Rough Idle Remedies for 4.2L FI Engines

If you have played around with the 4.2L FI engines fitted into the XJ sedans in recent years, you've probably noticed that it is very, very difficult to achieve a smooth idle in these engines, regardless of how well the engine runs in off-idle conditions. There doesn't seem to be a magic bullet to suggest in trying to resolve this rough idle issue.

One of our friends here in Atlanta, Ted Macklin, has spent much time (two years!) and money working on this problem and has detailed his steps in chasing this gremlin for us. We'd like to pass them along to you, with the understanding that we don't suggest that you will have to do all of these steps nor have we tried these remedies ourselves! The following is Ted's list of the steps taken, though not necessarily in the order given:

"Here is all that I have done to my 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 injectors plus replaced all fuel injectors that had cracks in the plastic "top hat" on the injector tips.
2) Replaced all rubber hoses (large and small) that connect to the intake manifold, including the aux air valve hoses, the little rubber cap at the bottom/back of the intake manifold, lots of little hoses that run under the intake manifold, and the brake booster vacuum lines.
3) Sealed the bolt threads that secure the aux air valve and the ignition amp to the intake manifold with teflon tape.
4) Made certain the cruise control bellows was not leaking.
5) Double checked for leaks all plastic Ts at vacuum fitting junctions.
6) Replaced the rubber bellows at the air flow meter and the two "rubber bands" in that same air path.
7) Replaced the o-ring in the oil filler cap, the seal in the oil dip stick, and the "coon skin hat" atop the breather pipe and all other rubber in that path to the throttle body.
8) Checked the distributor diaphragm for leaks.
9) Added a three-wire O2 sensor, which my 1985 XJ6 did not have, and added a ground wire from the O2 sensor shell to a known ground.
10) Performed compression test and valve lash adjustment, attempting to achieve .013-inch lash on all valves.
11) Verified all vacuum leaks had been plugged by decelerating from 60 mph and observing that the instant mpg read out on the trip computer jumps briefly to 99 mpg.
12) Confirmed that the catalytic converter was not plugged up.
13) Cleaned the throttle body and butterfly, and confirmed .002 clearance between the butterfly and the throttle body in the idle position.
14) Carefully routed the ignition wires using OEM wire clips to prevent ignition crossover.
15) Adjusted the air flow meter idle mixture about 5 turns out from bottom and tweaked by feel from that point.
16) Returned air flow meter clockspring under the plastic cap to OEM setting.
17) Installed new spark plugs (Champion), spark plug wires, distributor cap, and rotor. Verified ignition timing.
18) Redressed wiring harness (over water jacket) with fresh tape.
19) Installed synthetic oil.
20) Confirmed proper function of cooling system.
21) Installed Chevron 87 octane fuel in the tanks.
22) Purchased a TIFF vacuum leak detector to verify once and for all that all vacuum leaks had been eliminated."

As indicated, this list is not in any kind of order of attack. Ted does state that the very last step he took was to replace the bellows and boot between the air flow meter and the intake plenum and that installing these pieces finished the puzzle for him. We would comment that plugging any vacuum leaks occurring after the air flow meter seems to be a critical issue.

Thanks to Ted, and his friend Steve Randall, who was a co-conspirator in this campaign, for passing along this detailed information.