What Are Healthy M60B40 Cold Compression Test Numbers?

[jehu]

I was under the impression numbers below 190 indicate low compression like at the end of an engine's healthy life so to speak.

It'll still run even idle smoothly but lack the output of anything like near new or otherwise healthy examples.

If you get numbers like 150 on a COLD comp test is this a healthy engine?

Are the reports I read about number near 200 and above 190 as being considered normal all HOT test?

[moroza]

Yes, those are hot numbers, and compression is invariably specified hot for good reason: cold tests tend to be inconsistent, both with hot tests and with cold tests done at a different time. They're more prone to variation due to ambient conditions (how long it's sat, ambient temp...) as well as more subtle variables, such as chemical and physical properties of the oil (think Lucas oil stabilizer), or what pistons ended up at TDC the last time it was shut down. Cold compression tests can be useful for a specific diagnosis, but to know the general state of an engine's internals, you need to test it at the conditions under which it's designed to operate, that is, at full operating temperature and with minimal time allowed for oil to drain from the galleries. Temperature is the big one; I've seen 10% improvement in compression on cylinders 1 and 6 of a straight-6 just from replacing a stuck-open thermostat. That can sometimes make the difference between acceptable variation between the cylinders, and determining an engine is junk because there's too much difference.

150 on a cold test is nothing to worry about, tells me the engine isn't junk, but nothing more. 150 on a hot test tells me the engine is worn out (if an M60 or similar with 10+:1 compression).

Once worked on a newish Jetta 5-pot that ran perfect, except for a CEL showing a (imperceptible to human senses) misfire, that only happened after sitting a while. Hot numbers 190-210 for all five. Cold numbers after sitting ~3 days something like 180-170-90-190-180. Local VW dealer charged the poor customer $4k+ for work including a new cylinder head; in the stack of paperwork there was no mention of a compression test.

[jehu]

Thanks for the info. Having a hard time getting the mechanic to respond. The seller gave up and he's not t sure what was done to it but thinks he may have left the cold test number written down. He mentioned he thought two were low and I said if they were adjoining cylinders that would suggest a head gasket breach to me where they are closest. Can't do a hot test until the hard fuel lines get replaced.. apparently they were being replaced due to looking rusty and the work was never completed.

Would you be concerned about other wear to a stock M60B40 that had had the head gasket breached? The oil on the dipstick looks clean but may have been changed and until I can talk to the mechanic I'm hesitant. The mechanic is a kind of BMW legend in NH he has a yard with bunches of 2002's

https://www.bmw2002faq.com/forums/to...ipee-nh-today/

http://www.mye28.com/viewtopic.php?p=248295

Yes, those are hot numbers, and compression is invariably specified hot for good reason: cold tests tend to be inconsistent, both with hot tests and with cold tests done at a different time. They're more prone to variation due to ambient conditions (how long it's sat, ambient temp...) as well as more subtle variables, such as chemical and physical properties of the oil (think Lucas oil stabilizer), or what pistons ended up at TDC the last time it was shut down. Cold compression tests can be useful for a specific diagnosis, but to know the general state of an engine's internals, you need to test it at the conditions under which it's designed to operate, that is, at full operating temperature and with minimal time allowed for oil to drain from the galleries. Temperature is the big one; I've seen 10% improvement in compression on cylinders 1 and 6 of a straight-6 just from replacing a stuck-open thermostat. That can sometimes make the difference between acceptable variation between the cylinders, and determining an engine is junk because there's too much difference.

150 on a cold test is nothing to worry about, tells me the engine isn't junk, but nothing more. 150 on a hot test tells me the engine is worn out (if an M60 or similar with 10+:1 compression).

Once worked on a newish Jetta 5-pot that ran perfect, except for a CEL showing a (imperceptible to human senses) misfire, that only happened after sitting a while. Hot numbers 190-210 for all five. Cold numbers after sitting ~3 days something like 180-170-90-190-180. Local VW dealer charged the poor customer $4k+ for work including a new cylinder head; in the stack of paperwork there was no mention of a compression test.

[moroza]

Headgasket failure on an M6x is rather uncommon, so yes, I'd be concerned about how it got that way (overheated?).

Consistency between cylinders is about as important as actual numbers. More so if you don't quite trust your gauge. However, I would take cold numbers with a good heaping of salt; if we're talking 150-140-0-140, well sure, something is not right with #3, but less extreme examples (like the Jetta above) are simply inconclusive. I would use a little less salt in evaluating cold compression test numbers where a little oil was added to the cylinder.

What are the numbers, anyway?

[jehu]

Headgasket failure on an M6x is rather uncommon, so yes, I'd be concerned about how it got that way (overheated?).

Consistency between cylinders is about as important as actual numbers. More so if you don't quite trust your gauge. However, I would take cold numbers with a good heaping of salt; if we're talking 150-140-0-140, well sure, something is not right with #3, but less extreme examples (like the Jetta above) are simply inconclusive. I would use a little less salt in evaluating cold compression test numbers where a little oil was added to the cylinder.

What are the numbers, anyway?

The seller thinks he wrote them down and left them in the car. He's now living in another state but the car is 90 minutes from me. I may go see it this weekend.

I talked with a forum member who owns a repair shop and when I told him this seller states he gave the mechanic $800 to fix the "white smoke" problem which he did that $800 is not likely to have been a head gasket repair job . He'd bill about $2k for that done right all in. So I think if when I examine the fuel lines and see if I think I can manage to get the car to a point we can start it I'll do a hot compression test and examine the plugs and look at the cylinders through the plug bores for any sign of steam cleaning or obvious evidence of the heads looking cleaner than you'd expect from heads that had never been removed.

I want to trust this seller. I've dealt with him several times over the past few years its just he hadn't really paid super close attention to the thing since he bought it as he moved out of state around the same time so I'm just trying to uncover the truth.

[moroza]

Ah, the difference between Trust and Rely.

Depending on how much prep work a customer did and their relationship with the mechanic, $800 could be a headgasket job. I've sometimes charged less, mostly where the car was towed to me largely disassembled, and all I was to do was R&R the head and reset the timing, the "fine touch and expensive failure" stuff.

Rusty fuel line? Bypass it with ~12 feet of 3/8" (if memory serves). Good enough to test.

[jehu]

Got to talk with the mechanic in question a few minutes ago and he said he didn't do a HG on this car.. replaced some Ignition Coils, Injectors and Valve Cover Gaskets and thinks he did do a comp test but it was almost five years ago but he's pretty sure he hadn't found it needed a head gasket. Probably gonna check it again this weekend and see exactly where the fuel line issue stands and maybe bring a hose as you suggest if the tank is otherwise hooked up.
Why would a clutch pedal be all the way to the floor? disconnected slave?

Ah, the difference between Trust and Rely.

Depending on how much prep work a customer did and their relationship with the mechanic, $800 could be a headgasket job. I've sometimes charged less, mostly where the car was towed to me largely disassembled, and all I was to do was R&R the head and reset the timing, the "fine touch and expensive failure" stuff.

Rusty fuel line? Bypass it with ~12 feet of 3/8" (if memory serves). Good enough to test.

[moroza]

Clutch pedal is on the floor because the pressure plate isn't transferring its return force to the pedal. There are a few things in that path that could be leaking, broken, missing, or disconnected.