Weird back firing issus

Hey guys, strange problem on the gto (1st gen 1992) twin turbo

At idle it seems to idle fine, however if you rev from idle it’ll back fire, miss fire and just generally runs rough, then idles normal.

If you try to drive it the idle sits high at about 1600rpm but as soon as the speedo drops below 20kph it instantly idles normal again.

If driving even small amount of acceleration causes a lot of misfire /back fire.

I assumed at first it was timing but I’ve had cambelt done and retired and it’s exactly the same.

All vacuum lines I’ve checked and used a vac tester and all is good

Spark plugs also have been recently changed.

Hello. Have you fixed this issue yet?

Timing is immediately where my mind goes, there is cam to crank timing that sounds like you rechecked, but there is ignition timing too. It sounds like any time the engine would advance the timing the engine runs horrible.

Quick side story, I worked on big natural-gas fired engines on the pipeline for many years and they used magnetos to create the spark to fire off the engine and the only time they would ever backfire is if someone stabbed the mag in wrong and the ignition timing was way off.

So what I would do is enter the diagnostic modes for the CAS and the idle speed calibration. I have a second gen and I haven’t started working on it yet, gotta finish my truck first, but on a second gen you would find the brown wire in the corner of the engine bay by the wiper motor and ground it out, basically telling the car it’s in service mode and a technician needs to look at the timing, so don’t adjust anything. Then shine a timing light at the front pulley and see if it’s on spec. I recently read up on this whole procedure out of boredom just learning about my car as i haven’t had a chance to work on it yet. Anyway, i’d look up again what spec is and see if it mentions anything about advance timing.

If you keep that brown one grounded and then ground out the diagnostic port terminal, whichever one I forget, then the car knows to enter idle set mode. And i’d run that check.

Beyond that, I would quickly check the TPS sensor since it idles good and only at throttle chops up, and the camshaft sensor (is it called a top dead center sensor in Britain? I see it called that). I see on first gen models the camshaft sensor and the CAS are one unit. These are both inputs to the ignition system and have influence over it’s performance, but also individually they could cause issues like you’re having.

This is a good resource for anyone wrenching on these cars :
http://stealth316.com/2-ignitionsystem.htm

For all I know it’s already fixed, i’d love to hear about it though. Not on here much but i love talking shop.

Thank you for the detailed reply I have checked with the timing light and grounding cable to put the ECU into service mode the timing was pretty much perfect on idle, I’ve also checked the cambelt that is also fine.

There is a possibility that it is the camshaft angle sensor I would need to source one to test it however I’ve got a feeling that my main ECU has developed at fault despite it being a remanufactured unit I sent it off for testing just to make sure thank you for the information and the link I’ll have a good read through it