Culprit in bucking SUV may be sensor
Q: I wonder if you can help me figure out why my Chevrolet Suburban has begun to buck sharply at times while driving. It seems to happen at a specific point when I'm going up certain hills. It happens only for an instant, then it's fine again. Ideas? I'm pretty good at working on things, but this has me baffled.
A: This is a tough one, without being there and running some tests. You didn't mention the “service engine soon” light illuminating, so I'll assume the fault occurs too briefly for the onboard diagnostic system to catch it, or it's a pre-1996 model with less smarts.
Your clear description of the symptom makes me wonder if your throttle position sensor could have a glitch. This gadget is attached to the passenger side of your engine's throttle body and sends the engine computer a varying voltage signal that's proportional to throttle opening. With time and mileage, these sensors can become scratchy.
As you open the throttle, the signal voltage is supposed to rise smoothly between about 0.5 and 4.5 volts. Should a brief voltage drop-out occur as the sensor reaches a certain point of travel, the engine computer thinks you jumped off the throttle and cuts fuel injection commands, causing an abrupt power loss, almost as if someone cycled the ignition switch. This sensor is also important for transmission control in 1993 GM SUVs and up, so odd things can happen with shifting and torque converter clutch operation (1981 and up) as well.
Here's a trick that can help when a sensor is suspected of being faulty — unplug it. Modern vehicles are very smart and can cook up a substitute reading for many sensors if the part fails completely or is deemed incompetent.
Unplugging the throttle position sensor is easy, but please do so with the engine off and cold. Upon startup, you'll be greeted by an illuminated “service engine soon” light, but don't worry — you engineered this. Try driving the Suburban in a variety of modes and see if the bucking subsides. Don't expect the engine to run perfectly. Transmission shifting may be a little odd, as you're substituting one problem for another, looking for a difference.
If the bucking stops occurring, renewal of the sensor should fix the problem, although I'd check carefully for an erratic voltage signal first to be sure. If your sensor has elongated mounting screw holes, the replacement part will require a simple adjustment. If the bucking continues, or you're not sure of these procedures, it's time to make an appointment with a sharp technician to get to the bottom of this. Be sure to tell him/her that the diagnostic trouble code in memory, if it's the only one present, was a result of your tinkering.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org; he cannot make personal replies.