Tom's 250 Engine...compression v. leak down

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Steve Meltzer
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Tom's 250 Engine...compression v. leak down

Postby Steve Meltzer » Fri Jan 20, 2017 7:21 pm

First of all, rocker stand nuts, tho' missing, are highly overrated! Seriously, could you have skipped putting in the starter, and, since the harmonic balancer and crank bolt were right there, why wouldn't a leak down test, in lieu of the compression test, been better and faster? I'm trying to understand if one is better, or when which test is indicated, tho' here, the results would have shown the disaster you quickly discerned. I appreciate the schooling. thanx steve
steve
meltzer, "“With a Ferrari, whatever it is, it’s a $1000. If that’s what it is.”"
'66 330 GTC, '72 Daytona-red, did they make any other colors?

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tyang
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Re: Tom's 250 Engine...compression v. leak down

Postby tyang » Sat Jan 21, 2017 10:55 pm

Hi Steve,

I think setting up to do a Leak down versus compression test in this particular case was about the same trouble, and would have netted the same inconclusive results!

None of these two tests are better than the the other, but rather both help in their own specific way to give you an idea of one area of health of the engine. Normally, a compression test will give you a quick idea if the cylinders are holding some kind of compression. If you start here, and you get good strong numbers, you can feel that the valves and pistons are doing their job. If I were performing a Pre-Purchase-Inspection, low numbers would cause me to question if it was worth exploring the inspection any further, or tell the potential buyer to move on. If I wanted to explore the problem further, then a leak down test would be the next step, but takes a little more time. A leak down test will pressurize each cylinder and I can see how much pressure is leaking out of that cylinder. By listening carefully, one will be able to hear if the hissing is coming from the carbs (bad intake valves), the exhaust pipes (bad exhaust valves), or crank case (bad rings). These clues will help make a plan for the repair. Used in tandem, they give a better picture.

These are not end-all tests to determine engine health. None of these tests tell you anything about valve guide wear, bad valve stem seals, or oil pressure.

Since we're on the topic of compression tests, I'll tell you a story about how a compression test and leak down test can be misleading. We had an engine in the shop that had low compression on one cylinder with all the others showing good even compression. When we did a leak down test on all the cylinders the one with the bad compression numbers showed good leak down numbers. Why? When we pulled the oil pan to take a look inside the engine, we found the bad cylinder had a bent rod! The engine had a stuck float in the carburetor, and hydro-locked that cylinder. The bent rod caused the piston to come short of TDC, causing the lower compression numbers, but at TDC, the valves and rings held good air pressure during the leak down test. If we had only done a leak down, we would have never known the engine to have low compression numbers!

Confused?

Tom
'63 330 America #5053

Steve Meltzer
Posts: 773
Joined: Mon Sep 06, 2004 10:31 am
Location: with Barney the Beagle boy and Enzo 8995

Re: Tom's 250 Engine...compression v. leak down

Postby Steve Meltzer » Sun Jan 22, 2017 11:33 am

Thanx so much for the info. Great story....I guess the dynamic nature of a compression test gave info not available in a fixed environ. s
steve
meltzer, "“With a Ferrari, whatever it is, it’s a $1000. If that’s what it is.”"
'66 330 GTC, '72 Daytona-red, did they make any other colors?

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tyang
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Re: Tom's 250 Engine...compression v. leak down

Postby tyang » Mon Jan 23, 2017 2:13 pm

I find diagnosing problems as a mechanic very similar to how a doctor works, only MDs work on one make and model,...or is that two?

Tom
'63 330 America #5053

tim
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Re: Tom's 250 Engine...compression v. leak down

Postby tim » Mon Jan 23, 2017 3:04 pm

Greetings... And the model(s) doctors work on are running at the time...

Tim
1964 330GT 5769 -the big yellow taxi 61&66 Morgan +4's Daimler SP250 and some other dull stuff plus a brand new Mini Cooper S Clubman JCWS

Steve Meltzer
Posts: 773
Joined: Mon Sep 06, 2004 10:31 am
Location: with Barney the Beagle boy and Enzo 8995

Re: Tom's 250 Engine...compression v. leak down

Postby Steve Meltzer » Mon Jan 23, 2017 3:41 pm

Tom....shhhh! Don't tell anyone about the incredible similarities in Dx and treatment. And, in medicine, we're well into fabricating parts and they too, are expensive! Systems that often overlap and confuse, cranky customers/patients, "rules and laws" of science and sometimes, unexpected catastrophic system failures as well.

Tim, you're right, too, except for the pathologists!

s
steve
meltzer, "“With a Ferrari, whatever it is, it’s a $1000. If that’s what it is.”"
'66 330 GTC, '72 Daytona-red, did they make any other colors?


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