I've got an '88 SuperDuty with the 460 and it runs well until about 2400-2500 RPM and then loses power, and will miss or buck until about 3000 RPM and then it will take off like a switch was thrown. Any ideas?? I've checked the plugs, timing and put a new fuel regulator in. Same thing.

Thanks.

 

Well, to push the train in the right direction, I would suggest taking a gander at your carburetor, and ignition system. The Haynes and Chilton manual have trouble shooting sections on how to test various components of your ignition system, and throwing a kit at your carburetor wouldn't hurt anything either.

Or as the old joke goes..

A man goes to the doctor, says "doc, my arm hurts when I do this." (rocks his hand back and forth.

The Doctor replies, "Then dont do that!"

 

Does it do it at any throttle position or just full throttle? What else is happening when it happens? Will it do it out of gear reving in the driveway?

 

First of all, it's an EFI. It will only do it under load in the top gears. If I hold the throttle long enough it will spit and sputter up to about 3000 RPM and take off. That's the part that has me baffled. If I take off and go full throttle in every gear it doesn't always go into the ritual.

 

This one is a liitle weird but I think we can figure it out.

Have you pulled the codes yet? (I should have asked that first)

With problems like this it is always best to start with the ignition. So if you haven't done it yet/recently, it's time for a tune up to start with. Good normal style plugs (not platinums) and some wires plus a cap and rotor. I can give some suggestions on the best stuff to get if your interested.

That brings up another question, had you done any work on the engine just before this started?

If/when your confident all the ignition parts are good. The next thing to try is to shorten the plug gap, this will reduce the chance of an ignition related miss. If it gets better when the gap is closer then we know it's an ignition problem. I think your gap spec is .054 try it at .045 and go for a drive.

 

I have nearly the exact same problem but have not tried to run it past 3000 Rpm except at idle.

Here is what I know:

1989 460 4X4, w/C6 auto
Code 11. Everything checks out and no codes were set. There are no codes for a bad C6 vacuum modulator or a neutral safety switch.
Runs excellent until about 2000 RPM when it starts to cut out. It only does this under load on any type of hill. This is also at the exact point where it will normally shift down.
Will easily run to 5000 RPM in neutral.
Replaced the fuel pump (High Pressure, The two low pressure pumps in the tanks seem OK) and it curred it for two days. This is also where I realized that it wanted to sift down at 2000+ RPM under load.

So:

it will not sift down and instead wants to short out the ignition.
It seems to have good vacuum at the manifold at idle.
It shifts fine through the gears as long as each shift is below 2000 RPM.
I replaced the cap and rotor with a good used one. No change.
I checked the ignition wires. Seem OK but need to get it on the scope to be sure.
It does have double platinum plugs installed 10 K or two years ago.

I'm going to start by:

Replacing plugs, wires, cap and rotor.
Get a vacuum gauge and check manifold vacuum.
Plug the vacuum line to the fuel pressure regulator to see if that is it.

Other than that I need your HELP.

Whats next?

 

What it does in neutral means nothing as the fuel requirements are next to nothing compared to the demand under load.

 

It may seem obvious but how bout the fuel filter?

It sounds like a good tune up should fix your problem, weak/worn ignition problems show up at max load/cylinder pressure which is when your problem is occuring. I recomend to everyone to use normal autolite or motorcraft plugs, MSD cap and rotor, and Taylor 8.2 thundervolt wires. The cap, rotor, and plugs because they are high quality and work vary well, The wires cause they are much better then stock replacements and will increase the amount of power that gets to the plug.

If a good tune up doesn't fix the problem that alone will tell us alot about what is wrong, and a tune up is never a bad thing to do and is relatively inexpensive.

Sorry Bear, but what it does in neutral means alot precisely because the fuel requirements are next to nothing compared to under load. If it does it in neutral it tells us real quick and simple that the problem is not load sensative. And thus is most likely ignition related with fuel starvation a close second.

 

It is very obvious that it is either and ignition issue or fuel supply issue. From the report problem it is obvious that neutral will work fine because under load it revs to at least 2000 which takes in the neighborhood of 15 to 20 times the fuel and spark that a WOT neutral rev would.

 

Where in the above quote did it say anything about load or WOT. There is no mention of under what conditions it happens, only what RPM. If the problem was mechanical the symptoms would most likely not be load sensative, and would show up in neutral also.

Or are you a mind reader, if so what am I thinking?

 

If he is driving the truck down the street and accelerating, then the engine is under load. Or is there some magic universe where you live that the act of acceleration and under load do go together?

 

So sorry Bear, there's no mention of driving or accelerating either, only RPM change. Even if we assume he is driving it could be happening going down hill also. In this universe we have this thing called gravity that allows for acceleration going down hill without placing the engine under load.

These things are important, and not to be assumed. You seem to assume alot Bear, what is it they say about people that assume?

 

Problem is under load. The truck is a SuperDuty with a 12 foot mechanic's box, loaded with tools, rigging, fuel, etc. To reitterate, the truck pulls fine until 2500 RPM, stumbles until I nurse it to 3000 RPM and then pulls like an SOB. NO codes.

 

Yeah, I assume when a guy says that his engine looses power he is trying to go and not coasting. But hey, your fantasy world seems to suit you, so you live in it and not try to get me to live in it with you. I've done this engine troubleshooting thing for a long time, while making a good living at it, so I do know how to read and understand what is inferred.

 

To continue from my first post.

The truck cuts out anytime it is under load like on a flat or moving up any hill. This mostly occurs at 1900 to 2300 RPM.

I replaced the small frame rail filter about a month ago. No help.

I plugged the vacuum line for the fuel regulator. No change.

I just today, changed plugs, wires, cap and rotor with NAPA/Belden, Eclin parts. No change. The Motorcraft double platinum plugs I replaced looked fantastic, but maybe a little lean.

I guess I now move to vacuum, but need a gauge first.