Hi Boys and Girls,

Presently off in the middle of a long camping trip. F250 is loaded to 8600GVW with a slide-in camper, plus towing our Jeep. New engine (351W) from last winter has over 1000 break-in miles with zero problems, but this is the hardest I’ve worked the new engine. The old engine, even when worn and tired, handled the same heavy load adequately.

Ran into a little trouble yesterday. Covered about 225 miles. The first part went great. As the day went on, the engine seemed to struggle a bit on light grades. Would still cruise 70 no problem on the flat sections, but light climbs caused me to drop down around 50. Never did that before. No bucking or surging, though. Wasn’t quite sure what to make of it.

Today was worse. Only covered about 15 miles and could barely maintain 50 on the flats. Had a couple of light backfires, in the exhaust I believe. Pulled over to safely and broke into my spare parts. New distributor cap and rotor, no help. Spare coil, no change.

I’ve got a Carter electric pump, as detailed here:

https://www.ford-trucks.com/forums/1...tallation.html

I’ve also got a fuel pressure gauge, with the sender directly at the carb inlet, downstream of the fuel filter. Pressure was rock solid the whole time, at 5.5 PSI. The gauge has numbers so I know beyond the shadow of a doubt it’s accurate.

I didn’t think fuel delivery to the carb was the issue due to the steady pressure reading, but I decided to change the filters because that’s what I had available. One filter at the pump inlet (per Carter’s instructions) and a second in the stock location at the carb inlet.

Lo and behold, I found the fuel pump was leaking. Nothing major, but the outside should be dry. Judging by the stains, it had been leaking around the electric terminals on the bottom. Working on the premise to fix known problems first before digging deeper elsewhere, I changed the pump and both filters.

All seemed good and we backtracked the 15 miles to where we stayed last night. Not a hiccup.

The big question on my mind:

Did I really fix it? Could the root cause have been heat-related, whether fuel or ignition? Did the real culprit cool off enough during the 90 minutes while I changed the pump? I’m still skeptical of a fuel delivery problem due to the steady pressure reading, but maybe I found a gotcha?

Any thoughts? Still a couple of hundred miles from home, so my diagnostic capacity is limited.

 

One other clue I forgot to mention:

When the problem first appeared and the engine was struggling on hills, the temp gauge climbed slightly. Normally it points between the O and R (Hi Dave!) but the needle pointed straight up. Back on level ground it returned to the regular spot. Not an overheat by any means, just a slight difference not previously present. Maybe it will mean something to somebody.

Also got to wondering about a vacuum leak. Ran into this about 3 years ago, but the symptoms were different, especially at low RPM:

https://www.ford-trucks.com/forums/1...e-failure.html

Still got my fingers crossed the new fuel pump did the trick, even though it doesn’t make total sense. One big concern is if the fault returns, I’ve got nothing left in my bag of tricks until I can do some deeper diagnostics.

 

Your fuel PSI reading was static not under load where most of the fuel would be used and a bad fuel delivery would show up.
Any way to see what the filters look like inside to make sure the fuel is not contaminated?
Running a filter sock on the tank pickup tube?
Dave ----

 

Huh? You lost me. I have a Holley 26-503 electric gauge permanently installed. The sender is teed in at the carb inlet, downstream of the second filter. The gauge is on the dash. I’m seeing a live value under load while driving:

https://www.holley.com/products/fuel...s/parts/26-503


That’s what has me so confused. If I had a bad pump or any kind of restriction, shouldn’t I have seen a drop in pressure? The gauge was rock solid the whole time.

The filter have a solid metal body. No way to see inside without cutting them open. I’ll check them when I get home. Picked up some spares, too.

 

Something else to look at maybe the ignition control module. Depending on the truck you have it would either be on the distributor or bolted to the fender or inner fender. Not sure which side of the truck. It’s a bit of a long shot but they are a part that will fail and cause trouble. Moisture and heat will affect the module if it’s failing.

Good luck. Hope you get it sorted out

 

Can you give us an idea of what the outside air temp was during your drive ?

 

Karl, what I meant was you were looking at the gauge with the motor idling, no load on the fuel system, so the psi may be ok.
Now put that motor under load pulling the weight of the camper and gear for the weekend up a hill and a fuel system the is JUST making 5 psi could fail to as it cant keep up.
Dave ----

 

Ambient was around 90F the first day, maybe 85F yesterday. AC was on.

Last month we had a few 100F days so I drove the truck for a heat test. No problems, but the truck was unloaded at the time.

I should mention I’ve blocked off the exhaust crossover passage in the intake manifold, to reduce heat reaching the carb. Also added an insulated shroud around the front of the carb, with cold air piped in from the AC system for good measure. That was several years ago. Prior to those mods, I did have some severe trouble with heat affecting the fuel, but the symptoms were very different. The engine would die, as if turning the key, and would not restart until it cooled a while.

For monitoring, I’ve got a remote thermometer on the exterior of the float chamber. During all this, the max I saw was about 115F. That’s a pretty typical reading I’ve seen for years, so nothing out of the ordinary there.

 

It’s the Duraspark system, with the remote module on the fender. I forgot I have a spare, so I should try that, too. Thanks for the reminder.

Anybody else have similar symptoms as a module started to fail?

 

No problem. Not guarantee but definitely worth a look. I’ve seen plenty of the go bad. Just not sure of all the symptoms.

 

No progress to report. Ran into a lack of power at the same point on the highway as yesterday, about 15 miles. Long gentle upgrade and could not maintain 55 MPH in 4th gear. Wasn’t steep by any means.

Decided to cut trip short and slowly head home. Figure we’ll still get some camping in, just closer to home. Of course engine now behaved and we pulled 70 no problem, including similar uphill grades. I have a level attached in the cab for parking the camper, so was able to confirm the grade was the same.

Any thoughts? I’m beyond frustrated. I’m glad everything seemed to be fine after deciding to turn back, but would still like to know why. The mechanic in me wants to make sure it’s not some intermittent fault that will return to bite me. Definitely not confident enough to head off from the beaten path, though.

Some more details:

Tried both tanks, no change.

Fuel from two different gas stations, no change.

Tried spare Duraspark ignition module, no change.

Fuel pressure at the carb inlet, as measured with my spiffy dash gauge, was 5.5 PSI at all times.

My latest crackpot theory is some crud got past the fuel filters and was partially blocking the carb inlet. I say this only cuz it might splain why the fuel pressure gauge was steady even though the carb may have been running low. Just a hunch, am open to all ideas.

Figure when I get home I’ll check the volume from the fuel pump, but have no means to do that now.

 

Yeah, I'm thinking your running out of fuel from the carb. Might be time to open the carb and look it over.

Maybe once your home and unloaded, take the truck out for a good hard run, do some WOT a few times and see if it feels sluggish. Maybe find a good long uphill on the highway.

 

Accelerator pump maybe…???

I don’t really have any solid evidence. Just trying to toss out ideas

 

One other detail I forgot. After turning back, we took a smaller side road to avoid the freeway. Unbeknownst to us, it turned into a washboard gravel road. The drive was so pretty we kept going, but probably spent about 45 minutes having our fillings rhythmically knocked loose.

Like any lazy mechanic, I’m trying to make the clues fit my predetermined conclusion. We got back on the freeway immediately after that, and all was good again. I’m wondering if all that vibration broke up some crud restricting fuel flow, be it in the filters or carb.

Just a thought, trying to make sense why the engine started behaving itself again today.

 

I’m no carb expert. To me, a carb is a calibrated leak sitting on the intake manifold.

The engine seemed to accelerate okay, but then it hit a brick wall and would rev no higher. Could that be a power valve? Does the 2150 2bbl even have one? Is there a way to test it if so equipped?