99 Town Car Gauge showing overheating

[thomas.melbye]

Hello world,
Town Car 1999 120” limousine.


Having problems with temperature gauge jumping to max, coolant warning and loss of power/vibrations. Feels like limp mode.


Happens after driving anywhere from 5 minutes to 30 minutes.


After restart temperature shows normal and car runs as normal.


I have seen the temperature gauge jumping a little so I suspect electrical issues.


Before I saw the gauge jumping i suspected faulty thermostat or water pump.


Removed thermostat without any change.


Thought the upper radiator hose had too much pressure so I also replaced expansion lid.


I have replaced coolant temperature sensor on the bridge and cylinder head temperature sensor behind the alternator. No luck.


Tried disconnecting one sensor but same result. Temperature gauge did not show temperature when cht sensor was disconnected but still jumped up to max.


With cht sensor disconnected the car would also stall.


Strangely enough it showed a little rise in temperature when I disconnected both (before a new restart).


I’m going to replace the water pump anyway, but not sure if that is the problem. Radiator and hoses get warm so guess it’s circulating.


Unfortunately I’m not able to connect to Obd2 port because someone have tried replacing the port and all the cables are mixed up. Colors on cables differ from schematics I have found.

So…
What to do?


The car is not overheating as far as I know.

Is there more than two temperature sensors on the car?

Could this (god forbid) be a bad cable somewhere?

Any way to measure the cables to determine if they are good?

Any advice?
I’m not a professional mechanic in any sense, but I know what a spark plug looks like…

 

[oktowncar]

Start with the simple things first.
Have your battery load tested to see if it is in good shape.
One bad battery cell can bring in very odd electrical gremlins

 

[Action]

There are multiple sensors for engine coolant
One for radiator fans or PCM to signal fans
One for temp gauge or warning light
One for HVAC

A way to test the water pump would be to drive over 45mph. If the coolant temp drops, the water pump is not totally working.
But without a functioning temp gauge that can not be done.

Action

 

[driller]

Could this (god forbid) be a bad cable somewhere?

Any way to measure the cables to determine if they are good?

Yes, it could be an issue with the wiring harness.

Most every diagnostic procedure for troubleshooting issues will ultimately lead to testing for opens or shorts in the wiring. Specific procedures, connectors and pinouts are called out in the service manual.

Worrisome to me is the OBD2 port wiring issue and the fact that it is a limo which typically involves modifications otherwise not found on stock vehicles. The other oddity is it apparently reverts to normal following the event after restarting?

Blindly throwing parts at such an obscure problem rarely resolves it. I would recommend obtaining a service manual first and resolving the OBD2 connector issue to begin proper troubleshooting. Or find a competent shop with the necessary resources.

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[thomas.melbye]

Replaced CHT sensor and coolant sensor. One behind the alternator and one on the bridge of the intake.

Also replaced thermostat and water pump.

Still getting strange behavior where temperature is not changing on the gauge, rapidly goes to normal. Suddenly it jumps all the way to the top and the coolant temperature warning goes off. It then goes into some limp mode.

After stopping and restarting it most of the time goes back to normal temperature.

I’m suspecting an wiring problem with the sensors.

Even when I disconnected both sensors the car showed normal temperature when I started the car. Is there a third sensor somewhere that connects to the gauge?



Also had a blow out of the coolant. The level in the expansion tank was normal and when I revved the engine it suddenly blew out lots of the coolant. Typical behavior with blown head gasket if I’m not mistaken.

Added a bottle of Bar’s head gasket “stop leak & block repair” in trough the thermostat housing and filled up with coolant. (it lost a lot).

I have not experienced the blowout after that, but still problem with indicated overheating.



I’m kind of lost now.

 

[Action]

Also had a blow out of the coolant. The level in the expansion tank was normal and when I revved the engine it suddenly blew out lots of the coolant. Typical behavior with blown head gasket if I’m not mistaken.

Typical of a cooling system that has air in it.
That air could have come from a head gasket. However more likely it occurred when the cooling system was opened up to replace sensors. (Water pump and/or thermostat)

Really the issue should be diagnosed first. Then replace parts

A wiring diagram would be necessary. There are over a dozen for sale on eBay. Two for less than $15 plus tax.
If you are not up for using a wiring diagram to trace circuits, it may be time to pay a professional.
With the wiring diagram, you can get the OBDII connector correctly wired as well.

Action

 

[driller]

Typically the SHOC engines are less prone to trapping air in the coolant system than the DOHC modular engines of the era. But it could still be a problem.

If you suspect a head gasket problem, you may want to test the coolant for combustion gases indicating a head gasket breach. It's a simple test you can DIY or have your mechanic perform.