1998 f150 4.6 4x4 auto 180k

Runs 100% other than a strange and inconsistent overheating issue.

On cold startup truck will warm up.then the temp gauge drops to practically 0. After 15-20 seconds gauge shoots up to max and then falls back to normal after 15-20 seconds. Then it'll go back to driving normally. At some point it pukes.


What do you think?

 

" At some point it pukes " So ... Does it boil over ? or what ? What do you mean by "pukes " .

 

Yeah it blows out of the reservoir. Also the heat isn't functioning properly.

 

what type of coolant are your running. on a cold start up and shut down is their any noticeable amount of pressure in the cooling system.

 

there is an air pocket in the coolant system. when the temp probe is surrounded with air instead of water, it spikes up rapidly then drops. The puking of fluid is due to the air bubble building up pressure and popping the coolant cap. You can get a bubble from lack of circulation due to the thermostat vent hole being plugged. The hole allows coolant to bleed thru slowly and warm the radiator as the engine warms up. If the hole is plugged then you get a spike in temp as the thermostat finally opens and hot water goes to the radiator and cold water exits to the motor...
A second possibility is a hole in the head gasket allowing coolant to get into one cylinder and exhaust gas to get back into the water jacket.. When you startup in the morning do you have any stumble or rough running for 20- 30 seconds ? If so, that can be a small amount of coolant in once cylinder......... If not, you might pull the thermostat and look at it, maybe replace.

 

My vote is for a bunch of junk plugging up your coolant system.







Why? Because: 1. Coolant not flowing through heater core. 2. Sudden shifts in temp at temp sensor.

 

Exactly the same symptoms I had on my 98 when I had it. Once the thermostat is open I bet all is good and you have heat...right? Mine was a head gasket issue allowing exhaust gas into the coolant. Smell your coolant, does it smell odd like combustion? I had very high miles on mine (well over 200,000) and felt it was not worth fixing. What I did was take the thermostat out and snip off the jiggle pin and put it back in. That's like drilling an 1/8th in hole in the flange. What that does is allow the combustion gases to pass by until the thermostat opens. It stops the puking, the gauge stays consistent and you get your heat. It will take a little longer to warm up and get to operating temp.....but it will. Maybe takes another 5 miles or so longer for OD to drop in, but it like a new truck. Keep an eye on the coolant level...I ran mine about an inch below the low mark so it could rise up some until it settled in. Check the level every week. I got another year out of that truck and sold it to a guy with full disclosure. I assume he made it home.