Corolla_FX16

Hey all, new to the forum, and I'm hoping to gain some valuable insight on a problem my friend's 85 300ZX non-turbo.He purchased it almost 2 years ago for $300, manual trans, 108,000 original miles, only put 1000 more since.

The car runs rich as hell and experiences a shudder(stuble,stalling, whatever) from about 1500-2500 rpm's, when cold. Past 2500 or so and it runs fantastic. When warm, it runs fine but still runs lean. I'm very familiar with the problem, but I need help diagnosing it. I will provide all pertinent info and diagnosis I have accumulated over this two-year headache.

Engine Mechanical:
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everything seems to be functioning correctly. Engine has great vacuum test readings at idle and when the throttle is goosed, 20 inches of vacuum on the dot, with no needle flutter or strange deflections. cranking compressions and running compression are perfect. Oil pressure specs ok. No mods or tuning performed

Engine Breathing:
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No vacuum leaks detected with either the smoke tester or the can of WD40, no restrictions in the exhaust, an aftermarket cone filter connected to the stock intake piping (problem was present before the intake was installed)

Ignition System:
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I used a DSO to test the secondary ignition system, no direct problems, but the relatively high firing lines and the higher spark duration seem to indicate a lean air/fuel condition (verified below), no bad readings with the primary ignition system, battery brand new, all relevant grounds check out ok

Fuel System:
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tested pressure and flow rate, fuel pressure checks out (regulated and unregulated), FUEL FLOW is very low, barely a trickle at normal idle (should normally flow 750ML in 15 seconds), idle suddenly surged, similar to the 1500-2500 stumbling condition and BLASTED OUT FUEL while doing so, returned to normal with fuel barely trickling out. Replaced fuel pump, fuel filter, and blew out the fuel line with shop air to check for a restriction. Power and Ground to the fuel pump are fine

I verified the lean condition by testing the car with both a dyno smog pretest and an emissions analyzer (I'm studying to become a smog tech and I have access to the machines), both tests reveal an overly lean condition with high HC's, high 02, and high NoX readings

I am now stumped. I've physically removed the MAF and inspected it, seems fine. I also used the DSO to sweep test the TPS, no strange readings either. O2 sensor is functioning normally. Not suspecting a coolant/ambient air temp sensor, both would affect

I don't want to throw parts at it, I'm suspecting that the issue lies with a false reading to the PCM through a bad sensor, or a defective PCM itself, but what would cause the fuel pump to provide almost ZERO fuel flow?

Any ideas or thoughts would be greatly appreciated, and I also hope my considerable amount of first-hand diagnosis can help someone else out...

rogerz

something in the fuel tank blocking the pump suction. Change the chts it is one of the main causes of what you describe but the low fuel flow on top of that is a real problem. did you test the maf per the fsm. YOu DO have the fsm Don't you. free download from xenon and all your answers are in there. you could check the codes on the ecu but I have never found them to be much help. How is the battery and cables? A Z don't like low voltage. is the battery grounded to the frame or thru the starter. Starter grounding not reliable. clean the connectors to the ecu, tps and maf. Check for cracks in the rubber intake piping between maf and throttle body. Inspect very carefully. WD40 not so good on vac leak detect. unlit propane torch mo bettah. what about the AAC valve. you are too invested in all that fancy diagnostic machinery while working on a car that was new when dinosaurs roamed the earth. I do however commend you on the fact that you actually tried to diagnose the problem before posting on the forum.

Corolla_FX16

I've suspected the CHTS was bad, but I didn't have an economical (read:fast and cheap) way to test it, as I've heard its in the valley under the intake manifold. I already own the actual, 500 page paperback service manual for the 1985 Z, but I'll give the Zenon download a whirl I guess. I agree about the ECCS, it being helpful is about as rare as hen's teeth, plus its a pain in the @ss to get to lol. Battery is about 6 months old, load tested it with the AVR and it checks out ok. Cables are new. I'm pretty sure its grounded thru the starter, but I'll remake the ground to the frame, but the car has never had problems starting. I'll check all the rest later when I'm in the shop, and no, all that fancy equipment isn't mine, and yes, it is required for diagnosing newer OBDI&II vehicles. it also makes diagnosis, fine tuning and adjustments on older cars that much easier.

I appreciate the knowledge and the praise, its me and my friend's DD and I've been ironing out all the faults since he brought it home

rogerz

the ground strap if original has a square copper tag that bolts to the frame (M8 bolt) behind the battery. if that is gone at least run a couple of 10 gage wires to an M6 bolt on either side of the engine room. there are plenty to choose from. your problem isn't starting it is running the electronics with a faulty ground. believe me I still get grief from the wife unit because I didn't properly ground the battery on an 85T (too excited to drive the new toy)
attached is a pdf on how to replace chts without taking everything apart. Just blow off the oval connector atop the engine and solder a new FI injector to the wires (junkyard has em on Volvos nice). connectors are just another point of failure and there for assembly convenience. You won't have the car long enough to need another chts. first one probably lasted 20 years. when you put in the new unit don't torque it down it just needs to contact the head and be snug enough not to back out. goes in a dry hole so no worries mate. the fsm has a way to check it with a vom. from the ecu I believe but every z31 i have owned benefited from a new one. on the maf check do it on the bench use a hair dryer to blow air thru it for the test. YOU got a fuel delivery problem work on that. chts easy replace and not expensive.