Strange warm up behavior 308GT4

Harry, it sounds to me like just an abrupt thermostat opening you are seeing. The thermostat stays closed until the water temp reaches a set point, and the coolant stays within the block. Once the thermostat opens, the coolant can circulate all the way to teh radiator. At that point, a load of cool coolant from the radiator gets mixed into the cycle, and the temp drops for a minute.

Birdman

 

You might want to burp the system to see if there is air pockets. Park with front end a bit lower than rear, then open the rad bleed screw. Even if cold you will purge air that way.

Gerrit
http://dino308gt4.com

 

Hi Harry
I usually do this when cold. Because there is no pressure in the system I am asking gravity to help move the water downhill. This pushes whatever air is in the rad out.
As with many things there appear to be multiple ways to do this purge. I used to do it when engine was hot, e.g. after a drive, but got tired of almost burning my fingers each time.

Gerrit

 

I agree with Gerritv, those are the classic signs of air in the system. I usually have the car's nose up higher than the engine & expansion tank. Have the heater set to "Hot" as well. You may have to bleed the radiator a few times to get all the air out. Make sure that the expansion tank is full (when cold) before start up. Again, it may take a few cold/hot/bleed cycles to get it all of the air out.

 

I agree with Gerrit.
With a cold motor, park with the nose down (below the level of the radiator expansion tank), open the radiator filler cap, and open the bleed valve on the radiator. You need to open the fill cap otherwise you will pull a vacuum, and not all air will be burped out. Also gives a good opetunity to check fluid level.

For me this is a ritual every time I take out the car.

Opening the heater valve is also a good idea once in a while or with trouble some systems.

 

There are a variety of opinions regarding cooling system bleeding. I like the idea of bleeding a cold engine but what's the point of removing the expansion tank cap? With a cold engine the thermostat is closed. Seems to me this would negate any reason to remove the cap.

FWIW I bleed the system with cap in place, heater fully on hot, open the bleeder valve ( I often hear the hiss of air escaping )then start the car. As the car warms, coolant expands forcing more air out. Shortly coolant bubbles out. I have a coffee cup modified to hang on the A/C lines by the bleeder valve catching the coolant. Once the engine warms to where the thermostat opens and there is a STEADY stream of coolant ( no bubbling or spurting ) flowing into the cup I close the bleeder valve knowing all the air is gone. I discard the coolant in the cup or add it to my daily driver's cooling system.

Yes, I keep an eye on my coolant level and top it off when neccessary

 

The reason for removing the cap (when cold) is to prevent or remove the vacuum that builds up as the air leaves through rad vent. The thermostat has a bypass hole so that some water will flow even with the thermostat closed.

Gerrit

 

ah, ok makes sense

 

I agree with Gerrit, I have had this problem before, nose down for air removal and open the system at the rad and expansion tank.

The behaviour is classic air pocket in the system have seen it a few times on my own car.

 

On a gt4 any immediate needle movement on a gauge should be cause for concern.....

 

COOL!