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Old 04-29-2009, 10:06 AM   #1 (permalink)
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What usually fails?????

Working on an LT1 powered buggy, and it hasn't run right since a hard roll on Wrecking Ball during KOH, then water logged two weeks later on a trail ride. Before is died in the water, it ran...just not very well. Then it got wet, and died...

After we got it back to the barn, it would fire everytime, then just die. Jogging the throttle rapidly would keep it running, but still not for long. I swapped out the MAF sensor, cause it fit the problem, and now it runs better, but still will not stay running by itself. We also swapped out all other sensors that we had spares for, including the MAP, and the TPS.

When it runs, as long as you keep moving the throttle, it seems to stay running, but if the throttle stays in the same spot for a couple seconds, then it will die.

I have decided by use of a noid light, and a timing light, that fuel is shutting off causing the stall, not spark....

Could I have a pad PCM? On a GM product, does the PCM usually fail, or the PROM? I can get a PCM for $115, but the PROM is a custom burn that cannot be replaced before this weekend when the rig is supposed to compete in the CalRocs season opener...

Any help would be great.

Also, I cannot get any response from the PCM with a scan tool. The buggy has a standard 16 pin OBDII looking connector, but it only has 4 wires going to it as per the wiring diagram that came with the harness....Would the vehicle run at all with a bad pcm? Is the PROM still okay? Could it just be the scan tools (Yes two). If its an OBDI system, would an OBDII scan tool still work on it?

Thanks for any help... If we can't fix it, its going to get propane
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Old 04-29-2009, 12:05 PM   #2 (permalink)
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i had a friend with a simialar issue to what you are describing. after months of troubleshooting, he discovered that he had exposed wiring that was shorting and damaged the PCM. although, his ran. It was a little sketchy and it was rough, but it idled.

You could always look for a used one at a salvage yard. It would be a hell of a lot cheaper then buying a new one, and it would allow you to figure out if thats that case.

But, that is assuming that its the problem.

The likelyhood(sp) that you used two bad scan tools is very low.
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Old 04-29-2009, 12:57 PM   #3 (permalink)
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True about the scan tools, I was just thinking that they were both OBDII scanners, and the system is OBDI...Ihave no idea if that matters.

So did you buddy need a new prom too? Or just the pcm?
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Old 04-29-2009, 12:59 PM   #4 (permalink)
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Did the Optispark get moisture inside??
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Old 04-29-2009, 01:01 PM   #5 (permalink)
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Quote:
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Did the Optispark get moisture inside??
Maybe, but it would be dry by now.... Plus, it runs great at upper RPM as long as you are giving varied throttle input....I don't think its an optispark problem, but I will be pulling it out tonight regardless
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Old 04-29-2009, 09:35 PM   #6 (permalink)
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Also check for smooth variation in the TPS. If the voltage is jumping around its gonna get grumpy.
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Old 04-29-2009, 09:46 PM   #7 (permalink)
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The PCMs rarely go bad but it does happen. It's probably OBD-1. What year LT1? I would work on getting codes out before spending more money on parts.
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Old 04-30-2009, 08:53 AM   #8 (permalink)
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Okay, borrowed a Modis last night, and was able to pull codes, and read data. The only code was 43...knock sensor related.....checked sensor resistence, and repalced the sensor....no change in how it ran, but the light turned off

Pulled the optispark out and cleaned it... there were signs of moisture, but not bad. put it all back together, and it still runs the same

Now the good/wierd news....

Using our trusty we-rock regulated "ball valve" on the return fuel line, we closed the return line that spiked fuel pressure to 80 psi on the gauge (I think that is as high as the gauge goes), but it ran great!!! Throttle snappy, smooth, etc.

So the pump is gonna get swapped out, and the lines blown out, and see what happens....hell, we got plenty of time...don't have to tech in til 8am saturday morning
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Old 05-03-2009, 09:54 AM   #9 (permalink)
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Quote:
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True about the scan tools, I was just thinking that they were both OBDII scanners, and the system is OBDI...Ihave no idea if that matters.

So did you buddy need a new prom too? Or just the pcm?
from what i understood, he just replaced the PCM. I don't thing he did anything with the prom.
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Old 05-03-2009, 11:40 AM   #10 (permalink)
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throw the ECM into a bowl of rice and let it dry out for a couple days...
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Old 07-14-2009, 09:12 AM   #11 (permalink)
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Okay, so months, and four comps later, it still runs like crap

When we were in Reno, we pulled the fuel rails, cause there was a definate drop in temp measured at the headers....debris was found in the injector screens We swapped out all 8 injectors, and it seemed to run a lot better...for awhile....

Now it still runs like crap, ezspecially when cold, but won't run any better until shut off and restarted when already hot.
In other words, fire it up cold, and hold at 2000rpm, misfire out the tailpipe randomly for a couple minutes, if you snap throttle, it will pop out the intake... Once operating temp is reached, it still runs like crap...BUT....if you turn it off, let it sit for a couple seconds, then start it back up, it seems to run way better..still not 100%, but close to 85%...

Also, new symptom....the rig will glow the headers if held at 2500 rpm for two minutes..we never noticed this before.....I don't think the timing can be that far off, cause it still has good power...could it just be running way too rich?

Also, the optispark has been replaced..again.... along with the fuel pump, etc....wtf else could it still be?

Does anyone know a computer guy we could send this one to?? I would prefer someone local to the Bay Area, but at this point, anyone trustworthy would be great...
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Old 07-14-2009, 11:58 AM   #12 (permalink)
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glowing headers=lean condition.
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Old 07-14-2009, 12:48 PM   #13 (permalink)
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Old 07-14-2009, 01:58 PM   #14 (permalink)
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Is it possible to also be a rich condition?? Too Much fuel for the plug to fire, but it ignites 8 inches down the tube??
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Old 07-14-2009, 04:59 PM   #15 (permalink)
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too much fuel will actually cool the engine. too lean will burn the tubes. if its backfiring, popping, making the headers glow, running like poo, its too lean.
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