Twin turbo gto major misfire problem when hot! 🥵

Hello all my name is Leon. I have a 1991 gto twin turbo. I’ve owned it for over 19years.

Back in September I took it for MOT. Upon collecting it the garage said it had started to misfire.
I only just managed to get it home as every time I revved the car it would bog down.
After researching loads i thought it has to be the iac valve.
I changed that and the problem still persists.

I can drive it when it’s cold. (Just about) then when it’s warm I can’t rev it. Sounds like fuel starvation but the fuel pump is new and the car revs ok when cold.

So far I’ve changed the
Tested the throttle position sensor
Changed the coolant temp sensor
Checked thermostat
Ecu was checked by ecu doctor
Fuel pump only been fitted about a year (700miles)
Battery charged
Earths checked
Boost leaks checked
PTU sensor changed
Maf checked
Just can’t get my head around it.

I changed the PTU for a second hand one. The car ran perfect for about 20mins. The. When I accelerated it went almost back into limp mode again.

No engine light is illuminated during this problem.

Any help would be greatly appreciated

Also here’s a pic of my pride and joy which I’m very upset I’m unable to drive now!

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I had a similar issue with misfire. It sounds electrical so try and start from the engine and work backwards:

Spark plugs
HT leads
Coil packs
Wire/ harness

Good luck

Just curious as to know why was the PTU changed? and what diagnostics were used to pronounce it DOA :cry:

Just pretty much tried everything buddy and from numerous searches the PTU kept coming up as a possoble problem due to heat killing them. :cry:

For those readers who may be interested on the topic of Engine Heat ( currently my research ) generated by the combustion process allow me to share some thoughts and a few ideas. As you Leon8105 has mentioned heat killing of vital electrical components such as the factory PTU ( power transistor unit/ coil pack). I have experimented on my build and have researched tons of articles in how to combat and minimizes heat that is in fact one of the most detrimental elements affecting engine power and performance, especially these engines that are so tightly stuffed inside the bay leaving less than no roam for the heat to escape. A clean engine is a cool engine that will produce power and performance. On top of that you have an excessive carbon deposits within the combustion chamber, valves face ( which is just bad enough for loss of power) and whatever gets carbon coated will contain the heat generated by combustion process and in turn produce excessive heat affecting all electrical components or even frying it out of existence. AND so you have a hot engine and hot issues to deal with.

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Hi guys. Still no luck with this. Anyone in south wales able to help out? Really want this out for the summer :cry:

Just a few ideas that you can try; change out your existing PTU with a different one from a good running GTO and see if the problems still exists. If it is an electrical issue your ECM will send out fault codes and thus your engine light indicator will start flashing ( that is if the bulb is not burnt out ), so a code reader needs to be used. There are many reasons for a misfire but this happens when your engine reaches OT, a bad knock sensor? clogged or dirty fuel injectors? a million things to consider and to properly diagnose. If you do not have the factory two part manual this is the time to get a hold of one.
Good luck

I just re-read your threads and you mentioned that no warning light was triggered ( check engine light flashing ) so what I don’t understand is that how do you know that the symptoms you are having is indeed a misfire? loss in power during acceleration and in this case when the engine reaches OT could very well be clogged or dirty fuel injectors I had a similar experience and had to remove the fuel rail and do an injector test and there were two clogged shut injectors so I had them all cleaned out and re-calibrated which fixed my issue of sudden loss of power during acceleration. Check you injectors even if you just installed some brand new after market or OE ones. Also your spark plugs using a spark plug tester and ohm meter for the wires.

Thanks pal I will try that. The fuel pump is new but the tank was quite dirty so could well be that. The fuel filter however is not that old. But definitely worth a try. The misfire is now at idle also even with a cold engine. Will report back

Have you checked timing?

Your welcome, and FYI, checking injectors for proper spray pattern and resistance is a bit complicated if you do not have the proper equipment to conduct such testing. There are a bunch of DIY YouTube videos on injector cleaning but I strongly suggest they should be all removed and taken down to a shop that cleans and calibrates fuel injectors. I have done it before and just to discover that one of my OE injectors had a bad electromagnetic valve which I was told by the shop technicians so it had to be replaced. Also check the O2 sensors and make sure they have proper resistance. The super dirty fuel tank will do this especially when you first crank up the engine, all that microscopic dirt and debris goes very quickly to the injectors, so it does not matter what you do after, like cleaning out the tank and flushing out the fuel system and adding a new filter. If your GTO is smoking out the exhaust that should be an indication of an issue depending on the color of the exhaust gas. My GTO had 2 clogged open injectors, so the smoke was black, so it was running super rich with significant loss of power at full throttle. Without knowing, I drove it that way for a while until the engine kept on dying at idle. On initial investigation on checking and removing stuff I was lucky to discover oil in the intake pipes which translated into the rear turbo seal failure due to excessive heat generated from cylinders 4 and 6, the cylinders with clogged open injectors. With that being said, one issue led to several issues and in the end I decide to pull the entire motor for a complete rebuild from scratch.
I hope your issue is a simple fix like cleaning and re-calibrating your injectors and you will have your GTO back with full power this coming summer. keep us posted :wink:

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Typo error I meant to say 2 clogged open injectors :laughing:

We’ve had an issue over here on one of the Irish cars .
O2 sensor and ecu Faulty.
Diagnosed by an other owner.

Hi guys. Took the front 3 injectors out today. They did look dirty to be fair. Run them through a cleaning cycle and still runs the same. Now my oil pressure gauge has stopped working!! This car is doing my head in.

Is there anyone in South wales area that can either take the car and I’ll pay them to fix it or come and diagnose it on my drive? I don’t mind paying as I’m never selling the car. I just want it running. Thanks

The rear bank injectors also needs to be checked and cleaned. Also Check your O2 sensors or buy new ones. If you have OBD1 then run and see if you have any error codes.
The worst case scenario is that you may have a blown head gasket that will cause your engine to run poorly with loss of significant power. You can do a compression and leak down test, and there are many videos on YouTube. I know one member that lives in Newport and is a member on UKGTO and owns a GTO perhaps he can be of assistance.

I’ve got a mate dropping off a compression tester so will try that. If that’s all good then I’m going to strip the inlet down change plugs again and all the components I can. Might even try a second hand throttle body with iac and tps already attached to see if that’s helps

If you do have they guys details from Newport that could be good :+1:t2:

Do you have an email address? I can get a hold of him and give him your email.

For blown head gasket there are a few unorthodox ways to check, but you can never be 100% sure but it will give you a starting point. YouTube has many videos and one of the easiest ways is to pop the coolant cap off run your engine until hot and continuously observe for bubbles coming up the coolant housing, these are combustion gases trying to escape from a blown head gasket. You will start to have overflow of coolant when the pressure builds up in the system so have a bunch of rags ready. You could also have bad piston rings that is allowing excessive blowby. Also check your PCV valve.

If you do not have an OBD1/OBDII scan tool this is the time to get one, so no more guessing games on electrical components and sensors. If it is an electrical issue and the engine is not running to factory specs there will be an error code and you just decode it and it will tell you exactly what sensor to check. Also if you already do not have one this is the best time to get the Mitsubishi 2 part service manual. The electrical section gives you tons of information on each sensor and how-to diagnostics.