A fairly common symptom of head gasket trouble is wild fluctuation of engine
temp. I don't know why that is, but it's seen all the time. I rely pretty
heavily on the shadetree test for head gasket trouble: with the engine cold,
remove the radiator cap. Start the engine and pinch off the tube from the
radiator neck to the reservoir. Place the palm of the other hand over the
radiator neck for a few seconds. If you feel a steadily rising pressure or
(worse) fluctuations as the engine turns, the head gasket is very likely
bad. This test has some false negatives - the head gasket may not leak when
cold idling - but I haven't seen a false positive. The most widely accepted
test is a chemical test for combustion products in the coolant.
I often focus my attention in overheating cases based on the way the system
behaves. Bad radiators usually show up as a slow buildup of heat that never
wants to go away. Air flow or water flow problems show up at idle but the
temperature drops rapidly when the car gets moving; water flow problems
return to normal within seconds while air flow problems take a minute or
It really doesn't matter if the fans are running while driving; air flow
should be fine then. I hate to say it, but I fear for your head gasket at
this stage. Overheating while going down the road and cooling down when you
pull over and idle make that a prime suspect.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.