XKE 3.8 Compression Test Result

[Airbus]

Hello :-)

I am new to this forum, proud owner of a 1964 3.8 model. We rebuilt the car over the last ten years, and it is on the road for over a year now. Since the beginning the car was suffering from a very rough idle, along with quite some "pinking" when driving it with low RPM and opening the throttle(s). Ignition timing check is to go, also some serious carburettor tuning.

Now I am at the beginning of "troubleshooting". I did a compression test to start with, and found some worrying numbers. The test was done with the engine reasonably warm, throttle fully open, spark plugs out etc (comments / suggestions are welcome);
"Dry" "Wet"
Cylinder 1 203 203
Cylinder 2 185 191
Cylinder 3 162 171
Cylinder 4 174 171
Cylinder 5 191 205
Cylinder 6 205 214

I was shocked to see these numbers; it would eventually explain the "pinking" during acceleration, nevertheless the difference between the individual cylinders seems worrysome (the test will be repeated just to verify the above numbers).
Before going any further I will go and check for valve clearances on some cylinders. What would you suggest with regard to those high numbers? Exchange head gasket for a thicker type?

Thanks for any advice.

Joe

[Heuer]

Is it a 9:1 or 8:1 cylinder head? Normal readings are 180/155 lb/sq ins respectively. Seems two adjacent cylinders (3 & 4) are low which might be a clue. I would expect pinking if you are running anything other than super-unleaded 98+ RON.

[Airbus]

Hi David

It is a 9:1 cylinder head...I am exclusively using 98 or higher gas, in order to prevent pinking as much as possible.

I will do another test in order to confirm the readings and see what it says...

Thanks

Joe

[cactusman]

Pinking..or knocking....can also be caused by too much ignition advance so check the static advance too. If the distributor is worn or the springs holding the weights are weak or there is something amiss with the vacuum advance this too may cause excessive advance under some engine conditions and lead to knocking too, especially under load accelerating up hill for example. As heuer says use 98 + Octane fuel .

There does not seem to be a significant difference between dry and wet tests which suggests either a valve or two are not sealing or possibly a failure of the head gasket between no3 and 4 pot as these are the low ones...

[PeterCrespin]

There is no inbuilt CR difference between heads.

It may have been milled a few times

9:1/8:1 is a piston parameter.

The numbers show every valve was shutting properly IMO, under test temps at least. Even a few thou gives zero psi very quickly.

Just back off the timing until it stops pinking and check for weak mixture. Then re-assess the settings vs spec and the performance.

[abowie]

You have 2 separate problems. Your overall compression is too high. You also have a problem that is causing a failure of compression on 2 adjacent cylinders.

Too much compression is usually a result of head milling / block skimming over the years, and and rebuilding with with 9:1 pistons. We always do rebuilds with 8:1s.

In my experience you'll struggle to get it to stop pinging with compression that high. I tried everything I could on an engine with 195 psi. I failed, and in the end had to fit a 120 thou head gasket to bring the compression down.

As to the low numbers on #3 and #4 your guess is as good as mine, but that's not good. Head gasket failure between pots, as mentioned, is a possibility that is at least easily fixable. Other possibilities like block or head cracks, ring or bore damage etc are a lot harder to fix.

Were it my car I would ensure the distributor was working correctly and that maximum advance was no more than 32 degrees (16 degrees of mechanical advance as measured with a protractor with the distributor in a vice), and that the timing was set statically at no more than 10 degrees. Then I would tune the carbs to give 5% CO in the exhausts at idle, with the caveat that whatever is wrong in the engine may make it difficult or impossible to achieve a good state of tune.

When it still pings, you can try less advance, but this is not really addressing the actual problem. Set it somewhere between 5 and zero degrees and see if this helps. On my car I fitted an EDIS system and played with advance maps for months with no success.

Cometic make a 120 thou head gasket. Personally I would get one and replace yours. It will lower the compression ratio enough to make your car run properly, and if the other problem is the head gasket then it will fix that too.

While the head is off I would get it disassembled and crack tested. If you can, get the deck of the block crack tested as well in the area between #3 and #4. 3.8 blocks are less prone to cracking than 4.2 but it's worth doing.

Good luck.

[abowie]

Also, check your valve clearances, just in case they are too tight on #3 and #4 and are interfering with proper valve closing. Unlikely but possible, and another easy fix for that problem.

[christopher storey]

I'm afraid I am not persuaded that there is any significant abnormality in these readings. I am rather doubtful that they are indicative of a leak between 3 and 4, which in my view would produce a considerably greater drop in both cylinders than the overall 20% between highest and lowest, and about 15% drop if the two doubtful cylinders are compared with the average of the other 4. A leak down test would be very useful

As far as the pinking is concerned, there are quite a number of things to consider :

1. These engines always had a tendency to pink in the 1800 -2500 rpm range, even in the days of 100 octane fuel

2. Timing is critical : you need to check with a light that a. total advance ( vac disconnected ) of no more than about 34 degs BTDC is occurring ; b. that the mech advance curve corresponds with that shown in the manual , bearing in mind of course that those figures are distributor figures and thus need to be doubled to get crank degrees - as already mentioned weak springs will lead to excessive advance at low speed - and c. that the vac advance is not seized in an advanced position i.e. with vac connected the total advance should drop markedly when the throttle is opened sharply

3. Is the engine burning oil ? A significant degree of oil burning will markedly reduce the effective octane value of the fuel

4. Are the carb needles correct ( UM ) and is the mixture strength correct- something like 5% CO at idle is correct for these engines - because weak mixture is a prominent cause of pinking and all XK engines prefer mixture on the richer side

[Airbus]

Hi

Thank you for all that good input :-)

I will try to work systematically and see what the problem is. To make matters worse, the car developed an interesting engine clatter between around 1700rpm and above, when under load (I already used the search function in this forum)..it sounds horrific. I will check the timing chain tension and see what the valve clearances are.

Cheers

Joe