Compression Test Disaster - Help!

[Billy Smalls]

Hi All - thought I'd post here too as it's the same engine of course...

I have been having trouble with my B20A for a good few months now.

It feels like a misfire, sort of. It usually happens when I ease off the throttle and then open it up, like coming out of a corner or ascending a climb. That said, it didn’t do it at all this morning on my 15 mile journey?! There is a noticeable lack of power uphill (very hilly round here) though that has only been recent. On the flat it goes along fine.

It was pinking under load recently but I twisted the dizzy 10mm anti-clockwise and adjusted the idle which cured the pinking.

I’ve carried out the following:-
set new points, fitted new condenser, HT leads, dizzy cap, and rotor arm are sound, I assume the coil is okay

Carb taken off, thorough clean, new diaphragm, needles are sound, piston drops nicely, ATF in the dashpot to correct level, float chamber is spotless and tiny filter on the valve is clean, temperature compensator is working fine, new gaskets to the manifold. Idle trim is one a half turns out (though I read that on an old engine, you might as well screw it home?)

Valves are set correctly

Now comes the disaster…..

Did a dry compression test 2 years ago, all plugs out but forgot to wedge the throttle or choke open. Got readings of (1 to 4) 150, 135, 140 & 153

Did a dry compression test 10 months ago, all plugs out but forgot to wedge the throttle or choke open. Got readings of (1 to 4) 145, 130, 130 & 140

Did a dry compression test last night, all plugs out and this time did wedge the throttle and choke open. Got readings of (1 to 4) 150, 150, 125 (but then a tablespoon of oil brought it up to 150) & a disastrous reading of 25 on cyclinder 4. Adding oil to 4 made no difference.

Oh bum!

However, I should point out that all the spark plugs can be described as having a grey brown powdery appearance on the electrodes and the gap is set correctly. The thread and ceramic insulator nose is sooty and dry. This is the same for all plugs. Would a piston fail produce an oily plug?

I am now thinking maybe the misfire is a back-fire of some kind. The inside of the carb did have a very light sooty stain, slightly more on one side of the piston, if that helps???

Is the issue with pot 4 and a misfire connected? Why such a dramatic drop in pressure in a relatively short period of time? I only do about 5000 miles a year, and don’t thrash it – much…

Am I looking at an engine out job? Mega bucks? Etc.?

Any advice folks? Thanks a lot and sorry for the long post

[Derek UK]

25 lbs is a very low figure so something major has happened. My guess would be a that a chunk has been broken off or burned off the exhaust valve. Broken rings unlikely to get numbers that low. Just possible that the piston has a hole or crack caused by the pinking but these pistons are tough.
This is another test, a leak down test, where you pressurise the cylinder with a controlled air line and listen to where the leak is coming from. Both valves have to be closed for this. Leak will either hiss out of the exhaust or out of the engine. If the valve cover is on the leak will be mainly noticeable from the oil filler or dipstick hole.
If it's a badly burned or damaged valve the minimum to fix it is a new valve and a top end gasket set. Not too spendy.

[Billy Smalls]

Originally Posted by Derek UK

25 lbs is a very low figure so something major has happened. My guess would be a that a chunk has been broken off or burned off the exhaust valve. Broken rings unlikely to get numbers that low. Just possible that the piston has a hole or crack caused by the pinking but these pistons are tough.
This is another test, a leak down test, where you pressurise the cylinder with a controlled air line and listen to where the leak is coming from. Both valves have to be closed for this. Leak will either hiss out of the exhaust or out of the engine. If the valve cover is on the leak will be mainly noticeable from the oil filler or dipstick hole.
If it's a badly burned or damaged valve the minimum to fix it is a new valve and a top end gasket set. Not too spendy.

Thanks Derek; and can a new valve and gasket be fitted with the engine in the car? I think it will be slightly beyond my capabilities, so trying to work out man-hours for local garage. Thanks very much, I did suspect a major issue.

could it be that the valve is simply sticking? - I've read some stuff about unsticking with a piece of rope in the plug hole and plusgas / WD40 and a light hammer / slow power drill on the valve? or is that nonsense

thanks

[4v6]

Youll have to remove the head in order to remove the valves, but before you do that carry out a leak down test as suggested by Derek.
If you position the suspect cylinder on tdc with both valves closed and pressure applied to the cylinder via the spark plug hole, youd definitely hear a leak from an intake valve in the intake manifold, especially if the compression tester only gives 25psi, if its leaking from an exhaust valve, youll more than likely hear it coming out the exhaust pipe, put your hand over it to detect pressure rising.
If its the pistion/rings that have gone pull out the dipstick and listen/feel for pressure rising out of it as Derek suggests, itll save you a lot of time and possibly money in the long term.
Im presuming stuffing some rope into the cylinder and winding the engine over slowly onto compression is to cause the piston to push a stuck valve back out, but if its stuck once, itll likely stick again, it may be due to carbon buildup jamming the valve in its guide which would need the head off to rectify.

[Billy Smalls]

Hi – I set the valve clearances last night and whilst doing so measured the gap in between the coils of the springs, when valve is fully open (spring shut) and they were all approximately the same at 1.40mm. So I don’t think the valves in 4 are sticking. The car’s driven every day for the last 4 years, so that’s unlikely. When it was ticking over without the rocker cover all the valves and springs were doing the same thing.

Would a lead-booted blast up the motorway clear any crap out? Or would I run the risk of more serious damage?

Once I did the clearances, the previous reading of 25psi did rise to 50psi – still terrible but thought I’d mention it.

Remind me how you get TDC? And by definition both valves will be closed (springs extended fully) at TDC? I have a compressor, so I suppose I’d have to fabricate a coupling to stick in the plug hole?

Thanks

[arcturus]

Pull out the plug and use a wooden dowel to gauge physically. Pistons 1 and four are both at tdc at same time so use fly wheel marking checking dizy

[Billy Smalls]

Originally Posted by arcturus

Pull out the plug and use a wooden dowel to gauge physically. Pistons 1 and four are both at tdc at same time so use fly wheel marking checking dizy

Thanks - quick lesson for me - so TDC on piston 1 (and therefore piston 4) is when the rotor arm electrode is lined up central to plug number one's HT lead on the dizzy cap?

Thanks

[volvogv]

I had the same problem. At engine overhaul I found the #4 exhaust valve burned down to a knife edge and the pushrod was bent. In my case, the compression was ZERO!

[billysugga]

I had a couple of burnt valves on mine but parts for this ED 3.7litre engine are very hard to get. So I bought a long time ago, a cheap Chinese endoscope
(camera on the end of a wire) and stuffed it up the inlet and exhaust. Mine was that bad I could see damage, so it gave me confidence to take the head off.

But taking the head off yours shouldn't worry you too much.

[Triple-S]

We had very similar to what Derek described above; after far too long putting up with the pinking it suddenly expired in a lay-by whilst cruising sedately.
After head removal I found No 4 piston holed, No 1 nibbled away at the edge and both those cylinder's rings broken therefore damaging the bores.
I really think you'll be better off taking the head off and looking at your innards........ That isn't an engine-out job though: a couple of hours (mainly on exhaust studs....) with some decent sockets.

Paul