Compression testing

[Ahmedsinc 414]

Wanted to give the project M54 one last look over before I start meddling with it last week. Borrowed a reasonable quality German tester from the FiL, set it up and got 210psi from cylinder 1. Great start, or so we thought.

Cylinders 2 - 6 varied between 7 - 39psi. Seemed strange to get such a poo result, so initially wondered if the head had majorly cracked.

Went back to cylinder 1 and this time recorded 63psi Cyls 2- 6 ranged from 19 - 75 lb. Took rocker cover off & cranked engine a few times to observe free movement of cams and oil geysering up from the front of the head.

Took the tester back to the inlaws place, set it up on his old pushrod 4 cylinder engine and came back with 180 psi +/- 6 lb across all four pots, so we're sure the tester isn't faulty.

Is there something I'm missing here? (besides compression??)

Given that the engine has sat in the shed on a stand for 8 months (turned over manually every two weeks), and that it came from a car that took a fairly heavy hit to the front left corner would there be any reason for such an odd result? PS pump and reservoir damaged in crash, radiator, fan and associated hoses toasted but no visual signs of damage to block, head or intake.

We're both scratching our heads over this, got any ideas guys??

[yng_750 247]

Was the throttle fully open during testing? Was the cranking slowing down due to battery drain?

[yng_750 247]

I have also been told to take plugs out. Reinstall them, run the engine for a min then remove again. This apparently clears any carbon that flakes off when removing plugs which can slightly keep valves open. Ive never bothered due to the time it takes but just a thought

[yng_750 247]

And finally i tested a motor with a cracked head. Read 185psi on good cylinders and 30-40 on the bad ones. So a major variance of over 150psi

[trottsky 24]

Squirt a little bit of oil down the spark bores to get the rings wet again?

[Allanw 1071]

It will have pumped up the lifters probably. Cold thick oil at high pressure can push the lifters open, holding a valve open. (Oil pressure is SUPER high on cold oil, and flow is very low - that's why cold atarts are bad for engines).

I can usually make our M54 do it by starting it dead cold, running it for maybe 20 or 30 seconds at idleish speeds (mving innto the garage for example after a wash) then shutting it off.

When I go to start it next time, it'll sometimes sound like there is no compression at all! I always runt the engine for at least a minute now, and give a few revs to get some heat into it - seems to sto it happening, though it'll sometimes start rough. Never happens if it's run longer, or if the engine is already warm when it's done.

[hotwire 352]

^^^^ Yep to above. On that note - I go out to many "flooded" cars on AA call outs. Almost all have occured when a car has been started, backed out of garage & turned off to to wash it, started, returned to garage & turned off. Next day.... no go. An explanation to the member of the reasons for & they usually get the picture.

An absolute pet hate of mine is to start & stop a cold engine.

[Ahmedsinc 414]

Thanks for the input guys!

So bearing in mind that the engine is on a stand, how the hell would I get the lifters back down?

[yng_750 247]

They will retract in time. From dead cold do test again starting at the other end. If you can get your low cylinders showing higher that should give you enough piece of mind

[Ahmedsinc 414]

It will have pumped up the lifters probably. Cold thick oil at high pressure can push the lifters open, holding a valve open. (Oil pressure is SUPER high on cold oil, and flow is very low - that's why cold atarts are bad for engines).

I can usually make our M54 do it by starting it dead cold, running it for maybe 20 or 30 seconds at idleish speeds (mving innto the garage for example after a wash) then shutting it off.

When I go to start it next time, it'll sometimes sound like there is no compression at all! I always runt the engine for at least a minute now, and give a few revs to get some heat into it - seems to sto it happening, though it'll sometimes start rough. Never happens if it's run longer, or if the engine is already warm when it's done.

What you're saying makes sense. Oil would have been about as cold as it could get. Definitely explains the good reading on cyl 1, poor readings on 2-6 then poor on all 6 on second pass. All spark plugs were removed before testing, throttle butterfly wide open.

But wouldn't the springs force the valve back to closed?

They will retract in time. From dead cold do test again starting at the other end. If you can get your low cylinders showing higher that should give you enough piece of mind

Will give me a little more confidence in the engine but I'm probably not going to be happy til I can see all 6 within an acceptable range.

Thanks gents, will have another go this weekend and report back

[Ahmedsinc 414]

Following on from this, is it possible to bend a valves while running a comp test?

Took the vanos off this morning to prep for cam swap and couldn't rotate the crank to reach TDC.

Again, have I done something dumb? Clues as to why would be great

[yng_750 247]

Youbcan only bend a valve if the timing is out. You wont bend it by hand. When rotating an engine with the plugs in you often come up against compression but letting it sit still for a min or so will allow the pressure to ease and can continue. if the plugs are out it should spin freely. Well often still two hands on the pulley to turn. But should be easy with a rathet or power bar. Are you turning the right way? Possible the timing system doesnt like rotating backwards but im not sure on bmws

[Ahmedsinc 414]

Am a little concerned that Allan's explanation of hydraulic pressure holding the lifters and valves open when the oil is cold might be the problem here. Spark plugs were never reinstated after the comp test, so that isn't it. Prior to cold comp testing the engine turned freely with a power bar, am wondering if something may have happened during the testing. Really scratching my head over this one! Nothing appears to be fouling the flywheel end of the crank either

[Allanw 1071]

You could take off the rocker cover, and see what's happening. It may be a valve held open, but I don't know if the hydraulic lifters had that much travel (Though I'd never actually paid attention to the lifters!) Don't try too hard yet

hopefully nobody dropped anything in a sparkplug hole

[Ahmedsinc 414]

You could take off the rocker cover, and see what's happening. It may be a valve held open, but I don't know if the hydraulic lifters had that much travel (Though I'd never actually paid attention to the lifters!) Don't try too hard yet

hopefully nobody dropped anything in a sparkplug hole

Had already taken the rocker cover off to keep an eye on the #1 lobes aligning. It feels like the crank is locked solid, have tried nudging it anti-clockwise just to see if it moves any but no luck at all.

Utterly perplexed!

[yng_750 247]

very weird. is the starter stuck in the engaged position?
when you say you removed the vanos, is the cam chain still connected and turning the cams?

[Ahmedsinc 414]

very weird. is the starter stuck in the engaged position?

when you say you removed the vanos, is the cam chain still connected and turning the cams?

Starter is about 14 feet away in its box on the shelf so it ain't that either

Vanos is off the head, timing chains still on cam sprockets, cams still in place. Have unbolted the plungers from the splined whatsit that mates up with the end of the cam but the whatsits are still in place. Have given them a good wriggle but they didn't seem to want to come out - could this be the problem?I've no reason to remove the head so I'd like to have the crank at TDC before I pull the cams out and start swapping them for the new one.

With rocker cover off the cams aren't budging even a little bit when I'm trying to turn the crank with my breaker bar. Obviously I'm not throwing my considerable weight behind it on the chance something is wrong - even I have to admit that forcing it isn't always the answer!

[Silver Fox 43]

Did you fit a different crank sensor pickup wheel on the front of the crank, or take one of the belt pulleys off?

The clearance for the 6 bolts which hold it on is quite fine and they may be binding on the front cover and stopping the crank from turning, I know this from experience.

[Ahmedsinc 414]

Did you fit a different crank sensor pickup wheel on the front of the crank, or take one of the belt pulleys off?

The clearance for the 6 bolts which hold it on is quite fine and they may be binding on the front cover and stopping the crank from turning, I know this from experience.

I have removed drive belts, water pump, AC pump etc but haven't meddled with the crank pulley. It does feel like it's jammed on something, can't work out what it is though

[Ahmedsinc 414]

Little update, have removed the cams and timing chains today to swap associated bits to their new homes. Still cannot get the crank to move.

Head off time to inspect for a dropped valve? Can't think of any other explanation

[B.M.W Ltd 950]

If you cranked the engine over to do a compression test prior to removing the vanos unit. The problem you now have is either cam timing, which has bent a valve into a locked open position or you have introduced a foreign object into one of the combustion chambers. Unless there is something else that you have done that you have forgotten about or didn't think it was relevant.

[Ahmedsinc 414]

If you cranked the engine over to do a compression test prior to removing the vanos unit. The problem you now have is either cam timing, which has bent a valve into a locked open position or you have introduced a foreign object into one of the combustion chambers. Unless there is something else that you have done that you have forgotten about or didn't think it was relevant.

Thanks for the quick response Glenn.

Is a compression test supposed to be done with the Vanos removed? Probably a dumb question but it'll be good to know for next time!

I've removed the intake and exhaust manifolds, and as far as I can see all 24 valves are fully closed and the valve shafts seem straight to me. Cams are out of the engine.

Obviously my eyes are not as trained as yours but nothing look bent to me so far. Is there any way I can fish for foreign objects in the cylinder bore without removing the head?

Starting to panic a bit now

[will 169]

Compression test should be done with plugs removed, throttle wide open and then cranked via the diagnostic socket so that no power goes to any other circuit, just the starter motor. The vanos should be left undisturbed/no power to it.

Seeing it is on a stand with manifolds and cams removed (so valves should all be closed), a quick way to check if you have bent valves will be to put some paraffin/diesel into each cylinder, replace the spark plugs and then invert the engine. if the valve/s are bent, the liquid will escape rapidly. Dont forget to drain the oil first. Alternatively you can use a leak tester. If this shows bent valves, then off with its head...

However, from your description, I suspect something has gone wrong with the chain/tensioner causing it to lock the crank from turning in either direction. (top end failure will normally not lock the motor solid in either direction unless it is a catastrophic failure, your work on it should not have this result). I would firstly remove the sump and have a look from the bottom. See if the chain is hooked up somehow. (It should be loose as the cams are out).

Let me know what you find.

[Ahmedsinc 414]

Compression test should be done with plugs removed, throttle wide open and then cranked via the diagnostic socket so that no power goes to any other circuit, just the starter motor. The vanos should be left undisturbed/no power to it.

Seeing it is on a stand with manifolds and cams removed (so valves should all be closed), a quick way to check if you have bent valves will be to put some paraffin/diesel into each cylinder, replace the spark plugs and then invert the engine. if the valve/s are bent, the liquid will escape rapidly. Dont forget to drain the oil first. Alternatively you can use a leak tester. If this shows bent valves, then off with its head...

However, from your description, I suspect something has gone wrong with the chain/tensioner causing it to lock the crank from turning in either direction. (top end failure will normally not lock the motor solid in either direction unless it is a catastrophic failure, your work on it should not have this result). I would firstly remove the sump and have a look from the bottom. See if the chain is hooked up somehow. (It should be loose as the cams are out).

Let me know what you find.

Cheers mate.

Comp test was done with battery jumpered straight to the starter with the engine on the shed floor, no way that the vanos solenoid could have been energized.

I have privately wondered what could cause an engine to lock completely; guesses so far are foreign object somehow interfering with timing chain, piston ring/s jamming against cylinder bore/s, oil pump nut unthreading and disengaging sprocket and evil pixies casting magic spells on my dreams of more horsepower.

I suspect some of my theories are more realistic than others.

Bearing in mind this is my first ever attempt at anything more complicated than a rocker cover gasket so I apologize for the relentless (stupid) questions and wild theories.

I'll grab some diesel on my way home tomorrow and have a go at your suggestion. Wish me luck!

[thorburn 121]

Have you removed the ac pulley from the crank? As silver fox said those bolts might need replacing with shorter bolts