Missing compression

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SpringDon

SpringDon

Well-Known Member
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Somerset
Ive got a 12j where the compression seems to come and go and wondered if anyone had any idea what could cause this.
Story so far...
I've got a recon engine from quite a well known company. I couldn't start it and after checking various bits, found it had no compression.
On removing the timing cover, I found that the cam was on its way out of the engine. This turned out to be a missing thrust plate on the cam.
So it was sent back and when I received it again it seemed to have compression. Installed in the vehicle - checked it still had compression.
So tried to start it . Didn't start so checked compression and only found one cylinder with compression.

Since then, I've checked the valve tracts with a pump and found no leaks. I'm 99% sure that the timing is right. My question is whether missing or badly installed rings could do this or a crack somewhere?
Also when cylinders did have compression it was only 300psi and seemed slow to rise. I assumed that was because of new bore and rings. Does the compression rise as the rings bed in?

I plan to take the head off this evening for a look. (Apologies for the essay)
 
My old 2.5 NAD got 280psi on a cylinder with a bent rod.
400 psi each cylinder roughly after I rebuilt it.
Teaspoon of oil down each bore will help tell you if its pistons.
Valves maybe sticking open or bent and jamming?
Bent conroads?
Broke cam?
Cracked pistons?
 
SpringDon said:
Ive got a 12j where the compression seems to come and go and wondered if anyone had any idea what could cause this.
Story so far...
I've got a recon engine from quite a well known company. I couldn't start it and after checking various bits, found it had no compression.
On removing the timing cover, I found that the cam was on its way out of the engine. This turned out to be a missing thrust plate on the cam.
So it was sent back and when I received it again it seemed to have compression. Installed in the vehicle - checked it still had compression.
So tried to start it . Didn't start so checked compression and only found one cylinder with compression.

Since then, I've checked the valve tracts with a pump and found no leaks. I'm 99% sure that the timing is right. My question is whether missing or badly installed rings could do this or a crack somewhere?
Also when cylinders did have compression it was only 300psi and seemed slow to rise. I assumed that was because of new bore and rings. Does the compression rise as the rings bed in?

I plan to take the head off this evening for a look. (Apologies for the essay)
Click to expand...
1 cylinder had compression and the others nothing or ?
 
This is the problem. I've seen compression in all cylinders then next day after putting the crank pulley on, there's compression in only one cylinder. The other don't even register on the gauge.
Suspected the gauge was faulty but if you put your hand next the glow plug hole then crank the engine, you feel strong pulse of air from the cylinder with compression but very little from the others.
Rechecked tappets and checked that there are no leaks when both valves are closed.

Doesn't make any sense to me so just wondered if anyone had heard of such a thing. Guess it's pull the engine apart time.
 
is odd, even with all ring gaps lined up youd get some compression, if valves were leaking that bad youd hear it in manifolds
 
1. Valves must be stuck open for some reason, if the cam timing were out you would have similar values for each cylinder if there were no other damage, if the valves were stuck closed you would still feel the suck and air pulse through the glow plug hole.
2. I assume you are cranking it on the starter rather than hand turning the crank pulley?
3. If the company just forced the cam back in place and fitted the thrust plate then there could be various other valve train issues (bent / dislodged pushrods)
Time to get the valve cover of and check everything is moving correctly, also worth a quick timing check if you haven't done that yet.
 
Thanks for the ideas. I'm pretty sure it's not valves as they all appear to open and close and I can pressurise the valve tracts when the valves is closed.
Timing marks appear spot on and I assume if it was timing the compression would be off the same for each cylinder.
I'm thinking rings but why it would come and go is a mystery.
 
SpringDon said:
Thanks for the ideas. I'm pretty sure it's not valves as they all appear to open and close and I can pressurise the valve tracts when the valves is closed.
Timing marks appear spot on and I assume if it was timing the compression would be off the same for each cylinder.
I'm thinking rings but why it would come and go is a mystery.
Click to expand...
How are you pressurising the valve tracts with the head still fitted?
What mechanism are you using to ensure that it is only the valve tracts you are pressurising?
Intermittent correct operation of the valves and/or valve train would cause inconsistent compression test results. Pressurising a valve tract when the valve should be closed but is in fact open would still cause a pressure reading as the piston would be near or close to the top of it's cycle and therefore give the impression the valve is sealing correctly. You could measure for pressure in the valve tract while pressurising the combustion chamber when the valve should be closed, any increase in pressure shows an incorrectly sealing valve.
Also, rule out a possible variable in the test equipment by having it tested and/or calibrated. At the very least, check it on a known good engine, in the interests of accuracy.
 
Sounds like it is time to contact the company and ask for advice and potential refund / replacement. Doesn't sound like they have done a particularly good rebuild (assuming they actually have rebuilt it) as you would spot the lack of cam thrust plate when you did the cam end float test as it would fail quite dramatically.
 
As Dan says you could still have a leaking valve, if you leave the glow plug out when you pressure check the inlet / exhaust that will prove the valve sealing.
 
Pressure tested by :- removing glow plug , turning crank until both valve then using a vacuum/pressure hand pump and a big rubber bung to pressurise the valve tract. Only 10psi but would that be enough to tell if the valve was stuck open, wouldn't it?

The company have offered to take back the motor and get it started on a dyno. However since it has already been back once, I want this to be the last time I take it out, I'm not sure they believe my description of the problem and I'm not (yet) 100% sure I haven't done something moronic so I'll have a go myself.
I did ask for a refund and a core engine that could be exchanged elsewhere but that's not a goer a the moment.

As for the camshaft end float , I had a pretty big clue after taking off the timing cover and finding the cam sprocket had been merrily machining it's way through the cover. And that was just after cranking tests, God knows what would have happened if it actually ran. So in a way it's lucky it has no compression. Lucky me.
 
Know anyone nearby with a leakback tester? With that you should be able to hear where the air is going (airbox, exhaust, out the oil cap, into the coolant...)
 
That looks exactly what I need to see where the compression is going, ta.
Problem now is that the compressor in the workshop is 80 yards from the engine in the garage.
 
An update. Thanks to mr.4x4dog, I tried a leak down tester and found it was the valves.
Casting sand ,or whatever it's called, from the valve tracts was holding the valves open. This had to be scraped out and the valve seats recut.
There was an oil leak from the back of the motor so I had to take the engine out anyway. The leak was because the sealant was not in the right place.
Stripping the motor there was no gasket on the oil pump and the bore/ piston clearances measured between 0.1mm and 0.07mm, compression ring gaps were all over the maximum.

According to my Haynes manual the max clearance for a 12j is 0.05mm, so does that mean it has to be rebored?
The rings are marked 020 so I guess that means +020 oversize, so do I have to go 040 now?

Think I need to contact the engine company again!!
 
sloppy job ,was it bored after measuring the pistons to be used or just bored to nominal size,,either way its not right,you have to +40 or fit liners then back to std
 
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