At about 190,000 miles, I started experiencing the typical puff of smoke on the first start of the day. I ignored it for about 10,000 miles, but then I started to experience severe preignition (a/k/a valve ping), under even normal acceleration--usually after the engine was up to operating temperature and heat soaked.

I was running 91 octane, which I purchased from a number of different gas stations, to make sure it wasn't just a bad supplier of gas. I tried adding two bottles of octane boost to my tank with no improvement. Then I thought maybe it was a clogged injector, so I started running two quarts of Chevron Techron per tank and using only Chevron 91 gas.

I had to be very careful to back off the throttle and not load the engine up to where the preignition could be heard during this period. After about the third or fourth tank, the preignition completely stopped. Not only that, there is no white smoke anymore on the first start of the day.

Since then, I noticed that if I skip using one quart of Techron with every fillup for more than a few tanks, the symptoms return. This has been this way now for 45,000 miles.

I presume that the initial problem came from oil leaking through the intake valve seals down the intake valves and collecting on the tops of the cylinders when the car was parked at night. The oil eventually accumulated and "coked" up, creating a separate, glowing source of (pre) ignition when it became hot. That's my theory, anyway.

I can understand how the Techron could clean the injectors and the tops of the cylinders of carbon to stop the preignition, but how it conditioned the intake valve seals to stop the smoke in the morning has always been a mystery to me.

Chevron claims that it cleans the valves, but I can't see how the intake seals could be reached by the Techron.

BTW, I never go more than 2,000 miles without changing oil and filter, because of the possibility that the amount of Techron used might adversely affect the properties of the oil.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles