Weird temp gauge readings

2000 mercury grand marquis drives fine and then suddenly temp gauge in dash shoots straight up and then comes straight back down. This lasts for about 3 seconds. The engine will hesitate during these 3 seconds. Then it drives around fine for awhile (an hour or so) and then suddenly shoots back up again. I have replaced the coolant temp sensor and thermostat and coolant level is normal. There is no coolant in my oil or any signs of a leak. My alternator was charging a bit high the other day at 15.4 and I am not sure if that would cause the temp gauge to act up. Anyone have any ideas? I am running out of them!

It may not be in the cooling system. Could the serpentine belt be locking up or slipping momentarily? The temp will spike like that if the water pump stops turning. The water pump runs on the smooth side of the belt, so a bad water pump bearing or something jamming the water pump may allow slippage with little noise. A weak tensioner could also allow slippage. At 15 years old, I certainly wouldn’t discount anything happening here.

This does sound like an electrical glitch.

I agree with Capri, I can’t see the actual temperature changing that fast.


The reason I posted what I did, I had a water pump seize on me on my way to the shop in my old Honda. I was doing 40 mph when the pump seized, belt fried with a bone-chilling squeal, and the temp gauge shot up like a rocket. I instantly shut it down and coasted to a parking lot to get it off the road. The shop sent a truck for me. I also couldn’t believe how fast that temp gauge moved, even tho I saw it with my own eyes.

I will check the pump. Hoping to not have to take thing apart! At 180,000 it could be time though.
The electrical issue seems like that could be it. I read a post about another car doing this and they never found the problem. Maybe a PCM update or something. I have no idea. I guess just drive it around until it produces something that will throw a code or be steady enough to pinpoint. Thanks all!

My gut feeling is that the temp gauge spike is related to the alternator voltage as that number is too high.

The alternator voltage. can spike higher if there is a questionable connection between the alternator and battery (amps down/volts up and vice-versa) so maybe checking the alternator plug for burning and battery cable ends for corrosion or scale would be a good idea.

Update. The car started doing it really bad the other day. I looked in the fuse block under the hood and the 20 amp fuse was melted and fried. I replaced it and it has run like a champ for the past 3 nights. It is a taxi and used a lot. Not once has it done it. The fuse was for the fuel pump. Dont know how the 2 are tied together, but glad it has cleared up! Thank you all for your input!

That’s good news

However, you may be looking at future problems with the fuel pump, from the sound of things

The only times I’ve seen fuel pump fuses damaged and/or blown was when the pump was drawing too much current, and/or it was shorted

Good luck, in any case

Fully agree with db4690 about a pump problem being the cause of the fuse burning.

A long time ago VW had issues with high current draw from fuel pumps and the fuse would not give up. Instead an underdash wire connector would burn or the fuse block/wire block connector would burn.

The fuse was probably to the cooling fan. The fact that it was melted and the voltage spike suggests that there is something else going on.

It’s possible the heat from the fuse (before it got replaced) was heating something else in the same fuse block and that was causing the engine hesitation and the gauge jumping around. So it is entirely possible replacing the fuse will fix the problem.

But it’s also quite possible the fuse problem will return, b/c the fuel pump is starting to malfunction and drawing excessive current. A mechanic could measure the amp draw on the fuel pump, that would determine the fuel pump status for certain.

Edit: 15.4 volts from the alternator seems a little on the high side, but probably not out of spec. It could indicate the battery is about to go on the fritz though.

I’m leaning towards the electrical glitch theory. I had a belt break when I was driving the TR6 about 15 years ago. It took about 2-3 minutes before the engine really started getting hot. I was less than a mile from home when the belt broke and I was able to get home safely before the temperature gauge made it to the final hash mark before the “H”

If the fuse blows again we will replace the pump. We had issues with the pump about 6 months ago, but they cleared up with some work. Do you guys know if the current hits the fuse first or the relay for the pump? There is a black relay also for the fuel pump. Could the relay be going? Hopefully all stays good. It has been 5 days of heavy driving with no problem again. Fingers crossed!

Do you guys know if the current hits the fuse first or the relay for the pump?

The current changes simultaneously (well, at the speed of light) throughout the circuit.

What were the issues with the pump 6 months ago? Does this mean the pump was replaced or did someone bang on the tank to kickstart it?

Power should flow from the fuse through the main circuit in the relay and then to the pump although some Fords use a variable speed pump operated by a Variable Load Control Module.
I haven’t pulled anything up to verify if your model uses the VLC Module or not.

If the fuse blew that means there’s an issue somewhere and I’d be tempted to check the current draw of the fuel pump even if the engine is running fine. A high amp draw could mean the filter is clogging up or the pump is dragging.

The car would choke out when starting. If I held down the gas it would get going and run fine. I was told it could be the fuel pump going or just debris in the fuel tank getting sucked in when the fuel ran low. I usually would run it down until the fuel light came on then fill it up. I ran cleaner through it and it would clear up until a month or so later and then repeat. Finally I did a fuel cleaner and kept the fuel above a quarter tank and it stopped acting up. That was about 6 months ago. I was thinking it could be the fins in the fuel pump getting weak, but it seems to be ok now. I keep the fuel filter changed ever 2 months. I am going to check the current on the fuel pump. I have a feeling that think is the next to get replaced.