So I have been chasing down a problem since I bought this truck which is an 01 250SD with a 5.4…in addition to a rough idle, the vehicle shakes at high and low speeds but only when it shifts into about third and the RPMs drop…in addition, sometimes the truck has trouble starting…even when coming to a stop, the needle drops a little like it’s going to stall and then pick back up…sometimes it will turn over a few times before it fires and it’s fine…also the 7th plug fowls after a period of time….I’ve gone through plugs and coils and wires and bearings, intake manifold and gaskets, injectors…and now I’m about to cut off the factory Catalytic Converter….i thought it could be ball joint or u joints, but the vehicle doesn’t shake when it’s going at high speeds at all unless it’s given gas…so I’m going to cut it off and replace with high flow cats….they seem to be inexpensive and I’d probably like to go with Magnaflow but my question is…in order to have the proper back pressure do I need one or maybe two in tandem followed by a side exit pipe…as of right now I’ve got the exhaust cut just behind the cat and it covers my brand new skyjacker shocks with black carbon and junk, so I’d like to have a side exit pipe…has anyone done this and maintained proper back pressure? Does this sound like a reasonable diagnosis for what I’m experiencing? Thanks again, for all the experts available for sharing knowledge!!

 

Years ago I had a car that the cat came apart on the inside, and was plugging the exhaust with parts from the cat. Was told to run a wire up through the exhaust pipe and move it back and forth as the car was running, and it blew out the parts that were causing the problem. The car would barely run at slow speed. Barely make 40 mph. Does that sound anything like your truck?

 

The truck shakes when idling…but does fine when accelerating and at higher RPMs…it’ll be a smooth ride at 80 unless I let it get into 3rd or 4th…when I barely touch the gas and it’s at a low RPM(below 2,000) it’ll shake…I was on a huge hill going downhill the other day and everything was smooth without any gas involved…I’ve replaced everything short of head gaskets and the light comes on only to tell me the 7th plug is fouled, I replace it and reset and the light goes off and it’s better…the OBD printout at auto zone says one head is running leaner than the other…the one running lean is passenger where the previous owner broke the exhaust manifold studs…something that I haven’t dealt with, and makes a little noise until warmed up…all I can figure is the cat is preventing the flow of emissions…the truck sometimes has difficulty starting (takes a few cycles to fire), idles rough and shakes at low rpms when at high speeds…

 

If the cats were clogged you wouldn't have good acceleration and certainly wouldn't hit 80 mph. If it shakes at idle, I'd check the short term fuel trims. A power balance test would be wise, too, since it'll show compression issues.

-- Dave

 

Doesn't sound like a cat issue. I'd be closely examining the IAC valve to start with, then looking for a possible vacuum leak.

 

I thought it may be IAC but was told unless it goes up and down when idling then it’s fine…like constantly going up and down

 

Couldn’t a malfunctioning cat affect the O2 sensor and then change the fuel/air ratio via the fuel trims? I am not going to pretend I know a lot about this at all…it just seems that for some reason the truck wants to drop down to a lower RPM and when it does this there’s vibration and an obvious issue, but it doesn’t occur during acceleration at all…

 

Vacuum test might help, back pressure test for sure:

https://www.aa1car.com/library/converter.htm