View Full Version : Excessive oil vapour blow by on 84 S&S

01-27-2008, 02:31 PM
My 84S&S runs pretty damn good. Its used every week and gives us next to no trouble but its creating a lot of oil vapour out through the oil filler cap which inturn is sucked into the air intake. However I was under the impression it should be sucked in via the PCV at the rear of the rocker cover and the front oil fill vent should be where clean air is drawn into the engine?

Oil pressure is pretty low too (10psi at idle when warm, 25psi at 3000rpm).

It doesn't really seem to burn that much oil and sounds smooth, no knocks or taps.

Is this likely to be blow by past the rings? Is there anything else that could be causing it?

01-27-2008, 03:55 PM
10psi and 25psi are pretty light- can you use another sender to see if your's is just reading low? Also, remove yours and make sure the inlet hole isn't clogged with gunk, Teflon tape or something else.

If you have a puff of air when you remove the large cap on the valve cover, you may have blow-by. If you decide that it needs freshening up, it's an easy motor to work on, if you're so inclined. Parts are really easy to find and shouldn't be terribly expensive, VAT or other charges that may apply notwithstanding.

01-28-2008, 07:30 AM
There's like a constant puff puff puff of oil vapour from the filler cap when its removed. Even at idle its noticeable.

The oil pressure gauge is actually a mechanical one. It might be in accurate as the capillery tube has a few sharp bends in it which may be restricting slow.

Thing is there's no blue smoke from the exhaust (just steam but it is very cold over here at the moment) and no oil actually coming out the cap or collecting in the air filter. Plugs are clean as a whistle too.

I've taken the PCV valve out and it rattles when shaken so that means it should be good right?

Was thinking maybe its worth pulling out and doing one of the rebuild kits (rings, mains, oil pump, gaskets etc). Should be not too long a job and I do have an engine stand. Seen rebuild kits for about $200 so even with shipping, duty and tax to the UK it wouldn't break the bank.

That said it still runs fine so perhaps I'll live with it a bit longer.

01-28-2008, 07:57 AM
I would do a compression and cylinder leakdown test. If you see a puff, it definitely sounds like blow by. What color is the oil after about 10 hours of running? If it's black, it may be a bad head gasket. At this point, I would replace all of the bearings, not just the mains. They're not very expensive and you don't want a rod bearing failure after tearing it down.

If you're going to do a rebuild kit like that, I would check the cylinders for runout/grooving and at least hone them. If they're bad, honing and oversized rings will take care of that. If you want a bit more pep, have the deck and heads milled slightly. Once about .007" is removed, the compression starts to increase and if you don't go too high, it won't need much adjustment to the timing, if any. Making it breathe better will help, too and this would be a good time to think about that.

01-28-2008, 11:46 AM
Cheers. Will borrow a compression tester and check.

Just done an oil change at the weekend. Oil after 50hrs was dark but not black. Pretty much what I'd expect on a 50hrs service.

Having the heads skimmed would be a good option for us though as the engine is due to be converted to propane in the next few weeks (propane is 1/4 the price of gasoline in the UK) and the extra compression would help. (check my pictures for my propane converted 190)

I know a machine shop that would hone the block and skim the heads for not a lot so i reckon a little freshening up is the order of the day.

Oh I'm using 20/50 oil. From what I understand thats a bit thicker than the more usual 15/40 which my newer mercruiser 350 takes. Over here in the UK pretty much all oils are multi weight. 20/50 seems to be used on older motors and 15/40 on newer ones.

The boat is used all year and has no down time so doing a full rebuild isn't that appealing. A quick overhaul that can be done mid week would be much better. At the end of the day its a really old boat which cost us next to nothing so I think we would rather just buy another boat than spend a few weeks without a motor!