R32 Skyline GT-R low power

Question

EDIT: Added info from comments to bottom of question.

My friend recently imported a right-hand drive 1990 Nissan Skyline GT-R (R32) with the RB26TT motor into the states. It's beautiful, and I'm extremely jealous. The car is mostly stock except for NISMO aero parts, an auxiliary oil cooler (in addition to the OEM cooler, which has a small leak), and an aftermarket boost controller (unknown brand).

The car was driving "fine" for the first couple weeks before it lost power. Essentially, the car feels like it's only making ~150 hp at anything more than half throttle, or any speeds above ~60 mph. At half throttle, everything seems normal, and it is pushing a few PSI of boost. The car idles fine before and after the symptoms occur, but will spit and backfire as the revs drop after. The engine does not want to rev from idle when you give it full throttle (in neutral).

What has been replaced or verified to be working (using multi-meter or known good parts):

• Timing belt (replaced)
• Timing verified before and after the problem occurs
• Coolant (flushed)
• Coolant temp sensor (replaced)
• Spark plugs (replaced)
• Fuel filter (replaced)
• Fuel pump (replaced)
• Mass AirFlow sensors (MAF)(both test with known good parts)
• Throttle Position Sensor (TPS) (checked with multimeter)
• Engine Control Unit (ECU) (tested with known good part)
• Turbo system gaskets (replaced)
• Boost controller (swapped back to OEM, verified)
• Wastegate (verified via pressure test)
• Blow-off valve (inspected)
• Intercooler (verified via pressure test)

All 6 plugs look the same after ~150 miles of driving since this problem began, which should rule out injector related problems. He also removed the injectors and ran Seafoam through them, which all seemed similar. He unplugged both the o2 sensors, which made no difference. While there is a very small oil leak, the engine has hardly any lost oil in the 600 miles/4ish months he has had the car, which had been running ~10 lbs boost with the aftermarket controller (before the problem began).

He has not yet removed the aftermarket boost controller, but he has rerouted the system back to the OEM controller. After replacing the gaskets he has tested the turbo system and it is able to hold at least 5lbs boost (previous time the pressure tester shot off at ~6 psi)

We've been discussing possible problems, and plan on checking the fuel pressure next, thinking it might be the fuel pressure regulator. He has also ordered an OBD-I diagnostic tool/Nissan software that should be in before this weekend.

Can anyone think of what else could be causing this condition? Could the small oil leak be dropping oil pressure enough to throw a red flag at the ECU, cutting power (despite the minimal oil loss)? Would a clogged cat/exhaust cause this problem without overheating the cat and making things smell?

Thanks for reading!

FROM COMMENTS: OEM boost controller opens wastegate at 9lb boost, current pressure checker blows off at 6lb, but car builds boost until problem occurs.

SteveRacer suggested it could be a communication problem between the TPS and ECU, though both are individually reading nominal (AKA could be a wire gremlin).

Per Paulster2's suggestion we checked the pre-post cat temps, but the thermometer range was too low. It is worth noting that while the car was warming up the pre-cat temp was 150°F higher than post cat, possibly indicating a clog cat. Exhaust is completely welded so we have not tested that theory. Interesting read on exhaust temps vis-a-vi catalytic converter can be found here.

AaronLavers suggested we try to get our hands on a wideband o2 sensor for a better idea of what AFR the engine is seeing. He also suggested bad coil packs. We are holding off on replacing those, as a full set is ~600 USD.

OBD/CAN network diagnostic tool is unable to read data from ECU, multiple computers and phones were tried. All failed. Still trying to figure that one out.

Thanks again for taking the time to read this and think about it!

Answer 1

The problem, as @SteveRacer suggested, was caused by a faulty camshaft angle/position sensor. Everything we had read about that sensor said the engine would not start or would immediately die if it was bad, so we didn't think to check it.

After replacing with a known good part, the engine runs much better and builds boost, but is still running a little rough. New ignition coils are on order and should be in by the weekend, but all signs point to a healthy engine. The aftermarket boost controller was also poorly routed (~5' of extra tubing), which may have contributed to over/underboost situations.

This must be why they say if you can't afford 2 Skylines, then you can't afford one either.

Answer 2

Id be leaning toward an ignition problem. If theres an ignition control module, or something similar that controls the spark by taking readouts on various ignition and engine sensors, that's where I'd start. If the problem gets worse as engine warms, but seems fine when cold, that also indicates an electronic problem. As the part fails further, it will go from period symptoms, to more regular occurance when warm, but eventually happen worse and more frequently without heat until (possibly) the car won't even start.

The problem with testing parts like this, or coil packs (as previously mentioned), is that it's often the case that they'll read out fine until they get hot. So to get a proper test result, you need to take the car for a spin, tools in hand, and test during the time when the symptoms are occurring.