smallblock87

Here's my issue, I have an '86 Corvette that keeps overheating. I've read a few posts on what to check and here is what I've found/done so far (FYI my fan is wired to a switch so I had it running during all of the tests):

Water pump good? - Check. I just replaced it a couple hundred miles ago. I can visibly see the radiator level drop (temporarily) quickly with the cap off while starting the car. Shaft spins freely and without any play. (belt tension is also good)

Thermostat good? - Maybe. Upper hose doesn't get very hot with a brand new thermostat in. However, I removed the stat entirely (upper hose got much hotter) and the engine temp STILL rose towards overheating while at idle. If I got the car above about 30mph it would cool back down.

Radiator good? - Check. It's practically brand new and there are no leaves or crap clogging up the front.

Fan turning on? - Check. As I said above the fan was running at all times. It seems to be running at the same speed it was before I was having these overheating issues.

One more thing to note, the car has been sitting for a couple months. Not sure if that contributed or not. I'm kind of at a loss as to what could be happening. The only thing I can think of is an air bubble but the level seems fine every time I check it. I'm sure it's some rookie mistake I'm making.

Anybody have any advice?

AGENT 86

Could be an air lock, if nothing has changed since letting it sit for months.
Do you see coolant flowing with cap removed and engine running at operating temp ?

leesvet

What are you calling HOT ?

You may be surprized at what [I]Normal [I] really is to a Corvette...

Unless you are cruising at 240+ then you are probably closer to normal than you might think.,The C4 is not grandpas Oldsmobile...its the first of this newest generation of high performance cars that make big HP and get great gas milege....they run hot and were made to run hot.


Benefit of the doubt...lets say that your engine really IS running too hot...hitting 240 and climbing in a simple 65 mph cruise...not good.

You gotta check ign timing, lean fuel inj, real retarded timing, vac leaks (big'uns) EGR function, IAC function and leaking,
and did you do ANYTHING about diagnossing the fuel injectors themselves? they can/DO run lean when plugged with that ancient design thats in the C4...it needs an immediate upgrade to Bosch-III to get the operating spec back to 21first century. That will hel

Describe what YOU see on the TEMP gauge and lets see how far out of bounds that may be.
You might be surprized to learn what "normal really is based on yout l;ocation and ambient temps for the season.

mcm95403

Remember that the thermostat is an "on/off switch" for the coolant. It's function is to stop water from flowing into the radiator so that the water already in there can shed heat. When the water in the block gets to the opening point of the stat, it opens, the hot water flows out into the radiator and the (relatively) cool water from the radiator flows into the block. When that cooler water hits the stat, it closes and the process repeats. When you remove the stat you remove an important functional piece. That is why people who have "overheating problems" and then put in a LOWER temp stat thinking that it will reduce engine temp still have problems. A HIGHER temp stat will help to control the temperature better - other wise it's always open and the radiator can't do it's job because the water is rushing through it too quickly to shed heat.

Cliff Harris

First thing: what do you consider overheating? The original thermostat is 195 degrees, which determines the MINIMUM temperature of your cooling system. My car will run at 210 - 220 degrees and go to 230 degrees on a hot day with the A/C on. Normal.

The fact that the coolant level drops in the radiator should tell you something. That coolant is going SOMEWHERE, and where it's going is into the air bubble you have in your engine. There are a couple of ways to get an air bubble out of your cooling system.

First method: "packing" the cooling system. Let the engine warm up with the radiator cap off. When it's up above the thermostat opening temperature, run the engine at about 2000 RPM (I wedge a screwdriver in the throttle linkage to hold it there). Add coolant until the radiator is full. Put the radiator cap back on. Release whatever you used to keep the RPMs up. Make sure you put the radiator cap back on first, or you'll get a coolant overflow.

Second method: get the CORRECT radiator cap for your car and run through 3 or more cold to hot temperature cycles, which will purge the air from your cooling system.

The best way to go is to do BOTH. You shouldn't have air in your system if the radiator cap is working correctly. I recommend a new one.

smallblock87

Good stuff guys, I appreciate the help.

When I say hot i mean it rises to the red region on the gauge and I shut it off. So that's just shy of 260 if i recall correctly (gauges are from a TPI Trans Am, digital crapped out a long time ago).

Leesvet, I forgot to mention I've taken out the condenser. No more A/C in this car since I use it for the track only. Hence, the sitting there for a while... So i've got a pretty good view of the radiator.

I'll try "packing" the system tonight. I'll see if I can grab a new radiator cap too just for good measure. Any special way to tell for sure if it's the CORRECT one?

oldalaskaman

180 t-stat, robert shaw, lower temp. fan switch, manuals get to be a pita, 60-40 antifreeze mix, burp it, make sure your overflow is bout 1/2 full,when your t-stat opens, look in radiator and check for flow. I installed a radiator flush fitting in the IAC hose and bleed it there, no issues
bit more money, but an aluminum radiator. stocks are 30 plus years old and yours may be plugging

John Robinson

You need to get one of those infrared temperature readers. With the car to temperature then use the temperature reader to check the radiator at different spots. For instance each corner and across the radiator from the top to the bottom. What this will tell you is if the radiator is plugged inside. A hot spot would indicate that something is plugging the radiator at that point.

I know you said the radiator is fairly new but that does not mean that when you got it it may have already been plugged right from the manufacture.

oldalaskaman

I missed the 'fairly new' gotta get these spec's fixed

SunCr

What "Red" region on the gage? You mean the red warning light comes on? I thought you had an '86?

Start with a scan to see what the Coolant Temp Sensor is showing. Main fan should run at 226; a/c off, and cool it down a couple of degrees, but then you've got some fan switch you'll need to get out of the picture first. Anything above 35 mph doesn't require a Fan and it should be at the thermostat or around 195 with any steady cruise. If it has a functional a/c, fan will stay on longer at idle and around town and keep it cooler - 8 to 12 degrees - because it doesn't make cold air if it doesn't.

smallblock87

SunCr, my gauge has a "red region" because I'm using a Trans Am gauge cluster instead of the digi-dash. Mine crapped out a while ago so I replaced it with analog gauges.

Everything worked fine 2 months ago before I let the car sit. I'm just trying to keep this thing from overheating first. Then I'll worry about getting the fan's switching on at the right times.

Thanks for the input!

coupeguy2001

1. make sure the water pump is a reverse rotation pump. They have a threaded boss right on the top of the pump body. Sometimes people think just because they eliminate a lot of the stuff on the front of the engine, they can use a cheaper pump.
2. Look at the reservoir in the lower front right under hood area. wipe off the dip stick, and mark it with fingernail polish or some other water proof marker. If the water level is rising, you have a bad cap or hose from the radiator to the reservoir, and a big air bubble. Pack the radiator, then remark the dipstick.
3. Swap the temp sender. It is over 25 years old, and may be giving a false indication.
4. use duct tape, and tape a candy thermometer to the upper radiator hose. recheck temp.
THe candy thermometer should read around 220 or 240. The water going back in the engine should read around 160-180.
5. Check for the lower radiator hose internal spring. If it's not there, your water pump suction is collapsing the hose and shutting off your water flow.
6. without air conditioning, your car should run great temps with a thermostat.
It should warm up, the thermostat should open, and the normal thermostat body should restrict the water flow to aid in the water staying in the radiator long enough to do a heat exchange, and the water should be around 160 going back in the radiator through the bottom hose
.Doesn't matter what temp thermostat you use, the fan should come on at 225, cool to 215, and then come back on again at 225

IF your car is below 35 mph.

7. The ECM turns the fan on below 35 because that's the speed at which the congested nose of your car flows enough air to cool the radiator sufficiently.

Midnight 85

You've got to have a stat in the system or the water will go through the rad so fast it will not cool down sufficiently. If you don't want to run a stat then you will have to get a restrictor that goes in place of the stat. I know Moroso makes them, not sure of any other brand. All they are is a flat disc and there are three in a pack, all have different size openings. Start with the middle one & see what happens. If it is still not what you are looking for then go larger or smaller. OR, you could do what I used to do when I was racing, take an old stat and gut it, take everything off the disc leaving just the opening. I used one for years.

smallblock87

Thanks guys, I plan on running with a stock stat. The only reason i took it out was really to see if the pump was moving any fluid.

This is all really good info.