Compression reading question

[Musicars]

I am looking at a used replacement engine for my '06 Denali. I posted before that my truck lost pressure with less than 170,000 miles and I got the lab report back on the oil which seems to indicate I have a bearing issue. Ideally I have been looking for something from a '05-'06 with less than 150,000. There is a used engine at a yard close to me from a 2003 with 170,00 on the odometer. The engine is supposedly cranking 213 psi which is a lot higher than what I would expect. The lady at the yard even mentioned that the compression does not jive with the mileage so it may not be the original engine (or rebuilt) which means they are not sure of the mileage.

So, does anybody know what the maximum pressure would be on a low mileage 6.0L engine?

Is there a way to visually tell the difference in years on these motors?

 

[Tonyrodz]

I'm really curious about this myself. Have you tried googling the compression specs? How much are they asking for the motor? Since they're not so sure. I found this on another forum. I didn't read all of it, but it might be of some help. https://www.truckforum.org/threads/chevy-6-0-compression.14386/

 

[Musicars]

I'm really curious about this myself. Have you tried googling the compression specs? How much are they asking for the motor? Since they're not so sure. I found this on another forum. I didn't read all of it, but it might be of some help. https://www.truckforum.org/threads/chevy-6-0-compression.14386/

I have tried Google several times. All I get is general numbers on used engines from 150 psi to 185 psi. Have seen others asking similar questions and they get answers like "highest to lowest within 10% of each other". I want an actual number from someone that measured a low mileage engine. I came across one guy that was asking a similar question about an engine that ran 200 psi and he was told it seemed too high. I'm wondering if it is possible for a rebuilt or low mileage engine to have pressures as 213 psi without there being something to be concerned about. It can't be showing 213 psi at 170,000 miles, or can it?

They are asking $1100 plus a $200 core.

 

[iamdub]

Compression results can vary wildly as nobody really knows how accurate their gauges are. That's why everyone's numbers on the internet are all over the place. Well, that and some people don't know how to properly perform the test. Coming from a salvage yard, the gauge probably gets used a lot which means it's probably been knocked around once or a hundred times. On a newly assembled, blueprinted, broken-in, etc. gen 3 engine, the compression should be 180-200 across all cylinders with no more than 10% variance between them. Really, comparing all the cylinders to each other is about all a compression test is really good for. It's not so much WHAT the numbers are as it is HOW EVEN they are. But, to get an idea, a healthy used stock engine should be an honest 160-180 psi. GM's minimal spec is 100-120, but I'd say that's a nearly dead motor.

On a higher-mileage engine, you have to factor in carbon buildup in the combustion chamber and on the piston tops. This can easily raise the compression results a dozen or so psi. So, either that engine has 170K and their gauge is crap or it's fresher than 170K but has a lot of carbon buildup and their gauge may still be crap or it's a low-mileage rebuild with who-knows-what done (heads decked with more than just a cleaning pass?) that raised the compression 10-30 psi above expected spec... read with a possibly crap gauge.

I'd get a decent gauge and stop by a reputable engine shop and compare it against what they use. Surely they'd invest in a quality gauge. If you feel your gauge is accurate enough, bring it to that salvage yard and test the motor yourself. If it's still high, see if they have a borescope so you can look inside the cylinders. Or, add a borescope to your tool shopping list before you go to inspect the motor. You might have $50-$100 invested in some tools that you may rarely use, but they could save you from losing $1,000+ on a junk motor.

 

[Musicars]

Compression results can vary wildly as nobody really knows how accurate their gauges are. That's why everyone's numbers on the internet are all over the place. Well, that and some people don't know how to properly perform the test. Coming from a salvage yard, the gauge probably gets used a lot which means it's probably been knocked around once or a hundred times. On a newly assembled, blueprinted, broken-in, etc. gen 3 engine, the compression should be 180-200 across all cylinders with no more than 10% variance between them. Really, comparing all the cylinders to each other is about all a compression test is really good for. It's not so much WHAT the numbers are as it is HOW EVEN they are. But, to get an idea, a healthy used stock engine should be an honest 160-180 psi. GM's minimal spec is 100-120, but I'd say that's a nearly dead motor.

On a higher-mileage engine, you have to factor in carbon buildup in the combustion chamber and on the piston tops. This can easily raise the compression results a dozen or so psi. So, either that engine has 170K and their gauge is crap or it's fresher than 170K but has a lot of carbon buildup and their gauge may still be crap or it's a low-mileage rebuild with who-knows-what done (heads decked with more than just a cleaning pass?) that raised the compression 10-30 psi above expected spec... read with a possibly crap gauge.

I'd get a decent gauge and stop by a reputable engine shop and compare it against what they use. Surely they'd invest in a quality gauge. If you feel your gauge is accurate enough, bring it to that salvage yard and test the motor yourself. If it's still high, see if they have a borescope so you can look inside the cylinders. Or, add a borescope to your tool shopping list before you go to inspect the motor. You might have $50-$100 invested in some tools that you may rarely use, but they could save you from losing $1,000+ on a junk motor.

Thank you for the info. I went and looked at this engine. The yard guy was a pretty nice guy and tells me that they did have problems with their compression readings about the time they pulled this engine. It looks like an engine with 170,000 miles. I do have an inexpensive scope that works pretty good. The driver's side front plug was out so I started there. The piston must have been near TDC because I immediately saw wet oily grudge. I removed the the third plug on the same side and there was just a bit of dry carbon build up and I could actually see the recess in the piston.

So, I'm going to assume the original readings were wrong. The difference between the cylinder conditions tells me I should probably pass on this one. These engines are usually on kind of a boxed pallet with the starters already removed. Can't really recheck the tests.

There are some low mileage engines out of state I am checking on. The problem with those is I am trusting the info from the yard and apparently not all yards have accurate info as I have now found out. I have come across some low mileage LQ9 motors that have been tempting but I don't NEED the extra power and don't really want to deal with using higher octane gas. But would be fun!

 

[iamdub]

I've bought a few engines from M&M (mmauto.com) out of Fredericksburg, VA. They're quite reputable and, in my experience and others I have spoken to, honest. I was even referred to them by other salvage yards. They know it's too easy to research numbers and such to trace vehicle histories so they have no reason to tarnish their reputation by falsifying info. I'm in Louisiana so I couldn't personally inspect the engines, but they were always happy to send detailed pics as requested.

 

[Z28TWITCH]

$1,100 and a $200 core for a question mark motor with maybe 170K?...what nightmare fuel yards are you walking around in? I would run from a place trying to sell a motor and cant get you an accurate compression test.