What's wrong with my 1994 Corvette?

[girvin]

My 1994 Corvette (LT1/A4) runs great when cold, but once the engine reaches approximately 200-210 degrees F (oil temp), develops a rough idle and runs very rough - sputtering and backfiring (through the intake) if I give it too much throttle.

Mods include:

Throttle body coolant bypass
160* thermostat
Crane valve springs
Crane 1.6:1 roller rockers
LT4 knock module
Open air lid
Power couplers
High flow MAF housing (stock MAF sensor)
Air foil
Ford Racing matched 24# fuel injectors (“blue tops”)

Recent tune up included the replacement of the following:

Distributor cap/rotor
Spark plugs
Spark plug wires
Fuel filter
Air filter
PCV valve
(2) Front o2 sensors (did not replace rear)
Coolant temperature sensor
Ignition control module

When I pull codes, using the paper clip method, I get the following:

1. C12
4. H77
9. H72

As I mentioned, the car runs great when cold or below 200* oil temp. Any warmer and it’s like a switch going off - the car runs awful.
Any ideas would be greatly appreciated.

Thanks,
Greg

 

[toptechx6]

Runs rough hot

My only service manual (93) lists code 12 as "all okay" or the start of the diagnostic sequence, 72 is a gear shift selector error, I can't find code 77 listed, it may have been added to comply with OBDII.
One thought would be a weak ignition coil, I have seen them fade when hot. If you did the recent tune up work trying to correct the problem you might consider taking it to a dealer before throwing more parts at it, could turn out to be a bargain. Sorry I can't offer more help, good luck.

 

[John Robinson]

Check These

I see two possible areas to check. One the TPS switch. Two the valve adjustment. If the rockers are tight then when the engine warms up the expansion from heat could be holding the valve open causing the condition you describe. The TPS switch can do all sorts of weird things including not coding if it is malfunctioning. That I know for a fact because it cost me close to $600.00 to find out my no start was caused by the TPS. I assume you have run plenty of gas through the engine and have fresh gas in the tank.


Do you have coil on your system or are you using the Opti system for spark? I have had a coil do exactly what you describe also.

 

[toptechx6]

I see two possible areas to check. One the TPS switch. Two the valve adjustment. If the rockers are tight then when the engine warms up the expansion from heat could be holding the valve open causing the condition you describe. The TPS switch can do all sorts of weird things including not coding if it is malfunctioning. That I know for a fact because it cost me close to $600.00 to find out my no start was caused by the TPS. I assume you have run plenty of gas through the engine and have fresh gas in the tank.


Do you have coil on your system or are you using the Opti system for spark? I have had a coil do exactly what you describe also.

John, thanks for the insight regarding TPS problems, good to know. For your reference the Opti-spark system does use an ignition coil separate from the reference/distributor portion of the Opti unit unlike HEI distributors.

 

[girvin]

Since my initial post, I replaced the ignition coil. I thought that that fixed the problem, but the next day it acted up again. I am picking up a new (remanufactured) opti spark tomorrow. I wish that I would have replaced it while I was in there doing the cap/rotor change.

If the new opti doesn't solve the problem, I will look into testing the TPS.

Thanks,
Greg

 

[John Robinson]

Take a breath

Hold on Partner don't throw money at the problem. It may be the opti but don't let all the negative hype drag you into changing it. The opti is probable the most maligned part on the car. I would take the car to a good technician who understands the entire ignition system. Go to a few garages that work on cars and have the equipment to diagnose your car. Talk to them about your problem. If they won't talk to you about what could be causing the problem and how it relates to the whole system then move on. You will know when you have found the right person because they will be able to tell you without looking at the car what sensors control what sensors and what symptoms that produces with the engine. This is the person that will find the problem without breaking the bank. Good Luck with this problem

Throttle Position Sensor

The scan tool displays throttle position in volts. The voltage should display between .23 - .59, with throttle closed and ignition “ON,” or at idle. Voltage should increase at a steady rate as throttle is moved toward Wide Open Throttle (WOT) about 4.6 volts.

The ECM has the ability to Auto-Zero the TP sensor voltage if it is between .23 and .59 volt. This means that any closed throttle voltage between .23 and .59 volt will be determined by the ECM to be 0% throttle angle, and should display 0% when the throttle is closed. A failure in the TP sensor circuit should (MY OPINION MAY NOT )set a DTC 21 or 23.

Copied from the service manual for a 93 model

I have started a file of different system checks thought you might like to have this.

 

[girvin]

I wish that I could find a mechanic that would diagnose the Corvette without charging me an arm and a leg. I called around and spoke to 4 different shops (one dealership and 3 general repair shops) - they all told me the same thing. $200-$300 just to diagnose the problem. I figured that I'd come out ahead by replacing the most logical items, one at a time, until the problem was found. I was wrong. On a positive note, I did learn a lot about the LT1 engine in the process. I can now change an opti-spark in just over 3 hours, which I did tonight.

I am happy to say that the car now runs good. Looks like it was the distributor all along. The old one had definately been soaked at some time - there was significant rust on the internals. Receipts from the previous owner show that the water pump has been replaced. When the original went bad, maybe it leaked onto the opti, thus causing the problem.

Thanks for the replies,
Greg

 

[John Robinson]

Glad to hear

It sounds like you have found the problem. My comments about finding a mechanic were not meant to knock what you were doing. Here were I live I have found two good shops that have a technician who knows the systems. The one shop charges $60 for a diagnosis and the other charges based on the time spent at $60/hr so l/2 hour is $30. It sounds like the shops in your area are full of it. Since I only live about 3 1/2 hours from you and you can get to the opti so quick perhaps when I get ready to change plugs and wires you could come and remove my water pump to make the job easier.:L

 

[vee93]

My 1994 Corvette (LT1/A4) runs great when cold, but once the engine reaches approximately 200-210 degrees F (oil temp), develops a rough idle and runs very rough - sputtering and backfiring (through the intake) if I give it too much throttle.

Mods include:

Throttle body coolant bypass
160* thermostat
Crane valve springs
Crane 1.6:1 roller rockers
LT4 knock module
Open air lid
Power couplers
High flow MAF housing (stock MAF sensor)
Air foil
Ford Racing matched 24# fuel injectors (“blue tops”)

Recent tune up included the replacement of the following:

Distributor cap/rotor
Spark plugs
Spark plug wires
Fuel filter
Air filter
PCV valve
(2) Front o2 sensors (did not replace rear)
Coolant temperature sensor
Ignition control module

When I pull codes, using the paper clip method, I get the following:

1. C12
4. H77
9. H72

As I mentioned, the car runs great when cold or below 200* oil temp. Any warmer and it’s like a switch going off - the car runs awful.
Any ideas would be greatly appreciated.

Thanks,
Greg

If you haven't found the source of your problem yet, try this---
Put the gear in Park (Neutral if you have a 6 speed). Apply the parking brake and rev the engine up to 2000 RPMs for a coupla minutes. This simply act will clear out the oxygen sensors.
I had a similar problem with no codes and this cleared up the surging problem I noticed while sitting in traffic at stop lights.