Leak down Q

[Stevo]

Yesterday a friend came by with an "80" 240D which was running BAD. Compression test showed +-60 lbs on #3 so we decided to try a "leak down test" to try and determine if it was a valve problem or the 'bottom end". I removed the shrader (sp?) valve and gauge from the compression tester, added a valve to control the air flow. We hooked up the air to #3, adjusted the cam so both valves on that cylinder were closed and opened the air valve, hopping to hear air escaping from the air cleaner or exhaust pipe. But alas, we could hear it coming up through the timing case. When I got the head off the cylinder wall of # 3 was badly scored. Never having done a leak down test before I was wondering about a couple things...The engine rotated a little when we put the air to it and when I got the head off the piston was most of the way down, So my question is ....isnt the test spoz to be done with the piston at TDC to avoid "rotation"? How is TDC found, (Ah, unless you are doing #1 )

[sixto]

Ideally the crank will have a mark exactly 180* off TDC. If it doesn't have such a mark, you can use a cloth tape measure to determine where to make your own mark.

Sixto
93 300SD

[dkveuro]

There's a sweet spot at tdc for each piston.
You need a leak down test gauge set to do the job and record results.

Remember 5 cylinder engines do not fire every 180 of crank so you need to do the test at bdc..which is safer.

Trying to get the piston to stay at tdc can be fustrating and dangerous.
Do not use a wrench or breaker bar on the crank bolt when applying air pressure...use a flywheel lock.

Good engine = 2% to 15%.
Fair engine = 15% to 25%.
Poor engine = Anything over 25%.


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[Stevo]

Originally Posted by dkveuro

There's a sweet spot at tdc for each piston.
You need a leak down test gauge set to do the job and record results.

Remember 5 cylinder engines do not fire every 180 of crank so you need to do the test at bdc..which is safer.

Trying to get the piston to stay at tdc can be fustrating and dangerous.
Do not use a wrench or breaker bar on the crank bolt when applying air pressure...use a flywheel lock.

Ah, OK. So the trick is to be at "exact" TDC and that would be 180 degrees from #1 on my 4 cyl engine for # 3.
How does a 'flywheel lock" work? I would assume the starter motor would need to be removed to "jam" the ring gear? I knew there was more to this or everyone would be doing it

[sixto]

DON'T TRY THIS AT HOME ALERT - saner minds say there is more force at work than I imply! Please read the rest of this thread.

There's not a lot of force trying to turn the engine. I'll bet if you wedge something between the belt and power steering pulley (both sides), that'll keep the engine from turning.

Sixto
93 300SD

[OMEGAMAN]

There is a lot of pressure trying to turn that engine. Lets use a 4 inch bore with 50 pounds air pressure. Thats 628 pounds of force.

[sixto]

DON'T TRY THIS AT HOME ALERT - saner minds say there is more force at work than I imply! Please read the rest of this thread.

Much less than that if the compression test shows only 60psi

Seriously, whatever the force is imparted by 50psi, most of it is compressing the connecting rod which will stand up to it all day (this isn't a 603.97, after all). Very little of it is pushing the piston sideways to get the crank to turn. With the injectors out it takes very little effort to turn the engine, but likewise it takes very little effort to keep the crank from turning. You can hold a belt in your hand to keep the crank from turning. Either that or I've only done leakdown tests on very sick engines :/

Sixto
93 300SD

[Stevo]

Originally Posted by sixto

There's not a lot of force trying to turn the engine. I'll bet if you wedge something between the belt and power steering pulley (both sides), that'll keep the engine from turning.

Sixto
93 300SD

I would think you are correct. Even jamming the ring gear wouldn't keep it from turning the other way. Also in a case like mine where the compression was REAL low and the compressor was cranked down to, say 50#, I wouldn't think holding the engine from turning would be that difficult. Keeping in mind however that large marine diesels (Atlas, Enterprise) are started with "air"

[dkveuro]

If testing an engine with a leak down set...NEVER try to hold the engine stationary with anything other than a flywheel lock.

Using your hands on the belt will tear your skin off or worse if the engine is just a little off tdc.
Using a wedge under a belt will not stop it turning either, nor would putting it in gear if it has a manual transmission.

When the gauge is set up you'll be applying 90 to 100 psi to the cylinder if it has 2% or so % of leak down.

Even as stated, a pressure of 50 psi...which is 50% leak down ( Engine would not run at this kind of leak.) will generate an angular force to the crank that would be equal to cranking the starter.

At 100psi, there is an adverage total pressure on the piston at tdc equal to 1000 lbs.

The way I get tdc is, to balance the engine where it seems to like tdc and apply air...if the egine rotates, reset and try again...usually you'll hit the tdc mark for that piston in a couple of tries.

Having read and recorded tdc leak, test at bdc and see if there is a better reading...if so, this indicates tappered bore or wear at tdc allowing the ring set to increase it's end gaps......the difference is the total amount of wear.
The difference for a good engine would be 2% to 8% with 12% as a maximum.

All used engines I get to install are first tested this way to make sure I have an engine that is healthy.
Using the TDC O/T mark on a 240 will get is right most times.
Practice a little...it is suprising how it gets easy to do.

What ever way you work on your vehicle...stay safe.
Which means having jack stands to support it, not just a hydrulic jack, fire extiguisher....within reach ( even if your under the vehicle.) tell someone where you are, wear face protection when grinding/welding/cutting/heating or hammering . NEVER use an incandesent drop light.


PS.........For those of you who think you could 'hold' your engine at tdc, try this = apply shop air to cylinder at bdc....with a crank bolt socket and long breaker bar, try to turn the engine back to tdc.....let me know how you do !

ATTENTION : For the above test of strength....Make absolutely sure the cylinder is at BDC......you'll not get a second chance.


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[Stevo]

dkveuro

Thanks much for the great explanation. I think you have answered my question and then some, thanks again
I will try it on an engine on the stand

[OMEGAMAN]

Originally Posted by dkveuro

PS.........For those of you who think you could 'hold' your engine at tdc, try this = apply shop air to cylinder at bdc....with a crank bolt socket and long breaker bar, try to turn the engine back to tdc.....let me know how you do ! [/COLOR]

ATTENTION : For the above test of strength....Make absolutely sure the cylinder is at BDC......you'll not get a second chance.


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Aymen!

[sixto]

Thanks for correcting any bad information I posted.

However, during more than one leakdown test I noticed that the crank pulley would start to turn as air was applied and I prevented it from turning furter by holding a fan belt with my hand. Faulty engines? Faulty shop compressor? Seems like it was more than a guardian angel at work.

I don't expect to be able turn the crank against compression but I don't recall the crank pulley turning at starter speed or anything greater as the air filled the cylinder with the piston beyond TDC.

Again, I don't want anyone to get hurt so heed the advice of those who know (vs me).

Sixto
93 300SD

[dkveuro]

Originally Posted by sixto

Thanks for correcting any bad information I posted.

It's not that it's bad info' just miss-informed. Let's hope nobody gets hurt by thinking there's little to be held back.

Originally Posted by sixto

However, during more than one leakdown test I noticed that the crank pulley would start to turn as air was applied and I prevented it from turning furter by holding a fan belt with my hand. Faulty engines? Faulty shop compressor? Seems like it was more than a guardian angel at work.

It is possible that a valve began to open as the engine started to rotate.
It is also possible that there was not a lot of air volumne too, although I'd rather not test that theory .

Most leak down gauges require 100psi calibration pressure and the secondary air gauge will show a build up to the primary gauge setting if there's little leakage...ie; less than 5%.

Originally Posted by sixto

I don't expect to be able turn the crank against compression but I don't recall the crank pulley turning at starter speed or anything greater as the air filled the cylinder with the piston beyond TDC.

The crank will not initially turn at starter speed...but...if the other pistons are unloaded, ie: injectors out or spark plugs removed, then as it turns past 20 degrees atdc or btdc, depending where the lack of tdc is pre or post tdc, it will accelerate very quickly to bdc.

Originally Posted by sixto

Again, I don't want anyone to get hurt so heed the advice of those who know (vs me).
Sixto
93 300SD

Indeed, your comments are noted.....having been doing this kind of thing for 40 years now, I have learned to be safe.
I would always advise that mechanical locks or careful attention to detail is viewed at a distance, as body parts are not readily replaced.

I'm very glad to be able to offer advise rather than hoping everything will turn out okay.


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