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What is causing the cloud of smoke after a long downhill?

99 F-150 V8 - I live in Colorado so during mountain driving after a long downhill I get a cloud of smoke when I get back on the gas. Appears to be darker smoke not oil like blue smoke. Only other issue I have with the truck is an occasional cloud of blue smoke when I start it which I believe is just aging valve seals. What is causing the cloud of smoke after a downhill?

Aging valve seals.

It smokes when you start it in the morning because the oil has had a chance to seep past the valve stem seals and settle in the cylinders.

It smokes at the bottom of the hill because as you’'ve cruised down the hill with your foot off the gas, and probably used the engine to help you keep a modest speed, you’ve sucked oil past the worn valve stem seals and into the cylinders.

Think of it this way: your engine is like a pump. As it runs, the pistons continually pull air into the cylinders at every intake stroke. When the throttle plate is open, the air goes in easily. When you take your foot off the throttle, the pistons are trying to pull the same amount of air in, but the path os now blocked by the closed throttle plate. So a high vacuum develops in the cylinders, drawing oil past the worn valve stem seals. When you step on the gas again it burns off.

You might have some excess fuel being fed into the cylinders too, and perhspa oil passing by worn rings and/or cylinder walls as well, based on the description of the smoke.

In short, you valued '99 pickup is tired. I’d bet a compression check would be revealing.

Thanks for the great response. If it runs good otherwise, anything that I need to be concerned about or can I ignore it?

Just keep your eye on the oil level and you can keep on going for a long way.

Agreed. Ultimately the oil burning will probably cause your upstream oxygen sensors and your cat converter to have an early demise, but sincethe only solution is to rebuild the engine I’d just keep an eye on the oil level and keep on driving it.

Year, make, model mileage?

'99 F150…this might be one of the few cases where it might be worth trying one of those ‘high miles’ oils, they’re supposed to swell oil seals, I don’t know how well they work.

A thing to keep in mind with high mileage oils is, once you use it, you’re hooked. It’ll swell the seals, but if you change back to regular oil, they might shrink & leak worse than before.

You might want to check for a leaking fuel pressure regulator. The regulator is mounted at the back of the left fuel rail. Remove the vacuum hose from the regulator, and if gas leaks out of this connection, that’s what causing the smoke at start up and the smoke after driving down hill.


You have a V-8 so you can use thicker oil in the Summer. Straight 30wt is thicker oil and it may stop some of the problem. I used it on a 73 302 that put out a cloud all the time. With the 30wt it would only leave a cloud when the engine started. Your problem is not that severe, so maybe the engine won’t smoke when started. I used five quarts, you might need two with your regular oil.