MisFire Causes?

[DeMasi1990]

My 90 305 tpi is misfiring like crazywe cant figure this one out so im gonna ask,

What causes a car, when started cold. To violently shake in its engine bay and to backfire under load?... I noticed that if I rev it to exactly 3,000 RPM the engine runs smooth, and when i bring it down, it will start misFiring again...Today when I took it out, i heard it making a PUT-PUT sound out of the exhaust...the whole upper end of the motor is new..plugs, wires, distributor, distributor cap rotor etc.. all new.. new fuel pump, filter, IAC sensor, Everything, New..I was told that it has a dead cylinder or cylinders..But how could that be?? everything is new.. And it was doing this before any work was done to the engine... Only thing left original on that car is the catylinic converters, and the fuel injectors...Oh also the Throttle position sensor is original, the knock sensor is original and the oil pressure sender and water temp sender is original

[mtperformance]

Man thats not an easy question to answer with out being in front of the car but heres what i would check the fuel pressure regulator or a vacuum leak (this in my opinon is the problem) things like the egr valve,vacuum lines,bad intake housing after the MAF sensor but with the way you discribe the back firing until you get to 3000 rpms it sounds like your car is trying to compensate for to much air by adjusting the timing but its hard to say with out being there but i hope this helps. I also wouldnt say it has a dead cylinder because of the fact that it smooths out at upper rpms if you had a bad cylinder it would miss all the time. Got a CHECK ENGINE light?

[five7kid]

A '90 TPI doesn't have an MAF sensor.

Do a compression check. A dead cylinder can seem to smooth out at higher RPMs. The "PUT-PUT" sound could be a leaking exhaust valve, or, a cylinder not firing for whatever reason.

Could also be a dead injector.

And, the answer to the SES light question is vital.

[DeMasi1990]

Right, My car does not have a MAF sensor....To answer the question, No their is no Service Engine soon light at all. I also want to add the the heads where checked at a machine shop and partially rebuilt " it had bad valve guides.. Im leaning towards a dead injector.. these injectors On the car are from 1990 and have 191,000 miles on them.. Id also like to add that, there is a black thing over by my coil, i think its called a PURGE SOLINOID I think, But it connects to the EGR and to the Throttle Body..I was told that when cold, it keeps the egr closed, But when the engine warms up, the computer will tell it to open the egr by allowing vacumme to it..well, when cold, it sucks air, when its not supposed to?...I think I have the vacuumm lines messed up, does anyone have a vacuum diagram on how everything goes?, It may be messed up :(......Sometimes she runs good, sometimes she dont, but atleast the rich problem is gone..damn

[mtperformance]

A '90 TPI doesn't have an MAF sensor.

Do a compression check. A dead cylinder can seem to smooth out at higher RPMs. The "PUT-PUT" sound could be a leaking exhaust valve, or, a cylinder not firing for whatever reason.

Could also be a dead injector.

And, the answer to the SES light question is vital.
Iam not trying to start an argument but I have never seen a Tune Port Injection motor with out a Mass Air Flow sensor .I know for a fact that the 87 5.7's had them because i had one.

[DeMasi1990]

Correct...But my trans am Is not that model year...I have a 1990 SPEED DENSITY TPI, thats not based on a MAF but a map.. According to what I ready, after 1989 it was speed density.

[mtperformance]

Is that the way it came from the factory .And if so when did they switch over and was it all tpi motors .

[five7kid]

TPI went to speed density in the 1990 model year.

'85-'89 were mass air flow.

EFI V6's and TBI were only speed density in 3rd gens.

[DeMasi1990]

When I Reinstalled the Engine Harness a few months ago, Changed the location On a few Grounds... Their are a few grounds located behind the engine on the engine... the drivers side I beleve were 4 grounds, They all broke off during removal ( very brittle ends )... I replaced them, and ran extra wire to a spot on that platform above the engine mount, also on the passenger side their were grounds aswell, I relocated them On the firewall wee that small Metal belt like ground wire went from the engine to the firewall...I cleaned the contact points perfectly...But I have a feeling that this is my problem. Somethings not grounded properly. I want to try this out before I go the next step and buy new fuel injectors..Ya know, I can honestly say, If anyone ever needed to know where something is on this car I would be able to tell them im a flash... This car came very far.. From being a total shell into being a running driving car again..Now I just got to get her to run right! hahah:roflmao: