The patent: 1994 Corvette Coupe LT-1, Auto trans, 147,000 miles and no performance mods. The Vette is now a weekend car and was last driven 10 days ago with no problems.

Installed new Opti, plugs, wires, o2 sensors and EGR last summer.

The symptom: Intermittent “service engine soon” light. The light will come on when driving and 9 out of 10 times will go out when I have come to a stop light and the engine is idling in gear. The light will reappear once I start driving or if I depress the accelerator slightly.

The Conditions: bright sunny day, 92d F air temp and high humidity. Stop and go traffic and city driving.

There is no noticeable loss of performance and everything operates normally. The only thing out of the normal I did notice is that the engine temp was reading higher than normal 215- 220 vs. 210-213.
I’ll cure that issue next Saturday with a complete flush and fill of the cooling system.

I’ll let you know what codes I pull out later this afternoon.
Thanks for the assistance…..
What are your thoughts?

 

Regardless of the circumstances of when you see the light, or whether you ever do see it again or not, there has been a trouble code set, that needs to be checked and corrected. The code only indicates the sensor or circuit that sent the info that triggered the light. It doesn't list the cause of the light, but it gives you a place to start looking. Finding the cause of the light can only be done by systematic trouble shooting. I don't know enough about the later C4s to give you specific instructions. For the trouble shooting charts, you will need the FSM. The codes can be pulled by shorting terminals "A" and "B" of the ALDL. There may be alternative methods for the '94. Pulling and deciphering the codes can be found by doing a search on this forum, and of course, it will be in the FSM.

RACE ON!!!

 

Here is what showed on my DIC when I jumped the 4 and 12 teminals on the ALDL

1. C12
---

4. H27
---

9. ---

A. ---

then 1.0 showed until I shut the key off

I had no flashing, just what I posted above including the dashes.

Thanks for the help.

 

Here is some usefull info I found from a past thread:
"On a 94 you can check the codes with a paper clip as follows:

You short pin "12" to pin "4" and turn the ignition switch to "On" without starting the engine.

The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:

Module "1" is still the CCM module.

Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.

Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).

Module 9 is the ABS/ASR module number from 1994 through 1996.
Finally, Module "A", the DERM (Dynamic Energy Control Module --- (the air bag control module) will be requested to show any codes.

Just like the 1990-1993 display, you read the codes on the speedometer (big) and read the module number on the trip odometer (small).

A series of dashed lines for instance “ - - -“ indicates no more stored codes for that module.

A code showing as “C12” for the CCM module (module #1) means no codes are stored. So C12 is a good thing to see.

Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.

Turn the ignition off and remove the short to restore normal operation.

To clear the codes,

1. With ignition "off" ground terminal #12 of the DLC (use bent paper clip to connect pin 4 to pin 12)

2. Turn ignition "on".

3. Press trip/odo button on the DIC until 1.7 appears in the trip
monitor area of the instrument cluster.

4. Press eng/met button on the DIC & hold until "---" appears in the
speedometer area of the cluster, this will clear CCM DTC'S.


To clear PCM/ECM codes do the following:


1. Press the trip reset button on the DIC until the desired system is displayed. In this case it will be 4.0.

2. Press trip/odo button on the DIC until the desired diagnostic mode(4.7) is displayed on the trip monitor area of the cluster.

3. Press & hold the eng/met button on the DIC until "---" is
displayed in the speedometer area of the cluster, this will clear PCM codes.


To clear ABS/ASR codes do the following:

Same as above but you’re looking for “9.7” on the trip odo

And here are the codes

1994-1996: Diagnostic Trouble Codes
DTC #11: Malfunction Indicator Lamp Circuit.
DTC #13: Bank #1 Heated Oxygen Sensor #1 Circuit:
DTC #14: Engine Coolant Temperature Sensor Circuit Voltage Low.
DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High.
DTC #16: Distributor Ignition System Low Pulse.
DTC #18: Injector Circuit.
DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #23: Intake Temperature Sensor Circuit Voltage High.
DTC #24: Vehicle Speed Sensor Circuit.
DTC #25: Intake Air Temperature Sensor Circuit Voltage Low.
DTC #26: Evaporative Emission Canister Purge Solenoid Valve Circuit.
DTC #27: EGR Vacuum Control Signal Solenoid Valve Circuit.
DTC #28: Transmission Range Pressure Switch Assembly Fault.
DTC #29: Secondary Air Injection Pump Circuit.
DTC #32: Exhaust Gas Recalculation.
DTC #33: Manifold Absolute Pressure Sensor Circuit High.
DTC #34: Manifold Absolute Pressure Sensor Circuit Low.
DTC #36: Distributor Ignition System High Pulse.
DTC #37: Brake Switch Stuck On.
DTC #38: Brake Switch Stuck Off.
DTC #41: Ignition Control Circuit Open.
DTC #42: Ignition Control Circuit Shorted.
DTC #43: Knock Sensor Circuit.
DTC #44: Bank 1 LF Heated Oxygen Sensor #1 Circuit Lean.
DTC #45: Bank 1 LF Heated Oxygen Sensor #1 Circuit Rich.
DTC #47: Knock Sensor Circuit Or Module Missing.
DTC #48: Mass Air Flow Sensor Circuit.
DTC #50: System Voltage Low.
DTC #51: EPROM Programming Error.
DTC #52: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #53: System Voltage Low.
DTC #55: Fuel Lean Monitor.
DTC #58: Transmission Fluid Temperature Sensor Circuit Low.
DTC #59: Transmission Fluid Temperature Sensor Circuit High.
DTC #62: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #63: Bank 2 RF Heated Oxygen Sensor #1 Circuit Open.
DTC #64: Bank 2 RF Heated Oxygen Sensor #1 Circuit Lean.
DTC #65: Bank 2 RF Heated Oxygen Sensor #1 Circuit Rich.
DTC #66: A/C Refrigerant Pressure Sensor Circuit Open.
DTC #67: A/C Pressure Sensor Circuit Sensor or A/C Clutch.
DTC #68: A/C Relay Circuit.
DTC #69: A /C Clutch Circuit.
DTC #70: A/C Clutch Relay Driver Circuit.
DTC #72: Vehicle Speed Sensor Loss.
DTC #73: Pressure Control Solenoid Circuit Current Error.
DTC #74: Traction Control System Circuit Low.
DTC #75: Transmission System Voltage Low
DTC #77: Primary Cooling Fan Relay Control Circuit.
DTC #78: Secondary Cooling Fan Relay Control Circuit.
DTC #79: Transmission Fluid Overtemp.
DTC #80: Transmission Component Slipping.
DTC #81: Transmission 2-3 Shift Solenoid Circuit.
DTC #82: Transmission 1-2 Shift Solenoid Circuit.
DTC #83: Torque Converter Solenoid Voltage High.
DTC #84: 3-2 Control Solenoid Circuit.(Auto Only).
DTC #84: 2nd And 3rd Gear Blockout Relay Control Circuit.
DTC #85: Transmission TCC Stock On.
DTC #90: Transmission TCC Solenoid Circuit.
DTC #91: One To Four Upshift Lamp(Manual Only).
DTC #97: VSS Output Circuit.
DTC #98: Tachometer Output Signal Voltage Wrong.

Check and see what you have there, code C12 is fine, and you should get this code. 'H' infront of a number means 'history' or past issue which is not recently present, 'C' means 'Curent' or still an ECM known present issue."

I found all that info in a past thread...so you newbies...start doing a search for your problems...and you'll find a ton of info

The good news is I ran the car for a short time after I changed the oil and reset the gauge...guess what...no "service engine light"
I'll drive it tonight when the weather gets cooler and see if I have and thing show up. I'll also check the EGR solenoid

 

I had the same problem on my 90 with the intermittent SES light. I had my mechanic run a diagnostic code and it said something about the EGR valve. Something not too important unless youre passing emissions allegedly. Anyway, since then, the light hasnt come on and Ive been driving for about a week. I wonder if the diagnostic code disables it somehow after you read it the first time. We were planning on replacing the gaskets on the EGR pipe, but I just havent gotten to it yet. Wierd though, before you wrote down those codes I was gonna mention the EGR since its probably the one code for the SES where you dont really notice any performance issues while the lights poppin up frequently.

 

When you replaced the O2 sensors did you check your cats? I had less than 60K when the passenger side failed? It showed similiar symtoms to what you discribe.

 

your symptoms and code do makes sense.

There is a chance that the EGR solinoid is failing or has failed.

be sure to inspect the harness leading to and from the EGR solinoid. Also be sure the vacuum lines are good.

the solinoid is located on the left side of the engine, on the intake manifold, near the rear pair of injectors. It's bolted to an intake manifold stud.

 

I pulled the same exact codes from my 94 approx 1.5 years ago. Just changed the EGR Vacuum control signal solenoid valve. Delete codes in system by turning off battery. Good to go. Let me know if that helps.

 

Update: Problem solved when I replaced the EGR solinoid

 

FYI the history codes can be very recent history from the last time the key was turned off, to very old history the last time the battery was disconnected or the memory cleared.

 

Im having the same problem on my '90. We ran a diagnostic and it said the EGR valve was the issue. Same deal here?

 

In the case of 69mako he change the solenoid, which is monitored by the computer. The actual EGR valve is vacuum controlled and not monitored by the computer. So if the EGR valve is defective you actually have to troubleshoot it manually and check it to know if is operating properly.

So your code would refer to part monitored which is the EGR solenoid.
Best of luck.

 

The solinoid and the EGR valve are cheap parts to replace,
if the EGR won't hold a vacuum with your gauge on it...it's time to replace.

An EGR going bad will also give you a bit of a stutter once the motor warms up and the loops change

 

I found my EGR valve bad not holding vaccume either when looking for another problem over a year ago. The diaphram had a tear in it and would never open.

I'm not a EGR expert but I beleive they open when you take your foot off the gas. I don't they play a part when you accellerate. When I repacled mine I saw no difference at all. But then I think that the way it should be.

If I am off target someone let me know.

 

Service Engine Soon just means your car is turned on.







seriously though..the EGR is easy/cheap to replace. It made a world of difference when trying to troubleshoot mine. I had the original one still installed when I was trying to get mine timed, so my broinlaw pulled the sensor off and we noticed the valve was stuck shut, the screw for the valve was completely shot. We got a new one down at Vatozone and immediately the car idled better and wow it made a world of difference! If its the original one, just replace it.

 

This morning on the drive in to work, I drove between two truckers and noticed one of them was operating his CB radio.

Must have been some pretty powerful radio equipment he was packing because his transmissions came over my Bose amps and it caused my SERVICE LTPWS lamp to snap on my DIC.

 

I have had CB radio on my old Bose, too... scared the **** outta me the first couple of times.

 

The first time an amp Blows at volume it will scare the stool outta' you, too.

Remember that scene in the WW II movie Memphis Belle were you can hear the death throes of that doomed B-17 and her crew personified by the screech of her radio as she enters a dive from which no bomber of that size can recover after being hit by German ack-ack?

That's exactly what that shet sounds like!

 

My SES light came on at the track today, after a short break and filling up with gas, when I went out on the track again the SES light did not come on again.

It has only been about 1,500 km (1,000 miles) since I serviced the motor, changed oil etc, but have had two outings on the track.

What I would like to know is does this mean that I should change oil again now?

Under normal road use when the SES light comes on to indicate that oil needs changing etc, does it stay on, or just come on for a short while like mine did today?

Thanks for input