Leak down Q - PeachParts Mercedes-Benz Forum

Stevo · #1 08-31-2006, 04:00 PM

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 · #2 09-01-2006, 04:20 PM

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 · #3 09-01-2006, 04:42 PM

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%.

.

Stevo · #4 09-02-2006, 12:59 AM

Quote:

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 · #5 09-02-2006, 01:03 AM

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 · #6 09-02-2006, 01:31 AM

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.

Stevo · #7 09-02-2006, 10:08 AM

Quote:

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"