blow-by

junkfixr · Jan 21, 2015

I just got an '88 with a 2.9. The previous owner put World heads on, crank & rod bearings. He said the bearings were .010 over and the cylinders were nicely crosshatched, leading him to believe it was recently rebuilt or a remanufactured block. The compression is good on all cylinders but if you pull the PCV out while it's running, it has excessive blow-by from there and around the dipstick. The only work I've done was to replace the fuel system (tank, pump, filter) and I adjusted the noisy lifters by the book.

Any thoughts on what could cause the blow-by? I'm thinking rings or possibly (but doubtful) a restricted exhaust.

Another issue is a leaky rear main seal. Can that be replaced in the truck or am I looking at pulling this engine?

adsm08 · Jan 21, 2015

I have learned that you can't tell how many miles are on an engine just by looking at the cylinder walls. My truck's original block had +200,000 miles when I pulled it and I could still see the crosshatch.

Some blowby is going to happen, a lot indicates issues with the rings. That's the only thing I know of that causes blowby. What were the compression numbers?

RonD · Jan 21, 2015

You can replace the main bearing seal by removing the transmission and flywheel or flexplate.
On some engines you must also lower the oil pan, not sure on the 2.9l

All engines have blow-by, metal rings against metal cylinder walls don't provide a great seal, just good enough, and yes as rings/wall wear the blow-by will increase.
Best way to test ring/wall wear is a dry then wet compression test or dry/wet leak down test.
Wet being you add a table spoon of oil to the cylinder and then repeat the test, there will always be an increase, the "how much of an increase" tells the story.

2.9l has 9.0:1 compression ratio, so you would expect about 165psi dry, at sea level on compression test.
This is with all spark plugs removed, throttle propped open and a good battery for cranking speed.
If you were getting 140-145 dry and then 165-170 wet I would say you have some ring wear.
If it was 160 dry and 170 wet rings would be normal.

Compression test numbers are variable because of equipment, if you got 130 dry and 140 wet then rings are OK, compression gauge would be in doubt, lol.

Leak down test is when you apply a know pressure to a cylinder and then see how much of that pressure is leaking down(out).
You put the piston at TDC, connect air hose and gauge to spark plug hole, apply known pressure.
Say you applied 100psi and gauge showed 95psi, that is a 5% leak down(newer engine), then you add some oil and spin the crank a few times, put piston back at TDC and redo test.
This time gauge shows 97psi so 3% leak down, rings are fine.
If you got 80psi dry and then 92psi wet the rings are an issue.
But if you got 80psi dry and 85psi wet the rings are OK, valve seats are worn

PCV valve equipped engines are sealed so when engine is running you should have blow-by out the dipstick tube or vent hose or PCV Valve hole if you remove the negative pressure(engine vacuum)
So what you describe doesn't seem unusual when you remove the vacuum pressure(PCV hose).

I would do a toilet paper test

With engine idling remove dip stick and place a piece of toilet paper over opening, it should just sit there maybe even get pulled down a bit.
If pressure is coming out of the hole I would first replace PCV valve and clean it's hose AND the Vent hose.

junkfixr · Jan 21, 2015

I can't speak for compression yet. The previous owner told me that a couple of cylinders were around 130 psi and others were around 160 but I need to check it and see it for myself. Judging from what I'm seeing, if I put paper over the dipstick tube, it would be blown away. There's some fresh oil pushing past the dipstick.

What really, really bothers me is that someone replaced the heads and bearings and didn't go the extra step of doing the rings. It sat in the weather with a faulty fuel pump o-ring and the tank was half full of rusty water when I got it. It's only now after replacing the fuel system that I'm finding all of his shortcuts. By his word, I thought it was a good solid engine. If it sounds too good to be true, it usually is!

adsm08 · Jan 21, 2015

4.0 time.

RonD · Jan 21, 2015

Yes, the 130 to 160psi difference would indicate ring or valve issues, but the fresh oil at the dipstick would lean it more towards rings, assuming PCV(positive crankcase ventilation) system is working.

blow-by

junkfixr

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adsm08

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RonD

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junkfixr

New Member

adsm08

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RonD

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