I have a 1995 Holiday Rambler motorhome with a Ford 7.5 L/245 hp V8 Gas Fuel Injection 460 engine, 48,000 miles. It runs perfectly at sea level. When I drive up to the mountains it’s just fine accelerating up the mountain (naturally performance goes down a bit when I get to 7,000 feet). Then, I have a steep (1,000 ft elevation drop) downhill where I downshift for ~ 2 miles. At the bottom of the hill, engine stops dead and refuses to run with any load. Chugging and choking, missing, etc. After about an hour, I can get it going again, and limp (chugging in lowest gear) into the campground. Two days later, I start it up to leave, and it runs perfectly again. Is the computer malfunctioning, dirty injectors? The fuel pump was replaced less than 5,000 miles ago.
My guess is that the problem isn’t the altitude, in the sense of air density, it is a heat problem caused by a long hard assent and a slow high RPM descent. Even if the engine isn’t overheating, I’ll bet the under hood temperature is off the chart. I would add a possible ignition problem to the list.
Has this same problem occurred repeatedly? My first SWAG is vacuum is building in the tank due to a loss of venting caused when the fuel pump was replaced or the purge valve is not functioning properly.
Thank you MTraveler and Rod Knox for your comments. I will share them with my mechanic. Any further thoughts are most welcome!
Yes. This repeats every year at the same vacation location…always at the bottom of the grade. This particular time it was worse than in the past.
At or near your sea level location, do you have any hills where you can duplicate the 2-mile downhill decent? If you could duplicate it near sea level, then it would tell you it’s not altitude related.
I’m curious if there is something unique about the extended deceleration that is provoking or exposing a problem.
Do you see any smoke out the exhaust during your decent?
I’m wondering if some of your plugs are soaked with fuel (and not firing) after your 2 mile decent. If you could pull them out and check that might be telling. The next question is what would lead to that condition.
Does this motorhome use a distributor with the little gray TFI module mounted on the side of it?
If so, the module could be acting up due to heat failure and that’s a common problem with those modules. Those modules were also phased out after 1995 as far as I know.
Symptoms can vary but coughing, chugging, or even flat dying on deceleration is one of them.
It’s not the deceleration that causes this; it can be the slightly abnormal heat from climbing a grade.
This problem is always worse in summertime heat and testing the module is usually a waste of time unless it’s a permanent, catastrophic failure and most are not.
It’s best to just replace the module rather than waste time going through the testing motions.
For info on this problem do a net search for “TFI Settlement” and that will explain it along with the class action lawsuit against Ford over these things.
It never was on the distributor on the 7.5. Should be out near the radiator or whatever on the F-53
There are two pumps or more…a very low pressure lift and a high pressure mounted on the frame. The lift pump can not work at all and cause this symptom…seen it several tiimes.
Thank you, JoeMario and ok4450 as well. Looking into all your suggestions. If anything else comes to mind – anybody – please let me know. Really appreciate the feedback!
Also thanks to kawasaga!
The only thing I can think of that would effect engine performance with change in elevation would be the Manifold Absolute Pressure sensor.
The MAP sensor replaces the ignition vacuum advance and the throttle pump that were found on older vehicles that had distributors and carburators. If the MAP sensor gets screwed up with the sudden change in elevation, the ignition timing and fuel delivery will be incorrect at the new elevation and the engine will run as you describe.
Tester’s idea of the MAP sensor is a good one. There may be something about the strong manifold vacuum during your 2-mile deceleration that is negatively affecting it.
I would definitely follow his suggestion.
Rod Knox’s comment got me thinking about a possible fuel delivery problem. Try going down the hill next time with the gas cap off.
The module was relocated on the later TFI vehicles but that doesn’t mean they won’t fail for the same heat related reason.
Loosen a non-distributor mounted TFI module, turn the key to the RUN position, and touch the back of that module a few minutes later. It will blister the skin off.
Just something to keep in mind anyway.
It might be the fuel pump relay. It’s worth a check anyway. Sometimes they will fail if they get too hot.