mackelby

Temp gauge goes from perfect to pegged in a matter of a second. One cylinder head is getting hot. Just a few degrees over the sensor trigger. Needless to say it starts cutting out cylinders. No leak to the outside of motor. Tech says it is positive for Carbon monoxide in coolant. The head gaskets where replaced recently by previous owner. This is when the problem started. Does it need to be done again? does the cab have to come off? any ideas.

jstockert

I would change both temps sensors. The 4.6 has one on intake beside the thermostat housing and one on driver side head under the intake manifold. Also check wiring to both sensors.

mackelby

Tech says sensors and wiring checks out to be good. Still hard to understand why temp gauge goes from perfect to pegged as fast as the needle can swing. One of the cyl head temps was getting to 250. Didn't realize 98 f150 had cyl head temps sensors. If it doesn't, do some scan tools have the ability to read a cyl head temp?

Carcrazygts2

The pcm kills cylinders to try to remove heat. If it has combustion gases reaching the cooling system obviously Simeon didn't know what they were doing. You could have a cracked head or it is warped and wasn't machine prior to installation. It needs to be done again.

mackelby

Keep taking it to work. One time it will peg the temp gauge the next time it won't.When it does do it I jump out and use a temp gun on upper water hose and thermo housing. temps are both 193. I check the radiator tanks. Hot side 178, cool side 115. Seems like the radiator is doing its thing. Motor doesn't make sounds like it is hot. Doesn't smell hot. Upper hose never has any pressure on it. Same temps when the gauge acts normal. So one of the senders is UNDER the intake manifold?

Jbrew

If it tests positive for exhaust gases, then it's done. Personally, I 'd junk the engine, but that's just me.

Since the sender seems to be acting goofy, let it get to operating temp then crack the ECT loose like you would diesel injectors. Just crack it loose until you see coolant, then tighten it back down, be careful of course. Sometimes there's a stubborn pocket of air that lives just below the ECT which is your Engine Coolant Temp sender. The CHT Cylinder Head Temp sensor is a dry socket, so that problem won't exist. But with both sensors, either sensor itself could be bad or you could have a harness issue as they like to sever inside the insulation on occasion.

Anyway, I'd straighten out the sender issue first, then get a second opinion on the exhaust gases in the degas tank (expansion tank). Unless your absolutely sure yourself that is happening.

TheUnforgiven71

I'd say check your water pump as well, it's possible your water pump blades have devolved by now if it's never been changed, I had an overheating issue similar to yours and turned out I no longer had propellers on the water pump. Just my 2 cents