I'm having a power loss problem on a bone stock 1989 E-350 with 460CI, C6 trans. It runs well on it's own, but when pulling a trailer it falls apart. With the heavier load, stepping into the throttle results in drastic power loss and it seems to misfire. Heat makes it worse. It runs better when cold out. The following has been done:

New plugs, rotor, cap, wires, fuel filter, fuel pump (pressures check fine on the fuel rail), muffler (cat. checks fine), ign. module, EGR valve + controller, and computer. Ran some gas additive cleaner through it. The only codes it registered was the EGR valve.

Pull the plugs after they've run a while under these conditions and they look clean.

Ideas?

 

Man, you've done went through everything I would have suggested.

 

Ya, I'm running out of ideas. When it acts up, a very light foot on the throttle, just enough to maintain a light load, will generate more power than giving it a bit more. Give it more pedal and it's like you let off on the throttle. The van slows rapidly. It's like it's starving for fuel, but running under these conditions with a gauge on the fuel rail shows all is well. Run it lightly for a while and things improve. I think the timing may be retarding way too much. The updated computer seemed to help some, and I understand the revised programming included less retard as temps rise. Could a faulty temp sensor give these symptoms with no code? I think the temp sensor for the computer and the dash gauge are separate, but I couldn't tell you where they were located.

 

Sounds like your onto something.

I'm watching this closely because I'm having a similar but less serious problem. Please keep the thread updated..

 

How about the catalytic converter have you replaced that?It could be pluged up.how many miles on it?

 

I was wondering the same thing. Rebus said that the cat checked out good, but how do you check a cat? Input vs output temps?

 

Well, the cat. converter did check out fine. It was in perfect condition. However, I recently took it to an exhaust shop and they ran a backpressure test again and even with the new muffler (I think I mentioned it had a collapsed internal inlet tube before) there was still excessive backpressure.

My theory was that excessive backpressure was not only choking off power, but when you really got on it the pressure was so high it would overcome the EGR valve spring and push it open, forcing exhaust gasses into the intake -- really killing power. This prompted my trip to the shop. They pulled the cat again and examined it, finding it in perfect condition. However, he declared that it was not original and was in fact undersized. You could see that someone had welded on it to adapt the lines from the air pump. I have no idea why it was not original. The van has less than 90k miles on it.

He pulled out a new cat for the Ford 460 and it was twice the physical size, plus had a larger 3" outlet. So I bit the big bullet and had everything from the cat aft replaced with new 3". It runs much better now under power and even starts and idles better.

It's still not perfect though. There is still some power loss when getting on the gas but it does make noticeably more power before that happens and the stumbling/misfiring is reduced.

Here's the kicker -- I am still getting the code 33 for the EGR valve even though the backpressure problem has been fixed. The valve body and electronic head were both replaced with new not long ago.

 

Base timing to far advanced.

 

Timing has been checked a couple of times -- once by me then by a paid mechanic. No problem there.

 

Does that year have an electronic distributor? Mechanical advance/vacuum advance problems maybe?

 

Electronically controlled ignition.

 

I neglected to mention the O2 sensor has also been replaced. The truck still has the same power loss problem when pulling a trailer or going up a hill. It makes noticeably better power until then with the new exhaust system, but still, once a certain amount of throttle is opened under rather heavy load, the engine seriously loses power like it's starving for fuel. The more one depresses the pedal, the slower it goes -- almost like letting off the throttle pedal completely. Fuel pressures are fine on the fuel rail. The only EEC code it generates is a 33, EGR not opening. (The EGR shouldn't open under heavy load anyway.) As I mentioned, all the EGR equipment was replaced yet I still get that code whenever I drive it.

 

My truck did that because the air cleaner had not been replaced for about sixteen years. So i ended up buying a K & N for it and it was a night and day difference. I have a 1990 F350 with the 7.5L.

 

It has a new air cleaner element, but just a standard FRAM. Do you think there is an appreciable difference with a K&N?

 

Check your muffler, resonator, or other downstream device. The intake side.
If I were a betting man, I would place a lot of money on something being plugged with the wire mesh and crud from a previously failed catalytic converter that disintegrated.

Your EXACT symptoms happened to me once many years ago, and I spent a lot of time and money trying to figure it out.

OK under light throttle, because the mesh lets the gasses pass through.
Power loss under heavy throttle, though, because the air flow compresses the mesh and it becomes a plug.

Good luck!