The Navel of Narcissus

I'm having a very strange problem with my Toyota RAV4 and thought those of you who are automotively inclined might enjoy figuring this out. I have a service appointment on Wednesday and am pretty sure they will find the problem this time. Post your thoughts on the Comment page and we'll see who comes closest to a correct diagnosis. Armchair mechanics, start your engines!

First, if I start the car and drive to work (40 miles) and then drive it home at the end of the day, I have no problems. However, if I drive the car until it is warmed up, turn off the engine for 15-30 minutes, and then restart it, that's when I see the problem.

When I restart and idle the engine, within about two minutes the tachyometer will begin to register a small, oscillating change in engine RPM. The swings get wider and wider until the needle is moving regularly between about 500 and 750 RPM. Sometimes the swing is even wider. As the needle moves, the engine is making appropriately oscillating noises as well. If the headlights or dashboard lights are on, they are dimming and brightening in synchrony with everything else.

After a few minutes of this, the amplitude of the oscillations decreases slowly and the frequency increases until the idle is once again steady. I can now drive the car and will not have any problem unless I park and restart the car as above. This is 100% repeatable now that I've figured out the pattern.

If I drive the car immediately instead of waiting for the oscillations to begin and subside, then at some point the car will lose all acceleration. That is, pressing on the accelerator has no effect at all--the car will continue to slow until I pull over and let it do its crazy oscillations. Because this was so dangerous, I did take the car to the Toyota dealership, but at that point I hadn't noticed the pattern and could only tell them I was seeing an intermittent loss of acceleration. They told me the car hadn't registered any trouble codes and spent about three hours trying to reproduce the problem with no luck. Now that I can tell them how to reproduce the problem, they should be able to diagnose it.

Last item: Someone suggested bad gas so I did add some dry gas to the tank, but it had no effect.

Comments:

My guess would be air intake. Likely a bad O2 sensor or MAF [Mass Air Flow] sensor. Just in the spirit of fun. I'll be sure to check back to see how I did!

Posted by Richard Rote on April 09, 2007 at 07:09 PM EDT #

Fascinating. No idea. Praying it's not a software problem.

Maybe it's trying to tell you to get a more fuel-efficient car for your daily commute? :-) http://en.wikipedia.org/wiki/Toyota_RAV4_EV
http://www.fueleconomy.gov/feg/noframes/17328.shtml

(Way off-topic, sorry, but this is what I've been using lately for my own (5-mile) commute: http://strida.com/ Neat, huh?)

Posted by Kai Carver on April 10, 2007 at 06:53 AM EDT #

Sorry about your car prob. It might be a bad oxygen senor? By the way, have you done tune-ups lately? It might need one. Just a week ago, I had problem with some of my vw aftermarket parts, I had to bring it to a shop. Thank god, after the tune-up, my car is fine again.

Posted by VW Parts Blog on May 03, 2007 at 09:51 PM EDT #