Hello,
I been having a problem with my truck for about a year or so, the truck is a 1993 F250, 460 gas, Manual Trans with 48,000 miles. Its not a daily driver and usually is driven about 2000 - 3000 miles a year (not much). I just had the truck smog checked and it passed, the truck idols fine and has good bottom end power. The problem is it has no top end power and will not rev past 3400 rpm in 2nd or 3rd gear full throttle, the truck just falls on its face from a half throttle to full throttle. The check engine light is not on and never has been. It seemed the truck slowly got worse as I drove it and not all at once. I'm going to take a WAG at this do you think my Cat could be plugging up? Has anyone had or herd of such a thing? Any help would be appreciated, Thanks
Dwayne

 

Cat could be plugged. More likely your fuel filter and/or injectors are plugged up. Get a vacuum gauge and hook it up to manifold vacuum. Hold the engine at about 2,500 RPMs and watch your vacuum reading. If it steadily drops then you likely have a plugged exhaust. If it stays steady then your exhaust is probably okay. If you have a fuel pressure gauge then hook it up to your fuel rail and watch the pressure as you drive. It should stay steady and climb slightly under load. If it drops below normal then you probably have a clogged fuel filter, or less likely a weak pump. You probably have dual gas tanks. Each tank should have it's own fuel pump so I would try accelerating with each tank and see if there's a difference. The fuel filter should be located on the left frame rail a little ahead of the front tank. Best of luck

 

Definitely check fuel pressure, if you don't get 40psi at the engine when idling and vacuum removed from the regulator you could have a weak pump or clogged fuel filter. I would think a clogged exhaust would be pretty obvious, it should not sound like air escaping from a tire valve, also a rattle in the exhaust indicates the cat has disintegrated.

 

Hello,
Thanks for the input, so here is what I did today, I hooked up a vacuum gauge and ran the truck at idle the gauge showed 20 inches, increased RPM to 2500 and gauge showed 18 inches, increased RPM to 4000 and gauge showed 13 inches. If I pump the throttle it goes to 0 inches. That to did not seem like enough throttle because it was barley cracked open at 3500 to 4000 rpm. I put the vacuum gauge on the windshield and then drove it around the block. In second gear 2200 rpm about 1/8th throttle vacuum 6 inches on gauge, went to 1/4 throttle 2800 rpm vacuum was about 1 inch increased throttle to 3200 rpm which it would not go any further and it was at 0 inches on gauge. Continued to drive the truck in 3rd gear, 1/4 throttle 2200 rpm and 0 inches on vacuum gauge. To make sure I was covering everything discussed I had good fuel pressure before and after I changed the fuel filter, I drove the truck and there was no change in performance. So the million dollar question is it a plugged Cat? I'm leaning that direction because the truck sits more then it runs. Before I shell out the coins on a new cat anything else you can think of? Thank you for your input!
Dwayne

 

What does the exhaust sound like? Do you get a lot of air(exhaust) out of the tailpipe when the throttle is stabbed from idle?

 

No its pretty quit, it used to make a hissing sound when I was full throttle now it does seem like the hissing has gon away somewhat.

 

That's no good you have a clogged exhaust, period.

 

That kind of where I'm at also! I will remove the Cat maybe tomorrow either way I will post the fix to close out this post. Thanks again for you input!
Dwayne

 

Also I forgot to post is the two lines that are going to the Cat I beleive there tied into the smog pump. Well the line from the smog pump that enters the cat will not allow air to flow into the cat. I took the rubber hose off the EGR valve going to the Cat and it held 10 pounds of air pressure so basically my smog pump is dead headed and not pumping any air into the Cat.

 

Two lines going to the Cat? If I recall correctly there should only be one (from the air pump). Your EGR valve does not go to the cat. It should be on the back of the throttle body and it should go down and connect to the exhaust manifold or a little below on the driver's side. Maybe you are talking about the check valve for the AIR (Smog) pump? That should have a one way check valve in it. It will allow air to go from the pump into the Cat, but not from the cat into the pump. That protects the pump from exhaust gases getting in there and destroying it internally. If that line is blocked up you need to fix that, as a lack of air to the cat could be what caused it to fail in the first place. Also you may be looking at a new muffler as well. Often times when a cat falls, pieces of it break off and plug up the muffler as well.

P.S. I took the plugged Cat off my old 460 with headers one time and drove around the neighborhood setting off car alarms and generally wreaking havoc. Have fun!

 

The EGR and Air injection are 2 totally separate systems so don't confuse things by mixing up terminology. There are 2 valves in the AIR system, the one near the pump is normally closed and needs vacuum to open so that could explain the results you got.

 

The Smog system on HD trucks is different in several ways including 2 lines to the cat... apparently to supply a higher volume of air.

 

I am having the same problem. Never noticed it before because I don't beat on the truck, the 460 has enough power/torque I never pushed it until the other day. Same thing no CEL or codes, if I push the pedal past half to full it just has no power and falls flat no power. I know it should have a TON of power! My 1985 460 dually would throw you back in the seat when you went full throttle!

So do you think my CAT is also plugged?? It is expensive can I just "punch the CAT out"?? The truck is now a "antique" so does not need to pass emissions.

 

Sorry for the slow update I forgot to post the fix to my problem. It ended up being a plugged Cat so for now I punched the cat out to make it like a straight pipe. Runs great, gas mileage is a lot better so I will buy a new cat and swap it out when its smog time again. Thanks everyone for the feedback.