OM603 intermittent blow-by - PeachParts Mercedes-Benz Forum

BenzTurbo · #1 04-13-2015, 04:44 AM

i have been very confused by my OM603's blow by behavior in my 1987 300td. i have done a lot of work to it including a fresh #22 head. it currently has 214,000 miles, gets good fuel mileage, and uses less than a quart of oil in 4000 miles. i always am peeking around under the hood and sometimes i notice there is blow by coming out of the small vent hole on the valve cover. if i open the oil fill cap, there is a good bit coming out and lots coming out of the hose to the intake if i remove it. i do not think there is anything wrong with the ventilation system in the cam cover because the hose to the intake is doing it's job. it just can't handle any more volume when it decides to do this. if i rev to about 2000 rpm, it almost goes completely away. other times, there is hardly any. this could be after a long drive or idling for an hour. completely random! i may be dreaming but when it has a lot of blow by, it seems to have slightly more shaky idle. cold starting it wont give me any blow by the times i have checked if it matters. could this be an issue with a ring? all cylinders looked very nice for a 200,000+ mile engine when i did the head. honing still present. i just don't understand what could cause such an issue to be intermittent? all 603's i have seen have no blow by so this is odd when i pop my hood and it's doing this. i have been on many long highway drives and get's to stretch it's legs almost every day on the highway.

on a side note- when i got the car a few months ago with 200k, it was in sad shape. #14 cracked head, blow by, awful injector nozzles, list goes on. i took it out of town to meet up with friends and we removed my OX- CAT. didn't know if it was the drive that cleared it out of removing that badly clogged cat but i think nearly all blow by was gone until it got the new head and all other service while it was down.

ah-kay · #2 04-13-2015, 05:38 PM

It is always bad news when you see blow-by or smoke coming from left back vent hole. Don't block it or it will blow the crank seals. Did you check the compression numbers? Shaky idle also means low compression on one or more cylinder. The cross hatch may be there but compression needs more than that. My bet is on stuck or broken rings.

BenzTurbo · #3 04-13-2015, 06:07 PM

Quote:

Originally Posted by ah-kay

It is always bad news when you see blow-by or smoke coming from left back vent hole. Don't block it or it will blow the crank seals. Did you check the compression numbers? Shaky idle also means low compression on one or more cylinder. The cross hatch may be there but compression needs more than that. My bet is on stuck or broken rings.

It is definitely noticeable when it's coming out of the vent hole but not an extreme amount I don't think. I'm definitely not going to block it. I have a metal om617 oil cap on it and it is able to push a little past the o ring when I loosen it but doesn't want to fly off. So a ring is definitely broken or damaged?

BenzTurbo · #4 04-13-2015, 11:38 PM

i'm sorry i missed your question. i did not do a compression check. most of the time the idle is perfect and there isn't much blowby at all. the idle thing may be in my head but i think the 2 issues are connected. when the idle is does that, it isn't bad. but you can pop the hood and see it slightly rocking. any tricks i can try if the ring is stuck without tearing it apart? if a ring were broken, wouldn't there be this blowby all the time? maybe marvel mystery oil sitting in cylinders? but i drive this car daily and can't really have it down for days at the moment. my w123 is down for now.

ah-kay · #5 04-14-2015, 12:41 AM

Mystery marvelous oil would not help much except very light coking round the ring. If you are comfortable with the slight uneven idle then just drive it until something serious happens. Check compression to give you a better idea. There is nothing much you can do except pushing the pistons out to do a visual and replace the rings. It is a lot of work. I think driving like this would not do much damage. Good luck.

BenzTurbo · #6 04-14-2015, 01:35 AM

Quote:

Originally Posted by ah-kay

Mystery marvelous oil would not help much except very light coking round the ring. If you are comfortable with the slight uneven idle then just drive it until something serious happens. Check compression to give you a better idea. There is nothing much you can do except pushing the pistons out to do a visual and replace the rings. It is a lot of work. I think driving like this would not do much damage. Good luck.

The blowby kinda makes the engine look like a pig when it happens but that's the only real concerning thing. Will continue to diesel on with regular maintenance and upgrades!

I'm going to try and block off vac pump line next time it happens just for grins.