[xj-s] Rough Warm Idle

The Car: 86 Jaguar XJ-S with 65,000 miles (meaning close to 67,500 service
time)
The Problem: When at idle, when warm, occasionally, maybe 10-25% of the
time, the idle is very rough… Almost like it runs half the engine, and
doesnn’t run the other half. The tach needle goes to 700 then to 400 to 700
then to 400… Then most of the other time is idles just perfectly. It
always runs smoothly and perfectly when moving.
The Solution?

Thanks,
John

John,

Sounds like either a vacuum leak or one cyl. misfiring (bad injector or bad
plug). In any event, when the oxy. sensor detects unburned oxy. in the exhaust,
it richens the mixture causing the other 5 working cylinders on that bank to run
rich. Once you get up to speed and the switch on the bellcrank kicks you into
open-loop mode the oxy. sensors are ignored.

Regards,

-Ron
1986 XJ-S
1989 VDP
various MG’s

MGMagnette@aol.com wrote:> The Car: 86 Jaguar XJ-S with 65,000 miles (meaning close to 67,500 service

time)
The Problem: When at idle, when warm, occasionally, maybe 10-25% of the
time, the idle is very rough… Almost like it runs half the engine, and
doesnn’t run the other half. The tach needle goes to 700 then to 400 to 700
then to 400… Then most of the other time is idles just perfectly. It
always runs smoothly and perfectly when moving.
The Solution?

Thanks,
John

While replacing spark plugs on my '84, I noticed that PO had left the
injector on cylinder 6a unhooked from the harness. Also at that time I
replaced a bunch of old leaky vacuum tubing within the engine compartment.
Boy, did that car run smooth on 11 cylinders!
Is it time to replace my O2 censors?

Also, someone recently mentioned of getting 23mpg from an '86 at 70mph.
Mine gets 18-19mpg. Is there any way I can improove the mileage?

Val
'84 HE, running on 12 cylinders, overhauled distributor and cooling system,
fresh tune-up, 32lbs of air in all four corners, no vacuum leaks (I hope).----------

From: Ron White rewcne@concentric.net
To: MGMagnette@aol.com; xj-s@jag-lovers.org
Subject: Re: [xj-s] Rough Warm Idle
Date: Monday, October 06, 1997 10:46 AM

John,

Sounds like either a vacuum leak or one cyl. misfiring (bad injector or
bad
plug). In any event, when the oxy. sensor detects unburned oxy. in the
exhaust,
it richens the mixture causing the other 5 working cylinders on that bank
to run
rich. Once you get up to speed and the switch on the bellcrank kicks you
into
open-loop mode the oxy. sensors are ignored.

Regards,

-Ron
1986 XJ-S
1989 VDP
various MG’s

MGMagnette@aol.com wrote:

The Car: 86 Jaguar XJ-S with 65,000 miles (meaning close to 67,500
service

time)
The Problem: When at idle, when warm, occasionally, maybe 10-25% of
the

time, the idle is very rough… Almost like it runs half the engine,
and

doesnn’t run the other half. The tach needle goes to 700 then to 400
to 700

then to 400… Then most of the other time is idles just perfectly.
It

always runs smoothly and perfectly when moving.
The Solution?

Thanks,
John