I've got a '92 f150. Somewhere around 250k miles (it's got 198k on the odometer and the previous owner - my father - says he had the speedometer unhooked for a few years LOL ) . Since I've had it (about 3 years), it has had a power loss at low throttle. I'm not sure how to describe it, but here's my attempt:

When at low throttle, it has power loss. If you mash the pedal, it 'kicks in'. It's most noticeable when you're close to the threshold; cruising at around 40 or so on a flat road it'll be right on the line, so it's constantly surging and dropping out.

I've tried replacing basically everything in the ignition system, but it hasn't seemed to help. No CEL.

This seems to have gotten noticeably worse over the past 3 months or so. Then about a month ago, the truck overheated pretty suddently (never had a heat issue before). I checked it out, and the water pump was leaking around the shaft, and the thermostat housing seal had cracked. At the time I thought that caused it to run dry and overheat...

BUT I replaced both, and now it still has a slow overheat issue, which leads me to think maybe the overheating and pressure caused the seals to break instead of the loss of water causing the overheat.


So anyway, now it has an overheating issue: When I'm on the interstate it usually stays way low on the gauge (the gauge has "NORMAL" written on it; it stays between the N and the O). Exceptions to this are very hot weather (when this started it was about 105-110, and then it would even creep up on the interstate). Driving in normal traffic I'm OK if I watch it and I'm careful about how I drive. BUT - if I hit a steep hill, it spikes every time.

At a glance I'm guessing that it's either the head or the gasket, maybe? I don't think it's the gasket, since I'm not losing any water (unless I let it overheat, which I only let happen that once), or oil, no water in the oil or muddiness in the water, no white smoke, that sort of thing. So my guess is that it's the head (burnt valve(s)?), and that would involve replacing the gasket anyway so it's covered either way.

What do you guys think? Am I overlooking any kind of obvious?


A few things I do plan to check before diving into something that deep--

-Fan - maybe there's an issue with the clutch. Looks fine, but I'll do the 'kill it warm' test to be sure it stops.

-radiator - I'll do a thorough cleaning of the radiator fins, flush the radator, etc to be sure it's not a problem (the fluid right now looks just as green as when I put it in a month ago, though, so I don't think that's a problem).

??? suggestions?


But anyway, some of those problems might contribute to the heat, but the head is the only idea I have for the 'surging'.

Oh, maybe spark plugs - I'll replace those as well to be sure they're not the issue.

 

Exactly which parts, and where did they come from?

The erratic throttle response sounds like a bad TPS. Read this caption & the links in it:



The overheating sounds like a bad fan clutch:

 

Just came in from test drive... I think you nailed it. The fan looked like it was working fine, and was making an audible roar when it kicked on, but I guess maybe it wasn't staying on or something. Either way, with the replacement clutch, I warmed it up to open thermostat, then tacked a known 'problem' hill 4-5 times back and forth - no spiking of temp at all. I replaced the TPS while I was at it, and I can't detect any surging whatsoever.

Of course it's much cooler than my normal afternoon run, so I reserve final judgement until tomorrow, but at this point it looks like that fixed it!

I'd have never thought to check the TPS, and I thought the fan was fine, so thanks for saving me several hundred bucks and quite a lot of work replacing the head!! (especially since it would have still had the issue when I was done!!!)