Hello all,

I've known for a while that the compression on my 3.4 mk2 was fairly dreadful - with it being kept so far away from my house, I've been taking smaller bits back to my house for working on them, but with a free weekend I thought I'd try and workout what was causing the low compression and therefore how much trouble I was in(!)

I know the fuelling is not set correctly and I haven't spent a huge amount of time fixing it as I wanted to understand the low compression first - but it does run OK, especially once warmed up, if a bit rich on the front 3. Oil pressure and running temperature stabilise at respectable numbers.

So - today I retested the compression, and then did a leak down test. Pressures were very close to 75psi on cylinders 2,4,5,6 - a bit lower in 1 & 3 (55-60psi). As I say, fairly dreadful.

I suspected/hoped that the timing was out, but actually wasn't able to hear anything from the exhaust or inlet on any cylinder with the leak down test so either the valves/timing is OK or it just isn't the biggest problem. I intend to take the cam covers off tomorrow and check the valve timing just in case as it's not a big job.

On every cylinder I can hear hissing from the oil dipstick hole - I think this means that at least the piston rings on each cylinder needs some attention (and with it that far apart it may as well be done properly). At the same time, in the leak down kit manual it said that some air escaping past the pistons was normal and I'd already done the compression test so I'm not sure how close to running temp I actually was by this point. Saying that, all the cylinders except cyl 6 were quite audible without a bit of hose so I think this is most likely the culprit (especially given the compression).

The other interesting/confusing thing spotted - when we tested cyl3 there was a large amount of air passing into cylinder 2. I could feel it blowing out of the spark plug hole! Testing cyl2, the same was not true (no air passing into cyl3). Is there a mechanical explanation for this, or is it just very likely that we weren't completely at top dead centre on cyl3? Perhaps one of the valves isn't closing properly on 3 and it's finding it's way into 2 where the valve is open?

Finally - as the car is so far away and the summer is coming (to the UK) I'm interested in how long I can run the car like this without doing some (more) serious damage. At the moment it seems like the head/block at OK so I'm wary of damaging these. It's unlikely to be used regularly, and getting it to somewhere where I can take the engine out is not something I'm going to solve quickly - perhaps my longest journey over the summer will be to a car show, which will be a few hours journey away.

Any advice very welcome .

cheers,

Baz

 

I have to say it does sound like overhaul time, certainly a head-off job, but probably re-bore and new pistons/rings. You might get away without re-grinding the crankshaft, but certainly clear out the sludge traps.

How many miles on the car ? Over 100k means, with the materials used in the 60s, it is time for an overhaul, i.e engine out and everything done.

However, a top-end overhaul may work wonders with valves re-ground in.

I can't see your problems being down to valve timing. This engine has two timing chains and even when seriously worn, they will not make the kind of symptoms you describe, unless previous owner made a pig's ear of a job.

As regards doing damage, it sounds unlikely unless you have dreadfully low oil pressure. So an occasional trip to a rally should be OK, but don't get cross with the car if you are left at the roadside sometimes !!

 

It sounds like we're on the same page Fraser - The car was 1960 but was SORN for a time around 1990 and had a large amount of work done when it was put back on the road. Exactly what I'm not sure (I've only got some of the documents) but pretty sure the engine was worked on. But that was several years (approx 10) ago now. I've owned it for about 18 months - although I'm not near enough to class it as regular use it has been good so far and the work I have done on it hasn't revealed any shockers. From what I could see when I did one of the tappets the timing is not miles off, so I'm inclined to agree that I just need to bite the bullet and go for the overhaul.

The only problem is I'd like to do it properly and as much as I can myself (at least dis- and re-assembly - not the bores or valve seats). Realistically it's going to take me a few months to find somewhere to do that near where I live, and with the summer coming I'd really like it to be on the road as last summer it was laid up whilst I re-did the underside.

The oil pressure is good, and the 2 oil changes I've done since I've owned it have both come out looking like new so I'm happy that there's no major damage occuring - I just don't want that to change when I know there's something not quite right.

It sounds like my time tomorrow might be better spent balancing the carbs as best I can and trying to make it run a bit better a low rpm when cold, rather than getting into valve timing.

B

 

Good oil pressure is a good sign and indicates the bottom end may be OK or has even been overhauled. In fact the whole engine may have been done, and your bad cylinder pressures are down to the 10 years standing. Piston rings can seize up in that time, so a lot of running around this summer may see those pressures recover. Unless you've got bills and can speak to the owner of 10 years ago, only stripping down can tell how good things are. I would suggest if the pressures don't recover, you take off the cylinder head as a start. This will then permit you to look at the bores to see if they are lipped, and if the pistons are oversize indicating a rebore. Also inspect the cylinder head components and the camshafts. But before you do any of this, just lift the cam covers and check the tappet clearances, and report here.

 

If you are only driving occasionally and are not racing it, a 25% difference max. in compression is tolerable. As said in previous posts, if no oil blow-by or smoke, a valve regrind may bring it back. Get the carbs tuned and ignition, this too can make a difference. I have seen worse. I once drove 2,000 miles and got decent fuel mileage only to find on return that I had one dead cylinder with a split valve. The engine ran smooth without a miss or hint of trouble. Reground valves and adjusted to specs and that is all it needed.

 

Thanks for the replies all - always good to here it's not necessarily a disaster!

Fraser - the tappet clearances were actually one of the first things I did when I got the car, as it had a noisy one. They were all spot on apart from that one which had a larger gap than required - this is now much closer (if not quite perfect - I should have ordered a range of shims rather than just 1).

From what I saw when I did this, the valve timing is not miles off - I can believe that the valves might need some TLC though.

I had a very basic tune of the carbs (idle screw, rough guess at mixture whilst erring on the side of rich) and certainly at idle it is much more behaved now.

Ignition timing is next on the joblist when I back over with the car (then probably another tweak of the carbs). From there I think I will spend some time driving it over the summer (it will be almost a year since I last took it out for a spin) and first port of call in the winter is head off and inspect. From there, as you say it may be clear just how much work is required.

 

Did you do a second compression test with a bit of oil added to the cylinders?


This test confirms its rings not sealing, or shows you whether it is another issue like a valve not sealing.


As for cyl 2-3 air leak, you're spot on, its either not at tdc on compression stroke or a valve isn't seating properly.


Age is a big player in rings getting stuck, maybe a good drive if that's possible, to get some heat and load in the cylinders along with an oil change might be enough to free up partially stuck rings. Its what I'd be doing if possible before stripping it down.

 

That large tappet clearance could possibly indicate a bent valve. What happens if an XK engine is left standing for a long time is the lubrication in the valve guide, (oil from the last time of running), disappears and when the engine is started the valves can be a bit "lazy" in returning closed, so a rising piston thumps one of them and Bingo, bent valve and loss of compression. This actually happened to me with a 4.2.