I have an 89 Formula with a 350 TPI, automatic. I have to prime the throttle body to start the car and it runs fine for about 5 seconds. The engine then starts to stumble, spit, and refuses to accelerate and develops a misfire on several cylinders. The car will sit and idle but when I crowd the accelerator it bucks and gradually comes up, spitting (pops through the throttle body) and resisting the whole way. When I shut off the ignition, I have to reprime to start again. I have checked fuel pressure, replaced and calibrated the IAC, checked voltage to the TPS, changed ECMs and Proms, checked for vacuum leaks, and checked all the obvious items such as plugs, wires, etc. These all check out in normal operating range. MAF is also new. Dual Cats are free flowing. Again, the ECM sends no codes. Any similar experiences or suggestions?

 

fuel pressure!!! i know you checked it, but what was it is?

disconnect your maf and tps and try then. Once those two sensors are removed in goes into limp home mode and all the tables are standardized off engine rpm.

 

Did you check the fuel pressure regulator? do a leak down test on the fuel system and see if to hold 20psi for 20 minutes. Should start out at around 36-42 and drop off to 20 and hold for 20min...



When all else fails....Light match and run..

J

 

My 89 did that and I had a bad injector. I have also seen the coolant sensor make them run like that . But I am just tossing stuff out .


BTW how was the fuel preasure?

 

The fuel pressure is 40 lbs. What about the ignition module? Could that cause the problem? EGR valve? Thanks for the replys and don't hesitate to add some more! I have a 94 Firebird LT1 also...............anything on this car I can use to test on the 89? Thanks again.

 

A bad injector (or 2) will cause a really rough idle and when slowly accelerating off the line it will studder but in a very regular pattern. On full throttle, you will barely be able to feel it except for the utter lack of power.

What you have sounds EXACTLY like a bad fuel pump. I have had a bad fuel pump and bad injectors and what you describe is exactly what happened when my pump was bad. Your fuel pressure might be decent for idle, but check it under some throttle. It will probably drop.

 

A bad injector (or 2) will cause a really rough idle and when slowly accelerating off the line it will studder but in a very regular pattern. On full throttle, you will barely be able to feel it except for the utter lack of power.

What you have sounds EXACTLY like a bad fuel pump. I have had a bad fuel pump and bad injectors and what you describe is exactly what happened when my pump was bad. Your fuel pressure might be decent for idle, but check it under some throttle. It will probably drop.

Agreeing with Joe,

My problem ended up being a compound of issues, Trashed injectors, Bad Regulator , Faulty pump and a leaky vacuum line, The leak down test I mention earlier can also help diagnose a faulty injector.

J

 

Okay, finally got around to checking fuel pressure again with the car running. While trying to accelerate, the pressure went from a steady 40 lbs to around 49 lbs, dancing around between 45 and 50 lbs. Again, when I first start the car, it runs perfectly for about 5 seconds, then starts sluffing off and finally begins the misfire and hard acceleration. How do you check the regulator? Also, there is popping through the throttle body when trying to accelerate. I'm convinced the pump is okay and the injectors as well. The throttle body also seems to suck a lot of air, much more so than my LT1 engine when idling. Sounds like a giant vacuum leak. Anything else before I haul the PoS to the junkyard?

 

You might have a bad IAC mtoor then , just by the sounds of a vacuume leak

BTW . When I had ONE bad injector the car could barley hold an idle ... any gas and it would die right there.

 

Just got done replacing my IAC 2 weeks ago after another stumble hesitate dies problem, I suppose after 15 years that thing can sieze up pretty good...took me 5 min at a local autozone to fix..car runs great fixed alot of other symptoms my car was having as well......IAC definate possibility....

As far as checking the regulator there is a procedure outlined in the haynes manual for both the vette and the Camaro/firebird which is fairly staright forward and easy to follow.....requires few tools but an accurate fuel gauge is a definate must.

Thanks
J