How much blow by is acceptable?

[zacky6661]

So the 86 I just bought, as I discovered yesterday, has quite a bit of blow by at an idle. I take the oil fill cap off and boom, lot of smoke. Rev the engine, it goes almost completely away. At an idle, the blow by is so bad its coming out of my oil dipstick tube, with the dipstick fully in....

YES I have used the search, but could find no results even related to what I'm asking. I'll get a video or picture up soon, but my question is, how much blow by until an engine is considered "unhealthy" , and will a good oil change and CDR replacement at least make this not as extreme, or are my rings too far gone? I notice the engine is REALLY clanky when it first starts, but according to the PO its been a LONG time since its last change.

 

[dsltech83]

The way to properly measure blowby is with a ford special tool, an adapter that goes on the filler neck, you attatch an inces of water gauge reading to it and if you have more than 6 in h20(IIRC) you do a compression check. If you dont have access to that equipment the most easily accessable is a compresssion gauge for most people. The smoke going away off idle means the CDR is working. Oil change never hurts, over 3000 mi on an oil change in mine and oil consumption goes way up.

 

[Agnem]

Blow-by is pretty subjective. If you run a road draft tube, then you may have an idea of what is "normal" The 6.9 I pulled out of the Moosestang consumed 2 quarts of oil every 500 miles, and would shroud the truck in blow-by at a stop light on a hot summer day. The Moose Truck on the other hand, consumes no oil and you only see it now and then at a light. Consequently, my belief is that blow-by and oil consumption are bedfellows. I'd be surprised if anyone had low oil consumption, but a lot of blow-by. I'd let your oil usage be your guide as to what is "acceptable".

 

[PwrSmoke]

I'm curious about the blowby measurement. I can't find it in my manuals, it's supposed to be in section 29, but I don't have the Emissions Diagnosis manual so it must be in there. Could someone who has that manual explain the test? I have a way to measure inches of water but I don't know if you are supposed to have the CDR connected or what.. plus what the spec H2O spec is.

 

[zacky6661]

Blow-by is pretty subjective. If you run a road draft tube, then you may have an idea of what is "normal" The 6.9 I pulled out of the Moosestang consumed 2 quarts of oil every 500 miles, and would shroud the truck in blow-by at a stop light on a hot summer day. The Moose Truck on the other hand, consumes no oil and you only see it now and then at a light. Consequently, my belief is that blow-by and oil consumption are bedfellows. I'd be surprised if anyone had low oil consumption, but a lot of blow-by. I'd let your oil usage be your guide as to what is "acceptable".

Thanks, I'm gonna change the oil soon within a couple weeks, I'm cancelling my insurance tomorrow so it will be just parked, not gonna harm it. We will see the results after the change.

 

[OLDBULL8]

This will BLOW, pun intended, your mind trying to read blowby. A compression check of each cylinder would tell you more than a crankcase pressure test at idle speed (650 RPM), you would have to measure a new engine with very little blowby, then compare the two. But what good that would do is anybodies guess.

1. measuring blow-by/crankcase pressure you can use the basic principle of Bernoulli equation,
where this is applicated to a manometer system.
P1 + h1 = P2 + h2...........(where the gravity & density of the water is be ignored
because the value of gravity & density in the equation is same)
2. the pressure of blowby which we measure is the SAME with the differential pressure between
P1 and P2
then,
P1 - P2 = h2 - h1 ......(h2-h1 = delta h/difference of height)
3. thus, the higher the pressure difference between P1 and P2, the difference in height
(h2 - h1) will be greater.
4. so, the pressure of blow-by is not indicated by how much water movement, but is shown
by how big the difference in height on the pipe surface water manometer.