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Rough Idle Fixes, 4.2 EFI

Rough Idle Fixes, 4.2 EFI

Ted Macklin

Here is all that I have done to the Jag in the past two years in my quest to find the cure for the rough idle on the 4.2L FI engine: (It has idled smoothly now for three weeks so I can now say I've finally conquered the beast.)

1)Replaced all the seals at the fuel injector plus replace all fuel injectors that have a crack in the plastic "Top Hat".

2)Replace all rubber hoses (large and small) that connect to the intake manifold esp the aux air valve hoses. And the little rubber cap at the bottom/back of the intake manifold. Don't forget about lots of little hoses that run under the intake manifold. Don't forget the brake booster vacuum lines = the rubber parts.

3)Seal the bolt threads that secure the aux air valve and the ignition amp to the intake manifold with teflon tape.

4) Make sure the cruise control bellows is not leaking.

5)Double check all plastic "T"'s at vacuum fitting junctions for leaks.

6)Replace the rubber Bellows at the AFM and the two "rubber bands" in that same air path.

7)Replace the "0" ring in the oil filler cap, the seal in the oil dip stick, and the "Coon Skin Hat" atop the breather pipe and any other rubber in that path to the Throttle body.

8)Check the distributor diaphram for leaks. 9) Add "three wire" O2 sensor.

10)Perform compression test and do valve lash adjustment if required, go for .013 . 11)Verify that you have stopped all vacuum leaks by decelerating from 60 MPH and observing that the instant MPG read out on the trip computer jumps briefly to 99 MPG. 12) Be certain that the CAT is not plugged up. 13) Clean throttle body butterfly and confirm .002 clearance.

14)Carefully route ignition wires using OEM "wire looms" to prevent "bleed over".

15) Adjust AFM idle mixture for about 5 turns out from bottom and tweak by feel from that point.

16) Return AFM clockspring to OEM setting if you messed with it. 17) Of course new plugs (Champion) and wires and dist. cap and rotor button. Verify

18) Redress wiring harness (over water jacket) with fresh tape.

19) Install synthetic oil. 20) Be certain cooling system is healthy. 21)Install Chevron 87 Octane in the tanks. 22) Buy a TIFF vacuum leak detector to verify once and for all that you have eliminated all vacuum leaks (if you still have a rough idle).

John (JAT1127@hotmail) has brought it to my attention that there is another area in the Throttle body assy that can be a source of vacuum leaks....... While the parts book shows ONE seal only on the TPS switch side of the butterfly, Flyer94a, a former lister, says there is another seal on the opposite side of the butterfly (from the TPS). Accessing the second seal requires removal of the shaft that the butterfly mounts on. (I have yet to tear mine down as the new seals are currently b/o at CVWest so I can't verify the facts yet.) But I will keep you posted. Flyer94a also advises: Be CAREFUL removing the screws from the butterfly valve - they are split and flattened on the backside. This job will be much easier if you just remove the whole throttle body assy from the intake manifold. I believe this now makes item #11 on the long list of potential vacuum leak sources that I have compiled over the last two years. (See the CVWest homepage for the Tip of the Month if you want the full rundown.) Regards, Ted Macklin


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