Dear WeatherCat faithful,
Alas, my trusty wagon has been suffering from some strange throttle "stuttering" problem since spring. Since the electronic fuel injection system has a logging feature, I decided to make a run with my venerable 2009 MacBook Pro to see if I could find evidence of the cause. This drive allowed me to confirm a hunch I've had for a long time and WeatherCat was the key to the confirmation. Thanks to WeatherCat, I have this graph of the ambient air temperature during the drive:
It was an unusually cool day with the air temperatures not exceeding 74˚ F the entire drive.
However, Biquette's throttle body has a sensor measuring the temperature entering from the air filter. That shows something
very different!
You can click on the graph to get a larger image. What you can see is as the drive continued the temperature at the throttle body climbed well above the ambient air temperature. This isn't a big surprise. Biquette has one of those open style air filter holders:
These are very popular, but they force the engine to pull air that has been already pulled past the radiator and over the heads. As a result, the air entering the engine was over 30˚ warmer than the ambient air. That's fine on a cool day like this, but imagine touring Route-66 with the air temperatures closer 100˚ F. Asking your engine to ingest air at 130˚ is definitely increasing the risk of premature detonation. Since I'm still hoping to haul a heavy vintage travel trailer with Biquette, I think I have no choice but to come up with some sort of cold air intake one of these days.
I had suspected as much, but now I have definitive proof thanks to WeatherCat!
Thanks Stu!! Cheers, Edouard
P.S. I did find an alternative hypothesis for the stuttering problem. When I started the engine the voltage strangely remained low.
Looks to me like the alternator has a faulty voltage regulator.