Blow By

[mack88]

Hi there,i have mack 350 i had new head gaskets put on.I had one back one that was leaking but just to the outside,this was last fall.So i waited until this spring to get that fixed know i now have some blow by. not a lot if i let it idle for say ten minutes it leaves a 3inch spot on the pavement.should i be concerned about this,or what might be the cause of it,or how i might fix it.the heads were fluxed so i asume they should be good.thanks

[HK Trucking]

Hi there,i have mack 350 i had new head gaskets put on.I had one back one that was leaking but just to the outside,this was last fall.So i waited until this spring to get that fixed know i now have some blow by. not a lot if i let it idle for say ten minutes it leaves a 3inch spot on the pavement.should i be concerned about this,or what might be the cause of it,or how i might fix it.the heads were fluxed so i asume they should be good.thanks

Piston ring & cylinder liner wear.

In my experience, Mack engines exhibiting this type of behavior usually need an in frame overhaul very shortly.

If it's actually blowing oil out the breather, that's pretty bad.

Also, bad exhaust valve guides can allow exhaust to leak by into the rocker arm compartments under the valve covers which also will pressurize the crankcase.

[mack88]

Piston ring & cylinder liner wear.

In my experience, Mack engines exhibiting this type of behavior usually need an in frame overhaul very shortly.

If it's actually blowing oil out the breather, that's pretty bad.

Also, bad exhaust valve guides can allow exhaust to leak by into the rocker arm compartments under the valve covers which also will pressurize the crankcase.

the person that did my heads said that it showed very little engine wear so i thought it must mean rings should be good. the truck has close to 500,000 KM. on it.

[Kav]

the person that did my heads said that it showed very little engine wear so i thought it must mean rings should be good. the truck has close to 500,000 KM. on it.

Hi jook24 - Have you ever changed the rocker shafts on the heads. We had trouble with two different motors where there was oil dripping out of the breather pipe even after the motor had a new piston and sleeve kit. We found excessive clearance under the rockers as they had grooves worn into the shafts. When we changed the shafts the oil dripping stopped. We found that worn rings and sleeves usually caused excessive fumes and blow-by down the breather. One motor that had blow-by past the rings as well as the worn rocker shaft was loosing half a gallon of oil down the pipe in a big day. Don't forget that with the Mack sleeves you can still have original honing marks in the bore at those sort of miles and at the same time have stuffed or broken piston rings. Best regards- Michael.

[Kav]

the person that did my heads said that it showed very little engine wear so i thought it must mean rings should be good. the truck has close to 500,000 KM. on it.

Hi jook24 - Have you ever changed the rocker shafts on the heads. We had trouble with two different motors where there was oil dripping out of the breather pipe even after the motor had a new piston and sleeve kit. We found excessive clearance under the rockers as they had grooves worn into the shafts. When we changed the shafts the oil dripping stopped. We found that worn rings and sleeves usually caused excessive fumes and blow-by down the breather. One motor that had blow-by past the rings as well as the worn rocker shaft was loosing half a gallon of oil down the pipe in a big day. Don't forget that with the Mack sleeves you can still have original honing marks in the bore at those sort of miles and at the same time have stuffed or broken piston rings. Best regards- Michael.

[Mike]

Yea, I go along with Herb on this one. I've been there before trying to just fix the heads. That dog won't hunt for long, not with that many miles. Good Luck!!!!!!

Mike

[jerrymac]

Have a compression test done. Should be at least 400 p.s.i or better on a sound E6 and about 500 p.s.i.on an E7 engine. We usually do a quick check first with the manometer. Shouldn't be more than 1" of crankcase pressure under hard pull.

Oil consumption is also a good indicator of engine condition.

As a side note. Although less common, a bad air compressor can leak air past the rings and create pressure in the engine

crankcase, as can a bad turbo that can force boost pressure down through the oil return tube and into the crankcase.

[mack88]

Have a compression test done. Should be at least 400 p.s.i or better on a sound E6 and about 500 p.s.i.on an E7 engine. We usually do a quick check first with the manometer. Shouldn't be more than 1" of crankcase pressure under hard pull.

Oil consumption is also a good indicator of engine condition.

As a side note. Although less common, a bad air compressor can leak air past the rings and create pressure in the engine

crankcase, as can a bad turbo that can force boost pressure down through the oil return tube and into the crankcase.

when i first bought the truck it had quite a few oil leaks. it had not been used very much in the last 4 years,when the other guy had it.so i got most of them fixed. I have run 40 hours since then also did an oil change oil has'nt gone down on the dipstick, before i would be half way down. so i asumed the engine might be okay.

[Mike]

I get 310,685.6 miles on that truck if the engine had not been touched enternally.

Mike

[Noid93]

Check the rubber hose on the breather pipe especially right by the valve cover were it runs horizontaly. I had the section of rubber hose on the breather pipe right by the valve cover collapse and caused the blow by to accelerate through the orfice and made it look like a lot of blow by and scavange oil with it. It also made the crank case pressure a little higher than normal pushing oil and fumes out the filler tube and dip stick. I have saw it on 5 truck with 600,000 mi on it. My truck was breathing heavy out of the dip stick and filler tube also. Replaced the hose for bout 3 bucks. Easy fix....

Just something to check.

Edited June 13, 2008 by Noid93