Hey guys, got a problem.

This is on my 2000 F-150 supercab 4.2L V-6, w 5spd, with 300,000 miles.

Cranks up super as always and will warm up, etc at normal idle. Now I hit the road, shift on up to 5th, come to a traffic light, neutral out and stop and the idle starts increasing.

This happens all throughout driving and pretty frequently. The engine sounds weird, almost like it has a blower on it when it does it. The idle just creeps up by itself.
The only way to get the idle down is to kick the accelerator pedal and rev it up and release the pedal quick.
Sometimes if I turn the AC off too.

I just don't know...I've changed the DPFE, changed the IAC, changed the vacum tube under the upper intake (the one with the little PVC tube in it). Changed the vacum line from the brake booster to the intake. What could this be? IMRC? IAC????

Also.....
Was there ever a 3.55 ratio rear axle offered with limited slip? Does anyone have a FX4 with 4.2L V-6 here???? If so I NEED the S-code off the rear axle.
I've finally worn a part out on this truck, the rear axle.
I'd like to replace it with a similar unit (3.55 ratio and drum brakes) but the salvage yards need an S-code for it. Does anyone have it??????
If not with the larger axle work? the 9" ring gear version?????

 

Not sure about the 4.2 but on my '97 5.4 the vacuum line from the PCV ends at the back of the intake where a pretty large vacuum connection tends to degrade with age. It took me a while to find it. My symptoms sound a bit like yours. I was spraying some starter fluid around trying to find the leak and finally came upon the connection that is hidden in my case by the TPS (I think).

Cheers,

 

Not sure if this helps with your 2000, but my 2003 F150 4.2 Extended Cab (Super cab) came with the 3.55 Limited Slip Axle. I have an automatic, so don't know if the number would help you.

 

Hmmmmm, I may have to investigate this. My upper intake is a big U shaped deal and it has connections on the backside.

 

HEEEEEY that sure does Do you have drum brakes???? If so could you get the S-code off the tag for me?

 

Any other ideas?????????????

 

Generally the causes of lumpy or high idle problems can be found in:

1) Vacuum leak: Can be found by spraying starting fluid around the vacuum lines. If the engine rpm changes and straightens out then you've found your problem.

2) Throttle Position Sensor: If you've cleaned the throttle body sometimes gunk can get into the TPS and make it stick. Clean it up and see what happens.

3) MAF Sensor: I dunno, different opinions on whether you should clean them or not using MAF cleaner, make your own decision here. You could check the resistance and check against the spec if you can find it.

4) Coils: Cracked coils or with no di-electric grease at tips may create problems. Switching out a known good coil to one cylinder after the other to see if the problem goes away may help. You can also check the resistance on these as well if you can find the spec.

5) Crank Position Sensor: Can create problems at times. I've never had a problem with mine on this truck so I can't say what the symptoms are.

6) Intake gasket leak. If you have any whit smoke at startup after the truck sits for a long time the gasketing between the water and cylinder intake holes may have been compromised. Also leaks to the outside air (probably very rare) can create a fast idle but that would probably be continuous.

Step by step problem solving eliminating possible causes is probably the only way you're going to find the problem.

Good Luck!

 

Thanks a bunch! I hadn't thought of the TPS. Now thare's an idea

I need to remove the upper intake again anyway. About every 50,000 miles since 150K I have been cleaning it and the IMRC butterflys because at that time one of the little plastic clips on the control shaft broke. It might be time to change the intake gaskets I suppose????

 

Are you getting codes P0171 and P0174?

I was getting those and my truck(99 F150 XL 4.2L 5spd) ran terribly. You could hear air leaking while it was running, and the idle after it warmed up was also bad. Felt like the truck was going to die. I had a vacuum leak right below the throttle body on the corner of a "L" shaped hose. Fixed it and found another near the firewall on the passenger's side. (Mechanic found it with the smoke trick)

Codes disappeared and truck ran great afterward.
Good luck!