Hello everybody,

Have a '98 4.6L with 200k on the clock - and it's having a weird issue(s) that I can seem to pin down. First thing that started was the temperature gauge suddenly decided to sporadically work, or more specifically, wouldn't show any heat until the truck was thoroughly warm and suddenly the needle went from 0 (below operating temp) to 100, normal operating temp. position. Thought it was time to replace the thermostat; but then noticed that my heater wasn't working either, no heat at all, then sudden heat for anywhere from 10-20 seconds, then the air would go back to ambient air temp while the selector was on full heat. It wouldn't go back to full heat until I turned the heat off and on, or until I got onto the freeway...Weird heater and temp. gauge behavior had me thinking either thermostat, heater core, or both.

As things started occurring I obviously payed more attention while commuting to work, and have since had a couple times where I have gotten onto the freeway on-ramp and noticed the temp gauge climb suddenly until I back off of the gas, the engine seems hot for about 10-15 seconds then seemingly goes back to 'normal.' The first time it happened I checked and saw a little coolant leak once I got home...looked around and couldn't find the source, checked clamps and tightened some cheap-o hose clamps that had taken the place of the real spring hose clamps - then I waited and couldn't see any more dripping over the next few drives.

Took it down to my brother who works at a local repair shop, coolant flush and fill, and he pressure tested it - no leak to be found, holds steady at 16psi....very little to no loss on the pressure tester for nearly an hour, maybe 1/2 a lb. which was attributed to the pressure testers cheap seal/wiggling. Edit: We've pressure tested a couple times now....searching under the truck for any leak...no dice.

Here's what I don't understand...I've replaced the heater core before: OBVIOUS smell of coolant into the cab via the heater when turned on, right? Well now I can occasionally smell coolant burning, but it seems like it's coming from under the hood and in through my window, nothing like when it was the heater core...but I still smell it slightly. I can smell it for just a second faintly once I step out of my truck after I shut it off. I can also get a whiff at a stoplight/stop sign through the vents pushing cool air until I take off and get fresh air once again. Brother mentioned this could possibly be due to an intake manifold gasket leaking ever so slightly.

Now here's the kicker....through the two coolant flush/fills the temperature gauge has magically returned to seemingly normal operation, working like a charm....this has my brother, the owner of the repair shop, his coworkers, and me all stumped...What say you F150Forum?

 

Pin hole leak in the water pump by-pass hose...can't see it and will never see the leak till it gets real bad...it pisses out onto the hot manifold and burns up...this only happens after engine shut down. That was my problem anyhow...good luck with yours...

 

Will have to check that...anybody else?

 

i agree.....

themometer, rad cap, water pump... i'd change those, and then i'd turn to f150 forum for help. but that's just me


what's the name of the repair shop? i'd like to know where not to take my truck.

 

We'll look into that - this is just side-work for free (nearly, compared to shop rates) from my brother, the kid is still learning to wrench - after-hour work that the owner is nice enough to let him do.