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Old 01-03-2018, 07:12 PM
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Brenden Schaeffer
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Default engine help!

i bought a 1992 LT1 coupe corvette for cheap knowing it had issues but Iím stuck. When it starts it revs any where between 1000-2500 and when you hit the throttle it bogs out then revs back up but it will not rev past 3,000 it sounds like a rev limiter if you hold it there. The service ASR light and check engine lights are on and I know it has the port to get the codes but I canít find it? Is it under the cover of the driver side? Any ways please help Iím at a wall.
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Old 01-03-2018, 07:33 PM
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It seems like you may have a severe vacuum leak to find or a really messed up IAC.

Post 18 should provide you some insight with pulling codes:
https://www.corvetteforum.com/forums...ps-w-pics.html

Edit: the ALDL plug should be at the bottom of the knee cushion
And, if you plan on keeping/working on the car find yourself a Factory Service Manual (commonly referred to as FSM), it will be very helpful for you in diagnosing your symptoms. Can find them in the For Sale section sometimes or online (I found mine on ebay). That said, forum is still here to offer advice. More info you can provide always better.

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Old 01-03-2018, 07:39 PM
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Originally Posted by jmgtp View Post
It seems like you may have a severe vacuum leak to find or a really messed up IAC.

Post 18 should provide you some insight with pulling codes:
https://www.corvetteforum.com/forums...ps-w-pics.html

Edit: the ALDL plug should be at the bottom of the knee cushion
And, if you plan on keeping/working on the car find yourself a Factory Service Manual (commonly referred to as FSM), it will be very helpful for you in diagnosing your symptoms. Can find them in the For Sale section sometimes or online (I found mine on ebay). That said, forum is still here to offer advice. More info you can provide always better.
I took the IAC off and it looked pretty rough, could that be causing this all?
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Old 01-03-2018, 07:42 PM
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Hard to say, pull the codes first. You can try cleaning the IAC and passages as well, the manifold that it bolts to bolts onto the bottom of the throttlebody. You can remove it entirely, clean, reinstall with new gasket. If you move the position of the IAC pintle you may have to perform a reset procedure.
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Old 01-04-2018, 02:45 AM
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Brenden Schaeffer
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Originally Posted by jmgtp View Post
Hard to say, pull the codes first. You can try cleaning the IAC and passages as well, the manifold that it bolts to bolts onto the bottom of the throttlebody. You can remove it entirely, clean, reinstall with new gasket. If you move the position of the IAC pintle you may have to perform a reset procedure.
How do I do the reset procedure? Would fuel pressure have sowmthing to do with this? I put a new fuel filter in and it starts much fast now
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Old 01-04-2018, 03:24 AM
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Originally Posted by Brenden Schaeffer View Post
i bought a 1992 LT1 coupe corvette for cheap knowing it had issues but Iím stuck. When it starts it revs any where between 1000-2500 and when you hit the throttle it bogs out then revs back up but it will not rev past 3,000 it sounds like a rev limiter if you hold it there. The service ASR light and check engine lights are on and I know it has the port to get the codes but I canít find it? Is it under the cover of the driver side? Any ways please help Iím at a wall.

Check the TPS (Throttle Position Sensor).
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Old 01-04-2018, 09:08 AM
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You say the check engine light is on - the car is trying to give you some hint as to what is wrong. Rather than continuing to take blind guesses instead retrieve the codes, that is your starting point to diagnosing.
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Old 01-04-2018, 09:10 AM
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I strongly doubt that the IAC could prevent the engine from revving over 3,000.

Clean things up - put it back together - and pull codes. Write them down - then erase the codes, and restart the car. Run it for a while, and then recheck the codes. The codes that quickly re-occur are the ones you probably need to worry about first.

As for fuel pressure - YES that could certainly be the problem or part of the problem - so buy or borrow a fuel pressure tester, and hook it to the port on the fuel rail. You should see around 43 psi with the engine running. If the pressure dips below 42 - 43 lbs with no real load on the engine - you clearly have a fuel supply problem. Since the car is new to you - I'd drain the fuel out, and take a good look at both the fuel that came out, and more importantly - the inside of the tank - if it's full of crud - just throwing a new fuel pump in is NOT going to solve the problem for very long.


As many who frequent this part of the forum are already thinking - if you have decent fuel supply - the problem could well be with the opti-spark distributor that the LT1 engines used.
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Old 01-04-2018, 11:15 AM
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Originally Posted by Purple92 View Post
I strongly doubt that the IAC could prevent the engine from revving over 3,000.

Clean things up - put it back together - and pull codes. Write them down - then erase the codes, and restart the car. Run it for a while, and then recheck the codes. The codes that quickly re-occur are the ones you probably need to worry about first.

As for fuel pressure - YES that could certainly be the problem or part of the problem - so buy or borrow a fuel pressure tester, and hook it to the port on the fuel rail. You should see around 43 psi with the engine running. If the pressure dips below 42 - 43 lbs with no real load on the engine - you clearly have a fuel supply problem. Since the car is new to you - I'd drain the fuel out, and take a good look at both the fuel that came out, and more importantly - the inside of the tank - if it's full of crud - just throwing a new fuel pump in is NOT going to solve the problem for very long.


As many who frequent this part of the forum are already thinking - if you have decent fuel supply - the problem could well be with the opti-spark distributor that the LT1 engines used.
Alright Iíll do the pressure test today and get those codes and let you Guys know, thanks
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Old 01-04-2018, 02:01 PM
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[QUOTE=Purple92;1596308176]I strongly doubt that the IAC could prevent the engine from revving over 3,000.

Clean things up - put it back together - and pull codes. Write them down - then erase the codes, and restart the car. Run it for a while, and then recheck the codes. The codes that quickly re-occur are the ones you probably need to worry about first.

As for fuel pressure - YES that could certainly be the problem or part of the problem - so buy or borrow a fuel pressure tester, and hook it to the port on the fuel rail. You should see around 43 psi with the engine running. If the pressure dips below 42 - 43 lbs with no real load on the engine - you clearly have a fuel supply problem. Since the car is new to you - I'd drain the fuel out, and take a good look at both the fuel that came out, and more importantly - the inside of the tank - if it's full of crud - just throwing a new fuel pump in is NOT going to solve the problem for very long.


As many who frequent this part of the forum are already thinking - if you have decent fuel supply - the problem could well be with the opti-spark distributor that the LT1 engines used.[/QUOTEOk


I ran the codes and got 1. C41 and H41 4. ERR 9. H92What does the ERR mean?

Last edited by Brenden Schaeffer; 01-04-2018 at 02:02 PM.
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Old 01-04-2018, 03:03 PM
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Originally Posted by Brenden Schaeffer View Post
Originally Posted by Purple92 View Post
I strongly doubt that the IAC could prevent the engine from revving over 3,000.

Clean things up - put it back together - and pull codes. Write them down - then erase the codes, and restart the car. Run it for a while, and then recheck the codes. The codes that quickly re-occur are the ones you probably need to worry about first.

As for fuel pressure - YES that could certainly be the problem or part of the problem - so buy or borrow a fuel pressure tester, and hook it to the port on the fuel rail. You should see around 43 psi with the engine running. If the pressure dips below 42 - 43 lbs with no real load on the engine - you clearly have a fuel supply problem. Since the car is new to you - I'd drain the fuel out, and take a good look at both the fuel that came out, and more importantly - the inside of the tank - if it's full of crud - just throwing a new fuel pump in is NOT going to solve the problem for very long.


As many who frequent this part of the forum are already thinking - if you have decent fuel supply - the problem could well be with the opti-spark distributor that the LT1 engines used.


Ok


I ran the codes and got 1. C41 and H41 4. ERR 9. H92What does the ERR mean?
http://tech.corvettecentral.com/2011...trouble-codes/

Turn it on but don't crank. What fuel pressure do you see? Does it hold after you turn the key off? Does it try to stay at about 43 psi when you floor it?
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Old 01-04-2018, 04:12 PM
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CCM not communicating with ECM is the ERR code.
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Old 01-04-2018, 05:02 PM
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I'll preface this post with the statement that I own a 1985, and I know that the later C4s have some pretty significant differences. I want to help, but someone with a late C4 might have to correct me. A couple tests that might also help you diagnose:

Smoke test-pull a vacuum hose off the system (on the '85 the brake booster is a good candidate-not sure on yours), light up a cigar, and blow smoke into the system (with the car off, of course). See where that smoke comes out-that's a good place to start looking for a vacuum leak.

Fuel Pressure test-you can rent the pressure gauge at Autozone for $150 (which you get back when you return it). Attach it to the fuel rail, turn the key. See what pressure it reaches. Turn the key off. Does it hold that pressure? If it does, turn the car on. Does it still hold pressure? What about if you rev the engine? If you see more than a slight pressure dip at any point here, that's a warning sign.

I doubt it's the IAC, but pulling the entire throttle body off isn't a terribly difficult operation. Scrub it out real well with a toothbrush (preferably one you're not planning on using again ) A new IAC isn't terribly expensive, but it's not the first place I'd start.

I don't actually know much about your Throttle Position Sensor, but that might also be a good place to check. It tells the ECM where the throttle is, which tells it how much fuel the engine needs. Obviously when one of these is bad it can really mess you up. It should have smooth voltage increase as you move the throttle (test with voltmeter).

The fact that you're getting a communication error trouble code is...troubling. Not sure I can help you with that one. I would definitely get that figured out ASAP though. Hope you find some of this useful, and good luck with your new (and everlasting) project!
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Old 01-04-2018, 06:30 PM
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I believe that Code 41 is "Electronic Spark Timing (EST) Circuit (Open or Shorted). Basically the LT1's use a "Coil Driver" its basically a big switching transistor that takes the low voltage low current signal from the cars computer, and controls the Ignition coil by grounding the coil at the appropriate times. The primary side of the coil is always "hot" when the Ignition is "ON". The coil driver grounds the neg side of the coil when the computer tells it to - thus completing the circuit and allowing current to flow. When the computer tells the coil driver to open the circuit - the magnetic field in the coil collapses and the spark is created.

Terminal "C" on the coil driver should be grounded.
Terminal "A" on the coil driver should have power anytime the Ignition is "ON".
Terminal "B" on the coil driver is connected to the ECM.
Terminal "D" on the coil driver is what is connected to the neg side of the coil, and is the wire that is switched.


The manual has a procedure to check the coil driver, but it may just be easier to replace it - the coil driver module should be less than $50. Don't just buy the cheapest one you can find - an number of CF members have posted that they have received defective ones from some of the big "chain" auto parts stores.

NOTE: The coil driver gets hot is use - there should be a conductive grease like material supplied with the new one - USE it - if you don't get the special coating with the coil driver - get some thermal heat sink compound and use it when replacing the coil driver.

Last edited by Purple92; 01-04-2018 at 06:32 PM. Reason: Correcting typo
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Old 01-13-2018, 12:58 AM
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Originally Posted by Bfenty View Post
I'll preface this post with the statement that I own a 1985, and I know that the later C4s have some pretty significant differences. I want to help, but someone with a late C4 might have to correct me. A couple tests that might also help you diagnose:

Smoke test-pull a vacuum hose off the system (on the '85 the brake booster is a good candidate-not sure on yours), light up a cigar, and blow smoke into the system (with the car off, of course). See where that smoke comes out-that's a good place to start looking for a vacuum leak.

Fuel Pressure test-you can rent the pressure gauge at Autozone for $150 (which you get back when you return it). Attach it to the fuel rail, turn the key. See what pressure it reaches. Turn the key off. Does it hold that pressure? If it does, turn the car on. Does it still hold pressure? What about if you rev the engine? If you see more than a slight pressure dip at any point here, that's a warning sign.

I doubt it's the IAC, but pulling the entire throttle body off isn't a terribly difficult operation. Scrub it out real well with a toothbrush (preferably one you're not planning on using again ) A new IAC isn't terribly expensive, but it's not the first place I'd start.

I don't actually know much about your Throttle Position Sensor, but that might also be a good place to check. It tells the ECM where the throttle is, which tells it how much fuel the engine needs. Obviously when one of these is bad it can really mess you up. It should have smooth voltage increase as you move the throttle (test with voltmeter).

The fact that you're getting a communication error trouble code is...troubling. Not sure I can help you with that one. I would definitely get that figured out ASAP though. Hope you find some of this useful, and good luck with your new (and everlasting) project!
Alright Iím sorry for not responding, but Iím gone over slot with the car recently. Fuel pressure is good, checked 7 differnt Grounds and cleaned them all, I cleaned all the fuel injectors just to make sure everything there looked good, I cleaned the whole throttle body and air filter. And itís still doing it but this time when I started it something wierd happend. I turned the car on after all this and let it warm up then all the sudden it just shut off and after that it wouldnít fire but would crank, next I was playing with the stereo with key on power and all the sudden all the sensors started clicking and the fans turned on so I tried starting it again and it started fine???? Iím sure itís wirring but what are chances itís just the ECM/memcal? I also noticed that the alternator was pulling a negative charge off my battery, getting that checked tomorrow. Any help at this point would be great. Oh yeah also got a code 13 after all this.
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Old 01-13-2018, 02:24 AM
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Originally Posted by Brenden Schaeffer View Post
Alright Iím sorry for not responding, but Iím gone over slot with the car recently. Fuel pressure is good, checked 7 differnt Grounds and cleaned them all, I cleaned all the fuel injectors just to make sure everything there looked good, I cleaned the whole throttle body and air filter. And itís still doing it but this time when I started it something wierd happend. I turned the car on after all this and let it warm up then all the sudden it just shut off and after that it wouldnít fire but would crank, next I was playing with the stereo with key on power and all the sudden all the sensors started clicking and the fans turned on so I tried starting it again and it started fine???? Iím sure itís wirring but what are chances itís just the ECM/memcal? I also noticed that the alternator was pulling a negative charge off my battery, getting that checked tomorrow. Any help at this point would be great. Oh yeah also got a code 13 after all this.
try replacing the engine oil pressure / fuel pump switch on the rear of the block; if it hangs up, it could explain at least some of the intermittent problems you're having, and when it fails completely it could explain the "after that it wouldn't fire but would crank".
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Old 01-13-2018, 09:02 AM
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Originally Posted by mtwoolford View Post
try replacing the engine oil pressure / fuel pump switch on the rear of the block; if it hangs up, it could explain at least some of the intermittent problems you're having, and when it fails completely it could explain the "after that it wouldn't fire but would crank".
I don't follow. How does that explain anything other than the main circuit failing and the backup working or not working? What do you think that switch does besides act as a backup? IIRC, mine will start with it off and reporting 0 psi.
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Old 01-13-2018, 01:29 PM
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Originally Posted by aklim View Post
I don't follow. How does that explain anything other than the main circuit failing and the backup working or not working? What do you think that switch does besides act as a backup? IIRC, mine will start with it off and reporting 0 psi.
I may not be following exactly what's happening, but:

"something weird happened. I turned the car on after all this and let it warm up then all of a sudden it just shut off it wouldn't fire but would crank....so I tried starting it again and it ran fine"

switch is normally "open" when engine not running.

When ignition key is in "start" switch is bypassed and power is fed to fuel pump relay.

This explains (at least one possible explanation) why your engine starts and runs with 0 psi

when ignition switch is turned back from "start" to "on", the ignition circuit no longer feeds power to the fuel pump relay.

switch is normally "closed" when engine running. when engine starts and oil pressure builds, switch closes and provides power to fuel pump relay.

If that switch is failing to close intermittently, or visa versa, goes open ...as in bad contacts or a bad diaphragm...it could go a long way in explaining your symptoms. In my experience, switches, especially pressure operated switches, fail intermittently over time, not catastrophically, all at once.

Last edited by mtwoolford; 01-13-2018 at 01:30 PM.
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Old 01-13-2018, 03:34 PM
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I am guessing you have a bad ECM, a known problem for 92's and affected my car.
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Old 01-13-2018, 06:53 PM
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Originally Posted by mtwoolford View Post
I may not be following exactly what's happening, but:

"something weird happened. I turned the car on after all this and let it warm up then all of a sudden it just shut off it wouldn't fire but would crank....so I tried starting it again and it ran fine"

switch is normally "open" when engine not running.

When ignition key is in "start" switch is bypassed and power is fed to fuel pump relay.

This explains (at least one possible explanation) why your engine starts and runs with 0 psi

when ignition switch is turned back from "start" to "on", the ignition circuit no longer feeds power to the fuel pump relay.

switch is normally "closed" when engine running. when engine starts and oil pressure builds, switch closes and provides power to fuel pump relay.

If that switch is failing to close intermittently, or visa versa, goes open ...as in bad contacts or a bad diaphragm...it could go a long way in explaining your symptoms. In my experience, switches, especially pressure operated switches, fail intermittently over time, not catastrophically, all at once.
My under, which is standing, is that you get a 2 second prime charge to the fuel pump. After which, it needs ignition cycles to warrant any more pressure. The pressure switch only sends power to the pump after the engine is running and giving 3 or 4 psi as a backup in case the main circuit fails.
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