Disheartening readings. Compression

Well i have been fighting a bit of a stumble and misfire across rpm range. Have just completed replacing rear mainseal both upper and lower halfs believe it or mot was successful in no more leaks. Replaced fromt crank seal. Just upgraded cooling system with rad from coolcat. New fans. Installed dual temp gauge and senders. Engine temps stay equal on both sides when running or idleing.
Engine did get hot prior but not loosing any coolant have tested for ex gas in coolant. None.
Decided to do a compression test again and my readings on the a bank have deteriorate since last check in april. See image.
Wet comp test on low cyls does not make really any change < 5 psi.
Plan on doing a leakdown test but my regulator on mine just died so new one on the way.
I noticed when checking timing that it seems to have more fluctuation than prior but just had dist also sent out and curve reset and vacuum adv put in. So another variable. Bit that in and of itself has no bearing on mech compression

Question im wondering if some how a bank cam timing has changed some how. I guess i can pull covers off and get cam gauges and see if alignment is correct at TDC. Anybother way to check integrity say of tensioner ?

Not the same car (Mk2 3.4) but the " movement" under the timing light (either side of 6 degrees BTDC) indicated tensioner/chain, along with some rattle. Changed chains and tensioner and light became rock steady. FWIW. Paul.

Check valve clearances.

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Yes after checking with leak down tester both hot and cold (im going to run comp test again cold just to get readings.)
Unless that leads to a conclusive issue , I will most likely pull the covers. B bank is pretty much unchanged so sort of leads me away from excess timing chain slack etc. Why i still lead to poss heat related issue , is the fact its the 2 outboard cyls 1-6 and the two center cyl 3-4 tht have the most deficient readings.

Well cyl leak down cold
Shows considerable +2O% on the offending cyls and appears to be blow by past the rings. This too then actually may explain why my rear main started leaking more and more over last few months.
Now the question is do i just pull it now or and drive it for a while. Its already been off the Road a couple months and the florida weather its getting nice. So perhaps ill drive it for the winter. And monitor compression every couple hundred miles.
Then pull it out and do a complete teardown next summer.

If this is the case, the question is what caused such a wear on the rings and loss of compression in just a few months…? And only in one bank? And why not on the two center cylinders?

If the engine starved of oil it would be on both banks.
If the head starved of oil it would effect the camshafts and valve train but not compression.
The only way this could happen so fast is if the offending cylinders had no spark, or a massive over-fueling, that raw fuel was washing out all the oil. And I guess this would have been very apparent.

The engine overheated, and even if you say that there is no gas in the coolant, I would not exclude the possibility that your head gasket is shot.
I am willing to place a bet on stuck rings as a result of the overheat.
I would also not exclude a valve timing/tensioner issue.
Did you check the valve clearances?

Thanks for the feedback, yes it is def most interesting that it is those cyls. Little additional info , as far as running rich , I was having an issue with the car running rich and at first unable to lean out , but… it was the B bank that was rich . black sooty plugs , A bank looked pretty norm brownish. during this last partial tear down . ( aside from rear main , front crank seal ) i had the intakes off , cam covers off and pulled the Air tubes and blocked off etc. at the time i did check the valve clearance and they were pretty cose all the way around perhaps one or two valves maybe .001 tighter than spec. but didnt think much of it.
back to the richness issue a while back in effort to determine , I rebuilt the carbs with new floats etc . but it still ran rich on B bank.
i took a second look and while rechecking carbs i decided to lower float level just a tad, basically on the high side ( upside down) of the 17mm setting height.
still rich after that even w needle full lean .
in thinking i decided to take a closer look at the choke on that side and sure enough , I never disassembled shaft etc from it prior but the B sides disc was 180 out. basically allowing fuel past choke always . easy sort and checked A banks and it was fine.
So long story its been the B bank that was rich all the time , where poss cyl washdown i would think would occcur.
that being said with the new temp gauge that measures both banks, i noticed at idle that prior to fixing richness it ran a good 5-10 degrees cooler at idle ( b bank) when it ran hot prior perhaps the richnees did keep that bank cooler and the heat did cause some damage to the rings on the a bank. poss sticking , but have seen on motors where the rings have lost some of their “spring”.
i’m not a fan of a fix in the can like trying Marvel to free up or see if it changes . but dont know if maybe doing so & letting sit for a day or two might be worth a try…
I may again drive it for a hundred miles or so and see if anything changes. or if it progresses
Oil pressure has always been good on this car and never low as long as i have had it.
appox 25 psi at long hot idle and above runs 50 -75 rpm dependant. plus while pan was down looked at mains and bearings were in great shape.
And i may take covers off order a cam tool and just recheck cams at tdc to see if something has changed .
I know your not suppose to turn crank backwards , and when i took off front crank dampner im pretty sure it didnt , but t has me thinking … altought while doing leak down w @ TDC number one i did rotate it back ever so slightly while watching chain thru filler and it moves almost instantaneously ( ill have to measure degrees) .
just thinking that if tensioner was loose i’d have some play ??

Not the outmost expert, but I think that even with the mixture overly rich, as long as it ignites, it would not wash the oil.
On the other hand your B bank rings must be by now full of carbon and soot and could be stuck because of this?
If your carbs and cooling system are properly sorted I think you have nothing to loose, put some marvel oil or whatever, drive it some long distance, do a good old Italian tune-up, then measure compression again.

Yes I think you would.

But its A bank w low compression


First, I would not run the motor any more. Secondly, you’re going to have to bite the bullet, pull the motor and do a tear down analysis. What the motor was subject to in the past can only be determined by examining each and every part. Piecemeal attempts will only lead to further frustration and doubt its reliability and in the long run cost more $$$. Further driving may damage “undamaged” components!
Been there, done that and it isn’t pretty but necessary from the figures you’ve detailed. A full tear down and analysis is, I"m afraid, your only answer and option IMHO.

To more Happy Trails,


In mine, too! Easy to say, wrt to the monumental task of pulling/ pulling down a V12, but…it’s the best option.

Well that may be the way i end up going a couple more checks this weekend , then I’ll make a game plan to pull it , Im half of mind if i do , too see about perhaps getting an XJS motor to put in with Fuel Injection looking at 88 engine have to see if viable . I will of course keep the orig engine rebuild completely if it comes out , but always been my goal to go to some sort of FI eventually . wondering if when said and done if the se 88 parts could then be put on my block and heads … guess i also have a lot of forum post to sift thru …

Yes sorry, I mixed it up, ment to say A bank.

If the tensioner is suspect definitely not.
Otherwise worn rings or bad head gasket are just that, and if the cooling system and oil pressure are good, it can’t do more damage.

Yes, if it’s not stuck rings there is no other option…

If it comes to this, I would go with a complete engine.
Even though (IIRC) the 1988 bottom end would the same, HE pistons and heads are definitely different, There will be also a lot of little bits and parts that would be quite difficult, and probably more expensive, to find them individually.
I don’t know if you could though use the pre HE injection parts on a carb engine.


To sort some of the unknowns first I would remove both cam covers and check the cam timing with the gauge. There is the possibility the A bank chain may have slipped a tooth or two. They may account for the low compression reading across the entire A bank. If that’s the case then you may dig deeper into the timing chain tensioner and also the JackShaft sprocket. That sprocket has a history of rounding off teeth.
If you decide to go the EFI (XJS) route I’d recommend obtaining from a copy of their manual - “How to convert an E-type V12 to Fuel Injection”. Also, if you do go that route renew ALL the fuel injection hoses!!! Several have experienced catastrophic engine fire caused by hose failure (due to age!). Cheap insurance.

Keep us posted on what you find (or don’t find).


I wouldn’t bother with the aj6 manual. All it actually says is “this is what an XJS setup looks like” and I see no mileage in fitting 30 year old semi life expired components and claiming it’s an improvement.

If a whole bank has skipped “a tooth or two” then that’s easy to check with a timing light. Just confirm your ignition timing for the A bank and then draw a second marker on the crank 60’ further around for 6B (in a different colour) and confirm the B bank ignition timing.

Take a look at other fuel injected v12 engines on this forum.

kind regards

Sorry Marek but I don’t understand this.
There is a single chain that drives everything.
If the crank or the distributor shaft skipped a tooth both banks would be off by the same amount.
If one cam skipped a tooth it would not have an effect on ignition timing.
Am I wrong ?

No, you’re not wrong.
Dick said “there is a possibility the A bank may have slipped a tooth or two”, not “the jackshaft may have slipped a tooth or two”. I can only think he meant the small vernier teeth, but frankly I’m struggling to see even that.

All I have suggested is that there is an easy way to check the ignition timing on each bank is consistent with the other if you can draw yourself a second line on the damper.

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I plan for the sake of checking what Marek has suggested, but honestly and logic to me would assume if indeed that compression would be off on all clys of that bank, No? My cyls 2a and 5a are the same compression as prior test months ago.
And honestly it runs really well , short of occasional idle stumble. ( which could still be carb sorting ) i have not try and fine tune them just yet. … basics first, timing compression et al …
I will say that the Vacuum Adv and Dist curve tht Rob from British Vac Unit def has improved the performance and midrange accel etc.
The plugs on these cyl show no signs of fouling. I did inject a small amount of oil mixed w BG EPR let sit then drove. Anther compression test showed approx 5 psi higher on ea cyl.
I also want to mention these cyl thru bore scope show no scoring, can even see cross hatch on walls.
Im am still thinking i will drive the car recheck compression about every 200 miles. The hardest part is on my back leaning over , the plugs are a piece of cake to pull. I may even run a treatment of BG EPR. Which isnt nesc a fix in a can but we have used in the marine field for many diesels when oil samples have had signs same issue and it works often.
I really believe chain is ok and not that worn. The eng was rebuilt previously according to PO i just dont know when.

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