90 F250 Ex Cab 4x4 with a 351. I recently installed a rebuilt 351. It has about 600 miles on it. most of the time, the engine has low power, does not rev well. By that I mean it sounds like it's screaming at 3000 RPM's, and does not accelerate well. It is not smooth. Then, maybe about 1/4 of the times, it accelerates very well, is extremely smooth, and revs very well up to 5500 RPM's. It's actually fun to drive when this is happening. The engine is a lot quieter at this time and is very smooth - It almost feels like when a turbo kicks in (of course, this is naturally aspirated). This actually is encouraging because I know the truck can run better but something is holding it back This is very puzzling. Here's what I know:

New plugs, wires, dist cap, rotor. Timing is right at 12 deg. Map Sensor works. I was throwing Code 31, which is EGR Sensor. I replaced that and also the EGR Vacuum Switch. EGR Valve functions. Check Engine Light is not on (not burned out either). I've run 4 tanks of fuel injection cleaner.

Any thoughts?

Am I cycling between Open Loop and Closed Loop? O2 Sensor? Dirty Injectors? Bad Computer? I think the P.O. put in a chip, but I'm not sure what kind.

Thaks for the help!

Jeff

 

Sorry but it sounds more like a transmission or brakes sticking problem than an engine problem to me.

Clear the codes and see if the 31 comes back, you've probably fixed it with what you've done already.

Popa Tim

 

I don't know about the screaming at 3,000 R.P.M.s. But the rest sounds like a pluged cat.Can you feel good pressure at the exhaust? I think you can smell a strange smell when they are pluged. good luck.

 

Yea . my thoughts to ,,FORD call this BRICKING , the Cat. breaks apart inside , an melts ,This is very common problem ,, I would drop the exhaust system rite at the Y pipe going to the Exhaust manifolds an take it out for a run an see what it does -- Dont confuse the loud noise with more power ----See if the engine will run better .,, Also a Faulty fan clutch will make it seem like the motor is screaming. But wont hoild you back on RPM's. An even if the Trans was stuck in 2nd gear you should be able to get more than 3000, RPM

 

Thanks for the replies so far fellas. Ramrod is right on the money with the clutch fan. It was stuck and I replaced it with a new one. That is what helped it smooth out SOME of the time. It's wierd. Do you think the new clutch fan will spin too fast sometimes and go back to the old loud jet engine sound coming from the engine?

Thanks for the posts about the cat. The P.O. put in an aftermarket dual system. Both sides go into the cat, and then split apart again. There is no muffler that is seperate from the cat. It is throaty. I pull pretty good vacuum at high RPM's in the driveway, which made me think it wasn't a plugged exhaust........

 

Throwing back a beer - Cool glad it was cheap fix for you , NO the new one shouldnt do that , unless it goes bad--, The only time the fan clutch should kick in is at your sitting still like at a red lite / drive thru etc, The rest of the time your driving down the road , an the air going past the RAD. should keep the eng. cool , the fan has a spring on the front of it bi- metal , When it heats up it kicks in the fan to turn , rest of the time the fan is free wheeling. Send only money to this site ,, LOL

 

The fan clutch is the majority of the problem I think. I'm still fine tuning, though. I think I may have a bad O2 sensor. It idles a little rough (shakes the gearshift lever), and there's popping coming from the exhaust when I decelerate. Any ideas on the popping? I'm probably way overdo for an Oxygen Sensor. I've only had the truck for about a year and a half, but there's 174K on the truck (only 600 miles on the new engine), so it's probably money well spent.......

 

most times poping in the exhaust is due to the ( anti back fire valve ) having a problem , check the air pump , if its working , there is a small black plastic valve , has a couple of vacumm hoses going to it , usually one of the vac. hoses is broke, pinched etc, or the valve is frozen shut an u gotta replace it , I thing on the newer vechiles is called a diviriter valve ..other thing that causes backfire is timing too far advanced , rich fuel mixture