Help! 1994 LT1 Service engine soon light turns on and off

[SnakeDr]

I had corrected some issue with my car, went through inspection and everything was okay.

Then about 500 mile, later. I fixed the electron dashboard HVAC module, removing it from the dash, dismantling it and cleaning the buttons with alcohol. Had the system recharged and it works good.

While going through the Lincoln Tunnel, the service engine soon light started turning on and then off. It was pretty hot that day and the car appeared to be running hotter than normal. I thought initially it was being tripped by the A/C and kept it off.


Today was my first chance to look at it. It appears the light comes on when the cooling fans turns on, and when the fan turns off then then the light turns off. This happens consistently with the A/C is off. I tried pulling codes and I see the following:

the number four flashes: the 43 and then 68 and then --- I believe. On my previous thread I thought this was all corrected. Not sure if these are old codes.

Both fans appear to be running. It will turn on and off while with light in sync. Happens while it idols and cruising.

Please help!!!

 

[John Robinson]

Look for a short in the dark green wire that runs from the A/C compressor clutch and also the A/C clutch relay to the C1 pin on the ECM. Take and disconnect the A/C compressor clutch and with a test light to ground turn on the ignition and probe the Dark Green wire. If it comes on then disconnect the A/C relay. If it still lights the test probe there is a short in the Dark Green circuit. If you do not get a light with the probe then you can start to look for a bad relay or bad ECM. A short at any place in the dark green wire or grounded relay control circuit or stuck A/C relay contacts can set a DTC 68.

 

[Robertwav1]

I had corrected some issue with my car, went through inspection and everything was okay.

Then about 500 mile, later. I fixed the electron dashboard HVAC module, removing it from the dash, dismantling it and cleaning the buttons with alcohol. Had the system recharged and it works good.

While going through the Lincoln Tunnel, the service engine soon light started turning on and then off. It was pretty hot that day and the car appeared to be running hotter than normal. I thought initially it was being tripped by the A/C and kept it off.


Today was my first chance to look at it. It appears the light comes on when the cooling fans turns on, and when the fan turns off then then the light turns off. This happens consistently with the A/C is off. I tried pulling codes and I see the following:

the number four flashes: the 43 and then 68 and then --- I believe. On my previous thread I thought this was all corrected. Not sure if these are old codes.

Both fans appear to be running. It will turn on and off while with light in sync. Happens while it idols and cruising.

Please help!!!

Strange you have the same code as mine but I only get 43 which is the engine knock sensor. My SES light will come on as well as the fans and then go out. I'll clear this code and it may not happen again for another year or at least several months in between. I changed the knock sensor at one time and the problem was away for several years then, it came back. I'm thinking a connection under the car with one of the knock sensors...next time I do an oil change I plan on looking.

 

[John Robinson]

Check the wire on the knock sensor. They have a habit of getting against the exhaust and shorting out.

 

[Robertwav1]

Thanks John I'll check that.

 

[SnakeDr]

Strange you have the same code as mine but I only get 43 which is the engine knock sensor. My SES light will come on as well as the fans and then go out. I'll clear this code and it may not happen again for another year or at least several months in between. I changed the knock sensor at one time and the problem was away for several years then, it came back. I'm thinking a connection under the car with one of the knock sensors...next time I do an oil change I plan on looking.

When I shorted the pins, it cycles through 1 through 3 with no error codes. Then it gets to 4 and then flashes 43 and 68. Does the 1994 LT1 VIN P have two or one knock sensor? I had a bare wire connected knock sensor wire grounding out on the heat shield and used electrical tape. I would have to jack it up and take a look.

 

[John Robinson]

I don't know about the 94 but my 93 FSM shows 2 knock sensors. The Chilton says that the V P has two knock sensors.

 

[SnakeDr]

I don't know about the 94 but my 93 FSM shows 2 knock sensors. The Chilton says that the V P has two knock sensors.

1994 has two knock sensors also. I finally had a chance to take a look at the car again yesterday and today.

Yesterday I changed the A/C relay. After changing it my check engine soon light stopped flashing on and off. Now it was staying on as of 8/11/2012 Saturday. No other noticable pluses were going on after it.

Today. 8/12/2012. I had a chance to jack it up. I figured the electrical tape I used to correct original knock sensor issue had failed. Checking either sensor left or right it was possible it was grounding out ( on the passenger side the tape was stuck to the 02 sensor). I went to autozone and home depot looking for high heat electrical tape. I found some tape in the 280 degree plus range. I also found some better tape online. I will order it but its rated for around 400 degrees. For now I used the off the shelf tape. I taped up driver and passenger side knock sensor and used a wire tie to tie it up neatly. Drove it around for a couple of hours and I have NO service engine soon light coming on.

So...the only issue I have is that the Corvette is still running hot after and is NOT throwing any error codes. The radiator doesn't look dirty. The only other possible issue could be a bad water pump or temperature switch??? Help!!!

 

[John Robinson]

Ok the first thing we have to determine is what are you calling hot? The second fan without the A/C on is not even programed to come on until it gets to 228 degrees. I believe both fans will be on when the AC is on. In traffic it is not uncommon for the temp to get up to 235 degrees. My normal temp on the road is 208. When I get into stop and go traffic the temp will creep up to the 235 mark. On rare occasions when I have been caught in a traffic back up on the Interstate in 90 + temp I have opened the windows and turned off the air and turned on the heater but only if the temp is going past 240 degrees.

This is only my opinion but it seems that if the temp gets up to 235 or above I get a code 32 for the EGR. In reading the code 32 diagnostic sheet it says that a code 32 can be caused if the ECM reads a high temp sensor.

Another observation I have been noticing with the temp is it will hang around 197 to 208 while the oil temp is below 220 but when the oil temp reaches 240 or higher then the temp goes up almost on a one to one basis with the oil temp. I am going to go out on a limb here and state that in my opinion some so called full synthetic oils do not dissipate heat as well as others do. My reason for that statement is that on my last oil change the shop doing the change used there full synthetic oil and not Mobil 1. I know that this will probable start a lot of comments but I have noticed that my oil runs consistently 10 degrees higher than it did with the Mobil 1. and that directly gets passed to the water temp readings. Wouldn't it be interesting if we could somehow track what temp are being seen by others and identify what oil they are using that would make an interesting graph.

I would suggest you take your car to a shop that can pressure test the system and also test the radiator cap.

 

[SnakeDr]

Ok the first thing we have to determine is what are you calling hot? The second fan without the A/C on is not even programed to come on until it gets to 228 degrees. I believe both fans will be on when the AC is on. In traffic it is not uncommon for the temp to get up to 235 degrees. My normal temp on the road is 208. When I get into stop and go traffic the temp will creep up to the 235 mark. On rare occasions when I have been caught in a traffic back up on the Interstate in 90 + temp I have opened the windows and turned off the air and turned on the heater but only if the temp is going past 240 degrees.

This is only my opinion but it seems that if the temp gets up to 235 or above I get a code 32 for the EGR. In reading the code 32 diagnostic sheet it says that a code 32 can be caused if the ECM reads a high temp sensor.

Another observation I have been noticing with the temp is it will hang around 197 to 208 while the oil temp is below 220 but when the oil temp reaches 240 or higher then the temp goes up almost on a one to one basis with the oil temp. I am going to go out on a limb here and state that in my opinion some so called full synthetic oils do not dissipate heat as well as others do. My reason for that statement is that on my last oil change the shop doing the change used there full synthetic oil and not Mobil 1. I know that this will probable start a lot of comments but I have noticed that my oil runs consistently 10 degrees higher than it did with the Mobil 1. and that directly gets passed to the water temp readings. Wouldn't it be interesting if we could somehow track what temp are being seen by others and identify what oil they are using that would make an interesting graph.

I would suggest you take your car to a shop that can pressure test the system and also test the radiator cap.

Tonight I changed the thermostat. It seemed to be a little better. I didn't witness any boiling over into over run bottle in the front of the car.

Here are some pictures of my temperature gauge. The high side is after driving on the highway and pulling over. I assume the high side is about 240 degrees.

The low side is on the return trip. It had crept up on the high side shown in the photo but then dropped down to 235 degrees when the fans kicked in. This is while the car is sitting there idling for a while.

 

[John Robinson]

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Tonight I changed the thermostat. It seemed to be a little better. I didn't witness any boiling over into over run bottle in the front of the car.

Here are some pictures of my temperature gauge. The high side is after driving on the highway and pulling over. I assume the high side is about 240 degrees.

The low side is on the return trip. It had crept up on the high side shown in the photo but then dropped down to 235 degrees when the fans kicked in. This is while the car is sitting there idling for a while.

If you have had boil over as you call it that would indicate to me that the pressure cap is not holding 16# of pressure. The purpose of the pressure cap is to raise the boiling point. Unpressurized water will boil at 212 degrees by adding pressure it raises that boiling point. If you can not find someone that can pressure test the cap get a new one. Don't buy a cheap one spend the extra 3 or 4 dollars for the top of the line cap. The next thing would then be to fill the overflow tank 3/4 full on a cold engine and then bring the car up to temp and let it sit until it cools off. Keep an eye on the overflow tank and check the fluid level before you start the car it should have gone down so add more 50/50 antifreeze and drive the car and each time for a few days before starting the car again check that fluid level. What that does is to purge the air pockets in the engine and when it cools it will suck fluid back into the engine to fill the air pockets. If that overflow tank is to low it can not replace the air with fluid.

 

[SnakeDr]

If you have had boil over as you call it that would indicate to me that the pressure cap is not holding 16# of pressure. The purpose of the pressure cap is to raise the boiling point. Unpressurized water will boil at 212 degrees by adding pressure it raises that boiling point. If you can not find someone that can pressure test the cap get a new one. Don't buy a cheap one spend the extra 3 or 4 dollars for the top of the line cap. The next thing would then be to fill the overflow tank 3/4 full on a cold engine and then bring the car up to temp and let it sit until it cools off. Keep an eye on the overflow tank and check the fluid level before you start the car it should have gone down so add more 50/50 antifreeze and drive the car and each time for a few days before starting the car again check that fluid level. What that does is to purge the air pockets in the engine and when it cools it will suck fluid back into the engine to fill the air pockets. If that overflow tank is to low it can not replace the air with fluid.

Received a new cap today and changed it. i will follow the directions above and post results in a couple of days.

 

[SnakeDr]

Received a new cap today and changed it. i will follow the directions above and post results in a couple of days.

Still having a slight problem but near as acute (I believe) and this maybe due to the weather has cooled. However each change below has shown a slight improvement.

So far I have changed the thermostat and lastly the radiator cap. Each change was made one at a time, within a week gap of time or more. My upper reservoir tank was showing green when I bought the car. Then after about 2k plus miles of driving it started turning slightly but it was still largely green. Checked the oil cap and it looked good, no sign of a blown head gasket or cracked head. Yesterday, I put some prestone flush in the system, drove it around and flush it out today with water. Took it one step further, following the FSM, with the radiator cap removed and both knock sensors remove, I completely drained the system cooling system. Ran additional water through it until cleared up.

I have 100% water in it now. I want to flush it one more time and then add back anti-freeze. Should I used Dexcool or Green Anti Freeze?

It seems to be running better/slash cooler. I'm thinking about removing my radiator also just to make sure there is no debris stuck in the front of the radiator. There appears to be no way to check it without removing the radiator?

Tonight its about 66 degrees and it took a while to warm up. Changed from running the heat or A/C and the highest it reached was 210 degrees cruising.

 

[Hib Halverson]

I'm coming into this thread late, but looking at the images above, I got to thinking that you may be trying to use the analog coolant temperature gauge. Keep in mind that the late C4 gauges are totally inaccurate.

You need to use your digital display (the gauges button) to read ECT.

You may find your engine is not running as hot as you think.

If you are running straight water, don't do it for long without adding a product like Red Line Water Wetter. Water by itself is actually fairly corrosive and, if you have not antifreeze in the coolant, you need the corrosion inhibiters which are in Water Wetter. Otherwise you need to run at least 25% Dexcool or other antifreeze to have adequate corrosion inhibiters in the coolant.

 

[SnakeDr]

I'm coming into this thread late, but looking at the images above, I got to thinking that you may be trying to use the analog coolant temperature gauge. Keep in mind that the late C4 gauges are totally inaccurate.

You need to use your digital display (the gauges button) to read ECT.

You may find your engine is not running as hot as you think.

If you are running straight water, don't do it for long without adding a product like Red Line Water Wetter. Water by itself is actually fairly corrosive and, if you have not antifreeze in the coolant, you need the corrosion inhibiters which are in Water Wetter. Otherwise you need to run at least 25% Dexcool or other antifreeze to have adequate corrosion inhibiters in the coolant.

Yes. I have been relying on the on the analog gauge. thanks for the advice. I have not even thought about using the digital gauge. So in a 1994 LT1, I should be using Dexcool anti-freeze?

It's true...it maybe not running as hot as I think it is. Is it common to see a little wetness on the top of the overflow bottle?

 

[Hib Halverson]

Once you compare the ECT seen on the digital display with what the analog gauge says, you will be surprised at the disparity.

As for antifreeze...I prefer Dexcool, but there are a lot of people who do not like it. The factory fill that year was "traditional" (yellow, green, blue or whatever) antifreeze. Actually, the main difference (besides the color, of course) between Dexcool and traditional antifreezes is not the antifreeze–they both use ethylene glycol–it's the corrosion inhibiters. Dexcool uses a calcium-based inhibiter package and traditional antifreezes use a silicon-based inhibiter package.

As long as you change your coolant every three years so, either works well.

If you tend to go longer between coolant changes...say 4-6 years, Dexcool is is the better choice.

 

[John Robinson]

Yes. I have been relying on the on the analog gauge. thanks for the advice. I have not even thought about using the digital gauge. So in a 1994 LT1, I should be using Dexcool anti-freeze?

It's true...it maybe not running as hot as I think it is. Is it common to see a little wetness on the top of the overflow bottle?

Lets revisit how the closed radiator system works. When the coolant gets to temperature it expands. When it expands it has to go somewere and that place is the overflow tank. The overflow tank is not a closed tank so if you have put a lot of fluid in it then the overflow will come out of the top and so yes you will see wetness on the top of the overflow tank. Earlier I suggested you over fill this tank just so the system would have some fluid to suck back in when it cooled off. It shjould only take 3 or 4 hot cold cycles for the system to purge all the air in it. Until you get all the air pockest out you could have an air pocket develope at the temp sensor and that would give a much higher reading than normal. My suggestion is to stop fussing with it and drive it a few days to see if it sittles down. As for cleaning the radiator I have found that if I put a crevice tool on my vacuum and lay under (dont even have to jack it up) the car I can reach up in front of the radiator and vacuum the front of it..

As for what is normal for one of these cars here is what I experenced last thursday on a 666 mile one day trip. My temp was consistantly at 208 Degrees per the digital meter. I had one spot that I got into a huge traffic jam on the interstate and sat for several min. The temp crawled up to 235 degrees I turned off the AC and it started to drop back even while sitting still.

 

[SnakeDr]

Once you compare the ECT seen on the digital display with what the analog gauge says, you will be surprised at the disparity.

As for antifreeze...I prefer Dexcool, but there are a lot of people who do not like it. The factory fill that year was "traditional" (yellow, green, blue or whatever) antifreeze. Actually, the main difference (besides the color, of course) between Dexcool and traditional antifreezes is not the antifreeze–they both use ethylene glycol–it's the corrosion inhibiters. Dexcool uses a calcium-based inhibiter package and traditional antifreezes use a silicon-based inhibiter package.

As long as you change your coolant every three years so, either works well.

If you tend to go longer between coolant changes...say 4-6 years, Dexcool is is the better choice.

You are correct. There is a disparity between the analog vs digital gauge. I took a spin during lunch time and noticed it right away. The car didn't finish warming up until the return trip.

 

[SnakeDr]

[/COLOR]Lets revisit how the closed radiator system works. When the coolant gets to temperature it expands. When it expands it has to go somewere and that place is the overflow tank. The overflow tank is not a closed tank so if you have put a lot of fluid in it then the overflow will come out of the top and so yes you will see wetness on the top of the overflow tank. Earlier I suggested you over fill this tank just so the system would have some fluid to suck back in when it cooled off. It shjould only take 3 or 4 hot cold cycles for the system to purge all the air in it. Until you get all the air pockest out you could have an air pocket develope at the temp sensor and that would give a much higher reading than normal. My suggestion is to stop fussing with it and drive it a few days to see if it sittles down. As for cleaning the radiator I have found that if I put a crevice tool on my vacuum and lay under (dont even have to jack it up) the car I can reach up in front of the radiator and vacuum the front of it..

As for what is normal for one of these cars here is what I experenced last thursday on a 666 mile one day trip. My temp was consistantly at 208 Degrees per the digital meter. I had one spot that I got into a huge traffic jam on the interstate and sat for several min. The temp crawled up to 235 degrees I turned off the AC and it started to drop back even while sitting still.

Not really fussin over it but keeping an eye on it. After draining it last night it clearly needed to be flushed or completely drained. I only revisted this issue after a co-worker asked about it who is a ex-certified mechanic.

Glad I took one step further.

 

[WhalePirot]

If you can not find someone that can pressure test the cap get a new one.

New to this area is O'Reilly's parts who will sell/take return on a testing kit; so, it's free as long you return it within 48 ( I think) hours. Their kit includes a cap tester.

If you are running straight water, don't do it for long without adding a product like Red Line Water Wetter. Water by itself is actually fairly corrosive.

Emphasis MINE, in case you need it; an amazingly SHORT time for corrosion to start, especially on those aluminum parts... like heads. I didn't think it'd be alone in there for long, then life happened.

Don't repeat my mistakes, please.