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LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

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Old 11-30-2017, 12:46 PM   #1
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Need 2nd opinion

Just some back story - most of it posted in this thread here:

Had to put this project away for a little while and just recently got back to looking at it. Engine still doesn't start - Fuel was replaced, put on a known working Optispark as well, still won't run. Car turns over, will sometimes try and fire for a few seconds, then dies. The car hasn't run in 15 months.

I did a compression check on 6 of the 8 cylinders:

1 - 90
3 - 150
5 - 50
7 - 60
2 - didnt test
4 - 170
6 - 60
8 - didnt test

Oil brings up the compression in all the cyliniders to the 160-170 range in the low ones. I've tried starting it after oiling the cylinders, but no luck. The test was done on a cold motor and at WOT, but given that a couple of the cylinders look ok, I have to assume then there is issues with the others.

I'm thinking the only way to proceed at this point is to get the motor out and opening it up to check it over, with the minimum being a re-ring/bearing job or a full out rebuild.

Any suggestions or thoughts?

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Old 12-04-2017, 08:48 AM   #2
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Re: Need 2nd opinion

I think it would be better to continue your old thread, for context and continuity’s sake. Injuneer or I can merge the two, if you like.

In the old thread, you posted a datalog that it seems Fred never got around to looking at. I’d work on finding someone that can do that before you go to the effort of pulling the engine.

Depending on what the data log says, some things I’d be interested in trying before pulling the engine:
1. Verify plug gap
2. Check for vacuum leaks
3. Bench test/clean the injectors
4. Test and possibly replace the ICM and coil
5. Replace any sensors not providing good signal according to the datalog
6. Try a known-good PCM
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Old 12-06-2017, 01:39 PM   #3
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Re: Need 2nd opinion

Sorry I missed the data log. Did look at it, posted in the other thread, copy of it here. Sounds like the rings are not working, maybe crudded up and stuck in the grooves.

Sorry I missed the data log. Not much to see, but it does show some problems.

First, what were you doing with the throttle? Three times you opened the throttle 100%, but there is no indication you attempted to start the engine. What were you doing?

Log shows fuel is enabled.

There are two attempts to start the engine.... RPM indicates the engine is rotating, signs of the sensors responding. But generally, doesn't look good.

In both cases, your system voltage is low... 10+/- volts in the first attempt, 9+/- volts in the second attempt. The second was was so low, appears the PCM was not able to control things.

In the first attempt, appears you still had the MAF connected. MAF indicates 2.45 grams/sec air flow, throttle is closed, MAP is abnormally high at 67 - 80 kPa. almost like the engine isn't sucking the air in. Would be consistent with bad rings. The injector pulse width is low, not sufficient for cold start.

Second attempt is a disaster. Appears that's where you unhooked the MAF. But the system voltage is so low that the PCM doesn't seem to be fully functional. MAP stays in the range f 94 - 98 kPa, even though the throttle is closed. No vacuum. When you disconnect the MAF sensor, the PCM defaults to speed-density to calculate air flow. The unreasonably high MAP causes the system to calculate a large air flow, and it sets the injector pulse widths in the range of 18 - 62 mSec. Way too much fuel.

When the battery is so weak, it affects the electronics and the fuel system.

Not surprised it didn't start.
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