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Old 11-01-2009, 10:53 AM   #1 (permalink)
EcoModding Lurker
Join Date: Nov 2009
Location: Ohio
Posts: 16

Red Storm - '93 BMW 318 iS
90 day: 28.51 mpg (US)
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"Hot soaking" for improved mileage.

I did a search on this, and apparently no one on the board has mentioned one of my favorite tricks for cleaning a motor and getting optimum performance.

It's the hot soak. A variant of it has been around since the 1960s, when a mechanic demonstrated it at the North American International Auto Show as a way of cleaning lead-fouled combustion chambers. I've done it on my car every 3000 miles since I bought it at 136,000, and it's running strong at 193,000.

The procedure is very simple indeed. First, locate your fuel pressure regulator. It's a small vacuum line running from the front of the intake manifold, in most cases. The major repair manuals (Chilton, Haynes, Bentley) all show it clearly.

You'll need a can of Seafoam fuel treatment for the soak. Other treatments work, but none so well as Seafoam in both my experience, and the experience of the BMW mavens on Unofficial BMW. I have used Lucas treatment in a pinch. Seafoam's worth the 10 bucks.

Start your motor, and disconnect the end of the hose from the fuel pressure regulator that has a vacuum. Touch the end to be sure there is a vacuum present, and put that end into your can of Seafoam so it's just drawing off a mix of liquid and air. The motor will struggle a bit, and if it does, let it draw air for a second. It still shouldn't take more than a minute or so to get the motor to ingest the entire can.

One of two things are going to happen when the can is nearly empty. One, the car may stall. If you're down to only a couple ounces left, reconnect your line and leave it be. If it's taken the whole can and hasn't stalled out, shut off the motor manually.

Now for the hard part. Walk away for 8-12 hours. Yes, walk away. Close the hood, lock the doors, walk away.

After letting it sit, go for a drive. It will smoke worse than you have ever seen, guaranteed. Vary your speed for a while, and eventually the smoke will clear. The motor is now clean.

Go get your oil changed, as the soak frees any deposits in the combustion chamber and dumps them into your oil sump. Now your motor is clean, you have fresh oil, and will probably see a performance gain- mine always does!

I'm tempted to say that the hot soak was key to the mileage gain I have seen. When I bought the car, it had been largely neglected for a year- low octane gas, no oil change for 13,000 miles, and so on. I got 14mpg out of the first tank of gas. After a hot soak and an oil change, I got 19 out of the next tank with the same driving habits.

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