1980 W126 280SE Crankcase Blowby Problem

[horeysep]

I was experiencing minor blue smoke out the exhaust and excessive blowby into the air filter housing on my otherwise excellent 300K mileage car. Had rings and valve seals replaced. Only honed the cylinder walls, did not bore them out. Used original pistons and valves. Now blow by problem is worse. Have driven almost 2000 miles, and oil is dripping out the bottom of the air filter housing. I'm using a quart about every 700 miles. The car runs fine, and has much better power than before the overhaul.

Should I do a leak down test? Before the overhaul, a leak down test was judged OK, but not really that bad.

I would really like to know how to troubleshoot this problem. I expected the new rings to fix it. Any help would be much appreciated.

[TonyD]

I know this thread is a few years old, but I'd like to know the answer, too. I have a 1981 280SE 126 body.

[eur280te]

I have a 81 280TE 123 body with the same engine and found the crankcase vent was clogged.

G

[TonyD]

Is there another vent besides the black hose that goes from the cam cover to the air cleaner? Most cars I've worked on have some sort of PCV valve that pulls fumes from the engine into the intake manifold.

[ctaylor738]

The early M110's had a vent on the lower left front of the engine which connected to a plastic line that led to a cannister in the trunk. In later blocks, this was replaced by an oil pressure relief valve and the vent hookup was moved up under the AC tensioner bracket.

I am not sure if this is specific to my 280C, or if it carried over into the later engines, but the banjo fitting and tubing are shown in the 280E parts book.

[eur280te]

The hose runs down the side of the engine btwn the air filter and the head. It connects to vaccum in the intake manifold. I removed the top cover and cleaned out the port with a large bore needle. To test for the clog I disconnected the vacuum line while the car was idling and noticed an immediate jump in the rpms. Now the car has a faster idle speed that I need to address.

G

[ctaylor738]

With the top cover removed, did you see a brass fitting in with a large slot in the top? On my 280C, you use a giant flat tip screwdriver to remove it for cleaning.

And BTW you need to remove it to get the cam box off.

The vent and line that I was talking about was in addition to this one.

[eur280te]

I did not find anything in the hole but the hole (if that makes any sense) Now if I could only fix the hole in my head that caused me to by the beast.

G

[ctaylor738]

Might make some sense to check out if there is supposed to be a filter element in your engine. Unrestricted vacuum direct from the intake may not be a good thing.

[horeysep]

The first thing one needs to check on this engine is the small orifice in the bottom of the cam box that connects to the vacuum hoses that connect to the intake manifold. This is easily clogged. I use a small drill bit to make sure it's complete cleaned out. You can clean it, you think, with a small wire only to have it clog again almost immediately if there's some sludge in your cam box. I plug the vacuum hose coming off the bottom of the cam box with a rubber stopper and look to see blowby coming out of the bottom of the cam box before I'm convinced I've got it cleared out.

In my case, this was not the problem. I had paid a reputable shop to do a ring and valve job on my engine, so I thought the rings would have been fine. I checked the compression, and found it roughly equal, albeit low (~110-120 psi) on all cylinders. I thought my gage might not be calibrated, but since the cylinders were within 10 psi that seemed OK. I ultimately diagnosed the problem by checking the level of intake manifold vacuum. I could not exceed 15" of vacuum at idle, no matter what amount of ignition timing advance I dialled in. This is a sign that I did not have good compression. The old rings were actually sealing better than the new ones, I think. All the cylinders had about the same amount of wear in the same place.

I removed the cylinder head and borrowed an ID micrometer from a machine shop. Low and behold, just honing the cylinder walls could not compensate for the 0.008" of wear in the cylinder walls. The maximum wear, as you might expect, was just below TDC where compression is highest.

My car is a grey market car that undoubtably had seen service as a taxi! I searched and found pistons that could handle 0.005" over, but not 0.008". When you look at the head gasket, you can see the bores are much to close together to bore them out more than 0.005". I then searched for another block that could use either my existing stock pistons, or the oversized ones I found.

I finally found a good used long block for $1500 and had that installed intact. Now everything is fine. The original rebuilder, who hadn't checked the bores for wear when they tore the engine down, did the right thing. I agreed to pay an additional $300 for their labor and consumables (oil, gaskets, antifreeze). They paid for the used engine. I'm running Castrol 5W50 100% Synthetic in the used engine for the past 20K miles. It uses about 1 quart in 3000 miles. It runs stronger and no oil in the air filter. I couldn't be more pleased.

I hope you find your vacuum hole clogged up, and not low intake manifold vacuum in neutral. Otherwise, you may be up for another motor.

Good Luck.