My LT4, with 96K miles, has an overheating problem that I can't figure out. The water pump was replaced about 25K miles ago and the heater and radiator hoses were replaced about 9K miles ago. Back in May the temps started fluctuating, up and down from 200 to the 230s. I replaced the thermostat with a 180 and everything was fine until a couple weeks ago. Now it's behaving in a similar manner, temps moving up and down, with a max of 240.

This morning I took the cap off and topped the antifreeze off to just below the filler cap. Started the engine, and when the temp reached somewhere around 140 the coolant started moving up and with some flowing into the overflow tank. The level moved up and down until somewhere in the 160-170 range, then it started bubbling out like crazy. Does this sound like a blockage somewhere? I suppose the thermostat could be bad already. Any ideas would be appreciated.

 

Assuming you know of course, that these cars were designed to run 240-250 - - I went all through the same overheating (sans bubbling overflow) on my LT4, 100k mile, 1 owner car. Could NOT get the thing to cool down.

After replacing almost everything exept the water pump, including a custom computer to turn BOTH fans on at 175, and eventually noticing coolant loss, I then realized that I should have paid more attention to the positive reading on the C02 test I had done on the coolant. Brilliant! Head Gasket failure. Take this valuable step, believe the readings, and save yourself a lot of time and money.

 

The thing that's changed is that after the thermostat replacement it was running 197-205 even in the Phoenix heat this summer, except of course when I was stopped. I'm going to contact my mechanic Monday and have him do a compression check.

Thanks for the reply.

 

No problem...not that a compression check isn't valuable information, and it's certainly up to you, but the C02 check takes 10 minutes vs. 1-2 hrs. Plus, your real problem is cooling related not performance (IOW - you could have low compression and still not know whether it's oil or water contamination). Good luck either way.

 

I've never done the CO2 test. Can I get a tester at someplace like Autozone? What does it tell me?

 

With the car cool enough to pop the radiator cap, you suck some coolant into a reservoir that you mix with *something* and if the color changes, you've got Carbon-dioxide "poisoning" in the coolant. This tells you that cylinder gasses are getting through the head gasket into the water jackets - something that should NEVER happen. Not everyone has the tester, it's kind of old-school I guess. You may or may not find it locally, I'd call first.

 

It is Carbon Monoxide (CO), Not carbon dioxide (CO2).

 

Don't overlook the possibility that the new thermostat is not opening properly ... cause that's just what it sounds like.

 

I've considered that. The overflowing from the surge tank occurred at about the temperature the thermostat should have been opening. I guess I could start by putting in another thermostat and see what happens.

 

Dave,

1.You have to burb and temperature cycle three time to get the air pockets out of the system. Do you have the FSM procedure ? If not I can scan and send to you.

2.Are the digital temp gage and the analogue moving together. If you have an air pocket, the analogue (rear sensor) indication will move more erratic than the digital which has sensor at water pump.

3.The 'air pockets' could be a caused by a head gasket leak. The block tester cost about $40, has a clear glass column to hold blue dye. Let vapor above the coolant bubble up thru the dye, if changes from blue to YELLOW indicates exhaust gas in coolant.

4. I had a head gasket leak but it was so small no indication by blue dye.

5. Pressure test the cooling system at 15# to see if it will hold pressure.

6. Pull a 25# vacuum on the cooling system. If it will not hold a vacuum, the head gasket is probably leaking very slightly. Several of us on the forum have successfully sealed these leaks with fine particle nano sealant.

BURB and cycle three times per FSManual to get air out

If that does not solve the problem, do the about to diagnose head gasket leak or "casting porosity" as GM calls it

Joe

 

Yeah, right...what he said DUh!!

 

Let the car run until the fans come on. Put your hand behind the fan, if the air is hot, really hot, the T-Stat is working. If the air is ambient, your T-Stat is bad.

 

Thanks for the detailed response!

 

In the summer, with the A/C on, the fans will run all of the time (except over 40mph). They will kick to high when the temp reaches about 238. This usually keeps the coolant temp in the 190-210 range, even in city driving.
In the cooler weather, or when the A/C is off, the fans don't run until the coolant temp reaches about 225. They will cycle off/on/high as needed depending on the coolant temp (when the A/C is off). When the A/C is on, they will have the added control of always on and high compressor pressure also setting the speed to high.
Does this describe your fans' operation? If so, it's normal.
The 'burping' procedure for an LT1 or 4, 95 & 96 is to open the brass bleed screw on the thermostat housing until all of the air escapes (with the engine running and pressure cap on). You may have to do this several times. Always catch overflow with a towel to keep it off of the opti. The pressure cap should be on at all times except when the engine is cool. And keep away from the rotating elements.
Cycle the temperature full up (190+) and full down (overnight cool off) at least 3 times before attempting 'aggressive' maneuvers. Yes, that's what the FSM says 'before attempting aggressive maneuvers'.
It also warns to observe the analog gauge closely and shut the engine off if the temp reaches 260 (that's the boiling point of 50/50 at 15psi). Reading between the lines here, this won't work with straight water as it will boil well below 260.
I've changed my coolant 3 times with a full drain (knock sensors out) and twice had no difficulty. The other time, it belched into the overflow tank but was ok during the next heat up cycle. During the 3 heat up/cool down cycles (takes 3 days) the analog temperature was very erratic but never hit 260. It got more stable each cycle and the 4th or 5th time out was well controlled.
Hope this windy explanation helps and good luck.

 

A couple of questions; is the car overheating while you are idling only or all the time?

If the car keeps cool while running; I would look at the fans. I had problems with both fans, more specifically the fan motors.

Also, make sure your front air dam is in good condition and that their is no junk in front of the radiator.

One last thing is to check that the fan shroud is in good condition. Most people won't experience this problem; but I had to cut my shroud when I put in a Be-Cool Radiator and accomodate the extra thickness of the radiator and my supercharger. I ended up going back to the stock radiator.

Anyway, good luck.

Glenn

 

Just finished doing this. Both fans kicked on right at 228 and were blowing air so hot I couldn't leave my hand down there very long.

Thanks for the tip.

 

96 Fan Set Pts Stock

Lo On-219 Off-207
Hi On-228 Off-214

 

Have you ever introduced any of the "stop-leak" products to the cooling system?

 

my car had very similar problems. my car blew a head gasket at like 93k. i think the previous owner had a little too much fun with nitrous. but it had made a small crank in my radiator that wouldn't drip cause it was high enough on the radiatior the coolant would evaporate before hitting the ground. for quite a while i thought that i had another bad head gasket or something wrong cause i could smell the coolant and after blowing a radiator hose and replacing the waterpump (that also started leaking) i finally had enough pressure in the system to show a leak. some ideas to look into.

 

As a previous post said, make sure you open the coolant bleeder valves and "burp" the system until water flows from them -- as per the manual.

Tom Piper