Compression test without the intake ,affect the numbers

[chasinthesun]

Have a car Im asking about for a friend ,the engine has no intake and turbo currently,if compression is done without the missing parts will it affect the test?He bought my 300sd and hauled it back to Austin ,I tested the compression before ne came up to get in july,car was running fine ,he know claims its only testing 230ish on some of the cylinders ,he pulled the intake and turbo off and not sure if it makes any difference during test.Could it be something else he pulled off?

[Diseasel300]

Shouldn't make any real difference. Compression test is done on the starter motor and the RPM is low enough that any restrictions in the intake/exhaust shouldn't have any appreciable effect on your numbers.

[t walgamuth]

Originally Posted by Diseasel300

Shouldn't make any real difference. Compression test is done on the starter motor and the RPM is low enough that any restrictions in the intake/exhaust shouldn't have any appreciable effect on your numbers.

Agreed, for sure.

[chasinthesun]

He maybe leaving the heatsheilds in during the test ,anyway thats a big contributer to low test #s .

[NZScott]

The only thing that would make low compression numbers was if the ports were physically blocked by something up against them or some sort of ingestion of a rag or something. Otherwise just standard wear and tear (valve adjustment plays its part as well of course)

[chasinthesun]

The turbo being off has me concerned ,the oil line that runs to it if not looped back or blocked off would pump oil everywhere ,this would be extremely obvious though, if low oil is the case could this show up for the test? My tests were done accurately back in July ,Ive owned over 15 617 cars in the past and I do a test as part of my first few months of ownership ,a christianing of sorts.I would like to find out the tester make also and whats been done since july to the motor.A nice G Wagen is the reciever of this motor and would like to see the outcome to be a happy one.

[Diseasel300]

Since you're cranking on the starter, the oil volume will be pretty low. The oil in the cylinders makes no real difference in the compression results, you're interested in how well the rings and valves fit and thus seal the air charge.

As far as the turbo feed - loop it into a bucket. You're not going to pump all the oil out doing a compression test.

[BillGrissom]

Intake doesn't matter. On a gasoline engine, it is best to open the throttle plate, but I haven't noticed a difference either way. A diesel doesn't have a throttle (ex. some newer ones), plus would be higher without a throttle or intake anyway.

Buyer is probably measuring compression incorrectly, and there are many ways to do it wrong. The adapters must displace exactly the same volume as the parts removed, which is especially important if going in place of the injectors. I use the heat shields, but didn't measure much difference w/ and w/o, using my HF gage. Ask how he is measuring and to give photos, and post on this site so we can critique. It is also important that the Shraeder valve in the tip of his adapter is correct and works. Some people get the bright idea to install a regular tire Shraeder, which will give false low pressure readings since the spring is much too strong.

If the engine was starting easy on 32 F mornings and gave minimal blow-by w/ oil filler cap off, it must have good compression. Why did he remove the intake and turbo?

[funola]

Originally Posted by BillGrissom

Intake doesn't matter. On a gasoline engine, it is best to open the throttle plate, but I haven't noticed a difference either way. A diesel doesn't have a throttle (ex. some newer ones), plus would be higher without a throttle or intake anyway.

Buyer is probably measuring compression incorrectly, and there are many ways to do it wrong. The adapters must displace exactly the same volume as the parts removed, which is especially important if going in place of the injectors. I use the heat shields, but didn't measure much difference w/ and w/o, using my HF gage. Ask how he is measuring and to give photos, and post on this site so we can critique. It is also important that the Shraeder valve in the tip of his adapter is correct and works. Some people get the bright idea to install a regular tire Shraeder, which will give false low pressure readings since the spring is much too strong.

If the engine was starting easy on 32 F mornings and gave minimal blow-by w/ oil filler cap off, it must have good compression. Why did he remove the intake and turbo?

Regarding the Schrader valve, I have never tried a tire valve in place of the one that came with the HF kit but does it really matter?

Tire pressure max is what, 45 psi? Compression in a diesel is 250 to 500 psi. Logically, you'd think a tire Schrader valve in a compression gauge should not cause a low reading but may have problems with high pressures (may not be designed for it).

[t walgamuth]

Originally Posted by chasinthesun

Have a car Im asking about for a friend ,the engine has no intake and turbo currently,if compression is done without the missing parts will it affect the test?He bought my 300sd and hauled it back to Austin ,I tested the compression before ne came up to get in july,car was running fine ,he know claims its only testing 230ish on some of the cylinders ,he pulled the intake and turbo off and not sure if it makes any difference during test.Could it be something else he pulled off?

According to my old chiltons a mb diesel needs 200psi cold to give reliable starting. What's up, is he trying to ding you because its low?