Compression test - bad news

[neil3965]

Hi all

Aircooled 2.0L CU engine.

To cut a long story short - got my T25 last summer, did some routine work to get her roadworthy, set the timing and idle speed etc. and did a compression test back in Feb 2009 - results were:

1 - 8.8 bar
2 - 8.8 bar
3 - 7.8 bar
4 - 9.0 bar

Number 3 a bit low, but seemed within reason. Passed MOT in April, been running sweetly for 1300 miles since then, mostly on two holiday trips. Began to go wrong a month back - was running slightly rough, and cut out on idle when stopped at lights, but would start again.

Checked fuel filter - clogged full of muck. Fitted a new one, but still cutting out on idle. Checked for dirty fuel (no), and water in fuel (none). Checked over ignition system, found that resistor in rotor arm (5400rpm limiting type) had overheated and burned a big hole in itself (no idea why - possibly wrong rotor, with 5000ohm resistor instead of 1000ohm? It was too knackered to check). Coil producing healthy spark. Fitted new Bosch dizzy cap and Bosch HT leads, and new Beru 1000ohm rotor. Plugs (NGK B6ES) were new in Feb.

Still cutting out on idle! So checked for vacuum leaks and, sure enough, the rubber T-piece at the vacuum take-off for the brake servo had worked itself loose. Refitted it snugly. Now the engine would idle!

But it was still rough, seemed to be running on three cylinders only. So pulled all the spark plugs. All looked OK except for number 3. Oh dear...




So did another compression test:

1 - 9.0 bar
2 - 9.5 bar
3 - 2.5 bar (!!!)
4 - 9.5 bar

Repeated number 3 a couple of times after putting a capful of oil in the plughole, and got 3.5, then 4.5 and finally 5.5.

Don't know much about the history of engine, except that it's done at least 125,000 miles. Got these notes from previous owner, who dropped the engine to do some work on it -

"There were signs of red sealant on the main crankcase joints which suggested that someone had been there before. The bottom end was fine and it's more than likely this was new/rebuilt. Starboard rear piston and cylinder was the main problem. Apparently this gets the least cooling air. New cylinder and piston. New rings on all pistons, no wear or scoring showing. A VW guy near Berwick, can't remember his name, did the heads. New exhaust valves, seats and guides as appropriate."

So, apart from learning that when one thing goes wrong on these wonderful old creatures, it's as likely to be three (or more!) things...

What's likely to need doing? I'm assuming from the wet compression test that it's the piston/piston rings/barrel that are the problem rather than leaking valves. If so, is it best to replace all four pistons and barrels as a set? Or just the faulty one?

Anyone done this themselves? How big a job (I know I'll have to drop the engine). Anyone had it done in a garage? How much did it cost?

Any advice at all warmly welcomed!

Many thanks
Neil

[axeman]

Had the same problem on my 1957 1200cc 30hp engine and just putting the whole thing back together along with a new gear box. On the beetle engine the barrols are all indavigial so I pulled it off and and got it rebored .5 mm larger than standard. Installed a new piston and rings to match the rebore. Cleaned the heads and have reground that valves. Gona try and get itall back togeather buy theend of the week. You will need an engine gaskit kit. I used aplace called sutton rebore and I highly recomend them. I amguessing that your engine will be similar to that of mine

Hope this helps neil

[Red Westie]

Not to point out the obvious but have you checked the valve clearances on this cylinder before ripping it apart.
Martin

[T'Onion]

Like Red westy says check the clearances

as for rebuilding it ,Its an air cooled so quite simple really , more than likely to be the rings that are buggered , which you can do with the engine still in ,a lot easier with it out (about 1/2 hour job)

[neil3965]

Thanks for the advice guys - will check the valve clearances and let you know how it goes...

[ghost123uk]

At least it's an Aircooled, sometimes I envy you guys

[VWCamperfan]

I wouldn't let the readings from adding oil in the cylinder lead you to too many conclusions... Here's why... When you add oil to a conventional in-line engine, the oil runs round the whole of the piston and rings, all the way round! When you add it to these engines, being a flat 4 design, the oil stays low in the cylinder and doesn't move any further. The valves have probably seen a fair bit though.

[neil3965]

Umm, forgive my ignorance, but how do you check the valve clearances on an engine with hydraulic lifters?

[ghost123uk]

Umm, forgive my ignorance, but how do you check the valve clearances on an engine with hydraulic lifters?

Not ignorance Neil - Yes it is a good idea to check them.

Details are in the wiki, along with some detailed info on why as well as how

[neil3965]

OK - popped off the rocker covers and had a look at the valve settings (hydraulic lifters). Looked like the adjusting screws on No 3 valves were turned in more than on No 4 - about two turns more.

Unscrewed them - estimated that they had been set to touching plus 3 or 4 turns (who would have done that?).

Reset them to touching plus 1.5 turns. Ran engine for 5 mins, still rough. Let it cool, did another compression test. Now I've got 9.5 bar on cylinders 1, 2 and 4, and number 3 gives - 8.2 bar!

Now I'm baffled. Could an over-preloaded hydraulic lifter result in the valves not closing properly? Weak or faulty lifter? Or is it more likely that the compression tester wasn't seated properly on the first test? But then, why was No 3's spark plug oily? Can oil get into the cylinder through the valves? Worn valve guide?

If I had any hair, I'd be tearing it out!

[ghost123uk]

Now I'm baffled. Could an over-preloaded hydraulic lifter result in the valves not closing properly?

Yes

Weak or faulty lifter?

Unlikely but not impossible

Or is it more likely that the compression tester wasn't seated properly on the first test?

Only you can answer that one really

But then, why was No 3's spark plug oily? Can oil get into the cylinder through the valves?

Yes it can via the inlet valve.

Worn valve guide?

Unlikely that 1 would have worn loads worse than the rest I reckon.

I reckon now you have good(ish) compressions all round, you have fixed it

Swap the plugs around so an OK one is in the cylinder that had the oily plug and take it for a drive, then look at the plugs again but I bet it will be OK now

Don't wind those plugs in too hard, you don't want to strip a thread !!!