Quattropower88

I searched the forums and found a lot of info on overheating... but it seems no one had these exact symptoms...

(Apologies for long post -- but it takes some explaining)

Edit: It is a 1987 951

To start:
About a month ago I noticed the needle going up into the Red when I idled on a hot day, but would go back down when I started driving. The higher the revs, the cooler the temp got until it went back to normal in between the 2 tick marks.

Solution:
I fixed this by burping the car as per instructions on this forum and Clark's. It ran fine for about 2 weeks/200 miles. Still got up to slightly above the 2/3 tick mark when in traffic for a while, and would shoot down when revved up to 3k. Fan also cycled on/off properly when necessary.

Problem again:
Two days ago while on the highway, Needle shoots up to red, but stays at the bottom of the red area. "!" came on, flickered, turned off, came on, etc. Temp did not go any higher and the car was running fine.
When I cranked heat up to the hottest it goes, on the highest blower setting, no heat was blowing out of the vents. Once exiting the highway and going regular speeds, heat started working again.

- PO replaced the Headgasket, waterpump, and T-stat within the last 10k so it shouldn't be any of these.
- Is the system plugged? -- but I seemed to have gotten all the air out when burping it

Any help is greatly appreciated. Thanks!

User 52121

Sounds like you've got air in the system.

Either it's always had it and you haven't bled it enough... or you're getting air pushed into the system via some other means (bad HG).

jeffrsmith

What shape are your upper and lower radiator hoses in? Sometimes if a radiator hose is in poor shape (soft) they can collapse at speed and cause a poor flow situation - kind of like what you are describing.

Quattropower88

I suppose its always possible HG wasn't replaced correctly... Will bleed the system again and if overheating issue starts again in 2 weeks then

Quattropower88

Didn't think of this...Will heat the car up and see if they are soft. Thank you

aj986s

Another oddity to check is whether your fans are blowing in the right direction. I had a situation on my race car where the car would idle fine, and when fans came on, the temps stayed ok. But at speed, the engine would get hot. While burping the system at one point, with the engine on, the fans came on. I was standing in front wearing shorts, and asked myself why were my legs feeling warm air. Turns out the fans were blowing out. At speed, when the fans kicked on, the air coming in was fighting the air blown out by the fans, apparently creating a bubble which prevented correct heat transfer. Once we flipped the wires on the fans, they blew toward the engine, and all was well.

Quattropower88

Thank you... the fans are spinning in the proper direction so it must be something with the water cooling part

VirginiaF1

Is radiator original?
If not, when last replaced?

kingston944

Check that the timing belt is proplery tensioned, if it is loose the water pump pulley will slip and not circulate coolent adequately.

PlayGun

I second checking your hoses for wear and make sure they are not collapsing, I would also check your heater core, sometimes they can flake and clog the return lines.

944_S_TYPE

Is it possible you actually just have an indication problem? Do you have any other electrical problems, Like door dinger, or lights etc. May be a ground wire issue. back in 04/ 05 I chased a faulty overheating condition for a whole summer until I replaced my ground wires....

Quattropower88

Nothing wrong with the lights, but the door dinger does constantly change tone/frequency/volume -- very possible that ground wires might be shot.

Another indicator of this is when the fan turns on when the temp is at the 2/3 tick mark, the needle immediately shoots up a few millimeters. Is this abnormal?

txhokie4life

What are the signs of a bad/failing water pump?

xschop

Blown headgasket or clogged radiator fins

Quattropower88

Wouldn't the timing be off if it was loose enough for the water pump not to work properly??

Thank You