Engine overheating?

WILLIAM ROGERS2

MEMBER
2001 CHEVROLET MALIBU
3.1L
V6
2WD
AUTOMATIC
120 MILES

Overflow tank boil over through discharge hose

1) Tank pressure cap replaced twice
2) Thermostat replaced- original found to be defective
barely opened at 200 degrees on a boil test (did not fix problem)
3) Replaced water pump - found to be good
4) Replaced radiator - found to be clear of any blockages
5) Replaced the temp. Sending unit
6) Performed the bottle combustion test - results negative
7) Oil dipstick shows no mixing of the oil with coolant
8) All 3 fan relays tested with a relay tester- all good
9) When sending unit is disconnected- fans operate
(shows that circuit to the ECU is good)
10) Grounded the pins from two of the relays that receive their ground signal from the ECU. Shows that both fans are activated on that test.

11) When I replaced the thermostat, I changed from the original 195 degree to a 180 degree. With the help of a scanner, I read the real time temperature being read by the computer. The temperature gradually increased to !80 degrees, reaching mid-point on the gauge. Everything was normal up to that point. Then after another 10 minutes or so, the temperature slowly starts to climb. Fans still not activated by that point. Upon reaching 200 degrees, still no fans, not even low speed, which should briefly turn on by that point.
High speed fans do not kick in until approx. 225 degrees Engine starts to boil over around 215-220 degrees, well before fans kick on.

12) Although the ECU is reading the engine temp. Correctly, could there still be a fault with it?

13) One other thing. The ac system is not working due to a system leak. So I could not use the test of turning on the ac to see if the fans kick on. Is it possible that because the ECU does not receive a return signal from the ac sending unit, that it might cause the ECU to not activate the low speed fan ?

14) Also, after changing each part, I thoroughly purged the system of air pockets. Continued the purge process until well after the thermostat opened up and no further air pockets were noted.

I am out of ideas on this. I am also a former mechanic but this has me stumped.

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
46,590 POSTS

When you say "Performed the bottle combustion test - results negative" is this a chemical test? If so, these tests are only about 75% accurate, I have seen a small head gasket leak cause these kinds of overheating without the oil and coolant mixing FYI. Also, is the cooling system holding pressure? I have not seen a bad PCM turn the fan on too late. With this old of a car the head gaskets rot out due to the aluminum heads and case iron block.

This guide may help:

https://www.2carpros.com/articles/engine-overheating-or-running-hot

Sometimes you just need to remove the cylinder heads for inspection. You might want to try having the cooling system tested for exhaust gases while running. Please go over this guide and get back to us.