I had a very similar problem. Ran fine until up to temp and then, bang, major misfire, felt like fuel or ignition cut. Checked and changed allsorts. Plugs, leads, fuel filter, injectors untrasonnically cleaned, filter on fuel pump,ā¦ In the end I booked it in to a specialist and, in preparation, put some fuel in, 99 Ron, and bingo. Misfire gone. I couldnāt believe that poor (Iām guessing poor) fuel would cause a misfire soo so bad.
Mine has ran absolutely fine since.
Worth a try.
Cheers,
Geoff.
Poor grade fuel will make the engine run erratically on all 6 cylinders due to the abnormal combustion process generated due to the gasolineās poor flash point and so the knock sensor will send information to the ECU and thus there shall be an error code.
Hi sent you an email explaining the theory of having poor quality unleaded gasoline. If you have taken poor quality petrol or perhaps the petrol was contaminated from that particular station you can drain the fuel tank and add some good quality petrol " super unleaded " ( E-5 ) from a different station and see what happens.
A high percentage of Ethanol in unleaded gasoline can cause an engine to misfire.
If you dont have this you need one
Sorry your emails went into my junk folder. Thanks for all the support. Whatās that there? A diagnostic machine?
Hi yes that is correct, It is the OBD1 to OBD2 scan tool. I am not very familiar with the electronics of these GTOās especially the first generation that I own and that are all equipped with the OBD1 that is connected to the vehicles console unlike the OBD1 so I am still in the learning process. This I bought from BlackStealth on ebay and he is well known in the USA GTO world. So once my engine is ready to be started from its fresh build, I will plug this in and monitor the factory settings. If there is an issue the scan tool will allow me to pinpoint the source by reading error codes stored in the ECU. It also provides real time information regarding the various systems of the engine and its performance when running. I shall be testing it out in a few weeks and perhaps make a video.
Hi guys. Long time since my last update. In the time Iāve been off here Iāve done the following
Rebuilt both heads
New heads gaskets
Valve stem oil seals
New could
New PTU
New aic valve
Tested all major components.
Another 2 ecuās tested in the car
The misfire is still Present!!
Ok using a diag machine it seems my coolant temp sensor is reading mind 24degrees. When disconnected it reads 99degrees.
How is this possible when Iāve changed the coolant sensor twice it still reads the same.
Iāve also tested the fuel pressure and installed a new fpr
Any ideas guys?
Have you been able to establish which cylinder is misfiring? And if so, have you tried swapping that injector with another cylinder to see if the misfiring moves or stays on the same cylinder?
When trying to aid in diagnosing a friends GT over here in Sweden earlier this year, he tried another ECU, had the injectors cleaned, put new plugs in and even a brand new OEM PTU and coil pack, just to mention a few things - without any luck. Misfire kept coming from the same cylinder.
In the end, it turned out to be a real simple issue. The injector on the misfiring cylinder didnāt receive the correct voltage. In one the harness connectors, one of the pins had been pried apart slightly and didnāt make enough contact. Gently squeezing it back together with a pair of small pliers solved the problem.
Worth mentioning however is that his car misfired upon starting it and not just when being warm. Still, could be worth checking what the connectors look like on the inside - at least it doesnāt cost you anything 
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There are lots of counterfeit or crappy new automotive parts, so maybe you got a bad coolant temp sensor. When my coolant sensor went bad, it sent bad readings to the engine ECU, and the ECU would dump excessive amounts of fuel making the engine run poorly. A cold temp will dump extra fuel.
Also, if you suspect bad O2 sensors, you can disconnect both O2 sensors and the car will go into limp mode. If the engine runs better, then you know where the fault lies. But it is also important to reset your fuel trims back to 100%/normal by disconnecting the car battery for a minute (do this when the engine is not running obviously). When the engine reaches operating temperature, the ECU will switch over to O2 feedback (closed loop), and if the O2 are bad, with bad fuel trims, you engine will run poorly.
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Just a idea i had same kind of thing with my 1991 Twin turbo , new fuel filter and injector clean ( @ totally blocked up ) and next on the list remove fuel tank and rust treat as when i did fuel filter red fuel came out . worth a look
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