Removed all of the hardware and air pump. Motor looks allot better and several components are now easier to get to. What a great difference. However,...... Just as I was told, made no distinguishable operating difference.
Motor still idles rough, especially in gear. (1050-725 RPM)
I've flushed out the fuel tanks (lots of rust), replaced tank screens/filters, replaced fuel pump, replaced fuel filter twice, replaced ignition coil, plugs and wires.
Any ideas would be appreciated. Jim

 

How rough is "rough"?

Owners of Ser III 4.2s have been fighting a light idle 'tremble' for as long as I can remember....with varying results.

Or is is more like an outright misfire/dead cylinder type of roughness?

Have you had the fuel injectors cleaned? Not a bottle-of-something that you put in the gas tank but actually removing the injectors and sending them out for professional cleaning? Doing so almost always makes a good improvement in idle quality.

Cheers
DD

 

Hey Doug,... Sorry I failed to mention, I replaced all the injectors, including the cold start, several months ago when I first brought this old gal out of storage. At the same time, the ignition amp(?). At first I had the local "expert" working on it til I ran out of cash. (Didn't take long.
It doesn't die but sounds like it could. I put it in neutral at stop lights. Seems to holds between 700-750 RPM in drive.
Thanks, Jim

 

Is the ducting between the air flow meter and the throttle body elbow in good shape and secure? There's the accordian-type hose to the elbow, and then a simple round band at the other end of the elbow where it attaches to the throttle body.

Cheers
DD

 

A couple of months ago, I noticed the bellows type hose looking cracked and dry-rotted so I replaced it. After that, I noticed the band you mention had come loose from the elbow. The car wouldn't run after that. After re-inserting the elbow a few times, I replaced the hose clamp with a standard U.S., very tight, clamp. and it hasn't came off again.
Struck me strange as to why it kept coming loose. I did notice some cold engine backfire and thought maybe the back pressure was blowing it off. I should mention, the motor has only 43,000 original miles on it.
Your thoughts?
Thanks, Jim

 

A backfire thru the intake system ....which can pack quite a wallop....often suggests 'not enough fuel'. Does it still happen?

Back to rough idle...

A rough idle that has you thinking the engine is about to die is pretty bad. Do you sense that the engine is almost choking out from too much fuel ? Any black smoke from the exhaust? Sooty spark plugs? If so, look here:

XJ6 Series - EFI Rich Mixture


Also, does this problem happen only on a cold engine, or all the time, or....?

If it happens only on cold engine, consider this....another over fueling thing:

XJ 6 Series III 4.2 - Cold Start EFI Overfuelling



Moving away from over fueling issues, are you sure your new injectors are all firing OK? A problem with the injector harness wouldn't be unusual at all.

Is your throttle body clean and the throttle blade gap set at the proper .002" gap? This is important.

Are your AAV hoses in good shape? You'll have to get under the car to check as the hoses goes underneath the intake manifold.

There are many other possibilities but ^ these ^ are the ones that fall out when I tip my head. I might have more after another cup of coffee.

Cheers
DD

 

In the process of checking all the stuff you mentioned,... and I'm not done,... All the plugs look good (electrodes amber), Tracking the plug wires, I noticed the distributor without a vacuum hose. (????) I guess my local shop missed something during the tune process. I need to backtrack and start checking stuff that was done previously. Anyway,.... After plugging in the vac hose I found dangling below the intake manifold, I gained 125 RPM on idle. Thought it wasn't supposed to change anything when cold. Anyway,... It does seem to run better but still idles rough. No fear of dying now, but still not right.

Will continue with the other items you mentioned.
Thanks, Doug
jim

 

One additional item I have found to influence the idle quality of the later fuel-injected XK engines is the engine oil dipstick tube seal.

 

Well, continuing on with my rough idle, occasional back fire when accelerating, and misfire,.... I cleaned the throttle body and the throttle blade gap is now set at the proper .002". The blade was closing completely. The adjusting screw-stop was not even making contact. Now idles at about 1600 RPM warm. (???) A little more when cold.
Was going to check timing but my light's not working.
Bad luck or no luck at all.

 

Yeah, the timing might be way advanced but also check your AAV. It might be stuck open.

Cheers
DD

 

Can you tell what the timing should be, vacuum connected, as well as disconnected and plugged? The manual is confusing.
Also,...AAV???
Thanks, Jim

 

On your USA-market '86 the timing should be 17ºBTDC, vac advance disconnected, trans in "P", at 750 rpm.

AAV = aux air valve. This is the device that gives a higher idle speed when the engine is cold. It simply allows more air into the engine. It's the odd shaped aluminum thing bolted to the underside of the water rail....a few inches forward of the throttle body. It has disc that rotates from almost closed (low idle) to wide open (high idle) to allow more/less air. Basically it allows air to bypass the regular throttle.


But, I had another thought. Maybe someone tried compensating for the improper throttle gap by monkeying with the idle speed adjustment screw.

After setting the timing but before digging into the AAV possibility try getting the idle speed down with the regular idle speed adjustment:

Idle Speed Adjustment

Cheers
DD

 

Ideas:

1. The "mechanic" did remove the vacum hose from the distributor to check the timing. OK, so far, but failed to replace it, once having done so. Not OK.

2. Turn the engine by hand or with a bump cord til the pointer on the timing gear cover points at the flywheel. Adjust to 17 BTDC. Remove the distiributor cap and note the relation of the rotor to a plug tower. Oughta be just a hair before one. Caveat, very rough aproximation.

3. Determine TDC on the cylinder closest to the bulkhead. Remove the plug. Insert a dowell and crank the engine, preferably by hand for control. Now, observe the position of the pointer and the dampener. Relation in degrees?

4. You can use a test lamp in series between distributor and coil to see when the signal is sent. Old days, points close. New days electronic version of same.

Or, and best, a timing light invest. From a simple Neon off/on to the variable ones. A great addition to anmy tool box with more than mereely setting timing. Also detects HT.

The manual set plus a power adjust has and is still done by amny. Get it close, adjust by advancing til it pings on acceleration, back off just enough to quiet the ping.

There was a life before timing lights!!

Carl

 

Well,.. after a few choice words for the Jag engineering staff, I was able to find and adjust the idle down to about 950-1000 RPM in park. The problem was, I had to turn the screw several turns and bottom it out to achieve it.
Bought a timing light adjusted the timing from 12* to about 17*, warm engine, vac line removed and plugged. Timing went to about 27-28* when vac line was re-connected.
Any ideas floating around before I continue with my TS list? Still idles very rough in drive.
Thanks, Jim

 

Yeah, check the compression. A weak cylinder or two will definitely cause rough running.

And, invest in a vacuum guage. Put it on manifold vacum at idle. Steady needle or waving back and forth? If the latter, something basic is wrong. The engine is an air pump. Some leakage is unavoidable. Too much and all the tuning in the world will not fix it.

 

Hello Guys,... I'm gonna start a new thread in for "Rough Idle" and list all that's been done thus far. Thanks for all the help, Jim