12V Compression Low

[cullen]

I am currently looking at a 98 12v, QCLB 4x4 5 speed that a local fella has for sale. He says the compression is between 250-270 across all six. He says his valves are holding the pressure, and he also is telling me the truck has very little to no leak down/blowby. Says they are all holding nearly 100 psi? How is this done, pressurizing through injector port? Considering the low compression,( 250/270 rather than 370/400) he believes the engine was rebuilt at some point,(???) and the "wrong height" pistons were used. It only has 176,000 miles on it, and the truck is clean as can be inside and out, and no rust! Why would the bottom end be rebuilt at such low mileage? Different height pistons? As well, the truck smokes a little bit, and it smokes continuously even after it is well up to operating temp. Whitish gray smoke. He says the P-pump has never been touched, so this may rule out overfuel in regard to the smoking. My first thought is that this truck has bad rings, which is creating the smoke, and killing the compression. Does anybody have any ideas? Thanks!

 

[GAmes]

K&N or other cheap filter? I don't understand the smoke thing. My oil rings were worn out at 800,000 miles and the engine used a qt every 800 miles. Other than when mashing the pedal at low boost and getting a big black cloud I never saw any smoke in the mirror or when idling.

I doubt the engine was rebuilt at that mileage. My understanding is that Cummins specs blowby pressure and doesn't do compression checks. The local Cummins authorized shop charges less than $50 to do a blowby check. It might be worth your while to have a real mechanic take a look at it.

 

[Bob4x4]

It’s a lot more work to do a compression test than blow by.I would br surprised if a compression test or leak down test was performed

 

[GAmes]

It’s a lot more work to do a compression test than blow by.I would br surprised if a compression test or leak down test was performed

I agree. Sounds like the seller is blowing smoke.