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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!

  • Comments are not for extended discussion; this conversation has been moved to chat. – Move More Comments Link To Top Jun 19 '16 at 19:48
  • @MoveMoreCommentsLinkToTop i think it is very deconstructive for this question to move the comments to chat at this point in time. – agent provocateur Jun 22 '16 at 17:21
  • @agentprovocateur There is good reasoning here: meta.mechanics.stackexchange.com/questions/1639/… I've been planning on adding the useful comments to the bottom of the question but I haven't gotten around to it. – MooseLucifer Jun 22 '16 at 17:30
  • To check to see if the cat is clogged pull a bank 1 o2 sensor and road test or do a exhaust backpressure test with a gauge. As a rule of thumb anything above 3psi is excessive and should be checked further. I'd also be interested in what ignition timing is doing and engine vacuum. Was the vehicle tuned? Do you have any way to confirm the ECM calibration? Aftermarket fuel controller? The the aftermarket boost controller makes me wonder. – Ben Jun 23 '16 at 1:13
  • @Ben Good point about pulling the o2 sensor. The car was driven with o2 sensors removed, which could cross 'clogged cat' off the list. The ECU was swapped with a working OEM ECU, which did not solve the problem. Fuel controller and injectors are OEM as well. It does make sense for the previous owner to only increase boost and oil cooling capacity, as the OEM fuel system is said to be able to handle 1 bar of boost. – MooseLucifer Jun 23 '16 at 22:11
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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.

  • Just curious is it a 2 or 3 wire sensor? – Ben Jul 6 '16 at 23:00
  • @Ben I'm not sure, I'll look into it further. – MooseLucifer Jul 7 '16 at 20:58
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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.

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    @MooseLucifer I'm starting to think ignition as well, but unlike Brian B's answer, I'm wondering if there is something up with advance. Severely retarded or non-functioning advance would create the driving problems and perhaps even the "rev in neutral problem". I'm betting advance is controlled by ECU map(s), but I have no idea what the inputs are to get the cell coordinates, modifiers, etc. (On a Subaru there are a dozen inputs, multipliers, scalers, offsets, etc - but obviously not relevant here.) I realize you have doube-checked the mechanical timing. Are there cam position sensors? – SteveRacer Jun 27 '16 at 2:25
  • Good suggestion Steve. I'm not familiar with these models and engines enough to comment on where to look, but if its one of the types that change spark timing on the fly, it's definitely worth checking out. – Brian B Jun 27 '16 at 2:35
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    If it's a turbo car it probably has to be one of those models. Precision control of advance is critical at high boost to keep the engine out of pre-detonation (knock) -- which tends to destroy turbo motors in a heartbeat. But I don't know anything about Skyline ECUs. A wonky cam position sensor might also cause similar, but I can't say for sure it has that/those either. A completely busted cam sensor might put it in "limp" where it defauts to base timing with no advance. – SteveRacer Jun 27 '16 at 2:39
  • I don't deal with boost much, so I can't really speak with authority on those matters. But I do know that there are a lot of variable timing systems out there these days even without boost, like mivec, vvt, the BMW method (whatever it's called), and others, and a lot of them focus on drastically changing ignition timing based on sensor data, even if theres no actual valve timing changes (like vtec). So boost aside, there might be some other systems at play also. But just the come-and-go problem, power loss in certain ranges, and backfiring, all sound exactly like ignition system problems – Brian B Jun 27 '16 at 2:49

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