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Old 11-12-2012, 08:32 PM   #1
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Default the latest lt1 mystery

ok so... bought a non-running car. it's a 97. it has a full 1995 engine swap, incl. ecm. wiring all seems fairly cleanly done.

the guy had a motor rebuilt and put in, with a lunati cam, roller rockers, and beehive springs.

it ran fine, and shortly after it died, he assumed because of crap in the tank.

i looked in the tank and agreed with that diagnosis, and bought it

the engine cranks over, and fires only intermittently. its like... crank crank crank fire... crank.. fire... and it doesnt seem consistent which cylinders are firing, or whatever.

there is no backfiring

if i hold it at 1/4 throttle and keep cranking, eventually it'll keep running, but acts like its running on 2 cylinders. barely goes.

i've gotten this far:

- cleaned tank and fuel system. newer pump in it. replaced fuel filter. 40+lbs of fuel pressure, including while cranking, and 20 minutes later it hasn't bled down any

- replaced injectors (new accel injectors), they were very gummed up

- pulled valve covers and looked at rockers moving, checked lifter preload

- plug wires are going to the right plugs. very strong spark while cranking

- injector wires are going to the right injectors. test lighted some injector plugs. they're definitely getting juice

now, this thing has had the motor re&re, so is it possible something has been missed during reinstall?

im fresh out of ideas at this point

any idea what else i can look at?
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Old 11-12-2012, 08:44 PM   #2
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Maybe the opti?
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Old 11-12-2012, 08:48 PM   #3
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yeah, i wonder..

i thought if injectors were firing, then optical side must be ok..

and i have spark, so high voltage side must be ok, right?

is it possible for an opti to die in a way that would pass those very primitive tests?
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Old 11-12-2012, 09:05 PM   #4
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Check fuel pressure.

If fuel system is gummed up fuel press should be low. May need to replace entire fuel system - pump, sender, tank, lines (front, rear, supply, return) filter, FPR, injector s.
Sorry for the bad news but the wrong fuel stabilizer(s) can destroy the entire system - i can testify to this.

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Old 11-12-2012, 09:36 PM   #5
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i did check pressure, and cleaned the entire system, at least to the extent where the tiny amount of junk i couldnt reach in the tank should be filterable.

it's holding above 40lbs at the rail when priming. cranking also stays healthy around 40lbs pressure

there was definitely lots of crap in the system, though, but with a relatively clean tank, new filter, and brand new injectors... im not sure how it wouldnt be running, even roughly, on all cyl.
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Old 11-12-2012, 09:44 PM   #6
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if i didnt know better, i'd think the opti was misinstalled or something crazy like that, so it's firing way out of time

but that's not really possible to do

there's no backfiring or other 'things happening at the wrong time' kind of evidence at all

so confused
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Old 11-12-2012, 11:59 PM   #7
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Holding it 1/4 way to the floor and it sound like its running on 2 cylinders sounds a lot like a very big vacum leak. Have you changed any sensors? Map,Maf? And have you looked into all the wiring grounds fuses? I ask because you said it was swapped
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Old 11-13-2012, 04:41 AM   #8
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i tried starting it with the maf unplugged, just to rule that out.. no change

does it even use the map sensor while cranking?

grounds look tight, but tried running an extra ground from the block to the chassis, and no luck
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Old 11-13-2012, 06:59 AM   #9
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What about coil and ICM, also are all the injector harness' plugged into the correct injectors? Make sure all ground wires are hooked up and clean. That one on the passengers fender often gets over looked.
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Old 11-13-2012, 07:08 AM   #10
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Quote:
Originally Posted by steveo View Post
i thought if injectors were firing, then optical side must be ok..

and i have spark, so high voltage side must be ok, right?

is it possible for an opti to die in a way that would pass those very primitive tests?
Have you scanned for codes? Two opti failure codes won't set the check engine light, DTC 16 & 36. DTC 36, opti no high resolution pulse, will still allow injectors to operate, and will fire the spark plugs. It should run with this code, but without the high res signal timing will be off and it will run rough.


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Old 11-13-2012, 08:58 AM   #11
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Also the opti can be put in on the wrong spot. There are 3 other indentations on the back of the opti that ham handed people have jammed the cam dowel pin into. Go to shbox.com for a pic of the back of opti. It's tough to get it in there wrong, but it's been done before, so I would check it. # 52 of component location section. While you have opti off, take a look inside. Check cap & rotor condition. Then take shielding off and look at optical section. Is full of oil, coolant, corrosion- it's on it's death bed, get another one. GM or MSD only. You will regret buying import or reman opti.

Also check the sticky threads of this section for the diagnosing the opti thread. Watch the video and do the tests. Shoebox has opti testing, and also coil testing. Take icm to auto parts store for tests and don't forget to put new heat sink compound on icm back and heat sink when you reinstall or put a new one on.

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Old 11-13-2012, 11:05 AM   #12
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its funny, i just came to ask if the opti can be installed incorrectly!

i had no idea you could do that

im going to mark the pulley TDC and timing light the sucker, if it's on wrong, that'd show me pretty quick without pulling it off, right?

the opti is fairly new GM kit, i doubt it's dead already, but since im not the one that installed it, i dont trust it
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Old 11-13-2012, 11:34 AM   #13
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Sounds like a Optispark to me. Here is why.. #1 a genuine GM Opti is remanned - no new ones are being made anymore. #2 The optical sensor that is installed in these remans are NOT the reliable Mitsubishi sensors. I just recently had one apart to confirm this.

The symptoms sound like a timing issue.. similar to a timing belt jumping a tooth.. though we have chains not belts. The optical sensor controls ignition timing, and it can fail in a way that it will still run very poorly. A dead one would be easier to diagnose.

Double check the spark plug wire routing. Double check the harness going from the opti to the pcm. Pull a few plugs and check gap and condition. Black or fuel covered plugs will be normal to see here with it running the way it is. When the coil fires is the spark a nice strong spark? Can it jump a 1/2" gap? If everything checks out, pull the Opti off and change it.
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Old 11-13-2012, 12:09 PM   #14
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i did check the harnesses, the plug going to the opti isnt in perfect shape, but it does fit positively and the terminals are clean, and i did wiggle it while cranking, there was no change

definitely double checked plug wire routing first too

spark is very strong on all, definitely no problems with coil/icm/plug wires..

i guess the opti might just be dead/misinstalled..
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Old 11-13-2012, 02:28 PM   #15
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MMine did that after new motor installed. Opti. Just did one for a guy would roll over act like it wants to start then nothing. New opti and he was gone
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