Had another look. The downpipe loooks like it's touching the valve cover instead of the firewall. I'm guessing that a muffler shop may be able realign the downpipe.

 

if it's the original pancake downpipe, ditch that thing and put a new 3" one in. that is if you plan on spending money on the downpipe to have a shop do something to it. you can do this yourself for cheaper than what a shop will charge you in labor. oem replacement downpipe you should be able to find for no more than $150 and a few hours of your time.

here's another WAG, one time i got a little crazy and bottomed out on the skid plate that is under the transfer case and bent it enough so that it was touching the case. that caused a humming throughout the truck while rolling. i'm not saying that's the problem but check all up and down the power/drive train and make sure there is nothing touching even the station parts of the drive line.

 

Thanks for the advice. I'm a mechanical moron so please explain what a 3" pipe will do for me that the factory pancake pipe will not. I assume it's got something to do with horsepower.

 

its more about effeicency than HP. a round downpipe will help the exhuast to flow more smoothly. its a supporting mod to HP but can also help a stock truck breath better. Plus an aftermarket downpipe is cheaper than a stock Ford downpipe if you have to replace it.

 

Thanks very much. I'll ask for one when it goes to the muffler shop.

 

Prettty much had enough of this crap. $700 later in additon to new tires, I now know what isn't causing the problem. It isn't the exhaust, catalytic converter, muffler, engine mounts, transmission mounts or driveline. It also isn't the transmission ot torque converter. I do know that the hum got worse after the catalytic converter was removed. What the hell do I do now? It's making me crazy.

 

How long have you had this truck? there's a certain amount of vibration that's normal with them, and that range kind of "hums"...

 

Can you tell us what hase been replaced or looked at already. Kinda start from the top.

 

I've had the truck for about 6 months. The hum wasn't there when I bought it (or at east it think it wasn't). I started to notice it about 2 months later and that's when I first posted here. The noise starts to crop up around 1600 rpm, gets worse at 1800 rpm and then fades out at 2000 rpm. I didn't seem to be doing it when the vehicle was stationary. After checking out a whole bunch of possibilities, it seemed like the exhaust was the problem. I took it to a muffler shop last week and they agreed that the downpipe was touching something. The cut out the catalytic converter to give themselves some wiggle room and played with the downpipe positon. After a couple of hours, I went to pick it up. They had just returned from a test drive and said that the problem was still there and that they were unable to resolve it. I payed the $70 bill and drove away. The problem was now siginificantly worse. At 1700-1800 rpm, I couldn't even hear the engine anymore. I took it rectly to a well- respected local mechanic who inspected the exhaust and agreed that it wasn't touching anything but couldn't explain how the problem had got worse after the initial exhaust work. He then inspected the motor mounts, transmission mounts and driveline. He found nothing but the hum was definitley occurring when the truck was not moving or in motion. He even tried running it at 1800 rpm with the engine partially lifted and it made no difference. As a last resort, he sent it back to the muffler shop and had them put a new muffler on the truck. It made absolutely no difference. $700 later, the truck runs great except for this really loud rpm hum and bad cab vibration centered on the floor near the gas pedal. (The vibration wasn't there befoe the exhaust work either). If you guys have any more suggestions, I'd sure be glad to hear them.

Could the problem still be the original pancake downpipe even though it's not touching anything?

 

if this problem occurs when the vehicle is NOT moving then the problem has to be in the engine or torque converter. obvious would be to say the harmonic balancer. As an experienced diesel mechanic and a 3 year powerstroke owner, I am ashamed to say that I do not know whether this engine has a crankshaft pulley mounter balancer (viscous or rubber) or if it has balancer shafts. I would guess it has a traditional non fluid filled balancer. I would go ahead and replace that depending on the price. Since you described the hum in terms of RPM's and not MPH's that would also support the idea of an engine balance problem. Someone should be able to shed some light on the type/price situation with the balancing.

 

No balance shafts, Harmonic on the crank only. they have gone bad before and will do what you described, find a new harmonic and replace that.

Diesle Rod

 

Rod, good call but isn't this rare?? I've not experienced a bad Harmonic balancer on a PSD. It is that noticeable? Some guys in the NBS (99-03) forums have replaced good ones with an Fluidampr Harmonic Balancer $429 + S&H ouch!

94-97 Ford 7.3L Ford Powerstroke Diesel Fluidampr Harmonic Balancer 720211

Maybe CO131 can get one of the used takeoffs.
Ask here:
1999 - 2003 7.3L Power Stroke Diesel - Ford Truck Enthusiasts Forums

 

locol diesel shop told me about it, I have never experianced it.

Diesel Rod

 

the only experience i had with a damper going bad was with a fluid filled one. usually you just replace the rubber ones when you start seeing the rubber coming out of the seams (at like 6-700,000 miles on big rigs). The one time i had a bad damper it was actually tearing up the gear train. we went through three HEUI pump (HPOP) gears on a medium duty CAT engine before we found that it was the damper. the fluid filled dampers are very sensitive to damage the smallest dent in the housing will cause problems. all that said, yes, it is rare for this to happen.