I hope someone here can help me with this problem. I have a 1992 Formula 5.0, 5 speed, dual cat, 3.42 posi Firebird. It only has 102000 miles on it. It seems I have a flucuating timing mark problem. I disconnected the timing sprout to time it and the timing mark still is all over the place up and down. I have changed the ECM, TPS, CTS, injectors, air temp sensor, distributor, coil, wires, plugs, IAC, battery. If you let it warm up and give it a little gas to bring it up to about 1100 rpm it is very rough. Give it a little more and it smooths out. I have been a mechanic for 30 years and this one has me stumped! Has anyone else ever had this happen to them? Any suggestions would be greatly appriciated. Also, I will get some pictures of my car posted soon. Also include a picture of the option list. Thanks!

 

When was the last time you replaced the timing belt?

 

Thanks for the quick reply. The engine has never been apart so I don't think the timing chain has ever been changed. It could be pretty slack or jumped a tooth for that matter. It would never hurt to change it. Thanks.

 

You say you replaced the distributor? Was it new or used?
Along with the above mentioned chain being sloppy (good chance!), I've seen a worn distro drive gear and/or cam gear cause the jumpy mark syndrome as well.

 

Are you possitive that the plug wire is on all the way? If it were loose that could cause problems. The outer ring on the balancer could have moved, but I've never seen it moving while running.

Have you checked it with a differnt timming light?

 

No timing belt in a SBC. They use chains.

Jumping timing like that is usually a result of a worn, sloppy timing chain. Unless you run a crank trigger, you'll always have some fluctuation but excessive fluctuation means you need to change the chain and gears. The factory chain is designed to run smooth and quiet but they do stretch over time and the gears get worn out. A good aftermarket double roller chain set is much better.