76vette22

I am a fairly experienced mechanic, but I just need another opinion, not having a whole lot of lt1 experience.
Here's what I've got...
93 z28 with lt1. Stock except for msd optispark. Can start the car and let it idle forever without it overheating, but drive around town and the temp goes a little above normal (gets to about 220) but get on the interstate, and it is borderline overheating (was 241 at highest point before got off the interstate) and when i stop, coolant boils from overflow cap

Here are the things I have checked so far...
radiator flows without issue, new thermostat, water pump is circulating coolant, the air dam is intact, no bugs in ac condenser (which is in front of radiator) both cooling fans functioning properly, the heat works, car runs great and has plenty of power, bled the coolant system for good measure...

Now heres the kicker! Hooked my scanner up to it to see whats going on internally, and temp sensors are both accurate, but the oil temperature sensor reads -40F (But unless im wrong, that should not cause the engine to overheat). The fuel trim is much more lean on the driver side than the passenger side, even though the oxygen sensor readings are very close to one another.

I was thinking catalytic converter or head gasket, but because of the fact of having plenty of power and leaner fuel trim on one side, im leaning more towards head gasket... Any insight is very much appreciated

Injuneer

Your Z28 LT1 doesn't have an oil temperature sensor. When there is no sensor, the scanner indicates -40*F. Only the Corvette LT1 has the oil temp sensor installed.

Did you check the radiator cap to insure it is in fact holding 18psi?

Is the radiator always full when you open the cap?

The long term fuel trim (LTFT) adjusts until it returns the O2 sensor readings to an average of 450mV. That's the purpose of the fuel trims - to control the A/F ratio at 14.7:1, which is indicated by the O2 sensor voltage swinging back and forth from rich to lean, with an average value of 450mV.

If you have something on the driver's side that is either actually making it run lean, or causing the O2 sensor on that side to indicate a "false" lean condition, the long term fuel trim on that side only will elevate. False leans can be the results of a faulty O2 sensor or wiring, leaks in the exhaust before the O2 sensor and misfires.

To verify it's not a faulty O2 sensor, swap the sensors side-to-side, and see if the issue with the LTFT's switches to the other side, along with the sensor.

Fatdog2

Might check the thermostat .Some after market stuff may be crap.(Some members installed them upside down to.)Just tryin to help out.My02.

Fatdog2

I think there is also a sensor on the radiator that can go south.Not sure though.My02.

Injuneer

The sensor on the radiator is the level sensor. Has nothing to do with overheating, nothing to do with the PCM, nothing to do with the way the engine runs, unless its actually showing low coolant level and it is being ignored. The level sensor often fails, but that has nothing to do with this problem.

76vette22

Thanks for the insight guys, I forgot to mention, I did not test the radiator cap, but I did replace it just to see if that could be the problem. I also hooked my pressure tester up to the radiator and took the car down the road (maybe not the most brilliant idea) and when the car started to get hot, I pulled off the road, popped the hood, and shut the engine off, and the pressure was about 24psi.

Thats good info to know about the oil temp switch... But my question is if there is no sensor there, then why is there a reading for it? Do the camaros and vettes use the same computer?

As far as my o2 sensors, even though my fuel trim was off, the sensors maintained about the same reading. Does that still sound like it could be a faulty sensor or wiring? There were no misfires detected, and no history of codes present besides a coolant level sensor code, indicating that it had been low on coolant previously.

Also, as I have previously said, the car will idle forever and the temp will never elevate past 114*F. Only while driving does it get hot, and only on the interstate it gets borderline overheating. At the moment, I'm not too concerned if I may have a bad oxygen sensor, I just made note of the fuel trim because I was thinking it was a possible indicator of what the overheating problem may be.

Are there any relatively simple ways to test head gaskets on these engines?

grabbem88

This happened to me a month ago. My wp bearing giving it up at higher rpm
Now the temp stays right at 150-160 no matter what driving style

Might not be your problem but food for thought

Injuneer

1) I already explained why the LTFT's can be different on both sides, yet the O2 sensors are reading the same on both sides. I also explained what could cause differences beteen the two banks of the engine. Please go back re re-read what I posted.

2) There is no misfire detection on a 93 ECM. That's why "no misfires detected".

3) There is no trouble code for a "coolant level sensor", because the coolant lever sensor does not even connect to the ECM. It only goes to the light on the dash. You can't have the code you say you have. Might want to scan it again, and post the number for the code, rather than your interpretation of it.

If it's showing 114*F for the coolant temp at idle, no matter how long it idles, either the coolant temp sensor is faulty, or there t'stat is stuck wide open. Under those conditions, it's not going to go into closed loop.

Have you checked the CTS in the water pump using Shoebox's instructions:

http://shbox.com/1/4th_gen_tech2.html#ect

The temp sensors are thermistors - resistors who's resistance varies inversely with temperature. High resistance tells the PCM the temperature is low. An open circuit (no temp sensor attached) will be "infinite" resistance. -40*F is the lowest temp the PCM can indicate based on the 8-bit binary range.

The Corvette PCM's are similar, same pinouts, but the 94/95 PCM's are OBD-1.5 with the provision for a single after-cat O2 sensor. According to Probst's "Corvette Fuel Injection & Electronic Engine Management" Manual (GM part # 12497977) they can set both the 2-digit OBD-I trouble codes and the Pxxxx format OBD-II trouble codes. Hard to understand how that would work, since the code listing for 94/95 appears to have both 2-digit and 4-digit codes for the same fault.

n2ceptor

Is the black air dam installed under the front clip???

Injuneer

...

Nickw059

What kind of scanner are you using??

76vette22

Thanks everyone for all the input so far, especially Fred

Grabbem,
that is a very good point... Im not hearing any noises or having any leaking, so i'm not much suspecting a bad bearing, but you did spark another idea in my head. I know on the old style gm water pumps, I have seen the fan come loose from the shaft, and therefore the water pump not function correctly at higher rpms.... Does anyone know if this is a known issue with the lt1 water pumps? That may be the next thing I look into.

Fred,
Sorry about the confusion regarding the LTFT and o2 sensors, I was just trying to clarify if that could in fact correct the fuel trim enough to make a faulty o2 sensor give a seemingly correct reading... I have reread your first post, and I think I now comprehend the scenario you are explaining...
Also, as far as the trouble code for the "coolant level sensor" Im sorry, I read the code wrong. I do not remember the number for the code, but my scanner (snapon mt2500) gives me a detailed description of the code, so it is in fact not my interpretation of the code, but rather my reading of the description that was incorrect. The code read something to the extent of temp sensor low voltage, which gave me the impression that the coolant at some point may have been too low for the temp sensor to register a reading. But as I said both sensors are giving readings that directly correspond with each other, and my cooling fans are functioning correctly, so I dont think I have any reason to suspect a faulty sensor as part of my issue.
As far as the coolant temp being 114 at idle, that was a typo. The actual reading is 214*F. Sorry about that. The engine is in fact reaching normal operating temp and going into closed loop at idle. Also, I have not checked the CTS using Shoebox's instructions, but only because the sensors are both giving the exact same readings.

Nick,
I am using a Snapon MT2500 scanner with the "GM-Chrys-Ford-Jeep Domestic Combination" Primary and Troubleshooter cartridges (although unless there is something I am missing, the troubleshooter hasnt provided much help with my particular issue)

Thanks again for the help, and if you guys have any new ideas, please feel free to post them on here, and in the meantime I will be checking my water pump fan to make sure it is good and secure on the shaft and boiling my thermostat in a pot. lol (Shoebox's Idea)

Injuneer

You keep indicating "both sensors are giving the exact same readings". Do you mean the number indicated by the analog dash gauge/sensor in the head matches the digital readout from the sensor in the water pump housing?

76vette22

Yes fred, that is what i mean by that. Sorry if i wasnt clear enough by the wording of it. Havent had a chance to get bak to the camaro, but will update my findings as soon as I do

grabbem88

I try to base most of my issues with common sense and the way I see
It. Water pump/radiator/stat all check out and no leaks with any hose
Fans work and air dam is in tact plus you got heat so basically
Heater core has flow

You said at idle for a period of time the temp holds but driving the temp climbs

What I would do is get car to start climbing pull over and bleed it while
Car is still running and see if temp drops and/or if there was more hidden
Air pockets. Chances are that radiator cap might be faulty as well. Hence pressure
Is different when car is running compared to just idling