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Old 06-20-2013, 06:01 PM
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91 corvette elctronic engine problems

Long story Any help will be appreciated
It all started because my air conditioner compressor wsnt working
After checking everything I knew I suspected the ECM so I bought a rebuilt ECM
Engine was running great before the ECM was replaced
I drove the car back to the parts store to return the ECM core engine ran great
I was very carefull while installing the PROM in the exchange ECM
As soon as I started the engine after returning the ECM I knew something was wrong,engine ran terrible
Nursed it back home when I got home I put my hand on the ECM I was very hot
They gave me a new ECM engine ran terrible again,gave me third ECM still engine ran terrible
At this point I decided something else could be wrong,I replaced all the wires,plugs,dizzy cap,rotor,coil no change
Engine ran but not good
Now it wont even start,timing of the wires is perfect,I have a good 1/2 inch of spark,48 psi fuel pressure when I turn the Ign on it bleeds off to 0 then when I crank the engine over its about 45 while cranking but NO start,so I sprayed some fuel into the Throttle body and started long enough to burn that little bit of fuel
I suspect the PROM but its not available or at least I cant find one
As act of desperation Im replacing the fuel pump and the Dizzy module tomorrow
I know its a long question ANY help will be appreciated Im hopping around like a one legged chicken here
Theres not ONE code flashing at me

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Old 06-20-2013, 06:08 PM
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Could be the Ignition module is bad.That would allow spark when cranking (start bypass circuit in module), Which would explain why it would run on a spray can.
I have seen the modules go bad and allow spark but not signal the ECM to run the injectors.
That would be my bet. Dont just throw parts at it. Take a few minutes , do some checking of the basics and most of all save some $$$$$$$

You need to verify that the ECM is firing the injectors as it sounds that it isnt. 4 ECMs in a row arent going to be bad, leading me to the dizzy.
No codes will set for that condition either
Easy to check for injector pulse.
Use a noid light .
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Old 06-20-2013, 08:00 PM
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not to knock you here . but i never could return electrical parts even once, you won the lottery here to do it 3 times. can i have you parts store number?
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Old 06-20-2013, 08:29 PM
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Do all the test`s

Hi guy`s


LA tech made some good points do all his tests.But I think you have more than one problem.You should have some rest pressure in the fuel system.And also check that alt. You may have a bad diode and chevys do not like AC in electric system.


2x4`s
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Old 06-20-2013, 10:50 PM
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is this a tpi motor?
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Old 06-20-2013, 11:26 PM
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Originally Posted by killeratrod View Post
not to knock you here . but i never could return electrical parts even once, you won the lottery here to do it 3 times. can i have you parts store number?
I did a good $15,000 to $20,000 worth of busyness ( before I retired) in a bad year with just one parts store let alone the others I have accounts with, and have never been told I cant return electronic parts, as long as they are clean and re boxed! Its usually the unprofessional walk in customers that have no leverage and can't return them! They don't want to offend the professional shops that spend the big money!

Its too bad its that way but then life isn't fair !!

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Old 06-21-2013, 12:18 AM
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Quote:
Originally Posted by topwrench View Post
Long story Any help will be appreciated
It all started because my air conditioner compressor wsnt working
After checking everything I knew I suspected the ECM so I bought a rebuilt ECM
Engine was running great before the ECM was replaced
I drove the car back to the parts store to return the ECM core engine ran great
I was very carefull while installing the PROM in the exchange ECM
As soon as I started the engine after returning the ECM I knew something was wrong,engine ran terrible
Nursed it back home when I got home I put my hand on the ECM I was very hot
They gave me a new ECM engine ran terrible again,gave me third ECM still engine ran terrible
At this point I decided something else could be wrong,I replaced all the wires,plugs,dizzy cap,rotor,coil no change
Engine ran but not good
Now it wont even start,timing of the wires is perfect,I have a good 1/2 inch of spark,48 psi fuel pressure when I turn the Ign on it bleeds off to 0 then when I crank the engine over its about 45 while cranking but NO start,so I sprayed some fuel into the Throttle body and started long enough to burn that little bit of fuel
I suspect the PROM but its not available or at least I cant find one
As act of desperation Im replacing the fuel pump and the Dizzy module tomorrow
I know its a long question ANY help will be appreciated Im hopping around like a one legged chicken here
Theres not ONE code flashing at me
HI AL

You've been real busy lately I take it

I had a late model Vette did exactly the same thing after replacing the ECM that had shorted out and the engine ran like ***** !!!! there were no codes when I checked with my computer aneliser. I checked thinking there were maybe other wires shorted and found nothing did a Major tune up and it still ran bad All the while it was holding good fuel pressure when checked! Finally it wouldn't start at all unless I squirted fuel into the throttle body then it would run for a half hr. at idle but as soon as you gave it throttle it would stall, prime it again and it would Idle for a while and stall!! but read good fuel pressure right in spec. I changed the fuel pump and it straightened right out! I'm not sure if it was the fuel pump to this day LOL I could have accidentally bumped the wires or jarred something when putting in the pump and hooking up the battery and taping up wires!LOL And I'm not saying that's your problem But it was mine at the time LOL Aren't the late models Fun I just love em! LOL

I know that I'm not much help and your ready to pull your Hair out LOL

Chris
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Old 06-21-2013, 01:40 AM
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replace the fuel filter, it might be pluged up. enough that you'll get fuel pressure but no flow(volume) so it looks like it's fine as you'll have 45psi but as soon as the injectors start opening the fuel pressure nosedives.
have you checked the fuel pressure regulator and the vac line hooked to it.
could be a weak fuel pump like said above, but I'd replace the filter and check the regulator before diving into the tank
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Old 06-21-2013, 02:44 PM
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Ok so the fuel pump didn't help,also changed the filter
So Im going with LA tech advise and do the module deal to see if that helps
I don't have a noid light to check pulsing I sent for one today
As far as electronic parts go I do a LOT of business with these people and they have been real good to me
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Old 06-22-2013, 06:16 AM
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Ok so the fuel pump didn't help,also changed the filter
So Im going with LA tech advise and do the module deal to see if that helps
I don't have a noid light to check pulsing I sent for one today
As far as electronic parts go I do a LOT of business with these people and they have been real good to me
check the regulator. the hose going to, etc. they are known to die
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Old 06-22-2013, 06:59 AM
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He said he has spark while cranking , but wont run. Runs when sprayed with starting fluid or whatever, but also has 48 PSI fuel pressure.
Time to verify injector pulse.
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Old 06-22-2013, 11:20 AM
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Ok its not the module I just put a new one on it
I have a noid light on order will have to wait til next week to deal with that
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Old 06-22-2013, 05:36 PM
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What is the fuel pressure doing with the new pump and filter?

When you turn the key on, it should pressurize the fuel system. If you leave the key there, it should maintain pressure. As you engage the starter, it should maintain pressure until the motor fires, and then the oil pressure switch takes over and keeps the pump running. The pressure should not drop to 0. If it does, it's either starving of fuel, or its dumping the fuel into the motor through the injectors. Lots of fuel on the plugs? One stuck injector won't drop the pressure that fast. That mean that the PCM is keeping the injectors "on." And we are right back to a bad PCM or a shorted EPROM.

Is this an L98 tpi, or is it the LT1? You said distributor, so I assume L98.

Does anyone know if the VATS disables the injectors, or the pump? Its been a long time since I've worked on a TPI car so I don't remember. On your scanner, does it say that the VATS is engaged, or are you just pulling codes?

Keep us posted on what's going on.
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Old 06-22-2013, 06:23 PM
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Lowflier has some good points.
The VATRS will disable starter and fuel though. No help there. Still good points to raise when dealing with problems such as these.
Starting to sound like melted wiring somewhere.
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Old 06-22-2013, 06:38 PM
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Quote:
Originally Posted by lowflyer View Post


Is this an L98 tpi, or is it the LT1? You said distributor, so I assume L98.

Does anyone know if the VATS disables the injectors, or the pump? Its been a long time since I've worked on a TPI car so I don't remember. On your scanner, does it say that the VATS is engaged, or are you just pulling codes?

Keep us posted on what's going on.

theres a few different things vats can do depending on the year , and im not up on what was done in each year but there is a work around on the vats system

first off if vats is active the security light on the dash will flash

the fix is either replace the ignition lock cyl or measure the resistance of the resistor in the key ( black pelet ) and get a matching resitor to solder into the 2 wires leading upto the key cyl

most vats system disable the fuel injectors and starter for a period of 2 minutes from the time u turn the key on , turning the key to crank resets this 2 minute timer.

some systems only disable fuel injectors/spark depending on the year and or car

so what u can do is turn the key on and crank once then leave the key in the run position and wait 2-3 min's, once the time has passed with the key still in the run position u can jumper the terminals on the starter to get the engine to crank over and then the engine will start

i have used this method on my own car when vats went active while i was out shopping to get the car back home

if i recall correctly after a certain year the timer on the vats system was increased from 2 minutes to 5 minutes
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