So I have searched the forums extensively and found a ton of posts about overheating. I have read them all and tried a few things but the issue has not gone away. I dont want to but I cannot find a issue in the forums specific to what I am seeing on my c4.

I purchased a 1996 LT4 6 speed Coupe about a few weeks ago. It seems to run pretty good overall. Has all service records back to 1997. Had a waterpump fail in 2006. Overheated the engine. Chevy dealer put in new WP, new opti, and a new radiator.

When I bought it the owner told me it runs hot, but that it was normal to hit 230 in traffic, which I had read about here. I had already been reading these forums for quite a while and felt I had a good idea of where it should run.

The car basically slowly heats up at moderate freeway speeds (55-65). It has been nice and cool in SoCal the last few weeks, with temps in the high 50's at night and mid 60's during the day. The temp on the car while Im in 6th gear slowly climbs it seems to 219. It then stabalizes as the fans low speed kicks on.

This does not seem normal in very cool weather. I drained the coolent out, was going to change t-stat to 160 but decided to put the 180 back in. Replaced the belt and hoses. Changed oil to Amsoil 0w30 and filled the cooling system with dextron (orange) and mineral water.

Started the car, got it to operating temps (above 180), turned it off, let it cool down, filled it with more water, started the car again, got it above 180 again and then filled it with water again, not as much this time. The third time I got it warmed up and cooled down it did not really need much water but I added to the overflow tank and now it seems to be just about right.

I have read on here where the LT4 has ONE bleeder valve on the t-stat housing, which is easy to find. I have not bled it tho as it feels a little to loose for my taste, possibly stripped out by previos owner? When it starts to loosen up (with the car over 200 degrees, under pressure, but engine off) It feels like it will just pop clear out. I also see what looks like plumbers tape around the treads. Does any of this sound normal? should it feel like it is going to come clean out of the t-stat housing? Should I be seeing plumbers tape around the upper portion of the bleed screw?

So I get on the highway and it heats up gradually to 219 unless I either downshift to 5th gear or get off the highway and hit surface streets, (not in traffic)which really does not make sense. Why would the car cool better at 35-45 mph than at 55-65mph?

Last but not least I do seem to smell a faint trace of coolent after driving it around town and after it has got up to 230-240ish. It always seems to have a very faint coolent smell to it. I know when cars get hot they can release into overfill tank but should I always be smelling coolent?

Thanks for any help.


Edit: Oh and I had someone tell me it might have a possible head gasket leak, I do not see any white smoke when I drive the car but it is hard to see back there when driving. I have been debating the idea of adding fine nanoparticle sealer to the cooling system to possibly shore up any cracks or leaks. Any thoughts on this stuff?

 

sorry, post to long for my AADD syndrome

Have you pulled the radiator and cleaned out the trash in there?

 

Rad has approx 20k miles on it. Old 70 year old dude was the last owner, lived in Laguna Hills. That is about 5k miles a year. Would not think it was that dirty? I did find some trash up in between the rad and ac cond. Fished it out with wire hanger and blew the leaves and whatnot out.

 

Unless you reprogram the fans to come on earlier it will almost always hit 230ish at some point. Id highly recommend that. At highway speeds it should cool down some, only at low speeds/traffic should it get to that 230 point.

Smell the exhaust once its warm and idling see if you smell the coolant from there. Pop the raidiator cap off and then do a bleed down test on the cyls.

I couldnt stand the high temps they run at, drove a 93 with all new everything for a few months. There is no reason in my mind why a car should run that damn hot unless its strictly for emiisions;It drove me crazy

Just doesnt leave much room for error, doesnt leave much time to catch it if it creeps up to 240, 250 or more while driving and the temp gauge is already pegged.

 

Sounds to me like you have some type of "flow" issue.
Either your not getting coolant flowing properly through the radiator, or your not getting enough airflow through the fins.

At any speed above 40 mph, your coolant temperature should not be going over 195 - 198

Lay on your back under the nose of your car, and look up into the radiator cavity. These things will suck up all kinds of trash and block the fins.
Also check the area between the radiator and the condenser, and that can also block airflow.

Added: That 230 temp is normal while at extended stops, but not at highway speeds.

 

One of the first things I checked: Got up on on ramps and with a flashlight I fished out crap from both in front of the rad and in front of the condenser. Then blew it all out with shop vac.

Does not seem like anyway to powerwash the rad without fulling pulling it out which is a bummer.

 

Had overheating issues in my 88 constantly. Stewart high flow water pump, and dewitts radiator. Now runs consistantly cool, maybe too cool. Always between 180 and 200 all year, any weather. But i dont have to worry about overheating any more.

 

Usually your symptoms are coolant or airflow blockage related. Given your timeline of events and parts replaced, that means you probably have deeper issues.

Given your info, I would look at a minor head gasket failure. Not enough to cause major overboiling - but enough so that its heating up the coolant more than is normal. A very minor head gasket issue can cause upward creeping temperatures, without giving much of the normal indicators of exhaust gas in the cooling system etc.

Thomas

 

First have the system pressure tested. If you believe the bleeder is loose this could be a bad spot. A cooling system with a small unseen leak will emit steam after the engine is shut off as a way of bleeding off pressure, which is why you smell coolant. It will also suck air while it is running under pressure causing the water to boil and run hot. Faster you go the more flow through the pump the more air is sucked into system hotter car runs. This is why you need to burp or pack the cooling system to remove any air traped which causes over boiling and higher temps. Test system then you will know.

 

LT1/4's take a special thermostat

makes me think they put the wrong one in.

also the system needs to be burped properly.

I had a fan die on my car

on the highway it was fine, the second I hit of off ramp WAMOO

 

Ok my FSM has not arrived yet, does anyone have the step by step to burp the system? I will unscrew the bleeder valve when the engine is cold to see if it is indeed stripped out and then tighten it back up.

 

Make sure the air dam is in good shape to deflect air up to the rad.

 

With the bleeder valve you loosen it to remove the air trapped in system tighten when you see coolant raise idle and refill. Or you can open the radiator cap squeeze the top hose raise idle and fill with coolant might need to do this a couple of times to remove all the air.

 

I cheat, when I replaced my thermostat I drilled a couple of small holes in it so some water always will flow thru it. When I fill it, I open the bleeder, pour water till it comes out the bleeder. Fill the tank, go to bed, in the morning top off the tank, put on the cap and go to work. When I get home and it cools, open and fill, check in the am. I do this for a couple days because my commute is short, by the second morning I'm not adding any water and temps are back in the 180 - 200 range for my commute.

I do need a new air deflector and fans to get it back to running 180 - 190 (my main fan is programmed for 190)

 

If the bleeder's like the LT1s, the part with the wrench hex (and tape) doesn't get loosened when bleeding. It should have a small round **** with a screwdriver slot in it. This is what you use to do the bleeding, not the hex.
When filling up the system, cold,crack the bleeder. Then warm up the car until the Tstat opens and bleed it again.
This does sound like an "air in the system" problem.

Next time you flush, pull the knock sensors. There's a lot of crud in there that doesn't come out by just draining the system.

 

Being known for my one line posts...thats what its called.

 

Go to your local parts store and buy a block tester kit. This will tell you if you have a headgasket problem.

 

Yeah, I'd do that. You can rent it at Autozone. It is located on one of their mats on the counter. Block tester. Then you look and you have to buy the fluid.

What I would do is replace the radiator cap, drain/flush coolant and replace the thermostat with an OEM unit as it takes a certain thermostat. It would probably be good to drain the coolant from the drain plugs in the block to make sure there isn't crud sitting in there.

Oh, and what does the coolant color look like? 96's have dexcool. It is possible they used the wrong coolant in the past and it has sludged up.

 

You said you used mineral water. Was that a typo?

The fans do not run over approx 45 mph.

 

It doesn’t sound like you ever replaced the thermostat. (Did I miss it?)

I had one that was tired and as I drove it kept getting hotter and hotter. In a pan of water I could see it was not opening up enough.

Do the simple things first. Replace it with a LT1 stat as indicated above.

There are 2 air bleed valves.