2000 ML430, overheating in a strange way.

[CoyoteStarfish]

Just picked up the above vehicle for practically a song and a dance, knowing the previous owner was having issues with the cooling system and high temperatures.

So it goes like this, PO said it overheated, had a shop change the rad, fan clutch, water pump. Still overheating, so he left the car alone and unloaded it on me.

Now, I wanted to reproduce the overheat condition to get an idea of what is going on. Took it for a spirited drive around town for about 15 minutes and the temp never climed above 85-90c. With AC on.

Electric fans are operating, fan clutch sounds like it is engaging, no bubbles or pressure in the coolant system when idling cold. Cool beans.

Now I go to drive it home (about 1.5hr away), watching the temp gauge like a hawk and everything seems fine until about 45 minutes in the temp rises to about 110 suddenly, then drops to 95 within seconds, then back up to 120, then 100, then pegs at 120 and stays there. So I pull over, no discharge from the overflow. No tell-tale overheat "smell." No ticking of lifters. No post-overheat ticking or gurgling after shutdown.

Not sure what direction to go in for diagnosis. Any tips?

[Joseph_Conrad]

Do you have an ir thermometer?

[CoyoteStarfish]

Originally Posted by Joseph_Conrad

Do you have an ir thermometer?

Yeah, somewhere. Will start looking for it. Where is a good spot to sample the temperature?

[Zulfiqar]

Test the temperature near the upper radiator flange. Its actually the thermostat and the coolant sensor is nearby too

[Hirnbeiss]

Agree, sounds like the sensor. Her's the part number if you need it 0005425118

[CoyoteStarfish]

UPDATE:

Took it for a drive today to recreate the apparent overheat. It showed an overheat on the gauge, did start gurgling and steaming from the overflow port.

So it is indeed overheating. So I tried an old trick, spraying down the radiator with cool water. The radiator became cool to the touch but I could still hear the coolant boiling in the head/block. So I started it up to circulate coolant (didnt hear the aux pump running either) and the water flow kept the radiator cool enough to touch, but the gauge did not drop from around 120 and I could see and hear steam bubbles in the resivoir.

I'm guessing, coolant leakdown test to confirm compromised head gasket then individual cylinder leakdown tests to find the bad cylinder/bank then headgasket? Never really done this diagnosis myself.

[97 SL320]

Was the thermostat changed?

The only reliable test for a head gasket ( combustion to coolant leak ) is to pressurize ( air ) each cylinder through the spark plug hole then look for bubbles in the coolant / level rising. You may need to have the cooling system overfilled in order to see the level rise.

The only tool needed is an air hold fitting that is commonly used when removing valve springs with the head on. Sometimes a compression gauge hose can be used if you remove the valve core with a tire valve stem tool.

The fluid that looks for CO is a quick test but won't tell you what cylinder has the leak.

[CoyoteStarfish]

PO says thermostat was changed. But who really knows. No service records or receipts.

Also, once it cooled, found sludge in the coolant reservoir.

I guess, my question now is, do one head gasket or both?

[mpolli]

I had a similar experience recently with my 1998 C230 and I learned something very interesting. It is kind of a long story but it boils down to this:

The software in the car "spoofs" the temp gauge so it reads falsely in between about 90C and 116C. The end of the story is I had a bad thermostat, but I couldn't understand why my gauge did two odd things:

1) It sat at 93 exactly, whether I went up a hill or down a hill
2) It would jump erratically from 93 to 115

How I sorted this all out is I have a OBDII scanner (ScanGuage 2) and I could read the engine temp there also, and it disagreed with what the dash guage was showing. Basically when the actual temp got to 93C or above the dash gauge would report 93. It did this all the way up to 115. Then above 115C it would "play catch up" so that by the point it was at 120C it was correct again.

I assume they did this to avoid complaints from people who thought their car was running warm at 100C or 105C etc. The problem for me was my thermostat was bad but I couldn't tell what was happening. I was running maybe at 110C but the gauge said 93. Pretty dumb in my opinion.

If you can get a hold of a scanner that can read the temp you can test this theory.


In any case sludge is no good...

[Hirnbeiss]

Was the "sludge" floating on the coolant?

[CoyoteStarfish]

Oil/water slurry, clinging to the walls of the coolant tank.

[Zulfiqar]

does this engine have an oil cooler bolted to the oil filter tower?

[CoyoteStarfish]

Originally Posted by Zulfiqar

does this engine have an oil cooler bolted to the oil filter tower?

Yes it does. Is it possible that the oil cooler failed, mixing fluids, and the oil buildup in the coolant is causing the overheating? And possibly not a head gasket?

[Zulfiqar]

Originally Posted by CoyoteStarfish

Yes it does. Is it possible that the oil cooler failed, mixing fluids, and the oil buildup in the coolant is causing the overheating? And possibly not a head gasket?

Yes there is a possibility of oil cooler failure. It has been seen, specially if the coolant was not changed on time.

[CoyoteStarfish]

So my next question; how can I test whether it is oil cooler failure or had gasket failure?

Edit:

Upon inspection of the oil cooler, I'm going to try this. Remove, cap off one end of the coolant passage, pressurize the other end with air and submerge in water looking for bubbles. If anyone else has another method let me know