Backfire in Overdrive Only: Please Help!

[mrfixit49]

I recently inherited an '85 GN, with stock everything. It was in running condition, but (of course) we took it down to nothing and rebuilt the engine (without performance upgrades). Complete Tune-up preformed. The vehicle was restored and is completely running.

2.5" exhaust system installed. Downpipe to 2.5" testpipe/cat to dual 2.5" Hooker Maximum Flow Mufflers.

Here's my situation.

The vehicle has a huge exhaust backfire in overdrive.
The cat is now installed but it happens with the test pipe, or the cat; no difference.

1st gear acceleration ok, deceleration ok, cruising ok, all RPMs ok
2nd gear acceleration ok, deceleration ok, cruising ok, all RPMs ok
3rd gear acceleration ok, deceleration ok, cruising ok, all RPMs ok
Overdrive acceleration ok, deceleration ok, cruising backfire, not ok at lower RPMs

As long as there is a positive acceleration, even if it is ever so slight, it's fine. Also, if there is hard deceleration, it's okay. It's the feathering at 70 MPH on flat ground to maintain that 70 MPH that causes the backfire.

For instance, Interstate 5 in the middle of CA (straight flat road), Cruise Control On.

60 MPH Backfires on Deceleration part of the feathering.
65 MPH Backfires on Deceleration part of the feathering.
70 MPH Backfires on Deceleration part of the feathering.
75 MPH Backfires on Deceleration part of the feathering.
80 MPH Slightly higher wind resistance, more throddle, less back firing.
85 MPH Slightly higher wind resistance, more throddle, less back firing.
90 MPH Enough wind resistance to require positive throddle, even if there is a slight downhill; minimum if any backfire.
95 MPH Not worth chancing an explanation with CHP.

I have seen many problems helped by the fine GN owners in this forum, and I am hoping you could help me as well.

Thank you very much.

 

[cool 84]

What chip are you running? I've had mine do this when the TPS was slightly off and it was running rich on top of that. Only difference is I could make it do it in every gear by feathering the throttle. Get it fixed quick, the turbo hates backfires. Also try unplugging the electrical plug at the trans. If the problem goes away you could have wires wires mixed up in the plug/trans or any of several computer/ground problems.

 

[mrfixit49]

Thank you for getting back to me so quickly. I have a stock chip:

Numbers imprinted on the big chip:
16032500
014 B8508
DELCO

Numbers printed on the sticker on the big chip:
DELCO
CMM
1865

Trust me, I have tried to duplicate the problem in the lower gears with no luck. I'll pull the plug off the transmission and try it. I have read a lot about ground problems too. We just painted the car, so I'll re-check my grounds. Can you recommend a good chip for my hot air, weekly driver?

 

[dadslil84]

hello i have a 84 with the exsact same problem any other suggestions

 

[T- Type Tim]

I had this problem when I got my motor back from a rebuild. I went through everything, and that backfire was LOUD. Anyway , the cam sensor was slightly off, I read enough on this to know it was either that or the TCC. Since the motor was apart that was the first place to check. I just loosened the mount bolt and slightly moved it clockwise. Took it for a run , problem gone.

 

[85mustangkiller]

I have the same problem, it seemed to stop after I changed out my coil pack. doesnt backfire in overdrive now, but it still pops every once in awhile oc a hard deceleration, but if i tap the gas it stops.

 

[dadslil84]

ok i just had it replaced i will have them take a look, other wise i have a great running car , i just started working on my dads 84 t type with sunroof and two tone, need new paint new interior, but the motor was rebuilt buy buick in 88 and they put a new block and top end as well as exahust, ect. then it sat for the last 22 years. with about 5-7000 miles on it, the speedo reads 66,000 but i have all documents, from dealer.my daily driver is a cayman s and i have left it parked for over a week now. i love my child hood car. any gn, t type clubs out in nor cal

 

[mrfixit49]

Backfire Finally Fixed!!!

Sorry to resurrect an old thread, but I wanted to advise of my progress.

Thank you very much T- Type Tim for your recommendation. My motor was recently rebuilt and the cam sensor was installed to where the car was running very very lean, but running! Today was my first opportunity to get under the hood and mess with my poor neglected car.

Yes, the cam sensor was incorrect. I adjusted it to the best of my ability and the problem was corrected. No backfires whatsoever. I invested in Casper's Cam Sensor Cap with the installation LED to assist in the exact location it should be installed and hopefully I'll be able to install it next week.

Again, thank very much for everyone that participates in this forum. It is the help of everyone that we can accomplish so much with these cars (especially the hot-air guys!)

 

[T- Type Tim]

glad to hear you got it fixed and thanks for the follow up post!