Aleph Bayer

I've got here a c4 1991 6speed on Brazil and I'm having some issues with it.
First of all, it keept getting hot, then I saw that the fans weren't working. I removed both relays and tested them, and both worked well. Then I tested both fans by connecting a 12v direct to them, and also worked. I tried to connect the A and B wires of a plug under the steering wheel, and the system worked (as a video from YouTube showed me).
After these tests, I noticed that the Idle was never going down, like every injected car. It kept on high rpm like it was trying to warm the engine, but it was already hot.
I've read some posts here and I saw that the ECM uses a sensor on the manifold (next to the pulley) called Coolant Sensor to switch the fan and make the Idle go normal after it gets hot.

My doubt is if this is correct and if there's a way of testing this sensor so I can be sure that this is the problem, because, in Brazil, Corvette parts are hard to find.

Cheers

belairbrian

Using the Gauge button, select coolant temp on the instrument cluster display, what does it read?


The high idle may be a vacuum leak.

Aleph Bayer

I can't find here the gauge button, I think my model doesnt have it.
See the photo below.



I saw on Google that vettes have this button where my "range" is after 92, mine is 91.

About the Idle problem, it's also a possibility, I'm looking for some leak here.

ctmccloskey

First and foremost, Welcome to the Forum!! I hope that you find the answers to your questions easily!

Testing the coolant temperature sensors is fairly simple. You have a specific resistance value that indicates a specific temperature. Does the idle ever return to the normal range on your car or is it idling fast all the time?

You mentioned that the car is running hot as well? Have you verified that the temperatures you are seeing are actual temperatures? You can use an infrared thermometer to verify the temperatures that you are seeing on the gauges. On my car the gauge could read 180 Degrees but the computer sensor could be reading 30 degrees which could cause some cranking issues. Even though I see normal temperatures I still verify what my engine "sees" to be sure to eliminate issues with the fuel system. This is the cause of long cranking times and numerous other starting/running problems.

Remove the cover from over the radiator and expose the radiator and AC Condenser. Frequently there will be lots of road trash building up between the condenser and radiator. There are posts where they show you how to make a screen cover for the air opening.

Do you have a Factory Service manual for this car? They show you how to test the temperature sensors in detail. My car has two temperature sending units, one for the dashboard gauge and the other is directly wired to the Engine Control computers to tell the computer what the engine's temperature is. Again, I am sorry to say I don't know if this applies to your car, this is why you need the FSM for your year Corvette.

Have you tried adjusting the TPS (Throttle Position Sensor) back to the factory settings? That would clearly cause the problems that you describe. I would check and adjust if needed both the "Idle Air Control" (IAC) and the throttle position sensor (TPS). The TPS can cause weird idling issues if it is out of adjustment

If the car is still idling fast then I too would advise looking for a vacuum leak someplace. The temperature sensors are important for cold starting but should not be doing what you are describing. If one of them fails then the engine could be seeing incorrect data, fortunately they are easy to change and fairly inexpensive.

I am not sure that you car has a cold start injector, I have a 1988 Coupe and it does have one. These sensors do not operate unless the ambient temperature is below a specific number. If you do have an cold start injector it would be on the drivers side at the rear of the plenum. The injector itself has a plug on it that can be unplugged to verify that it is okay. There are many people who block these off or disable them when they have trouble. Mine works but I had to buy four to get one working unit.

Check out the throttle body for vacuum leaks and then verify the operation of the TPS and IAC units and be sure that they are set correctly. These are likely the culprits that are haunting you and are easy to resolve.

Good Luck with your Corvette and I hope that you find the cause of the high idle.

Aleph Bayer

Thanks for the attention and the welcome!

Answering your questions:

The Idle never returns to normal, and it's kinda bad to turn on the car when it's already on normal temp.

​​​​​​When I said the car runs hot, I meant that it never switches on the fan. So, if I let the car on, it will overheat. I couldnt use the car yet because of this issue. Actually it doesn't run hot, it simply keeps getting hotter and hotter. That's why I think the Idle issue has something to do with it. It's like the sensor ain't working and it doesn't say to the ECM that the car is already hot and it can slow down the Idle and, eventually, switch on the fans.

My car also has only 2 temp sensors, one for the analogic gauge, and the one that goes to the ECM. Unfortunately I don't have the FSM, i have only the owners manual (it's not the same, right?).

About the IAC and TPS, before going there I wanna make the fans work, because I can only leave the car like 3 minutes on so it begins to reach high temps.

I live in Rio de Janeiro, it's very hot in here, I don't think the cold start injector would operate on these conditions.

If after solving this fan issue the car continues to Idle weirdly, I'll try this things you mentioned, thanks a lot!!

aklim

I'd get a scanner so he can see what the ECM sees and hit the sensor itself with an infrared thermometer. Both should be kinda close. Not sure what he means by "fast" and how he knows that but the gauges aren't the best. For whatever reason my 91 Firebird has more accurate oil pressure and temperature than the 91 Corvette.

You can't do that. There is no adjustment on a 91. I believe the earlier ones were slotted so there is adjustment but my 91 does NOT have slots. Just 2 screw holes. Unless it is far out of range, the startup voltage is considered the "baseline". Before adjusting the IAC, I'd check to see what the desired idle speed or command idle or whatever it is called is. If somehow it is set higher, we will be chasing ghosts.

I'd check the timing to ensure it is at 6 degrees or anything we are trying is going to have limited effects. As to whether it is high or not, again, scanner will tell.

I don't think he has a cold start injector. I thought they ended after 87 or 88. Generally speaking, I'd have that injector tested or get a tested one instead of getting one and hoping it works.

A good way to check the IAC and TPS is with a scanner. If he doesn't have one, she should get one since he will need it sooner or later. Get the car to stable operating temperature and shut everything but the motor off and see what the scanner says for IAC counts. If he cannot get it to about 20-30 counts and it is always at 0, he has an air leak somewhere.

aklim

What is your definition of normal? It is hard to say since I don't trust the tach. Get a scanner and life might be easier since you now see what the ECM sees.

How do you know it is getting hot? Just the gauge or did you do a thermometer on the sensor? IIRC it is the one under the TB.

Totally different.

What do you mean by "high"?

Definitely not since you don't have one on a 91. Mine doesn't.

How do you know the fan is good as is the relay?

ctmccloskey

Be sure to order a Factory Service Manual for your particular year. These are critical if you want to fix your car yourself.

What happens when you turn on the Air Conditioner? That should start the cooling fans up automatically. The fans do have a relay that operates them via the ECM. On my car it is near the back on the drivers side according to the manual.

Since it appears that your TPS is installed without adjustments then start looking closer at the possibility of a vacuum leak. When I bought my car the idle was going up and down and it turned out to be a broken vacuum hose near the cruise control parts. I also found a vacuum leak at the EGR solenoid on my car.

Being as old as these cars are the rubber parts are breaking down fast in most cases. I eventually bought enough vacuum hose and replaced most all of my hoses with new Hose.

Do you have an Check Engine Light on? Have you tried to see if you have any codes on the computer?

I realize it is getting warmer this time of the year in the southern hemisphere but I never heard of any car going from a cold start to overheating in three minutes.
That is not normal at all, I wonder if the thermostat is stuck closed as this might force the engine to heat up like you describe. Have you checked the Thermostat? While the car is running you should feel hot water going to the radiator's inlet if the thermostat is opening. If it is not opening the engine would just get hotter and hotter.

When was the last time you serviced the cooling system? Are you using the proper amount of anti-freeze? I have seen people use 100% A/F and wonder why their car runs so hot. In your climate down in Brazil you probably don't see many freezing temperatures so you could use less A/F and more distilled water. Water is more efficient at getting rid of heat then A/F is. The less risk of freezing would make me use more water in the mixture.

What are the temperatures (ambient and on the cars' gauge) that you are seeing when the car is overheating? These cars have high fan activation temperatures unless you turn on the A/C.

Has the car been unused for a while? Does it have the original radiator and shrouding? Do you have enough coolant in the engine and overflow bottle?

You can check the fans to be sure that they operate by applying power into the fans wiring connector. You might try this to be sure that the fan motors are good. Starting at the fans work your way back on the wiring and be sure everything is connected properly.

This is starting to sound like you have several items all failing together, check out that the thermostat is working properly and then go after why the fans are not starting or running.

The high idle is probably a vacuum leak but it could also be the other temperature sensor feeding bad temperature information into your cars computer telling it that it is very cold outside and since you do not have the 9th injector (Cold Start Injector) the computer would let the engine run a bit faster for warm up. Check the temperature sensor for the ECM, identify it and verify the reading is correct.

Please answer these questions and maybe one of us here on the Forum can have a "Aha" moment where we figure it out and help you get that Corvette on the road where it belongs!