dogleg

the mrs and i just got back from a weekend away in the 928 81 euro 5 speed,300 mile round trip great weather,nice hotel, great car.only issue i had on the trip was i noticed blue smoke from the exhaust in the mirror ONLY when coasting down steep hills 4th gear foot off the gas, usually after about 1/4 of a mile of this blue smoke would start to appear in the mirrors.i have just checked the oil level again on returning home and level is half way between the two marks ie top and bottom which doesn't seem too bad consumption wise.i've read our model will use a little.my gut feeling is this is a crankcase pressure problem and that i have a breather blocked somewhere!the car never smokes on hard acceleration even revving it to 6k through the gears.has anyone ever experienced this?any ideas?

hacker-pschorr

One guess is valve guides / seals.

Valve guides / seals are a wear item on all cars, as they wear oil will make it's way into the combustion chamber. Under decel the high vacuum is sucking oil past the guides. Check your spark plugs, good chance they are oil fouled.

The valve guides / seals were shot in my 81 by 80,000 miles, my 79 much sooner (it has been a track car most of it's life). The seals in my 81 were so bad, if I parked the car for a few weeks (winter storage) so much oil would drip into the combustion chamber you would swear it was in fire it was smoking so bad. My spark plugs looked like they were dipped in margarita salt.

The early 16V cars have a vacuum limiter (little space ship looking device) on the side of the throttle body. When these fail, it will allow excessive vacuum that can make this problem much worse.

What is the mileage on your car?

dogleg

hadn't thought of valve stem seals,but in my experience when they are worn they usually allow oil down into the cylinder overnight or after engine idling for long periods then when the trottle is opened you get a short burst of blue smoke slowly clearing away.i never get either smoke on start up or after idling for long periods.but i will check that valve,and maybe your right on this type of engine but i never seen it occur like this on ordinary cars!mileage is around 200k i think,odometer not adding miles in my time with it.but car is fresh no rattles seems to have been cared for over the years.

hacker-pschorr

Daily driving my 81 was smoke free, only decel & long periods at rest did enough oil to pool up to produce any smoke.

If the plugs look free and clear, I would search elsewhere.

I have six sets of 928 heads on my basement, mileage unknown and two of them EuroS, one 90 S4. All of them have valve guides beyond the allowable tolerances.

My 944S (almost identical head to the S4) needed guides at 80k. No signs of smoke from that car, the head was off to repair a broken cam chain (no valves were hurt).

dogleg

cheers hacker,only fitted new plugs in last month and none oiled up at that point.still have the old ones,haven't binned them yet.also this car was in storage and not started for 6 months prior to me putting it back on the road in march,and no smoke on start up even then,also started on the first spin of the key!i,m hoping its not seals but will keep an open mind,any more ideas on this anyone?????????

Trip..

Rule of thumb for 4 stroke motors.

If oil burns on start-up, the valve seals are bad.
If oil burns on de-accel, the guides are shot, will need to be knurled and reamed.
If oil burns constantly, the rings are shot.

dogleg

hadn't heard that one before,but its looking increasing like seals/guides no chance this could be blocked breather (crankcase pressure) causing oil to be forced in under high vac

robot808

dogleg, i had a blocked breather problem last year on the way to SITM. It expressed itself by blowing oil through the gasket and on to the manifold creating a smoke screen that would have made James Bond jealous. It happened when I was on the gas though, not off it.

IcemanG17

I agree with Hacker....sounds like valve seals-guides to me.... An easy check is the plugs of pop the intake tube and look into the port.....if a guide is leaking you will KNOW instantly....it should be clean, but if one or two have lots of carbon...you found it!

dogleg

will try and check this but assuming its not a piston ring problem im expecting the intakes to look oily as its the only other way in for the oil to get in!maybe i could run it for a while with the breather pipe recycling pipe unplugged from the air intake manifold just vent it to atmosphere see if the problem disappears ?

Lizard928

Actually in my 86 engine one cylinder had really warn guides, to the point when I was at the track I would avoid engine braking while braking as I was creating a smoke screen. Yet all the intake ports were crystal clean. Found the offending guides on the teardown.

Landseer

Can you give more detail about the blocked breather?
Where / what exactly blocked?
Thanks

robot808

Chris, it was pretty embarassing I must say. I dropped in a Euro TB in my car. When you do that, you have to figure out a way to get the vacuum hose to the U of the throttle body. Well, as you may have read before, I am crazy stoopid, and did not see that the hose that I was running there had kinked...

Trip..

It's possible, however doubtful. Try loosening the oil cap or just remove it and make the same drive if possible. See if the symptom persists. If the problem goes away, then you found your problem, if not... Well. Keep digging I guess.

I saw something similar to this happen on an old 80's Corvette because the owner filled it with 0W20 oil, the oil was too thin for his old motor. I'm not comparing the two vehicles, I'm just relaying a past experience.

I know old oil will burn even in a tight motor when it's past it's change. Royal Purple is one that comes to mind. Everytime I've used it, it starts to burn in about 3-4k.

Those "High Mileage" oils are the worst, Everyone I know that's used them have developed bad seals (smoke on start-up) I would dare to guess 20 people or so. I know this may rock the boat, but I don't know of anyone that's had good luck with any of them.

I like good ole Castrol, I've been using it since I was 16. My old Beater Blazer has 255K on her and it doesn't burn or leak a drop. Just my opinion of course.

When you get it figured out, let us know.

How many miles on your car?

Dave928S

Valve guides in a 928 are relatively short, and when they wear they allow wobble of the valve as the bore of the guide wears more at each end. My guides (82 Euro) were within factory wear limits at the defined valve lift, but I regarded that wear as excessive with 'waisting' of the guide bore, and fitted new guides and stem seals. Prior to this I was getting similar minor oil burn on the overrun (my motor had 230,00km on it). Some discussion on a thread I started here confirmed my thinking on what should be regarded as the wear limit (less than WSM).

At the mileage you have your guides are likely to have similar wear, combined with hard stem seals.