Hi all!
ive recently finished up with a head gasket replacement on my 1986 series 3 XJ6 for the first few weeks after all was well.

a few days ago however I noticed some weird "stuttering" while driving into work,
around 3000 rpm the car feels almost as if switching gears and the rpm will slightly drop before bouncing back up and down.
it happens not only in drive but also in park, ive already replaced the fuel filter thinking that could be the issue as it was completely gunked up from running the old gas that was sitting in the tank while I replaced the head gasket.

ill attach a video below to give a better idea of what's going on.
from what ive read it seems as though this is an issue that can be caused by a multitude of things and is really only solved by trying various things that could be wrong until one of them works.
any ideas are appreciated.

 

P.S I believe the coolant temp sensor to be bad as I pulled it and it read 0 resistance while cold which to my understanding means it is telling the ECU the engine is hot.
I would start there but dont believe this to be the root of the issue as I discovered the dead sensor prior to this issue starting and it happens regardless of if the engine is hot or cold.

 

Yes CTS is idle circuit only... stumbling at 3000 with or without load suggests ignition.

To reduce the number of rabbit trails offered, can you give us a brief history of ignition components: plugs, wires, ignition coil, ignition module in amplifier, cap and rotor.
If you have timing light that light that is triggered off #1 or #6 ignition wire (and a helper) check ignition timing at idle and 3000 RPM. Best to put a white paint dot on 33 degree mark as the engine compartment is quiet busy and noisy at 3000 RPM. 33 degree at 3000 RPM and correct degree at idle, will confirm your centrifugal advance is working correctly. At idle and 3000 RPM the vacuum advance is not a factor.
Rgds
David

 

sadly almost all of the ignition system is original besides the spark plugs and wires so there's not too many variables easily eliminated with the history alone.

I believe I have a timing light somewhere although ive never used one so it may take some time to figure out how exactly that works. ill do some research and get back to you on that

 

Have you checked the in-tank fuel filters?
They are a classic for getting partially clogged and the fuel pump cannot deliver enough fuel under extreme load.
This then reduces fuel supply to the injectors and misfiring.
Again and again, I have seen this in tank filter problem in both carby and injected engines and it gets worse as older rusty tanks create more crud in the filter intakes.
There is a small bung to drain the fuel and a larger one to remove the filter.
Worth checking!

 

I have not although I definitely should..
the fuel filter was very very clogged up and took a solid minute to drain all the dark orange liquid out of it. however I ran a cheap clear fuel filter after replacing it for a bit just to see if anymore crud came through the lines and couldn't see anything.
to be completely honest ive just been afraid to touch the tanks as ive heard countless horror stories of working on gas tanks gone awry and causing massive headaches.
are these filters easy to remove? or does it call for dropping the entire gas tank?

Edit: My only thought on the matter that makes me lean more towards ignition over fuel is the fact it happens at EXACTLY 3k rpm both with and without load, if it were starving for fuel I feel as though this would have changed when I replaced the nearly fully clogged fuel filter or when in drive, although this could just be an incorrect assumption and dont want to completely rule fuel out as a possible issue just yet (should probably get a fuel pressure tester to confirm/deny this theory)

 

Update: upon going to check the timing as doug suggested i noticed a small puddle of coolant on my disributor from the thermostat housing above and realized i had a small leak there which ive read can cause this problem.
anyways i tightened up the bolt that the leak was coming from, plugged the vaccum advance and checked the timing (seemed to be around 14 degrees BTDC at idle and 30 at 3000 not exactly spec but not too far off) and lo and behold the stutter was gone, i reconnected the vaccum advance and the stutter returned.
im pretty inexpereinced with timing so im not too sure what this means. do i need an entirely new distributor? is it just the coolant causing issues that will go away with a good cleaning?

 

Great progress!!!
30 degrees at 3000 is ok, and I too have had leaking upper radiator hose/thermostat housing fluid on distributor causing problems. You can just replace vacuum advance unit on the side of the distributor These folks will get you the right one for your model.

British Vacuum Unit
Canterbury, NH 03224
(603) 731-1788
​For orders or info, use our web "Contact Us" or email us direct at
​https://www.britishvacuumunit.com/lu...ibutors-1.html

Rgds
David

 

Glad to see you found the cause of your problem.

Re the fuel tanks. Under each rear fender there is a roughly 2 to 3 inch hole with a plug.
Remove the plug and in the bottom of the tank is a small hex bolt with I think a 9/16 head.
That is the drain plug.
The drain plug is actually screwed into a much larger nut which holds the in-tank filter.
That can be unscrewed to get access to the filter.

I have owned two Series 2 XJ6s and one Series 3 XJ6 and have never had any difficulty in changing filters

 

If you call British Vacuum company, you may want to see what it would cost to have them repair your distributor and bench test it, as they have all the test equipment and detail parts.
This will ensure reliability for some time. Go over with them your 3000 shudder with vacuum disconnected and then reconnected, they may also recommend changing the advance base plate mechanism on the inside of the distributor.
Rgds
David

 

If you are planning to work on distributor yourself, there is another possibility Inside the distributor; there is ground wire or a spring wiper that makes the ground between the moving advance mechanisms. If you have the spring wiper it may be having poor contact at end of its movement, a good cleaning of wiper tack with electrical spray cleaner and application of dielectric grease, may solve the problem. If you decide to change the base plate, SNG Barret has the pickup and base plate as an assembly. Pick-Up and Base Plate Assembly Fits XJ6 S3 and 1 more Original Equipment Supplier Product Code: AEU1500 $190.


Rgds
David

 

wanted to give this thread a quick update for those interested...

Firstly thanks to everyone for the help and suggestions.
per davids recommendations I gave British Vacuum Unit company a call, and while he did inform me he doesn't work on electronic distributors and wouldn't be able to be of much help he did give me a good bit of knowledge and a few more ideas of what to look for.
would absolutely recommend giving them a call for your pre electronic distributor needs.

Anyways, his main recommendation for me was to open the cap and give the distributor a good cleaning before going too crazy.
before diving down into it all I went to test the problem one more time both with and without vacuum advance and of course this time the problem was happening regardless so out the window went my idea of what was wrong with things.. regardless I figured I should check the inside of the distributor anyways.
as I went to do this I found one of the distributor cap screws to be nearly completely corroded at the top and it stripped away with a few turns.
without a spare screw on hand I called it for the day and was ready to find a new screw so I could cut the head of the old one off and slip the cap over top.

however... as I went to pull out of my back yard (work area) I decided to test the issue one final time, lo and behold.... gone. absolutely gone, the rpms stayed completely smooth past 3000 with no issue at all.
I couldn't tell you what I did, my best guess is either whatever coolant worked its way into the distributor worked its way out I really have no clue...

so im gonna do the right thing and pretend like nothing was ever wrong in the first place

again thanks to those who gave their two cents on the issue. always appreciate the help!

 

Good news!
Could have been coolant fluid dripping??
I would invest in a new cap and rotor for your glove box, new caps come with new screws. The screw is special and I have not seen them separate from a cap.
Rgds
David

 

I've also seen the rubber gasket under the filler cap (on the thermostat housing) go bad and drip coolant down onto the top of the distributor.