Ford 7.5L 460 loss of power when hot

[weschaney]

Hi guys, i just acquired a 1991 27' fleetwood class c rv. It sat for about 8 years before i bought it. I've since drained the old gas, new fuel filter, trans flush, motor flush and new battery. It fired up and ran like a champ for about 300 miles. Now I'm having a issue where after driving for about 5 miles and reaching running temperature it bogs down so bad it won't down shift and will eventually stall if you try to accelerate at all. After it sits over night it runs strong, for about 15 min.. then gradually bogs down till i have to pull over. revs fine in natural, so this is a issue under load. I took it to a trans shop, they said not a trans issue in their opinion, feels like a clogged cat to them. i removed the cat, still has has the same problem. any ideas?

THANKS FOR ANY HELP!!

[MEXICOWANDERER]

Jerrybo66 X 2

[Jerrybo66]

Thank you so much..
So many times folks ask for help, many members take time to give their advice then if it works, that's the last of the OP. There were so many options here that made sense that we needed the final result. You're a good poster.. Thank you..
Jerry

[weschaney]

Sorry for such a late update to this, but my assumption was right. Pulled the tank out and the pump was so rusted I can't believe it even ran. Installed a new tank/pump and now runs good as new!

[MEXICOWANDERER]

Back in the late 80's when folks begged me to cure repeated failures in the distributor mounted version of this nightmare I found Ford in their infinite ignorance of things electrical decided to use silicone GREASE as an interface between the aluminum module base and distributor. I replaced with TRANSPO manufactured modules and white zinc oxide thermal compound and that was the end of the issue. I was around 5 additional years to witness no further failures. Ford also used wires with crummy insulation INSIDE the distributor. have doubts? Look at the wiring insulation of the 2/0 diesel battery cables.

[Horsedoc]

Fuel line getting too hot and vaporizing fuel before it gets to the EFI maybe? Is the insulation still on the line?

[Gdetrailer]

weschaney wrote:
Hi guys, i just acquired a 1991 27' fleetwood class c rv. It sat for about 8 years before i bought it. I've since drained the old gas, new fuel filter, trans flush, motor flush and new battery. It fired up and ran like a champ for about 300 miles. Now I'm having a issue where after driving for about 5 miles and reaching running temperature it bogs down so bad it won't down shift and will eventually stall if you try to accelerate at all. After it sits over night it runs strong, for about 15 min.. then gradually bogs down till i have to pull over. revs fine in natural, so this is a issue under load. I took it to a trans shop, they said not a trans issue in their opinion, feels like a clogged cat to them. i removed the cat, still has has the same problem. any ideas?

THANKS FOR ANY HELP!!

Your engine most likely has the notorious Ford TFI (Thick Film) ignition system which is well known for STALLING, DIEING AND SPUTTERING and other drive ability issues WHEN HOT..

Also well known for wiring randomly grounding out..

HERE is some helpful info on the TFI ignition systems..

[time2roll]

Ignition will usually cause rough running or stumble pretty harsh.
Smooth loss of power still sounds like fuel.

[allbrandauto]

if fuel pressure and volume is good next step is looking at computer system do you have a cheak engine light on with ign. on engine not running does it go out when running if yes you don't have computer problem

[Gjac]

Wes this may be a long shot but some times coils fail when they get hot. What temp is your eng at when you lose power. Also when I first got my MH and was losing power I started the engine at night at saw that the ing wires were leaking to ground, there were sparks all over. These 460's and 454's generate a lot of heat, I end up going to 10.4 MM wires and headers to fix the problem.

[bigfootford]

weschaney wrote:
Jim, it bogs and bucks, in neutral i can control the rpm fine, only when under load (in drive) it starts to buck and bog. The more research i do I'm finding a difference between fuel pressure and fuel volume. in some instances you can have good pressure, but the pump can be so clogged the volume is compensated thus starving the engine of fuel. kinda weird and hurts my head. I plan on testing the pressure while driving today. Thank you for the info.

Do you have 2 tanks or just 1? 2 tanks has a pump in each tank. Two tanks two pumps it is easy to evaluate.

Both my trucks have the 2 tank thing... My 89 had 1 pump for both tanks and my 04 has 2, one in each tank.

Yes it does sound like it is intermittently encountering a fuel delivery problem...

Measuring the fuel pressure will not separate a failure between the regulator and pump.
Well there is more, it could be electrical but I rather doubt it because as soon as you relieve the demand for flow, back to idle the engine does not die.

So if you were to power brake the rig upon failure and you are sitting the engine will bog there too? Power brake= in gear, depress the brakes and give it the gas not floor it but like you are going to take off.


UGGGG.

[weschaney]

Jim, it bogs and bucks, in neutral i can control the rpm fine, only when under load (in drive) it starts to buck and bog. The more research i do I'm finding a difference between fuel pressure and fuel volume. in some instances you can have good pressure, but the pump can be so clogged the volume is compensated thus starving the engine of fuel. kinda weird and hurts my head. I plan on testing the pressure while driving today. Thank you for the info.

[bigfootford]

weschaney wrote:
The trans will shift manually, what it feels like is the motor starving of fuel and not reaching RPM high enough to shift on its own.

Ok...

Does it feel like it is bucking/engine, power no power or it just slows down like you let off the gas?

If it is doing this and you put the trans in neutral being careful not to over rev the engine can you control the rpm... up and down?

Well for now fuel pressure is something to check, like some of the guys are saying....

Hoses not long enough? If you have a smart phone with a camera that will do video you could record the gauge... Motor home's are not the easiest to get at the engine components... Uggggg.

I have the Harbor Freight Fuel Pressure tester... It works fine.

https://www.harborfreight.com/fuel-injection-pump-tester-62623.html

Here is a informative video...

Pay attention about the part about the vacuum line for the regulator. Check for cracks on both ends.

https://www.youtube.com/watch?v=g6rf9oPDq1I

Jim

[weschaney]

The trans will shift manually, what it feels like is the motor starving of fuel and not reaching RPM high enough to shift on its own.

[bigfootford]

Won't even shift down even manually?

Cools off then all ok?
I have a 94 f250/460 fI with the same trans... E4od....

Sounds like the ECM is in trouble...

A GOOD trans shop can put their computer on the system and take the rig for a run and monitor what is going on.... Why it gets to the point of not shifting down sounds like the ECM acting up.

A lack of fuel pressure, plugged exhaust system etc is not going to keep the trans from shifting!