BMW Repair: Overheating E38/M62, replacing head gaskets, head gasket

Question
QUESTION: Merry Christmas John.  My 1997 740iL continues to overheat.  I replaced the fan clutch.  Helped but did not solve problem.  Reservoir cracked due to heat and pressure, replaced with new filler cap.  Helped would run at normal temp for a while, then would get hot.  Replaced Thermostat, ran at normal temp most of the time for two months, then got hot and radiator burst at plastic sides 12/23).  Replaced radiator.  Runs at normal temp until I drive at high speeds, then gets hot.  Aux fan operates, unkown if it operates at all speeds.  Any insight would be greatly apppreciated.  Thank you.

ANSWER: Sounds like it might be a head gasket.  You can get it checked for <$50.....they sense exhaust fumes in the coolant.

Replacing head gaskets on these is rough and expensive (~$4200), if that's what it is.

(Let me know if it is).


---------- FOLLOW-UP ----------

QUESTION: Hi John,

Just drove the car 21 miles to Los Angeles at 60 mph and it runs at normal temp for a while, then goes to 3/4 on the temp guage, then back to normal for a while.  More normal than 3/4.  If it was the head gasket that was failing, wouldn't it just fail and run hot all the time?  I have also checked the oil and coolant and neither show signs of any mixture.  Please help me with your insight.  Thank you.

Answer
I bought an e38 last year as a project car.  After clearing a number of issues, I wound up with the temperature rising and falling similiar to what you are describing.  It can run hot then closer to normal, and back and forth.  All high, but moving.  A head gasket can put air into the system, and that bubble moving around can drive the sensor up and down.

You can get a digital readout of the actual temperature by entering a code through your dashboard.  Don't recall the exact sequence, but you can find it on www.roadfly.com.

I'd take it to a shop and get the coolant checked for exhaust gases.  I did on mine, cost ~$35, and was pissed I hadn't done it much earlier.

I wound up replacing the head gaskets, and the car ran beautiful after that.  It's a big job, but if that's what it is, and you decide to do it, let me know and I'll pass along some help.

If the test shows it isn't it, you might be losing coolant through the famed valley pan.  But that usually shows more as losing coolant than overheating.