Hi I have an 86 Fiero v6 auto and I need help figuring out a backfiring issue.

So basically the car starts and idles fine but when I head out on the road and try to accelerate it backfires like crazy. If I keep the rpms low when accelerating it’s fine but if I get on it at all it backfires then I’ll come to a stop and put it in park and it idles perfectly and revs just fine.

New parts
Spark plugs, wires, distributor cap, fuel injectors, 02 sensor, map sensor.

It doesn’t have any lights on and it hasn’t thrown any codes so I’m pretty confused. Any help is appreciated thanks

Did you insure that the wide timing mark on the balancer actually lines up with TDC of cylinder #1? (The outer ring of the harmonic balancer can eventually slip, thus throwing off the timing marks.)

And triple check that the spark plug wires are going to the correct cylinders.

Was the proper timing procedure followed to a T?

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
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Yeah I was super careful with the spark plug wires and car revs up perfectly in park so I think im good there.

How would I know if the harmonic balancer slipped?

I followed the procedure for the distributor timing pretty carefully but the backfiring problem got way worse after I adjusted it but the car also seems to run better than it did before so I was hoping it wasn’t related.

Would backfiring be caused by timing being too advanced?

Here is a drawing where the factory damper notch is located in relation to the mounting hub notch.



This is when the balancer slips. Notice how far the notch has slipped compared to the drawing of the stock TDC mark?

[This message has been edited by IMSA GT (edited 08-02-2025).]

quote Originally posted by ViperRedFiero: How would I know if the harmonic balancer slipped?

Take out #1 spark plug, stick something like a plastic straw into the hole down to the top of the piston, and then turn over the engine by hand to find TDC. If the balancer hasn't slipped, the wide timing mark on the balancer will be lined up with 0° on the timing scale.

quote Originally posted by IMSA GT: This is when the balancer slips. Notice how far the notch has slipped compared to the drawing of the stock TDC mark? CLICK FOR FULL SIZE

Heh heh... that's my photo of my '86 GT from 2008, although the image has been rotated and cropped.

quote Originally posted by Patrick Here: Finally got a harmonic balancer to replace the one which had “spun” on my GT. Here’s a picture of what I saw after I took the crank pulley off. Notice where the main timing mark and keyway are. Now compare the relationship of the timing mark and keyway with this image of a good balancer. Had a heck of a time finding a long enough metric bolt at various hardware stores to use to pull the replacement harmonic balancer back on!

Are you letting the car get up to operating temperature (above 180 degrees)? If not, then you won't get several potential check engine light codes.

Have you checked fuel pressure? If it is low, it will idle fine, but go really lean with any type of load and cause misfires and backfires.

Have you performed a compression check? It will let you know if you have any weak cylinders and valves not sealing, which can cause misfires/backfires.

Have you checked the exhaust for any cracks or leaks? Allowing oxygen into a hot ehaust with unburnt fuel will also cause backfires. Misfires do this, as well as cracks in the exhaust.

As others mentioned, it could also be timing.

Thanks for the help. I’ll try and check all those things this week.

Yes I’ve let it warm up before driving it and I’ve had it out on some cruises around town and its fine as long as I don’t get on it.

I’m still confused on one thing does the fact that it will rev up to around 5000 rpms while in park rule out things like timing issues and low compression and exhaust leaks or could those still be the culprit despite this?

Also the car doesn’t seem to lose power or misfire when it backfires.

Thanks.

quote Originally posted by ViperRedFiero: Thanks for the help. I’ll try and check all those things this week. Yes I’ve let it warm up before driving it and I’ve had it out on some cruises around town and its fine as long as I don’t get on it. I’m still confused on one thing does the fact that it will rev up to around 5000 rpms while in park rule out things like timing issues and low compression and exhaust leaks or could those still be the culprit despite this? Also the car doesn’t seem to lose power or misfire when it backfires. Thanks.

Revving while parked is one of the easiest things for an engine to do. It takes very little power and fuel. Revving to 5000 rpm while pulling the Fiero along at speed takes a lot more power and fuel, which will bring any issues with ignition and fueling to light.

quote Originally posted by ViperRedFiero: Also the car doesn’t seem to lose power or misfire when it backfires.

I strongly suspect a dyno would tell you otherwise.