Temp gauge 99 4.0L

[backroad101]

So my temp gauge red lined on the way home today. Then it drops back to normal then redlines again. It does this over and over. It does it at speed and it does it at a dead stop. It will be fine then all the sudden it will climb, red line, sit there then drop back to normal. Sometimes it idles a lttle rough, sometimes its fine. What would cause this? I just replaced the thermostat but that was not the issue. Thoughts? I want to tackle it again in the morning and get it resoved before it snows. Temp sensor or sending unit? Other causes? Don't want it to overheat

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[sholzy]

Bet you have air in the cooling system.

 

[Doofy]

Couldn't tell where you live but if it is below freezing there, make sure your anti-freeze is at the proper mixture. Make sure you burp the radiator good. Get a good aftermarket temperature gauge.

 

[Original_Ranger84]

Yeah I would start with your coolent protection level

 

[backroad101]

just to offer a little more history, the gauge acts up after the truck warms up. I could be wrong, but the heater does not seem to blow as hot has it should. The coolant was flushed earlier this year. Also, I checked the oil last night and no coolant there. No visable leaks anywhere else and no pools of coolant on the ground. Also, the previous owner had the head gaskets replaced and the heads resurfaced. The coolant resevoir was almost dry when I check yesterday

So far I have topped off the coolant and replaced the thermostat. I want to jump back on it as soon as the parts store opens. Any and all helpp appreciated

 

[Original_Ranger84]

MIne started leaking very VERY slightly from the thermostat gasket, you might try picking up some of that UV dye and running it through the coolent for a few days then use a black light and see if you can find a leak.

 

[backroad101]

Here is where I am at. Replaced thermostat. Gauge bouncing, red lining dropping. Replaced temp sensor and sending unit. Nothing. Took to NTB. On the way there it was staying closer to red line as it bounced back an forth. They said replacement thermostat was defective and replaced it.

Appeared to fix the problem but again, the gauge rose, redlined then dropped to normal (mid point). Now it is not doing it as exxcesively as it was before and for the most part the gauge is in the mid point spot and good to go, but a couple times it red lined.

Thoughts? I would think if it was overheating it would stay red lined.

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[mosher41]

my 99 4.0 did the same thing I believe it was Plugged Cats, internal cumbustion chamber temps were getting so hot that it was boiling the coolant inside the block. It also degraded the head gaskets which eventually went bad too. There are 2 cats they run 500.00 and up.
After I changed the cats everything improved, MPG, throttle response, and the temp gauge is now steady and consistant

 

[backroad101]

Next update. Let the truck idle for 30 minutes while I washed my other car. Temp gauge steady in the middle every time a checked. As soon as I moved the truck to get my other car in the garage, it acted up again. Temp gauge redlined, idle a bit rough and the oil gauge flickered once or twice (had this issue once over them summer after 60 highway drive). All other gauges are fine. The Coolant resevoir was also completely full (is this normal?). There have been no codes tripped

If it is the cats I want to be sure due to cost.

I havent mentioned but we have had a week of around freezing weather this week followed by yesterday and today being in the low 40s

Thoughts? I'd really like to resolve tomorrow. Again any and all help is greatly appreciated.

 

[sholzy]

You have air in the system. Raise the front of the truck a little. Set the climate control temp selector to hot. With cap off, run engine until warm. Add coolant as needed.

 

[sholzy]

Guess where the air is at? It's in the heater hose. That hose is usually the highest point in the cooling system. That is why you don't have heat in the cab.. If you have one of those after-market flush kits installed in the heater hose, just loosen the cap until fluid comes out and then add coolant.

 

[greenpus]

i had a mustang do that. I ignored it cause the temp rose too fast and then dropped too fast.. anywho, it turned out to me a shorted wire in the dash causing it.

 

[backroad101]

You have air in the system. Raise the front of the truck a little. Set the climate control temp selector to hot. With cap off, run engine until warm. Add coolant as needed.

When you say set it to hot, do I set the speed to high? And when you say cap, i assume you mean radiator cap and not the resevoir cap? Driving up on ramps, too much? Otherwise maybe my driveway slant is enough?

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[backroad101]

Guess where the air is at? It's in the heater hose. That hose is usually the highest point in the cooling system. That is why you don't have heat in the cab.. If you have one of those after-market flush kits installed in the heater hose, just loosen the cap until fluid comes out and then add coolant.

I do have heat in the cab now though

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[sholzy]

When you say set it to hot, do I set the speed to high? And when you say cap, i assume you mean radiator cap and not the resevoir cap? Driving up on ramps, too much? Otherwise maybe my driveway slant is enough?

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Driveway should be enough. All you need to do is let the air have a high point to escape from, best place is the radiator cap. Just the temp selector, no need to run the blower. Yes, radiator cap, sorry.