Potential fuelling/ignition issue - Best order to troubleshoot

Troubleshooting
1

Hi all

1990 mk1 GTO, NA manual unmodified (to my knowledge)

I’ve noticed an absence of engine power (pull) past 3.5-4k revs. The revs continue to climb without issue but it feels like very little additional power is coming from the engine. The lack of extra power past that range is especially obvious when getting up through the gears joining motorways and the like.

I thought this odd because I’ve read that peak power on the 6g72 is around 6k revs regardless of being NA or turbo. It feels really quite sluggish since it naturally follows a lot of low end oomph from the car. No smells, noises or other changes seem to occur at the same time.

I’ve done some reading and would note the following:

  • Car thoroughly serviced last year, 60k service inc. spark plugs/fuel filter/all fluids
  • Idle is in the correct range once up engine is up to temperature
  • Throttle response feels good, no issue climbing towards redline gradually or full throttle
  • Can’t hear any obvious misfires, spluttering. No lurching or similar
  • No fuel cut off or stalling (as I’ve seen reported by others with the same loss of power in this rev range)

The following issues occurred relatively soon after the car was brought back onto the road, I have not noticed either in the past few months but may have some relation?

  • A handful of times when starting with a warm engine, the car cranked well, started and sometimes there’d be a slight 1-2 second chirp-like noise as it fell down just below the acceptable idle range after hitting peak RPM, it then stabilised up slightly to idle range and idled/ran fine.
  • The car ran a little rich at first but is noticeably better after some injector cleaner and runs on the motorway. I have not smelled it running rich for months now so am tempted to blame that on it being off the road for such a long time

Based on my symptoms, I’ve seen likely causes could be fuel pump, PTU or coil packs, and fuel relays and/or resistor. I’m tempted to exclude fuel filter and spark plugs as these are new. All engine work was done by a reputable JDM specialist with past experience of GTOs.

Do these symptoms point to any one thing? I lean towards a fuel relay causing a voltage issue or a degraded fuel pump but am happy to take advice as I appreciate the parts involved can be pricey if I go replacing them willy nilly. Any help would be greatly appreciated.

2

I would personally start with checking the fuel pressure in the fuel rail. These cars are outstanding when driven regularly but collect faults if left standing for any length of time. The fuel pump needs to be submerged in the fuel all the time or it will gum up or corrode inside if left for long periods. The filters throughout the system also become fouled up which includes the filters in the injectors. I can’t think for the life of me why the relay or resistor should be suspected but I am happy to be enlightened if anyone knows something I don’t. For some reason, people on the net blame coil packs and the relay /resistor for everything ranging from starting issues to a flat tyre.

3

Thanks Joe. I see there’s a section in the workshop manual on testing fuel pressure. Is there a specialised tool needed or would something off amazon etc. suffice? I’ve noticed significant improvement since running tanks of 99 octane through it so I’ll make sure the pump is well fed.

The argument I’d read on the relay/resister was something about there being 2 voltage modes on the fuel pump. The post said one of those electrical components degrading could cause the fuel pump to not switch into high voltage mode at higher rpm thus feeling like power was lacking when more fuel was required and the fuel pump wasn’t providing it. I’m not familiar enough to know if that follows on these cars.

4

I’m hoping that someone else will jump in here because I don’t think the n/a model has the resistor so this would not be an issue anyway.