's TBI. Odd. My 91 Formula is a TPI 305......

But you say it has a MAF...... TBI cars don't have MAF's.

Try disconnecting the O2 sensor and see what that does.

Well

Re: 1989 Firebird Formula rough start
 

Yeah, I'm confused as well. Could also be an ignition module getting heat fatigue I suppose. Would that o2 reading make a difference when starting in closed loop?

Re: 1989 Firebird Formula rough start
 

Well if it doesn't have a MAF I'm not sure what I unplugged. Hopefully I can try the O2 sensor today

Re: 1989 Firebird Formula rough start
 

The O2 probably won't matter in open loop, but it's hard to say from his description if it's gone into closed loop or not when the problem occurs. That's why I suggested to unplug it and rule it out. Easy enough to check.

If it's a matter of it needing to get into closed loop to run correctly then you're going to have to look at BLM and INT values and see if it's having to add or subtract fuel to get to it's happy place. What's your regulated fuel pressure?

GD