Slight miss at idle

I have a North American 4.2 liter 1985 XJ-6 with a slight miss at idle and only at idle,. Engine has good compression on all pots, great power, puled and checked all spark plugs no fouling–I can’t figure it out.
Chris

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Do you mean the common ‘uneven idle’, Chris - or something else…?

It’s a characteristic of the xk that it smooth out at increasing rpms - even with serious issues…

Frank
xj6 85 Sov Europe (UK/NZ)
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Frank, it is an uneven idle. And…thank you very much for your quick response.
Chris

Just a thought, try running the engine in the dark and check if there’s any stray sparks from any of the plug leads?

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Chris, since when do you have it? (Both car and uneven idle; and the slight unevenness is, sadly, normal on most of our cars)
Search for “rough idle remedies”, the ten point suggestion list, to get an idea.

David

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I just cleared this up with one of my Series III XJ6’s. Keep in mind, that this car had a host of issues. Cam chain slack which made it impossible to start sometimes. Anyway. This is what I did.

  1. checked the compression to ensure it’s even or close to even across all bores.
  2. Checked the spark level. Mine was weak orangish yellow instead of blue.
  3. Reviewed the burn pattern on all the spark plugs. mine were pretty consistent. the PO put Bosch platinum plugs in it. I’m not real confident the stock ignition works that well with non copper core plugs - I swapped out with E3 diamond spark plugs.
  4. Checked the idle speed with a dwell meter and compared it to the rev counter in the car. spot on.
  5. disconnected the vacuum advance and checked the timing; it was off about 4 degrees at an idle. fixed that.
  6. Checked the fuel pressure at the rail - 35psi within spec.
  7. Check all injectors were firing (NOIDS are a must have for this). all injectors firing.
  8. pumped some fuel into a container to inspect for yuck. fuel looked clean; and smelled like it should.

Still hard starting and I wasn’t pleased with the idle or the performance - a bit lumpy. not like a mild street cam lumpy but a pafoom pafoom occasional misfire lumpy.

  1. Checked the cap - it looked like new and so did the HT leads. so I left them.

  2. Replaced the coil with a MSD blaster 2 coil. Downside is the MSD is significantly bigger around, so it doesn’t fit in the stock coil holder - you can purchase the holder from MSD or Amazon. or remove the bottom screw and one of the screws in the water rail, and have stay there with one screw - like I did. It’s a tight sqeeze so it’s not going anywhere. Why you might ask would I do something half assed like that? The other #3 phillips screw holding the bracket on is stripped by the PO.

  3. I also dumped the ballast resister because I accidentally grounded it against the water rail with my screw driver and let the smoke out. I priced the ignition module - which was the better part of 500.00 for an OEM Lucas module. It had screws, so I decided to take it apart. Low and behold and much to MY suprise (not anybody else in the forum when I reported my findings) a GM HEI module - with GM embossed on the top of it, as you can see in the picture.

  1. I replaced the GM ignition module with a MSD multi-spark ignition module that matched the coil for the street fire HEI ignition.

The caveat to all of this is when I popped the distributor cap off to see if this addition caused any issues, I noticed the extra glob of something on the magnetic pick within the distributor - see below

I’m not sure if that was there before the ignition upgrade or not. I didn’t check it out first - I should’ve. I adjusted the gap on the magnetic pick up to be with in specs. I checked the timing, and the RPM’s all was still spot on.

The ignition upgrade was the ticket. Much to my surprise the XK6 fired up on the first click The car has great pick up - better than ANY of my other Series III with 1/2 the miles on the clock. I’m very pleased with the end result.

So there you have it. Something to ponder. oh, and listen to @Frank_Andersen He has been a huge help to me. When I joined this forum I’d just purchased a non running Series III. This one.

I didn’t know where to start as to figure out why it wasn’t running. With Frank’s help (and others of course), I’ve turned this $500.00 non running XJ6 into a reliable daily driver.

A shout out to all that have helped me over the years. Thank you!! without you folks, this forum would be like the other Jaguar forum. Please accept my sincere gratitude!

Mark

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Marl,
Thank you very much for this information–very comprehensive indeed. I will start on these steps with my son in law who is a very good mechanic. Thanks again, Mark.

Chris

Also Mark–your XJ is lovely!!
Chris

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