I (generally) understand what compression test is supposed to accomplish. But I am still not quite sure why all plugs need to be out (or not) when measuring the compression.

One can only connect the compression tester to one of the plug holes, right? This essentially "seals" that particular hole (for the duration of the test).

Why does it matter if the other plugs are pulled or not?

Also, on this V12, the two banks of cylinders are essentially independent (at least they are not sharing the same gaskets, right?) So, even if it is true that it is better to have "neighbor" plugs taken out, wouldn't it only have an effect on the six cylinders from the same side? This would then suggest taking 6 plugs from the same side out and testing them one by one, while not worrying about the other 6 plugs from the opposite bank. Or am wrong on this? (Sorry if it is obvious. Just trying to understand the process better.)

 

taking ALL the plugs out is more to do with placing less strain on the starter and gaining a faster more consistent cranking speed than would be possible with all or some of the plugs in place. If you leave plugs in the reading between first and last on a 12 cylinder motor will be biased in my opinion simply due to battery issues.

 

Always, ALL plugs out, throttle jammed wideopen, ign coil unpowered.

Been that way as long as I remember, and that is a while.

200psi would be a good figure for the Lo Comp engine your market has. Mine ran 235psi average, and we have 12.5:1 comp engines.

Really make sure you have NOTHING anywhere near that engine that is able to fall down a plug hole. I know its been mentioned many times, but I still hear of guys looking for that last washer, or small nut, and realising the inevitable, its inside one of the cylinders.

Not good for the blood pressure, or the sense of humour.

 

How very true.....


Circa 1961, I bought a bedraggled 1940 Ford pickup, sans engine and transmission. Cheap. Oh, no brakes a bent front axle and right side fenders straightened with a sledge!!! Wifey diagnosed me as insane....


I went shopping, on the cheap. Local junk yard gave me a motor off the iron pile. Cheap, it has a knock. But, hey, it is a 53 Mercury motor. 254 CI in lieu of the
Ford's either 221 or 239.


I hauled it home and proceeded to strip it. The bed of the truck was a fine place
for that. Aha, the little brass tip from a spark plug in one hole. But, yuk, it busted
out a chunk of piston!!!! Must have really PO'd the PO!!!


I freshened it up and cobbled up a transmission from the junk man's iron pile. Several to get one together.


It ran like stink......


Bit by bit, fixed it's other ills. A chain and a bottle jack straightened the I beam axle.
Steered much better with all wheels pointing forward.


Carl

 

Yeah, I got scared enough (about dropping a washer or a nut in a spark plug hole.) That's why wanted to explore whether it is absolutely necessary or "nice to have" to remove all plugs or some (say, by row). Since it is necessary, will proceed accordingly.

Had an idea: can I put a bolt (with big enough head) in the hole after pulling a spark plug (to prevent things falling in)? Or what other tricks people use to temporary cover the holes?

 

Nice big wad of old shirt etc, screwed into a thin end with the big unscrewed end big enough to not get sucked in. Works for inlets too with the inlet manifolds off.
Greg

 

You will not want anything in those holes as protection when you crank it for testing, coz they will become missiles.

Just be FUSSY, and look 20 times, then 10 more, and make sure all the bits you removed are on the bench, far away from the engine bay.

The first time is SCARY as hell, but if you are a methodical worker, and have a good dose of common sense (all V12 owners have this naturally) you will be fine.

DO NOT remove any spark plug until that V is CLEAN, and mainly around the spark plugs. Most of us find broken porcelain and other odd bits, wedged in the bottom of the spark plug well, and gravity being what it is, they fall inside as soon as you lift that plug clear.

You will be fine, REALLY.

 

Gentlemen, I still don't think you have to pull all the plugs out at once. Too many opportunities for other problems.

Battery should be on a charger so that you do not run it down and thus your cranking speed is similar.

And yes, disconnect the coil...forgot to mention that.

v1rok, You are looking for similar pressures so that you can identify any problems. Abnormally low pressure in one cylinder would point to a problem that needs to be looked into. It could be a bent valve, a dropped valve, a wire drawn valve (small crack), a broken ring. Also, when your cranking the engine keep your ears on to listen for a different pitch or sound. That is why you want the charger on so your cranking speed will be the same.

But remember, the whole purpose of this post is to determin what the sound is that you think is bad. Check that timing first.

And find someone else nearby with a V12 jag. Listen to another car. The V12 has a sound all its own.

 

My grandads quick and easy method was to run the engine and pull a plug from each cylinder in turn listening for a change in the sound the engine makes. If you pull one and the sound stops then you've found the cylinder that's problematic.

Generally this would reveal a rod that has high tolerances. No change is good.

Short the pulled plug to the block rather than hold it away. This prevents cursing and saves loading up the EFI module. On the jag it way be easier to get an extra lead and ground the plug end, then pull a plug from the dizzy. Put the grounded lead into the open dizzy port to consume the sparks.

 

Grandad might have found it a bit challenging to whip the plugs one by one out of a running V12 JJ!
Greg

 

I reckon the pacemaker would have taken a beating from the HEI system, and the whoosh of the 11.5:1 comp screaming for release.

It is NOT a complicated procedure. 3 revolutions per cylinder, write down the reading as each one is done (your memory aint that good), then refit the plugs and DRINK something alcoholic.

4 hours tops including R&R the plugs once that compressor is out the way.

 

Oh yeah, old tales abound. Way back Touch each of the four spark plug tops with the engine running. Get a zap means a good HT lead. Loss of RPM means that cylinder is pulling a load. Converse is true.


Dates back to horses in harness. A slacker might appear to be pulling it's share, but
only in appearance.


Carl

 

I heard this very same noise on a customers XJS V12 4 or 5 weeks ago. Dropped valve seat. Let's hope I'm wrong about your though.

 

Grandad could take a lickin'. Bucket of kerosene for cuts and scrapes. He was a Ford man and would have thought I was crazy if he saw the jag snake pit.

Definately do a compression test. It's easy(ish) and important. No question.

Grandads test for a noisy engine just reveals a rod that's knocking. If you're not sure what your sound is then you can eliminate this possibility. A knocking rod will stop sounding when its cylinder isn't firing. Pulling plugs from the dizzy is real easy. Hardly any blood involved.

 

Never did a test on a V12. Last one I did was circa 2002 on my OHC 6. Merely cranking it with all plugs in hinted at bad news. It was. Zero in all holes!!! I did take the precaution of using my shop vac in the valley before pulling the plugs. One of the easiest tests I ever did.


And, I never thought of a cylinder at a time in respect pug removal. All olut, then test. if any doubt, prep with a battery test and charge. Full up is best. I don't think the few amps afforded by a charger in charge mode help much. In boost, at about 50, yup!!!


Connecting rod knocks are variable. Quiet under load, but, rattle on throttle close. Rev engine just as tad, snap throttle shut. If the rod inserts are loose, you will get a staccato rattle. Been there, too many times and in the most inconvenient places!!!


Carl

 

I had a weird compression experience with my old 960. 5 cylinders had 10.9 bar. The 6th had 0.0 bar. Checked everything. No hole in the piston. No burnt valve. All that was wrong was a valve spring that was too soft. It wouldn't close the valve enough. Resulting in random compression loss, depending on air pressure outside, engine temperature and revs... Cold it had no compression. Warm it had a little (from 0 to 4 bar is at least some). One new valve spring and it was as good as new.

 

Yeah, Daim, at times, short of a "big" deal, other bits from my long past.


1. First "car", in 1946, a derelict 23 "T" Ford Roadster/pickup. In spite of all efforts, it would not fire up!! Til, school pal and "T" mentor counseled head removal. Its, not ours, ours hopeless !!!


Great, one intake open, always . Taped closed. Reassembled and it fired right up.


2. Decades later, gotta have a V8. Six won't get it !!! Four door 37 Ford on dealer's back row. WWII survivor. Not my ideal choice, but the only one in my "budget". Sold, $200 !!!


3. Ran fairly well. poor idle and clackety clack. Latter fixed by replacing a couple of busted valve springs. Extremely worn cylinders not so much. Or, other probably leaking valves, no at all.


Got a lot of service in spite of it's ills. Once up off idle, it ran quite well. Did a lot. Some good and some not so much.


Carl

 

simple...
buy valve cover gaskets and remove valve covers and then check your cam to tappet lash...

if all is good then it is not a dropped valve seat...
which is scary

 

The weekend is almost here...All tools have arrived. Compression tester, crowfoot wrenches, etc

Quick question. Or confirmation. I should make the engine "run out of fuel" by pulling the fuel pump relay, right?

Also, descriptions recommend doing the test on a warm engine. Given that I will be getting into and cleaning the Vee for the first time (including removing the coil and leads), I expect it would take a few hours By that time the engine would not be warm for sure. Still Ok?

 

Just do the best you can. No one can do more. Agree, disable the fuel pump. The inertia switch is a good place.


If you start work on the engine as it is warm and work steadily, it will be warm enough.


And, if not, not an issue. It is not necessarily the bar or PSI values that you are looking for, but the variance in values, cylinder to cylinder.


Lay out your plan and then execute... it will work out.


Carl