My 2000 F150 4.2 runs fine but like today I took a three hour test drive. The temp gauge pegs to H out of the blue and the oil light flashed on, then I'm assuming it went into limp mode. I turn the engine off for a few seconds and when restarting everything is back to normal. I'm assuming the engine isn't actually overheating. Then it seems fine for about an hour of driving but I do notice the temp gauge wavering a bit when usually it would stay in exactly the same spot.....then after another hour of driving the gauge would peg again and go into limp mode. Restarting....back to normal. The truck sat all summer and the battery was dead, I put a trickle charger on it and it seems fine but I read that a dying battery could cause this? I also read it might be a bad temperature sender. I'm going to test it as soon as I can find it. Haynes manual says put a jumper wire on it should make the gauge go to H. Could it be the water pump beginning to fail? I suppose I could rule that out since it's not overheating. The only other thing wrong with the truck is the AC doesn't work at idle which I read could be related to the fan clutch. Any tips much appreciated.

 

Guess I'll have the coolant flushed today.

 

Reading your post, at first I was thinking something electrical, like a bad sensor, bad connection or even the bad battery you mentioned.

But I agree, the water pump and/or fan clutch are also possibilities. Before you start spending big money, you might examine your fan belt, and make sure it is not worn out or slipping.

 

Get an infrared thermometer gun - and when the gauge pegs out use it to see if it's really overheating or not.

 

Thanks for the replies. I had my battery checked today and it is fine. My belts are good as well. I should mention the truck has about 115k on it with a lot of idling due to my job. I read in my manual there is a weep hole on the water pump I can look at using a mirror that might help me determine if it's failing. I also bought a laser temperature gun today but I'm not sure exactly where to point it on the engine? Also, picked up an engine coolant sensor today that I'll install tomorrow.

 

Point it at the thermostat housing.

 

I see the hole under the intake manifold where the coolant sensor is supposed to be but there is nothing there. I did manage to find the cylinder head temp sensor...so I ordered that and will install on Monday and then do the test drive and use the temp gun. By the way, that temp gun is cool. I've been shooting all sorts of things with it. Pizza...beer..etc.

Is there a way to check the old temp sensor with a ohm meter to check if it is bad or not?

 

If all else fails have a hydrocarbon test performed on the cooling system. A blown head gasket can generate the type of symptoms you are having. Good luck.

 

The fact that you have the problem, shut it off for a few seconds , fire it back up and it's fine again has me thinking a possible computer problem. It's doing it after a half hour in separate drive cycles. Each time you shut it down and start it begins a new drive cycle. since the oil light comes on too, I'm thinking bad ground somewhere.

 

I think part of the problem is the gauge is reacting to the cylinder head temperature so it doesn't really give an accurate idea of the coolant temp. At any rate, I installed a new cylinder head temp sensor and test drove it. After about 30 minutes the temp gauge pegged but I was on a busy road so I just kept driving. It stayed pegged for about 15 seconds and then moved back to normal temp. I pulled it over and pointed the temp gun about 8 inches from the thermostat housing and a specific spot and it read 168F. I then realized I was driving on a hilly road when it overheated so I drove to a road that I knew had a long fairly steep incline. Sure enough it overheated two times in a row. Both times I was able to jump out and quickly get a temp reading. It reached 205F for a few minutes before slowly returning to 168F. I noted that the gauge returned to normal temp before the coolant cooled down. So I guess I really do have a intermittent overheating problem that shows itself when there is a load on the engine. What would be the next thing to test? I'm starting to think it's the water pump but don't those just completely fail?

 

205 is far from overheating.

 

The impellers in water pumps can rot away and you would never know it.

 

I gather the temp I'm getting from the gun isn't the actual temperature of the coolant but the average temp of the surface area it is pointed at. I'm guessing it's probably reading lower than actual temp....especially since it is rather cold here in Wisconsin at the moment. The coolant temperature change in correlation to the the cylinder head temp senders reading in relation to load is what I was interested in. Is a 40 degree change normal? I'll inspect the weep hole on the water pump tomorrow, is there a fan clutch test I can try as well? What about thermostat?

 

Shoot the lazer right at the point the upper hose hits the radiator and the top of the thermostat housing. If you had temps there ranging around 240/250 I would be concerned. Outside temp doesn't have alot to do with it but a thermostat does. I have a new motorcraft 195 T stat, shot my lazer at the t stat housing and got 204 and at the upper Rad inlet got 200 T stat open. T stat closed a few minutes got 205 at housing and 193 at Rad inlet. Hope that helps.

 

If it's overheating, the fan should be cranking at full speed, rev it up and it will sound like a wind tunnel. Replacing the thermostat with a new OEM 192 wouldn't hurt. If you aren't losing coolant, looking at the weep hole will be inconclusive. Don't discount the possibility of an eroded impeller and/or a clogged radiator.