Here's the story. My father ordered his 86 TA during July of 86. 2 days after driving he noticed huge backfire and and horrible engine idle/acceleration. He contacted pontiac and they brought the car back to the factory. Prognosis was that they put the GTA ECM in an 86 Car and replaced the engine after tearing the car down to bare chassis. Car was put into storage for most of its life until i got to it in august of 2010. Yet, 25 years later it still backfires. Do you guys know what the problem might be?

 

Please describe these backfires in more detail. Are they multiple? Do they happen while pushing the gas pedal? or when letting off the gas pedal?

Does it backfire when cranking?
Does it start up and idle ok?

 

Backfire from exhaust is generally caused by unburned fuel entering hot exhaust from ignition drop out. Is it difficult to start? Check spark. Pull the cap and visually inspect the distributor for corrosion or other deterioration. Sitting for long periods is not good for a car. If it has original wires, cap, rotor and plugs, they need to be replaced, even if the miles are low.

 

Car starts on one turn. It idles fine even in cold starts. I tested it. Didn't start it for one week and still started on one turn.The backfiring starts maybe 2 secs after start and lasts all the way. Backfires after letting off the gas pedal. But, the thing is nothing is affected in performance or idle quality. Car was sitting for 13 years but my father took care of it and put stabil in the gas tank. Started right up. We changed all fluids. Thing was dad said the car always back fired right from the dealer but not even pontiac factory could fix the problem. I changed the plugs,wires. I have acdelco cap,rotor originally bought from 86 extra in my shed.

 

The first thing I would do is check for an exhaust leak, starting at the cylinder head/manifold interface, and then the AIR system.
Sometimes you can see leaks at gasket interfaces while revving the engine in darkness.
You can start eliminating the AIR system for troubleshooting by first removing the belt for the AIR pump, then capping off the feed tubes that go to the manifolds and CAT.

An exhaust valve not sealing will cause backfires as well, but that possibility is much less likely. Visually checking the valvesprings and doing a compression test on each cylinder are ways to check for this condition.

 

I would do a leak down test sounds like a exhaust valve not sealing as 305sbc said. May have a broke valve spring or valve seat.

 

After reading your more thorough explanation, ditto on the exhaust valve or exhaust leak.

 

But, this was from the pontiac dealership back in 86. You guys think it's possible for them to put faulty parts in there? I have a cylinder compression tester but it's as old as the car lol. Can't do anything atm because i have snow mounds and ice surround my car in the car port. Forgive me for the ignorant question but where can i find the valve spring and valve seat to check? I checked the air tube connecting to the cat and it seemed to be fine. When the rest of the ice melts from the front i'll check the manifold and cylinders, and air system. Thank you for the guidance.

 

Valve seats are part of the head and will require cylinder head removal to replace. Valve springs are under the valve cover and can be replaced with out pulling the head.A compression gauge may not detect the problem,thats why I recomended a leak down tester. If there is leaking air you can tell where its going,intake,exhaust,or crankcase.

 

Absolutely correct.

 

I'm going to buy the leak down tester. And finally the snow is melting so i'll be able to see what's going down. I'll check the valvle springs first since that's the easiest access point. I don't have a garage so i can't stay out for too long in the frigid weather.

 

Remember, 305sbc also mentioned exhaust leaks. Be sure to check carefully for any exhaust leaks before getting too involved in leak down tests. Also, check for a sticky(slow to close) EGR valve.

 

The only exhaust leak i have is from the cat which is rusted out.The air was coming through a hole in it. But, it was doing it from the dealership so i know it's not that. I checked and that was the only issue visibly. I will check the compression soon.

 

The cat is too far away from the exhaust valve to cause a report(backfire). At least I havent seen it. Aside from what's been mentioned already, one thing that should be given some thought is decel fuel cutoff. The ECM watches TPS rate of change to determine when decel fuel cutoff is appropriate. It then drops injector pulse until prescribed conditions are met when it restores normal injector function. It does this to prevent exhaust report which can be caused by excess fuel collecting in the exhaust.

You may have an error in your ECMs programming that is affecting this part of its operation. It may be worth researching PROM updates for a fix to this particular issue. I understand that your father had the car into the dealer for this when it was new. However, there have been a multitude of updates since then. Rbob in the DIY/ECM PROM

 

You may want to PM RBob. You'll find him on the DIY/PROM page of this forum. He really knows his stuff about OE programming and may be aware of a fix for your problem.

 

Thanks, I think you might be right about the ECM cause Pontiac stupidly put the 87 GTA ecm in my 86. I will PM Rbob. Thanks for all your help!

 

I don't know if you figured it out yet but you may want to try tightening your rockers. I had to do that to mine it was backfiring constantly too. do one head at a time take the valve cover off and put pushrod clips on to keep oil from squirting to the next town over then while the car is idling one by one loosen each 5/8 rocker nut till it starts "ticking" loudly then slowly start tightening it again until the sound just stops, then tighten one 1/4 turn more. then repeat on the other head. Hope it works for ya

 

I haven't figured it out yet. This car is driving me insane. Sometimes it'll do it, and other times it wont(at start up) But it always happens. I was actually looking to put 1.6 rrs in there. Would that alleviate the problem?

 

I wouldn't replace any parts until you get what you got right. Try adjusting the stock ones first. they could have not been tightened from the factory or overtightened. Also check compression on the cylinders like others have been suggesting. They need to be close to the same on each one within 10-20 psi

 

Alright how do i remove the covers to the heads ?

 

four nuts and spreaders on the valve cover. Remember you gotta have a way to stop the oil squirting everywhere either deflector clips http://www.jegs.com/i/JEGS-Performan...oductId=934237 or find an OLD valve cover and cut out just enough of the top so you can reach the rocker nuts. or stuff rags over your exhaust manifolds to keep oil from smoking

 

I'll get the clips, rather keep my oil in the car. Is there a specific tool I need to tighten and loosen the rocker arms?

 

nope just a 5/8 socket and ratchet