Engine running hot

[dustinhu]

Hi there,
My 928 GT engine temp seems to hover right below the red line after I've been driving it for about 30 minutes. I thought maybe the coolant was old/bad, so I recently flushed the system but that hasn't changed the outcome.

I guess my questions are:
1. Is it normal for the engine to run this warm? I doubt it but just checking
2. Would the problem be indicative of the thermostat not functioning properly?
3. My heater/ac always blows really hot air so I leave it off all the time. Could this be raising the temperature in the engine compartment because not air is getting blown out?

This is a relatively new purchase so I'm still learning about the car.

Thanks,
Dustin

[stepson]

Use an IR gun to tell you what the temps really are. Do not always believe the gauge in the pod.

Please post your results.

Passenger side top radiator hose
Drives side bottom radiator hose
Thermostat housing

[dustinhu]

Great. I'll buy a gun and start there.

[xcracer]

If it would be hot your fans would be running like crazy if they are working correctly. If they never come on at all then that could be an issue. A thermostat stuck shut could also cause high heat. Curious what you find out with the IR.

[dustinhu]

Ok. I was able to take measurements today. It wasn't as hot outside on my drive home so the temp gauge in the car was only at 3/4 way to red.
Upper house -178
Lower house - 181
Housing - 162

Also the heat is always coming into the cabin. Would that be indicative of a bad thermostat?

[stepson]

Originally Posted by dustinhu

Also the heat is always coming into the cabin. Would that be indicative of a bad thermostat?

More likely to be indicative of bad vacuum to the hot water valve on the back of the block under the air box. Test the hot water valve with a Mityvac. If it wont hold vacuum, it will open allowing hot water to flow into the heater box and voila: hot air into the cabin. Just replace it if it doesn't hold vacuum.

In addition, if you motor mounts/transmission mounts are collapsed, that will allow the engine and transmission to drop, breaking the foam seal and allowing outside hot air from the engine compartment into the cabin as you drive

[Mrmerlin]

since your asking these questions I think it is time for you to do some work on your machine.

Please replace the thermostat,
the rear seal and front O ring.
the coolant cap,
the heater control valve and the short hose that connects it to the engine.
the 4 way vacuum connector at the brake booster and the blue check valve.
radiator drain plug, radiator vent plug (on the top driverside).
NOTE these only screw in to snug they are sealed with an O ring.


Also if you have no idea when the last coolant change was add 2 gallons of Zerex GO5,
and 2 gallons of distilled water to this list.

NOTE the block drains hold about 1.2 gallons from each bank.
NOTE do not flush the system drain and refill.

the only part you can flush is the heater .
To do that flush with water into the heater control valve,
disconnect the 1 inch hose from the front of the water bridge,
and drain into a bucket also add some water to the coolant bottle.
Then connect the hose lines back

Please report back when these repairs are complete

[dustinhu]

Thanks everyone. More to come

[JK McDonald]

Hey Dustin - Like several others have suggested there are a number of troubleshooting steps involved in sorting out an indicated engine temp problem. Mrmerlin is one of our 928 gurus and has helped guide most of us through the expected laundry list of new ownership woes.

Many 928 owners have also had to deal with the same type heater shut off valve issues you are experiencing. The heater control valve is spring loaded to fall or fail to the "open" position. The typical symptoms are usually either -

1. The heater valve closes properly (vacuum control solenoid energized) when the heat is "OFF" or when the A/C is selected and the heater core has no hot water flowing through it when the engine is running - but the heater valve gradually creeps opens at engine shut down. This fills the heater core with unwanted hot water each time you park. The A/C must now cool off the heater core at every start up before you feel any cool air at the dash vents. --------OR---------

2. The diaphragm in the heater control valve has failed open and does not hold a vacuum or it's vacuum source is missing. This allows a constant flow of hot water into the heater core which means you'll feel heat from the vents any time the engine is running.

One solution or semi-fix used by some owners in the summer is to zip tie the heater valve arm to the closed position simulating an applied vacuum at all times. Even when parked, the zip tie holds the heater valve closed off. There are two draw backs - you must remove the air filter housing for access to the heater valve each time you apply or remove the zip tie and you have no heat or defrost if the weather suddenly changes.

I might offer an alternate solution once all your electrical and vacuum heater control components are working properly but still allows hot water into the heater core on engine shut down. You could install an additional cable controlled hot water "shut off" valve in series with the factory vacuum shut off valve. It is a little tricky but this new manual shut off valve actually fits nicely behind the fuel cooling cartridge against the center of the firewall.

Naturally the air filter housing must 1st be removed to begin this project. Then the short inlet hot water hose to the vacuum shut off valve is pulled off the fitting from the block. You want this leg of heater hose plumbing because it's the inlet side of the heater core. Once disconnected - this allows just enough working slack in the factory vacuum valve and it's 14" of the heater hose that continues on across the firewall toward the driver's side of the engine compartment. Just cut out about 1" of heater hose after the vacuum valve to insert the manual shut off valve. Before tightening the hose clamps - make sure to rotate the new valve within the heater hose until the arm of the manual control valve can move unhindered. Now attach a short length of flexible "manual choke" style control cable before everything is carefully placed back into position and the vacuum heater control valve is reattached to it's hot water fitting. One very IMPORTANT POINT there is a large square opening in the top of the bell housing that a 1/4" drive socket will disappear into if dropped. (Don't Ask) - Stuff a rag into this opening before getting too far along in the hose clamp handling stage. The socket is retrievable by removing the drive shaft pinch nut access plate - but what a PITA.......

Don't forget to check that the vacuum line is securely reconnected and in place to regulate the original heater valve. I ran my short manual "choke cable" behind the air filter housing and up near the radiator reservoir. Now each summer I can simply pull the arm of the manual shut off valve closed to eliminate any delay in my A/C blowing full tilt meat locker cold air out of the vents. In the winter the manual valve is left open allowing the automatic factory vacuum controlled heater valve and the normal temp regulation circuitry to moderate the heater and defrost temperatures.

Prior to this modification each time Miss Purdy was started I'd been telling my wife that she must be having some kind of short term hot flash.

https://www.google.com/search?q=automotive+heater+control+valve&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjyrfDHt6rOAhUB5iYKHUHVD6kQsAQIogE&biw=1138&bih=580&dpr=1.2#imgrc=IV4YgpV4u8sWbM%3A

Good Luck, Michael

[dustinhu]

Great suggestions. I'll post back once I've replaced some of the core components listed. I have already put fresh coolant in so at least that job has already been done.