Overheating Suburban - Black water, gage fluctuates

Got an overheating problem – need advice please….

Problem:
The engine overheats and the temperature is very erratic. The temp gage is fluctuating up/down sometimes from 160-210, others from 210-240. I do lose water… somewhere. Last night I saw it hit 245 and turned engine off to coast and sat on the side of road. As I sat there, it pegged the gage @ 260+ - no apparent damage though – I got to get this resolved!

The water in the radiator was black! It stained the recovery tank. I feel the gage is working as I got a check gage light last night when the temp hit 245ish.

I know I have done a poor job of maintenance on the radiator over years, as I had (have) a very small manifold leak. One extremely small leak is at the push connection on the front passenger side of the intake. I will address that this weekend as well. For years I have not used coolant and the inside of the radiator was black (literally) a few months ago.

What I Have Done Already:
I purged the cooling system a bunch, replaced the thermostat, and back-flushed the radiator. The water is still light brown at best. I again re-purged the system, and replaced the Thermostat again. I had temporary relief, but now the problem re-surfaces. These remedies worked good in the beginning, but lately, it sees way too high temps!

Details:
When the engine is running hot, the heater does not work. But, when the engine decides to intermittently cool properly, the heater works. (T-stat problem?)

Sometimes, from a cold start, the engine will heat up and act as though the thermostat never opened up. It goes right past 160…. up to 220ish and then you can see the temp fall back to anywhere from 160-200. FYI - even if this were due to compression leaking into the radiator, why wouldn't the thermostat open up and offer relief at the appropriate 160 is level and give at least some cooling affect. This makes me think its a T-stat issue again.

I do see bubbles come from the radiator, but felt it must be air pockets, not compression. ? With the radiator cap off, I see the radiator ‘overflow’ for long intervals, then I can re-fill it. Sometimes it takes it, sometimes it doesn't. When I give it gas, the water in the radiator sucks down. (T-stat problem?)

Thoughts:
At times I think it is a blown head gasket, then maybe a malfunctioning thermostat, or a clogged radiator/block. Maybe a water pump that works intermittently (is that possible?).

I have bought the stuff to add to coolant that can repair a head gasket leak, but have not used it yet. I heard of a chemical you can add that can tell if you have hydrocarbons in the coolant, but could not find it. I wonder if you need a detection tool as well?

Thanks

I am going to hazard a guess that you still have air in the system from reading you post. Had to change the water pump on my '98 K2500 7.4 a few years back and it took almost 2 weeks to settle down. Big thing is to keep the recovery tank level at the cold level when the motor is cold. Drive it and get it hot. Park it and and add coolant as needed the next day when it's cold. Another thing I did was park it nose up one day and nose down one day until it finally expelled all the air. A real pain!!

Their is a good chance you have a plugged radiator. Watch both the top and bottom radiator hoses temperature. Check by feel and make sure you have good flow through the radiator. Some radiator shops will clean and rod the radiator but some won't.

Thanks - I am going to replace the pump today (kinda noisy and its a lifetime warranty) and pull the radiator to have it rodded. I can better backflush the radiator and maybe the engine?

It just runs too good to be a head gasket - although I have to admit I am losing more fluid than the intake line is responsible for.

Also - the idea of air in the system has validity too... I filled the system at 5am when it was cold outside. I drove to work and it ran 230+. After work, I drove to get the new H20 pump and I sat in 75 degree weather and traffic and it ran cool !!!!!!!

So the black water doesn't bother you all?

Assess how you are flushing the radiator. Filling and draining the radiator and block is not sufficient. You need to force water in at the head and let it flow out the bottom. I have had good results with the Prestone flush n fill kits.

Your thermostat is likely toast. Replace it or remove and it place it in a pot of cold water on the stove. Heat it and see what temp it begins to open - it should be 195 degrees. If it is coated and clogged wit the black stuff you mention, remove it and do your back flush. Then replace with new.

Remember when you flush to set the heater thermostats to maximum heat.

As for the head gasket leak, you should replace the head gasket. The snake oil in a bottle is poor maintenance and will likely cause you other problems. If you suspect compression gases in the coolant, a shop can confirm that with a gas analyzer in the radiator fill opening. You might be able to smell it as well.

It is normal for the coolant level at the filler neck to go down when you rev the engine.

i don't know how many miles you have on your suburban but i strongly suspect that you have a intake manifold gasket leak. i just purchased a 97 suburban about a month ago. i too was loosing a lot of coolant and couldn't figure out where it was coming from. i was convinced that it wasn't the intake manifold gasket because the thing ran SOO well. i finally realized that the coolant was leaking out the back side of the engine and down to the ground. my mechanic said it wasn't effecting the operation of the engine because it was leaking to the exterior and not to the inside of the engine.

i think you should look into this as well as doing the things you talked about (flush and thermostat replacement)

If you have a pressure gage a leaking head gasket will normally pulsate the gage when reving the engine. Put about 8psi on the pump and start the engine and watch the gage reving the engine slightly.
You are going to ruin your engine without anti freeze or water pump lubrication with additives.
The gas analysis is more definate but I would pull my own heads with a pulsating gage. I always get my heads checked for straightness and shaved if necessary. I also pay for a pressure test but I'm retired and am not as fast as when I was younger. Parts also cast a lot more at this time.

Thanks again guys... OK - I replaced the water pump and backflushed. The thermostat was also replaced even though it looked good. I pulled the radiator and plugged the ATF and Oil Inlet/Otlets. I then flushed it backwards and was AMAZED at the crap that came out. I half filled it then laid it flat and shook it, when I did another clump of stuff would come out. It took 30 mins. Soooo... I know that was an issue and probably still is to some degree. I would assume that the motor is caked with that stuff and all the hoses look bad too (inside). I also backflushed the block as best possible through he water pump inlet/outlet.

BTW - if you replace the water pump, get a fan clutch tool - you need it to make removal of the big nut (which is right hand thread.)

Upon re-assembly, all went well. I took it for a ride and it does oscilate in temp, but from 160 - 200. After running a while, it settled down tonormal operating temps and the heater worked great - we need that here as temps were cold in the 50's.

One problem though - At 55mph, when I goosed it, I could feel a shudder. My old Monte Carlo did that for years until I finally figured it was the shroud/fan clearance and missing shroug bolt causing it. So for my 99 Sub, I took the top shroud off and R&R'd the water pump pulley and fan clutch. I did not see anything wrong and re-tested for spinning evenly /wobble. I saw none before and after re-installing.... ?????

Also - in wondering whether it was my fan clutch, I also wanted to check it for proper operation. I spun it when cold and it was stiff. When I spun it a bit, it would freewheel like a wheel of fortune. I even applied a heat gun to it to see if it would get stiff for hot operation. It did not. I thought I found yet another issue, but upon start-up and watching it, I see it did start to 'hook up'. Maybe it just needed centrifical force to enable lock up. ???

So - the end result is the car is running cool - mostly 160-190. Heater works great, even previously, the A/C was freezing (36-40ish) cold with a gage. I am still losing water.... if I recall correctly, Chevy had intake issues leaking at the rear as mentioned. Since - I had a few injectors replaced, so I assume that issue got resolved??

On the gases in the block, I may try the vaccum gae test and possibly a mechanic gas check. I refrained from adding the K&W Block Sealer, especially as I want to bypass the heater core if I do it - agree?

The 99 Sub has 188,000 miles and plan on a newer Sub in mid 2008. It has been a great, dependable car - EXCEPT for chevy's crappy door hinge system. And while I am at it - whoever designed the eggshaped center console (that is almost useless), should be shot.

As far as the intake leak is concerned, if you have the 7.4L 454, the injetors are inside the upper plenum. The whole lower intake manifold has to be removed to replace the leaky gaskets (much more involved). BTW don't go the cheap route on the gaskets. Buy the GM part. Also, don't trust the temp gauge on the dash, hook up a good scanner and get the temp form the computer. My dash gauge reads 160 but the actual temp is 198.
Good Luck.

Hi everyone

The issue was indeed the intake manifold - I took it to a shop for fixing - $850. ALl works well - especially since I cleaned teh radiator so well. The heater is not very hot - so I think the heater core probably needs flushed out. ;-( Anyway - it doesn't overheat now !!!

As a note - CLEAN THE RADIATOR OUT WELL - loads of difference !

also do not put a thermastat made in mexico in as it will not open in boiling water.