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Baffling Throttle Issue


Dear Cartalk,

I called you last week in regards to a strange throttle issue with my 1987 BMW 535is which has 210K miles. This issue is this: when I start the car and the engine is already warm, the throttle will sometimes buck, or will be partially or completely unresponsive. At altitudes near sea level and/or temperatures above freezing, I am able to pull off to the side of the road, depress the accelerator pedal for a couple minutes, and the throttle regains responsiveness, ie, I will be able to rev the engine in neutral between 3000 and 5000 rpm and the car will then drive fine. When the problem arises at higher altitudes, however, the throttle takes much longer to regain responsiveness, the engine will not rev above 3000 rpm and it takes much longer to regain responsiveness.

For perhaps a year or so, this issue occasionally occurred when driving the car in my town of Davis, CA, and seemed to happen more frequently during rain. Since it rains so infrequently here, and since I?ve been out of the country for most of the past year, the problem did not warrant much worry.

Recently, though, returning south on I-5 from Oregon, I reached chain control at the Siskiyou range in northern California, put on the chains, slowly reached the summit, and there, at the highest altitude and lowest temp with near 100% relative humidity, the throttle shut down. I was able to keep going by flooring the accelerator, releasing the clutch and sliding forward for about 100 yards. The throttle would only respond when I floored the pedal. I repeated this for nearly an hour, five times the throttle becoming unresponsive and five times regaining responsiveness. Once I reached clear road and the rain line, I took off the chains and the car ran great for the remaining 270 miles back to Davis.

So I took the car into the shop. They ran the diagnostic and found no faults and also did a vacuum test, but there were no leaks. Next weekend I drove to the peak of the Echo Summit at Lake Tahoe, this time with no chains, and the problem started again. I literally coasted down the entire hill into the lake basin. Luckily the house my 22 other friends and I had rented for a bachelor party was just off Interstate 50. I ditched the car by the house and got a ride to the ski hill from my friends who were following behind. I drove the car a day later while it was snowing and problem continued. Three days later it was dry while I drove home, and the car ran like a top.

I take the car to a second mechanic. He checks sensors such as the coolant temp, the fuel pressure and the spark: they?re all fine. But he tells me that it?s unlikely that the problem has to do with humidity. Could barometric pressure be the cause?

I have a long time mechanic in Sacramento who has encouraged me to do a number of repairs myself. He told me to gather as much information as possible regarding throttle issue before bringing the car to him. So after having two near breakdowns in Donner Party-like scenarios, taking the car to two baffled mechanics (neither of whom charged me), I come to you, Click and Clack, to see if you might be able to uncover the apparent mystery.

Please enlighten me with your knowledge.


James McCeney