Starting Issues & Poor Performance

Hi All,

Hoping to get some ideas as to what I need to be looking into before a start blindly swapping out every part I can think of, in an attempt to fix my issue - it’s a weird one.

I have a 1992 GTO NA, only 60,000 miles on the clock - original engine and gearbox.

The main issue is that occasionally (and most commonly in the Summer months) when I go to start the engine, on the first or second crank it will misfire (one of the cylinders seems to fire whilst the intake valve is open) - when this happens, the engine will continue to turn over until eventually it will start up (takes a lot longer than usual) and run on 4/5 cylinders until eventually it clears up and runs on all 6 - runs absolutely fine after. However, most of the time (especially in the cooler months - I live in the UK, so that’s most) the car will start up absolutely fine and run on all cylinders with no problem. We’ve recently had the rear bank head off and the cylinder walls, piston, head, valves, cams etc, all look in very good shape (should do at only 60K), so I don’t believe it to be a mechanical fault and something more towards electrics/sensors (compression readings are fine) - which is where I am not very knowledgeable at all and need some help. Before I go into detail of what we’ve done/replaced I’ll also say that there is a slight issue where the engine will not idle and will occasionally stall (usually when the RPM is decreasing and there is load through power steering at slow speeds) - for example, parking up.

Recent work
Cambelt - Timing and Tension is spot on (1 year/2000 miles ago)
Head Gasket (rear bank) - had to be removed due to a foreign object (washer) being dropped in through the intakes for the rear bank - yes an unfortunate & costly mistake - All gaskets for plenum and throttle body were replaced with Genuine gasket - (1 year ago).
New IAC (Stepper Motor) - This was purchased due to the stalling issues, however they remained.
Coil Packs - (2 years ago)
New PTU (1 year ago)
Spark Plugs (1 year ago)
HT Leads (1 year ago)
ECU Rebuild - Capacitors were leaking (1 year ago)

I haven’t replaced too many things yet, as I don’t want to waste money/time testing/replacing things that are unrelated to my issue, so hope that one of you may know the answer already. Most of the work undertaken was taken in an attempt to fix the idling/stalling and low performance issues that I have been having with it, however I believe that the misfire whilst starting issue is possibly related to the low performance issue too - Yes my second problem is that the car is very sluggish, even for the NA.

Any help/advice will be appreciated.

TIA, Harry.

Hi Harry
It seems like you already spent money on items that could have easily been checked. Without carrying out any tests, a forum is only going to give you a bunch of guesses as to the problem. I would start with the temperature sensor which can be tested or the MAF. Where are you in the country ?

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Hi Joe, thanks for the response.

Yes, we should’ve tested the parts prior to replacing - money wasted, lesson learnt. Are you speaking about the ECU (Coolant) Temperature sensor? I’m in Lincolnshire, near Grantham.

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Did you get this fixed? Sounds like a lot is going on.

Has this car sat? Is the fuel old? All other things equal, if an engine starts easy in the winter and hard in the summer my mind goes to fuel immediately. Or other things that could cause a rich mixture like a filthy clogged air filter.

Engine backfires are big clues and usually mean timing in my experience. Could be cam to crank but ignition is more likely. If that’s what you heard. Though if it runs and drives good after starting that seems unlikely.

I haven’t gotten the chance to start working on mine yet to see the throttle body design, but I fix cars for a living and many vehicles get hesitation issues at low rpm/ with load situations like you describe. The chevy pickups/tahoes/ etc with the 5.3/6.0/etc. get so bad the engine will shut off when you pull over and start slowing down. Some engine designs even with an IAC will call for the throttle plate to be slightly ajar when in the closed position, or some have a little passage way right next to it for an additional idle air circuit and when that gets gunked up and closed.
I could keep going but for all i know this is fixed or something. It’d be cool to know what happened or if you still need help. If so, what else have you looked at.

Have you entered diag mode for idle relearn and CAS timing adjustment? I’m not sure if that’s different between gen 1 and gen 2. What was your compression test results? Did you do wet and dry or they were all good? Low compression engines are notorious for being hard to start. Does it backfire regularly? Backfire is a huge clue.

Sorry for the late response, I’ve had a few other things going on so this was put on the back burner a bit. The ECU temp sensor tested absolutely fine. Thanks for the in-depth response 94caracas - The compression tests were carried out a few months back so I cannot remember the exact readings, but they were within the correct for the NA engine. In regards to fuel that’s what I am starting to this may be the cause of the issue. Spark seems absolutely fine, air filter changed very recently - timing has been double/triple checked - including CAS calibration/ignition timing. the only thing I can think of now is fuel. The car - as evidence by its low mileage (60k) has been sat around for a lot of its life, and most recently it was sat for a good 1.5/2 years. I also do have issues with hunting, so I’m thinking a dirty/clogged fuel filter or maybe poor fuel pump (the car has been in my family for a good 8 years and I don’t recall it ever having had a fuel filter - bad I know, but I only recently took ownership of it and it’s on my list) Already have a filter so may look at both whilst I change that over. Also smelling a lot of unburnt fuel when it does backfire and run rough on start-up, so maybe leaking/inefficient injectors? I think in the next year or so, once I have my own space to have this car sat, I’ll look at the entire fuel system and clean/refurb repair as much of it as I can to hopefully fix the problem. I’ll update this thread if I find the cause.

Thanks both