smokes startup after cold shutdown

jhuis · 01-19-2009, 06:00 PM

1979 911 sc 98000 mi, smokes lt blue /white on startup after shutdown cold but not after a hot shutdown and cold start even after sitting a month. If I drive it out of my garage cold, shut it down cold, after an hour, on a cold reatart it will often smoke considerably until the engine clears it out. Ideas, thanks, John.

yelcab1 · 01-19-2009, 11:16 PM

Have you considered a leak down test? That will tell you if it is rings, or valve stem seals

livi · 01-20-2009, 12:44 AM

If the smoke clears up after a few minutes driving you are probably not in a rush to a top end rebuild. Most of our old gals smoke a bit on upstart, but it should clear up after a short drive.

T77911S · 01-20-2009, 02:52 AM

Quote:

Originally Posted by yelcab1

Have you considered a leak down test? That will tell you if it is rings, or valve stem seals

i just bought a leak down tester from HF. how does it tell you if the valve stem seals are bad if the valves are closed when doing the test?

Rusty 356 · 01-20-2009, 03:54 AM

Leak down will only tell you if you have a valve that isn't seating properly or in a worse case bent or burned. You could use the leakdown to keep the valves closed and remove the exhaust springs and replace the seals, cheapest way to buy time and reduce the smoke if it is the guides.
Good luck,

kodioneill · kodioneill's Garage

make sure your oil level is correct. you can't believe how many people overfill the the oil tank. when warm (not hot remind you) just at operating temp. the level should just be in between the lines or near the bottom line. oil expands when it gets hot and will rise up to the correct level. some engine oils like cenpeco expand a lot.

jjrowe · 01-20-2009, 08:08 AM

This sounds like some wear on the piston rings but I wouldn't worry too much about it. The rational behind that is your oil pressure is higher when the motor is cold leaving more static pressure in the sump area when the motor shuts off. Oil then slowly leaks past the piston rings when the car sits. A hot motor will not have as much pressure after shutdown (especially if you let it idle for 30 seconds or so before shutting it off to "dry the sump") and so there will be less seepage.

- Jesse
76 911S

yelcab1 · 01-20-2009, 08:22 AM

Quote:

Originally Posted by T77911S

i just bought a leak down tester from HF. how does it tell you if the valve stem seals are bad if the valves are closed when doing the test?

If the leak down test confirms that the cylinder is tight and not leaking past the rings, not leaking past the valves, then the only source of oil left is the valve seals.

jhuis · 01-20-2009, 08:37 AM

Guys, thank you for your ideas. I have been careful to keep the oil level correct but will check it again. I bought the car a year ago and it had been over repaired ($7000) and underdriven, 20,000 miles in 20years. The service records show good compression except 60% on one cylinder, rest 90% plus. So, the advice to check compression and to watch for trouble in at least one cylinder is good. I am determined not to let professional repair gurus start on this thing to no end. I am pretty handy and value what you have offered as being in my range of skills to evaluate and address. This 911 follows my first 356 in 1962, 2nd in 67, 3rd and forth in 1970, 912e in 2005 (still have it, very strong rebuilt big engine) and I like all of the practical help I can get to make this 911 my last great porsche experience. Thanks, John.

T77911S · 01-21-2009, 07:03 AM

i would do another compresion test just for my interest, although it sounds like they may have doen a leak down test, i would do both just to be sure of what the condition of the motor is.
mine smokes on startup too. i dont worry about it as long as it does not keep smoking. what i think happens is when it sits, all the oil drains back into the block. i keep my oil a little low because of this. the oil can then seep past either the rings or the past the exhaust valve stem seals. i have always thought one way to check valve saels is to look for smoke when you let off the gas. the increased vacuum sucks oil in through the seals.
if it runs good, you dont suspect bad/worn valve guides, keep driving and start saving money for a rebuild later.

when my oil is up to the full line, the gage reads full, when i am down to the fill line my gage reads in the red, so i know my gage reads correct. when i start it up, the gage reads in the red, it does not start to show oil in the tank until it is near 180 deg. that is a long time to have extra oil in the case. how much is in there i dont know, i suspect a lot. i have also noticed it uses more oil when i keep it to the full line.

i would not worry about the smoke, but i would look into the low compression, which could be the cause of the smoke