me again, same problem. For those just tuning in, its a 2000 supercab 4x4, 4.0/auto that after it warms up it idles rough and bobbles and jerks quite a bit. Moslty noticible stopped (regardless if in gear or Park) or crusing at a constant speed (more so here w/ AC on). Gas mileage has dropped considerably from about 15/20 to around 13/15 (c/h). These have been done to it:

new wires & plugs
changed pcv
new air filter (needed it)
changed fuel filter
new fuel pressure regulator
changed grades off gas
ran seafoam through it
checked & cleaned MAF
throttle plates have been checked and are clean
checked for vacuum leaks (none)
made threats to it
lots of prayer (can't find a preacher or priest that will do an exorcism)

haven't checked:
O2 sensors (not posting any CEL codes)
intake or valve cover gasket leaks

Since the fuel pressure regulator was replaced, it doesn't like to start when cold or sitting for a few hours. Normally it requires cranking on in just a few seconds, but when cold or sitting awhile can require 10 seconds or more to start (cranking on it) then it jerks and starts. Shut it off & wait, and it starts fine. Its sitting back at the shop that replaced the regulator and they think it starts fine. I went up and started it while it was cold, then restarted it a few miinutes later. They did notice a difference in starting times but still have no idea, other than perhaps a fuel pump relay that may not be kicking in as it should.

Any ideas would be welcomed. I'm running out of patience and money.

 

o2's are about $100 just replaced mine...it sounds like a vaccum leak, but if you sprayed carb cleaner around the upper intake while it was running, and it did not rev. the idle, then obviously it's not. Did u pull the pcv valve while running and spray carb cleaner down the hole? That would check the lower intake mani. gasket...

 

as stated above, the pcv has been replaced (was in really bad shape). This has bugged me to the point I've spent most off the afternoon on here (don't tell the boss) and found a few other people with similar issues and gave me some new things to check.

intake gaskets
lower intake bolts
O2 sensors
have garage pull codes while they've got it (I don't have a code reader)

 

Mine misses a bit at low rpm's.
I changed the plugs and wires to no avail.

I'm thinking coil pack.

Have you considered that?

 

would the coil pack affect gas mileage?

It hit a new low this morning at fillup: 10.4 mpg. Now its getting slightly better mileage than a semi. This is rediculous. Will be stopping by a car lot on the way home tonight. I'm done with it. Well, almost. I'm very frustrated if you can't tell.

 

Well.. what you describe and given what you've done. I think it's the 02s.

What you describe will fit because when the motor is cold the PCM ignores input from the 02s. But if they were that far gone I'd sure think you'd see a CEL. Usually gas milage will get real bad just like your seeing.

Rich

 

We are going camping this weekend (and find out how well our Explorer likes to pull a 2000lb. camper... the Explorer is better for the kids on a long trip rather than jut stuffing them in the jump seats in the Ranger.) When I get back on Monday, I'm going to replace the plugs with Motorcraft plugs. I hope to find black, charred & burned tips. If that doesn't remedy things, I will look into replacing O2 sensors. Plugs were changed about a year ago. Don't remember what I put in. The old ones weren't in bad shape.

I had a Nissan 4x4 w/ 3.0 SOHC v6 that got crappy gas mileage (not this bad.) The timing belt slipped, sending the valves to their death crashing into the pistons (wonderful self destucting design) and required the heads to be rebuilt. They replaced the O2 sensor (that one only had a single sensor) and new plugs... ran like a brand new one. Mileage jumped from about 12mpg to 17-18 (in town mostly) and it ran much smoother thn before. I know comparing that motor & the 4.0 is comparing apples to apricots, but I'm hoping the end result will be similar.

 

...just for giggles: when you replaced the plugs, what plug did you install? I ask, because there's a thread here that details how the FoMoCo cross-reference, as well as the cross-reference at many (if not all) auto parts stores is wrong and will give you plugs that have a shank that is just a tad shorter than what the 4.0L SOHC originally called for -- the result is, slightly rough running, occasional stalling, poor (and increasingly poorer) gas mileage, etc.

For a demonstration, stop by your favorite auto parts store and take a look at the NGK TR55-IX -- this is the Iridium plug called for by the cross-reference charts (assuming you want an iridium plug, of course)... [it's also, btw, the same plug that's cross-referenced for the 4.0L OHV engine!]. Compare this plug with the NGK TR55-1IX, which is the proper NGK plug for the 4.0L SOHC. You'll notice the shank is 1/8" to 3/16" longer on the correct plug... The stock number for the correct (TR55-1IX) plug is 7316.