[QUOTE=jmgtp;1580596528]
Many of you are indicating that the problem can be the radiator (internal/external), thermostat or cap – but can anyone explain why a failure of any of those items would cause an overheat ONLY at/after WOT? Doesn’t make sense to me yet and I’m trying to understand it.

QUOTE]


My answer to this would be the engine is making alot of heat at full throttle or under a load. If the radiator has a limited capacity, ie. dirty stopped up etc. it dosent have the ability to remove the heat. At idle the heat generation is not as great, and the radiator could be adquate at that point. However, if you put a new radiator in 4 years ago, I doubt this being your problem.

The thing I would try next is a new cap. Water boils at 212 degrees but with 15 lbs pressure it will not boil until 250 degrees. If your cap is not holding pressure, it could be getting hot enough inside the engine to boil, and then push water out into the recovery tank. But again, its just a guess.

 

The conclusion I am arriving at is that there is a myriad of things that it could be. Regardless of the cause, the heads and head gaskets have been subjected to extreme temps/boilover and I can't say I trust them now.

I think the safe route is to tear it down. It's a lot of time but not a lot of money.

-I'll have the heads inspected
-replace headgaskets (and intakes, etc)
-new head bolts
-inspect steam tube and banjo bolts
-replace thermostat
-replace radiator cap
-replace upper/lower radiator hoses
-remove radiator and inspect for external issues and flush out

It won't cost me much at all to replace all the above, probably around $200. The bigger issue is time and and labor, labor being the tough one because I have a wretched lower back.

One last test I am considering is putting each cylinder at TDC on the compression stroke and pumping in 100psi through the spark plug hole to see if I can get bubbles in the expansion tank. Not sure if 100psi will be enough to expose a HG failure though, considering it takes WOT to bring it out.

 

Why do u have the fans programmed to come on at 200* with a 160* thermostat?

 

With the 160 stat the engine runs 180-190 most of the time. If I set fans lower than 200 theyd run continuously.

 

did any of this start happening after doing any work to the car? ie. say after installing the waterpump etc?

 

On @ 200°F/Off @ 185°F
Use w/180° TStat

This is what I use w/160° TStat mine don't run all the time.
On @ 176°F/Off @ 166°F

 

Is there a possibility that the water pump is shutting off during WOT runs? I know some GM vehicles actually shut off something like the AC compressor when you got WOT. Not sure where you are getting your power for the pump from though. Wild guess.

Other than that, my guesses are T-stat and radiator obstruction. I don't think your head gaskets are blown.

 

I also have an overheating problem, but mine is reverse - the temp goes down to 194-97 on freeway, jumps up to 237 on city street. was thinking it was a radiator blockage.... would you suggest i try a this as well?

 

PM sent!

 

To address some of the above since my last check in...

I haven't done any mods in about a year so its not anything I did recently. The water pump relay gets its power from the fuel pump relay, so anytime the motor is running so is the water pump.

I am following through on my course of action - which is basically to say I don't know what caused this so I'm going to address it all.

I got quite a bit done in 3 hours tonight. I have the passenger side cylinder head sitting on the bed of my truck (a nice temporary workbench while my actual workbench is occupied!).

I don't see any obvious sign of a blown gasket but it didn't come off cleanly either so its hard to say. There is all kinds of gasket gunk on the block and head, this is going to be a PITA to clean. The head looks great, no carbon buildup to speak of and no Grand Canyon size cracks. I have to see about finding a local place to pressure test them.

I know this car originally came with the green coolant. Somewhere along the way someone decided to use Dexcool and I kept with it. Any harm in switching back to the green so long as I flush out most of the orange?

Side note: I have been having this odd 'motor' smell permeate through the vents after ~5 minutes of run time. And today I finally found why. There is a 1" x 2" in the 'heater box' (or whatever you want to call it, but the enclosure that houses the ventilation fan) RIGHT by the passenger side header. Temporary solution is aluminum tape so I don't get anything in there while I am working.

 

I would actually highly recommend switching back to green, or even the high mileage yellow coolant. Dexcool is terrible. It gels up and crystallizes and blocks passages. It also gets very corrosive when it starts to harden. I had it do it to me with a car I had a while back. I would say remove the knock sensor and drain the radiator completely and flush with water then drain again until all the orange is gone, then refill with something else, using distilled water.

 

The cheapest and most critical part.....replace it!

 

You are experiencing typical C4 temps. If your uncomfortable with this, upgrade your radiator to a DeWitts.

 

I'd rather run straight water........

I know I know, GM says......

There is just too many "episodes" with Dex......

I'm with Seabright, there is a passage somewhere that is allowing superheated exhaust into the coolant chambers. WOT in 5th and 6th put a load on the engine and that is where you saw the spike in temps....

Think about it, why does a worn clutch begin to slip in 4,5, 6th gears when you mash on the accelerator? The load.......

The load is there on your highway runs.

Now that you've cracked the head off, might as well replace the gasket.

Look for a gasket that has a lot of graphite "Sheet" in it......it takes up a lot of gaps and seals on older motors.

 

I've been running dex in my L98 since it was built...10-12 yrs now and have absolutely NO issues of any kind. Green stuff turns to ACID after so many cycles or as it ages...dex does not. Green is terrible for aluminum. Thats why dexcool was invented...for the alloys and lt metals used in todays engines.

What I HAVE seen is halfass flushing of green and someone thinking good 'nuff and refilling with dexcool. That is where the CLOGS, GELS and other problems originate. Operator error, not product defect. But as we all know,. its ALWAYS the product.....uh-huh.

If dex were so bad, then why does every high-end car made in the world send their product out the door full of dexcool or a comparable orange long life coolant?

Just my $0.02 worth of hillbilly wisdom for the day.

 

Well, not quite what happened to me. WOT was only 1st and 2nd to get up to speed, after which I went to 6th and maintained speed (no WOT in high gear) at very light load. The temps rose at that time. I believe I pushed exhaust into the water during WOT and with the displaced water temps just skyrocketed despite the light load.

I'm going to remove the other head tomorrow. These heads have been on for about a year. I'm wondering if my problem was that I didn't clean the mating surfaces enough when I installed the heads and the gaskets let loose on me. I want to get them damn near perfect this time. My concern is, how do you clean the block REALLY well without knocking a ton of gunk into the bores? I know I could clean the bores up after but with the piston in there I feel like particulates will get stuck between piston/cylinder and then when the engine rotates the rings will gouge teh material into the walls? Right now, I can still see the original 'crosshatch' hone marks - no scoring at all.

I know I didn't chase the threads in the block either, I just blew them out with ~100 psi. Can someone confirm that a 7/16-14 thread chase is the correct size?

What all do you do for a top end rebuild? Safe to get the gunk in the bores and then vacuum it out and use compressed air to blow out whatever is left?

 

If there is crude in the cylinders, just use a large socket on the front pulley and rotate the motor...the pistons will push up trash enough for you to clean it......It doesn't need Class 10,000 clean room treatment.....it's a motor...it'll burn all that crap in the first few moments.

Blow it out or suck it with a shop vac.....

if your really weirded out about it, tape the edges of the piston and cylinder before you start scraping. That way no crude goes down in the gap to where your rings are.

 

Ah....the clean up, this is the part we all love so well, not really! But it is probably the most important part of the job. Crusty surfaces, nasty bolt holes, and oil in the wrong areas is bad news for any rebuild.

My block before;



And after I reconditioned the surfaces, chased threads etc.



I used 3M reconditioning pads http://www.sjdiscounttools.com/3m7453.html on a 90deg die grinder at medium speed-light feed, this also worked on the pistons at the full up position. I would us a scotch bright pad #7477 red by hand on the heads..cover up as you go, and vacuum debri.

 

I now have both heads off the engine. Like the other head, the gasket on this on the drivers side did not come apart cleanly. I don't see any obvious blow outs but I really can't determine if it was the gasket.

Both cylinder heads have clean combustion chambers (little carbon buildup), the spark plugs are the same color on each and the valves all look great.

I examined the heads more closely today and did find a possible area of concern near the intake ports. There is what appears to be a crack that both heads show and all intake ports show in that 'gap' area. But, this may just be a casting mark I don't know.

The first picture is me pointing to the spot I'm looking at that has the 'crack' so you have an idea of where I am looking. the second is a closeup. Casting mark or crack? If the heads were to crack from heat would it happen just anywhere? Right here? Or somewhere else. What i can tell you is that this crack is external, and there was no coolant loss from that spot.

edit: just looked at my stock cylinder heads to compare. They also have this so I believe it is a casting mark. Its just that on the ported heads the 'crack' appears larger. The stock heads are very dirty so they can be hiding the size some. Also, the below picture is of the one that looked the largest. It is the head that was on the drivers side.

 

I think this is worth running down, despite the fact you've already taken the heads off.
I had a similar problem with a Maxima (Hey, everyone needs emergency transportation!). The electric fans would only come on when the AC was on. I drove it (unwittingly) like that for months with no problem, but when my oldest son was teaching my youngest son to drive stick, he turned off the AC and kept the car at low speeds. Bye Bye headgasket.
Your problem is presenting a little differently, but wiring in a lightbulb to the waterpump might help eliminate that possibility. If the light goes off at WOT, or similar situation, you know your waterpump is taking an unscheduled break.

You are presenting an interesting 'puzzle', you need to consider these oddball possibilities that can't be solved by conventional solutions, like "replace the radiator cap,"

Good luck!