Subject: Smokin' at the Autocross
Date: Tuesday, 24 Aug 1999
Organization: Doc's Auto Clinic
To: Dave Abbott <dabbott@ix.netcom.com>Dave wrote:
Hi Doc,
I wish my Z was so fast it was just smokin' the competition, but that isn't all of the story. My car is a normally aspirated '91 300ZX with about 76,000 miles. I seem to be smoking up the place when I run the car hard (mostly 2nd gear) on an autocross course. The first time I took the car out, it smoked so badly about half way into the run that I thought something had broken. Certainly no flying insects could have survived in that oil cloud. I was so concerned that I didn't even do the last two runs, and took the car to the shop the next day.
They said that they did a wet and dry compression test and it checked out with all cylinders within about 4 psi. They couldn't duplicate any smoking when they claim to have taken it out and run it hard. There is no apparent smoking problem under normal driving. I changed to a heavier grade oil, 20W50, for the next autocross and while it isn't nearly as bad, it still smokes. It also smokes -- for a short time -- when I first start it back up on the grid. Surprisingly, other folks at the autocross say that "all these normally aspirated Z's smoke when driven hard." I find that hard to believe, but would leave that for your response. It seems to me like it might be valve guides. Do you have any recommendations?
Dave Abbottdabbott@ix.netcom.com(not a ZONC member yet, but if you can fix this???)
Doc replies:
Hi Dave,
Doc here, your description of the situation that you are encountering leads me to look at the valve guide/ valve seal area of the engine. Running the engine at high revs in second gear causes lots of oil in the valve area and a high manifold vacuum situation whereby the engine can draw the oil into the combustion area past the valve seals. Since it clears up and is not present in normal driving, and the compression test proves that the bottom end is OK, the valve seals are the original ones since '91 and there is a possibility that there is a new style silicone type seal available now. I hope that this helps you........
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