I've been having this problem for over a year and haven't figured out what's wrong yet (I was busy this past year with another project that mushroomed).

I have an '88 GTU. The engine was rebuilt at 126,000 miles (now up to 150000 and still pretty reliable), with cleaned and balanced fuel injectors. Never floods, regular oil changes, stored during the winter, etc. It has a Crane Cams Hi-6 CDI box (a never-used one), RB header, RB exhaust, high-flow cat, (the latter three really improved the power) silicone hoses, cone intake, and I've recently gone through and improved grounding with fresh marine-grade cable and terminals and used anti-seize; though I need a higher-output alternator.

The problem started after I installed the CDI box (or at least it hasn't helped). Basically, at part to half throttle, plenty of low-end torque, no hesitation, even above 3800 RPM and it always revs to redline (it also freely revs to redline in neutral). But, at WOT, the engine will surge (act like it has more power for brief moments then settle down), and it seems to misfire at idle; it idles roughly at 650-750 RPM, and currently idles around 1000 at cold start, but settles down after it warms up.

So far, I've put in new plugs and wires (it was over two years anyway), new fuel filter, an adjustable fuel pressure regulator (needed, since I have a walbro pump), and no difference. I've bypassed the CDI box, and while I've noticed a slight improvement, the hesitation is still there. The TPS sensor has been adjusted and re-adjusted (no bucking) to no avail (and the sensor was purchased new in 2005).

The coils and ignitor assemblies were disassembled, redone with new wiring, solder, and heat shrunk terminals (and I made sure to note wiring color coding and positions), and stainless fasteners with anti-seize. They appear to be well-grounded, and the coils are 0.5 Ohm Leading and 0.8/0.8 Ohm Trailing (within spec according to the FSM).

One thing I've noticed that makes me think that this could be an ignition issue is that with the timing light hooked up to the trailing coil, it shows the same timing mark as the leading, sometimes flashing the trailing mark. Otherwise, ignition timing seems to be spot-on.

The auxiliary ports did have two issues; one of the Pineapple Racing inserts came loose, knocking around in the sleeve and was re-secured and one of the actuators (the rotating assembly) had seized. The manifolds were reinstalled with fresh OEM gaskets; so I doubt there's a vacuum leak.

I've gone back and forth over what the problem could be, but I can say that grounding isn't an issue. The entire electrical system grounds were redone with crimped, soldered and heatshrunk terminals in the engine bay (heatshrunk marine-grade terminals and copper stranded marine or high-quality insulated cable).

This winter, I am going to pull the secondary fuel injectors an check for any debris, as well as replace my fuel pressure gauge and sending unit (the latter is malfunctioning, the replacement unit will be properly grounded) in case the fuel pressure is set too low or high.

So, I'm stumped. It could be secondary issues (I plan to convert to electronic control), but the supposed missing at idle tells me there's something else going on. Maybe there's more than one issue causing the problem.

Any ideas on what else I should check? Also, are there any recommended tools to help check the ignitors for correct operation?

Oh, and the tachometer has always worked perfectly. :-)

 

About the idle skipping a bit, when it's idling at approx 750rpm..........warm the engine up FULLY. Idle the engine. Go to the variable resistor and turn it towards rich, slooooowly. The *missing* at idle should clear up. Note the screw in the variable resistor has STOPS at full R and full L. Don't force the screw against the stops or try to overcome the stops.

 

get a couple dyno pulls and see what the AFR's look like

 

Thanks, I've written that down and will check that in a few months when it comes time to get the car back out of storage.

[QUOTE]
get a couple dyno pulls and see what the AFR's look like
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I'm not sure if that's doable; I'm in northeastern Michigan. :-)

I've got an AFR gauge, but it's tied into the same oxygen sensor as the ECU, so readings are either not accurate (a non-heated one-wire sensor) or I'm reading feedback from ECU. I guess now is as good a time to ask: should I have a second bung welded into the header and run a separate oxygen sensor for the AFR gauge?