Gadgedizzle

So ive got this 92 Toyota 2wd pickup with a 96 LT1 and its killin me.. When its cold, it runs hard and bounces off the limiter no problem just the way it should when youre gettin busy... when it warms up though, when you floor it from idle, it shoots straight to 5k and hesitates and struggles to go above and beyond and only barely hits the limiter.. almost seems like its ignition is going full retard and the engine loses all power to accelerate above 5k rpm.. no missing, popping, backfiring, nothing... runs perfect aside from this.

Tests ive done...
Its got no cats and new 3" exhaust.. back pressures not the problem.
Its got new plugs and wires. Thats not the problem.
Its got great fuel pressure.. 32-34psi at cruise, 40ish psi at wot while doing a pull to redline. That eliminates the filter, lines and pump.
I believe the ignition coil is new within the last year.
Only dtc's are for skipshift light, downstream o2 heater (no rear o2 on one connector), vats malfunction (started truck without my dongle to enable fuel once), and skip shift solenoid circuit (tried the resistor bypass trick and failed)

As far as i can tell, none of my dtc's would cause or be related to my problem.. im kinda stuck.. all data from my maf and iat seem correct. Does the pcm monitor knock when cold?

Thats the only idea i have left..

4th Gen 4 Life

This sounds like a optispark problem and seems really common. Research the optispark on the lt1s and you may find your problem. Should just need to be cleaned.

They say it can get water in it from a leaking water pump or moister.

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Gadgedizzle

it's not the opti. None of the signs of optispark failure are present. it is running solid mounts however. I don't have a way to prove it at the moment, but I believe this is going to be a false spark knock retard situation. I won't know for certain until I put my 95 pcm in and tune out the knock sensors. I'm just looking for other possibilities that would cause this without people just automatically assuming the optispark. I find it hard to believe that the optispark would be "bad" and not completely fail after doing it for the last 2 years. Its in a 2700# truck and there has never really ever been much need to redline the thing sincw its so stupid fast as it is right now. I want to get it running correctly because im getting slicks and want a real et at the track. Ya know?

Injuneer

I'd agree.... people feel the Opti is the cause of every problem on the LT1, and this one doesn't sound like it is. I think the thing that is throwing people off is the fact that it loses all power, but you indicate "no missing, popping, backfiring, nothing". A faulty Opti high voltage section would certainly cause misfires and irregularities. So would a heat soaked ICM. And the ICM and the optical module would be setting codes.

I'm sort of surprised you run it to redline when it's still cold.... but that wouldn't be relevant either. Does appear to indicate it's getting adequate fuel and spark. Just how hot does it get when it's warmed up?

As far as the knock, appears you are still running the 96 PCM. Do you have access to an actual OBD-2 scanner that would let you observe the spark advance and the knock retard? And what is the difference in operation between cold and warm that would cause "false knock" to become a problem only when it's hot?

4th Gen 4 Life

With these type of problems i have seen a million different solutions to fix and the cause. I would try to do the cheapest troubleshooting first. Cleaning. The opti would not hurt $5 worth of water pump gaskets. And like you said to tune out the sensors? Ive also read up on a post talking about the pcm thermostat going bad causing the engine to run rich or lean and causing it not to run right. Im currently having to do process of elimination on my 94 because i the pcm is not throwing codes for what ever the problem is.


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Gadgedizzle

Agreed... i have an actron obd2 graphing scanner and set it to graph total timing. Since i cant see spark retard or any of that with the diagnostic scanner that i have thats the best i can do.. i saw a total of 28-30 degrees approximately (according to the very low resolution graph on my scanner).. ive driven around with a fuel pressure gauge taped to my window and flogged the **** out of it and i saw 38-40 psi (hard to read when youre damn near sideways on the limiter.) So i know fuel isnt the problem.

I was looking at an $EE bin for a 94 F-body LT and it appears that it doesnt start looking at the KS till the pcm reads 176* coolant temp in the "burst knock retard vs coolant temp" map.. i know its not a 96 bin, but its something and kinda makes me feel better about my diagnosis. Ive also talked to force fed performance in sac ca. And they said they tune the KS out on LSx because of the same thing im talking about with solid mounts. I suppose i could just delete them with the resistor mod and run it again.. all i ever run in it is premium.

I wanna hurry up and get this fixed.. i just got my slicks.. im ready for that low 12-high 11 sec pass!!

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Injuneer

Burst knock is "theoretical" knock. The PCM sees operating conditions that MAY produce knock. That might be the result of faulty sensor info. The TPS sensor, coolant temp sensor, IAT sensor and the MAP sensor would probably be possibilities. I don't see that as related to solid motor mounts (which I don't think you mentioned you are running).

Gadgedizzle

Yeah.. i mentioned solid mounts in my second reply.. i didnt see it as a problem, but now im reading where solid and even poly mounts register as knock in some cars. Theres even a thread where exhaust leaks have been detected as knock. So im not ruling knock out as a possibility. The way a local tuner describes it, the engine makes noise, it echoes through the mounts to the frame. Then the frame echoes back through mounts to the block. He made the metaphor of speaking loudly in a tunnel. Im speaking clearly, but the tunnel echoes back my last word between my current word and next word basically doubling the frequency of my words. The knock could see that "doubled noise" as a resonant knock frequency. They apparently have the same problem with LS engines running solid. Im not saying this is what im up against or what the problem is, but im definitely not ruling it out..

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Injuneer

I understand how the knock sensor works, and I'm aware that solid motor mounts, located immediately next to the knock sensor, can cause the knock sensor to register knock. But that isn't "burst knock". Burst knock is a value calculated in the PCM when the PCM sees the possibility of knock, based on engine operating conditions. It has nothing to do with the knock sensor.