3.4 XK engine: Investigating lack of compression

[Chris Hawley]

New to this forum so please excuse if I'm not familiar with all the etiquette as yet.

Situation is: 3.4 XK engine last run about 10 years ago. Engine was in satisfactory running condition when last used. Doing a compression test cold, I'm getting 40 - 60 psi on four cylinders and absolutely zilch on the other two.

Might there be any suggestions for a logical way to proceed with investigation? Not too worried about details at this stage, more just a general approach. For example, might a reasonable first step be to remove the cam covers and have a look at the valve train?

Any suggestions much appreciated.

[Jimbov8]

Hi Chris and welcome. As the engine has been stood for so long, remove the plugs and put a teaspoon of Oil down each. Wait 5 minutes and then spin then engine on the starter for 5 - 10 seconds. Replace the plugs and retest the compression, if still the same then you start dismantling.

[LairdScooby]

Welcome aboard Chris! In additin to Jims advice, can i ask - first have you removed all the plugs before the compression test? Second, have you pulled the fuel pump fuse and made sure the carbs are empty of fuel and third, are you holding the throttle wide open during cranking?

Lastly, i'd vary the idea of the Oil down each plug hole by using a synthetic Medium Viscosity ATF such as Carlube ATF-U  or NAPA MVA ATF and i'd turn the engine over by hand so all pistons are as near the mid-point of their stroke as possible then add an eggcupful (~40-50ml or 10 teaspoons) of ATF to each cylinder, leave 5-10 minutes, turn it over by hand several times, leave 5-10 minutes more then lay a rag along the valley between cam covers over the plug holes before spinning on the starter for 5-10 seconds to expel the excess Oil.

I've used this method with good success when recommissioning engines that have been laid up for a time. Also i'd add ~0.5L of the same ATF to the engine Oil before you start the engine and leave it until you do an oil/filter change, then i would add another 0.5L as part of the fresh oil going in. I do this regularly with the oil/filter changes on the rest of my fleet (such as it is) and it has rewarded me with better running, improved performance/economy and emissions.

Another use of the ATF is in the dashpots of the carbs, most manufacturers recommend using engine oil i know but Volvo always used to recommend ATF. I tried it in other non-Volvo cars (several SD1s and various other cars with CD type carbs) and it has always given better performance than engine oil.

[Chris Hawley]

Interesting. Plugs are out, no fuel in system and, yes, foot on accelerator pedal.

I'll certainly try the approaches as suggested above later this week.

An extra snippet is that the engine had had its bores long term soaked with CocaCola (not by me).

Anyhow, I'll proceed as above.

Thanks

[LairdScooby]

CocaCola is great for coins and other things but i'm not sure i'd like to try it down the bores of an engine i was trying to rescue/recommission.

Will be interested now to hear how the ATF does, it's freed a few engines including one on a converted lawnmower (converted to drive a big alternator so battery charger/12V supply for lighting in an isolated workshop) where the owner had forgotten to refit the plug after "borrowing" it for his actual lawnmower and left the thing open to the weather over a particularly wet winter. Unsurprisingly the elements had siezed the piston in its bore but after soaking it in ATF for a few days, he managed to free it.

[Chris Hawley]

Outcome not good. Despite ATF and suchlike down the bores compression still zero on two cylinders. WIth cam covers off all valves can be seen to be moving freely and valve clearance are all reasonable at around 14 - 16 ish.

In hope rather than expectation I thought I'd see if the engine could be persuaded to fire up. Spark present, fuel present, but not a sausage. But that's a bit by the by.

Would I be right in thinking that 'head-off' is now unavoidable? But would there be any merit on doing a leak down test prior to that? And  what are the most likely candidates for a 'no compression' situation? Exhaust valves, gasket, bores/rings?

Given that I know this engine has been subject to neglect, abuse and then more neglect I'm picturing a chamber of horrors when opened up.

 

[Jimbov8]

If you are 100% sure the valves are opening and closing then the head will have to come off. I would suspect worn valve seats or piston rings leading to the lack of compression in the two cylinders.

[LairdScooby]

Silly question before you decapitate the engine, are you sure you have a spark and that it's happening at the right time? Sometimes if an engine has been laid up for a period of time they are reluctant to fire even with seemingly good compression because the spark is weak. Also is fuel getting through not only to the carbs but through the carbs? Sticky needle valves/floats can cause problems in this area as can gummed up choke mechanisms, especially those that don't just lower the jet on the carbs but open a rotary valve with different drillings for enrichment.

[Chris Hawley]

9 hours ago, LairdScooby said:

are you sure you have a spark... carbs...

That's a fair point. Few component on this car may have escaped the effects of long term standing.

Set up is: 3.4 8L engine, 45D6 distributor on conventional points, HIF7 carbs with AED converted to manual operation.

Timing was a 20 degrees ATDC so that's been pulled round to 10 BTDC (roughly, on static adjustment).

Spark at plugs is confirmed using a Sparkrite plug tester. But what I oberserve is that rather than one clean, birght, spark every two revolutions it's showing one main spark followed by 3 or 4 faint afterflashes. I don't understand that.

The float chamber on the AED can be observed to be filling (lid off). But guess carbs will have to come off for a close inspection and assessment.

The general dilemma is how much time to spend (?waste) trying to coax this engine back into a zombie like state of running given that it seems that head removal is inevitable in any case. No firm answer to that I guess: But my inclination that an engine at least fires up and runs (somewhat) is a better prospect for repair than one that won't.

All opinions greatefully recieved!

[LairdScooby]

8 minutes ago, Chris Hawley said:

Spark at plugs is confirmed using a Sparkrite plug tester. But what I oberserve is that rather than one clean, birght, spark every two revolutions it's showing one main spark followed by 3 or 4 faint afterflashes. I don't understand that.

That sonds very much like the dizzy cap is dirty and tracking is occurring - in other words you're getting a spark at the correct time and then as the rotor distributes sparks to the other plugs, dirt is providing a track across the cap to the one plug giving a spark. Have you also verified that when #1 cylinder is at TDC, the rotor is pointing to #1 segment in the cap and that the leads are on the cap in the correct order?

13 minutes ago, Chris Hawley said:

Timing was a 20 degrees ATDC so that's been pulled round to 10 BTDC (roughly, on static adjustment).

A closed points gap could also have caused this, the points may only just be opening just enough to give a spark but way too late - worth checking the gap and then re-checking the static timing too.

[Chris Hawley]

Good pointers, thanks. Will be a few days before I'm working on this one again - but plenty to explore.

Gets me thinking though whether I can even trust the timing marks on the pulley. So better confirm that as well.

[LairdScooby]

Forgot to ask, after soaking the bores in Oil, did the compression on the other 4 cylinders improve?

[Chris Hawley]

56 minutes ago, LairdScooby said:

after soaking the bores in oil, did the compression on the other 4 cylinders improve?

No change really, still returning about 40 - 60.