2002 Toyota Tacoma pickup’s V-6 engine stumble is more than an idle problem

Dear Car Talk:

My wife drives our 2002 Toyota Tacoma V-6 with 180,000 miles. She reports that the engine will sometimes die when shifting from park to reverse. It seems to be worse in hot weather and usually happens when the AC is on.

A quick bump to neutral and the truck starts backup fine. I bought the truck new and it’s in excellent shape otherwise, but now I’m worried about future transmission problems. Suggestions? — Ronny

I doubt it’s the transmission, Ronny. More likely, something is causing the idle speed to drop. And when you put the truck in gear, which puts an additional load or “demand” on the engine, the idle speed drops a little bit more and the truck stalls.

I’d check the operation of the idle air control. When you use a major accessory like the AC — one that also places a big demand on the engine — the computer is supposed to tell the idle air control to boost up the idle speed to prevent it from dropping too low and stalling.

The idle air control may not be working the way it’s supposed to when the AC is on. It could just be dirty.

More likely, though, is a vacuum leak, which is very common on older cars and would also cause the idle speed to drop. Because it seems to happen only when you shift into reverse, it could be related to how the engine twists when you put the truck in reverse.

If you open the hood and watch while someone shifts the truck from park to reverse, you’ll see that the engine actually moves a little in one direction. When the truck is shifted from reverse to drive, the engine will move in the opposite direction. While it only moves an inch or two, it can be enough to make a crack in a hose open more or close more.

To check for that in our shop, we have a tech plant a foot on the brake pedal and put the truck in reverse. And when the engine begins to stumble, we have another guy go around with a wand that’s attached to a cylinder of propane. And that second guy will shoot a very small stream of propane gas around the areas where we suspect a vacuum leak.

When the propane encounters a vacuum leak, it gets sucked into the engine through the leak and raises the idle speed. So, when we hear the engine go faster, bingo — we’ve found the leak.

A vacuum leak could be anywhere, but I’d definitely check the fat hose that connects the air-flow sensor to the throttle body. We’ve seen that hose leak before.

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