danalex

Had the VP replaced today (113K mi)... Mechanic noted on receipt "extensive blow by." When I ask him about it, he explained it as (my words) "...excess fuel gases from the piston pumping...the gases are expelled out the front-side of the engine into the "bottle". He said since I have about 113k miles, I shouldn't worry about it for another 100-150K miles; it's just a common thing for Cummins.

True?False?

WestTN

Blowby is leakage of cumbustion gasses past the piston rings into the grankcase. They exit via the crankcase vent tube.

torquefan

The first thing I would do is to discard the bottle and extend the breather tube so it vents well below the fan shroud area. The engines that had the breather attached to the gear cover were very bad for coating the front of the engine and the radiator fins with a greasy mess that caused overheating. The closer to the road that you can get the end of the tube, the better, as far as keeping your engine and undercarriage clean.

I have seen a couple of trucks with blow by so severe that there was oil dripping off the rear bumper, and everything else under the truck. Hopefully yours is nowhere near that bad.

Polaraco

Blow by is just what they said.

All diesels have blowby. That's why the oil turns black so fast. It's pretty hard to keep 500 PSI trapped in a cylinder with just 3 steel rings against steel. Their not that precise

It is possible the service tech is on some sort of incentive program to drive you into the showroom to buy a new truck.

If the oil bottle isn't over flowing between oil changes, and you're not adding any oil, I would disregard what he says.

My .02

VNV

Tate

A vacuum leak will also show up as blow-by. The vacuum pump exhausts into the timing case, so any excess volume comes out the breather tube.

patdaly

Don't forget, what is excessive blow by in a gasser is normal in our stuff.

Guess which type that service tech sees more of?

I wouldn't give it a second thought until she starts using oil.

swordfish

X2^^^
I noticed on my truck when I had my oil cap off when running (for testing purposes) I had blow by coming out the oil fill hole. It concerned me as well, then I read on here about vacuum leaks. Check all your connections for dry rot. Even if there isn't a "hole" the cracks can cause a leak. I replaced the connections that were cracked and I no longer see anything out the hole or relocated breather tube.

My .05 (adjusted for inflation)

swordfish

Polaraco

I forgot about that. It's been several years since I had a 5.9. Good call

danalex

Wasn't at a dealership, and he didn't entice me to have it fixed immediately. Thanks for all the replies!

Polaraco

OK. . . If none of the other conditions apply, then forget about it.

Sorry bout that

Diesel Dave2

As per your orginal post...My answer is True.

totalloser

I'll add that ALL internal combustion engines have blowby. Diesels blacken the oil because of the combustion byproducts. Tolerances are EXTREMELY tight. Unmeasurable for ring to bore, the only way you can measure them is by checking ring end gap, which many turbodiesels are full circle on the top.

The gasses leak past the gap in one ring, then around the piston to the gap in the next, then around again to sneak past the oil wiper ring. Combustion pressures are extremely high, preinjected might be 500, but after injection much much higher.

Sometimes if you overheat an engine, the pistons expand and force the rings into the grooves against carbon buildup causing them to stick. A solvent based "motor flush" will free them up, but failure to adhere to the instructions can cost you an engine (don't load the engine with that stuff in there).

I do a motor flush at about 100k on gas engines, and less for my 5.9. Gas engines the little oil holes in the pistons that go to the oil wiper rings get plugged up in less than 100k, which I don't like, but on the diesels there is another very good reason to have solvent run through the piston once and a while. The pistons are oil cooled, and buildup restricts the flow to the cooling jets which keep your piston from melting. High performance mods I'm sure cause heat that breaks down oil and causes buildup in these ports, so by that logic a cleanout once and awhile seems a good idea.

After an overheat I've seen a motor flush bring back the compression and stop oil burning. Sometimes it comes back and after a teardown the verdict is usually that the piston rings got too hot and annealed. This means basically that the "spring" in them that keeps them properly contacting the cylinder walls is gone. These rings you can slip them into the bores without a ring compressor due to them taking the shape of the bore when annealed.

Probably more than anyone would want to read, but words are cheap and I type fast.

Beefdoc

I have a fair bit of blowby on my 01 with 410K on it (I do have oil spots under the truck, from front to back). I agree with throwing away the bottle and extending the blowby hose, however if you drive in snow be sure to check the hose frequently when bucking a lot of snow. I have plugged mine twice and that usually means it pukes out a lot of oil either out of the front or rear seal. Both times I lost 4-5 quarts of oil, and neither time was I sure when it shot it out. My two bits. Matt

J BODY

130K on my 2001 5.9. I run 7k between oil changes and you can still read the hash marks on the dipstick through the gold colored oil. Truck has pretty much been used to tow, or long highway driving cycles. Lots to be said for proper break in procedure and making sure your air filter is sealed properly and clean. Stock air filtration on my 01 fyi and nothing but mopar filters.

Tallguy67

I would like to offer an alternate explanation for blow by. Blow by is what happens when a little weenie in a rice burner with those annoying rat trap mufflers messes with one of our CTD trucks thinking they can out-race us. We hit the pedal, pass them on the left like they are sitting still in a cloud of nice black soot and they wonder what just happened. That, my son, is blow by. (I tried to type that with my best Larry the Cable Guy voice).