The car in question is my '98 XK8 Vert, with 90k miles.

During the summer, I replaced the expansion tank on my car, reset all the codes due to low coolant, etc, and finally the coolant smell was gone. For a while everything was good.

Then one day, I was driving when I got a "Restricted Performance", and after a few miles the RP went away, and even the "Check Engine" light would go away. While the RP is on, the temp gauge stops working, but would resume working once RP is gone. (Probably the RP goes away when the gauge starts working)

For a few weeks this was the norm, but the last few weeks (since it got a bit colder) the RP comes on and stay on until I switch the car off. Restarting the car, the Check Engine light still stays on and the RP would come back after a few miles.

These are the codes:
P0116
P0125

The car is not losing coolant and the coolant level is OK. The coolant was replaced a few times this year. I replaced the coolant temp sensor situated close to the thermostat last week as well.

Some more info.. In May this year the car's head gaskets, primary and secondary timing chains, guides, and tensioners, water pump, thermostat (x2), etc. was replaced. After that, the car was running a lot cooler using the original gauge (yeah, I know, I know...) but since I don't use the car in heavy traffic, the needle would rarely make it to the middle. Since it got cooler, the needle now barely moves, and after a few miles it would still be in the blue zone at the bottom, and then I get the RP and the gauge stops working.

Apart from replacing the thermostat again that may be stuck open, is there another sensor somewhere? Could it be a suspect fuse, or something completely different?

Thanks in advance..

 

First clean the connection to the temp. sensor., It is possible that the replacement sensor is faulty, do you still have the old one? The thermostat does sound like it's stuck open, but if you can't trust the sender you don't know. Do you get hot air form the vents?

 

RJ - The new sensor did not make any difference at all, but I kept the old one.

The temp of the air from the vents depends on the temp I set. I have not noticed anything different than driving "normally." Apart from the temp gauge and the restricted performance, the car drives fine.

Although one of the recommended fixes is replacing the thermostat, I am not completely convinced it is stuck open. The temp gauge that stops working still holds the biggest clue in my opinion.

I was hoping that someone else may have encountered the same before... Any other ideas? Anybody?

 

Can your OBDII scanner tell you what temp your engine is running at? That would tell you if your gauge is running true before it drops out.

 

Here is a little test that will cost all of $1.50. Go to your local Radio Shack and get a pack of 270 ohm, 1/2 W resistors: 270 ohm 1/2W 5% Carbon Film Resistor pk/5 : Resistors | RadioShack.com

Unplug the temperature sensor and gently plug one resistor lead into each position the 2-pin socket. Now turn on the ignition...no need to start the car. If you are now showing center scale you have proven all the electronics are functioning properly (wiring, ECU, instrument cluster). Remove the resistor.

If the above test passed, then the problem is either temperature sensor or the thermostat. Now read the resistance of the temperature sensor itself with a multimeter set to ohms with the engine warmed up. If it is over about 300 ohms your engine is running too cool. It is highly unlikely both sensors are bad...they rarely fail.

That leaves the thermostat stuck open as a near certainty...which is a much more common and most likely failure.

 

Steve, that's great. I have made a note of the test for future reference.

 

I stopped by Radioshack and got the resistors.

Did the test, and the temp guage immediately went to the center scale.

My multimeter is in the mail, and hopfully I can do the second part between this weekend and next weekend.

Steve, thanks again!

 

Another Update.. Tried to connect the multimeter, but were unable to connect to the sensor properly.

However, I let the car warm up for quite a while, and then took it out for a ride of about 6 miles with the ODB connected. Outside temp was about 50F and running at 45mph the coolant temp was between 121F (coasting downhill) to about 136F. This was reading the actual ODB scanner data while driving.

During the whole time, no warnings or error codes showed up. I was hoping to get the restricted performance while driving to see the actual temp at the time.

However, after searching the forum, it seems that the temp I was reading is too low, and that the thermostat is probably stuck open. Before I do anything else, I will go ahead and change the thermostat. (And check the old one using different water temperatures, just to make sure)

While replacing the thermostat, would it be a good idea to change to a aluminum thermostat housing kit?

 

Yes, change the housing. If you order from Welsh Enterprise, the housing and thermostat come together. I would suggest removing the old housing by breaking it up, but be careful not to lose pieces inside. Then when replacing, use socket head screws at the back and use a ball drive to tighten.