Cameron's complete transfer of a D-Jet to an Amazon makes me hope he has the answer to a common D-Jet symptom. I have talked to several others on the web about the vehicle "bucking" when held at a constant speed with little or no load (which usually is 2000 to 2500 rpm). Like myself, most have tried the less expensive fixes first: checking vacuum hoses, points and plugs. Then its on to replacing temperature sensors. At a certain point, the price of tinkering goes way up, and I need to know which item is most likely to be the culprit. Is there a common fix, or at least a common series of steps towards fixing this problem? The plugs on my ES always indicate a rich mixture and the mileage is poor (15 town and 19 highway), so I don't think its starving for fuel.

Phil says: In spite of your rich-looking plugs, your symptoms still indicate (to me, anyway) a vacuum leak. This will cause a rich idle and rich running under some circumstances. Sounds backwards, but it's not -- let's just say the ECU gets confused. In the "little or no load" scenario you describe, the ECU cannot compensate for the leak at all (this is your highest manifold vacuum situation), and you get the "bucking" associated with an overly lean mixture, just as with a carbureted engine.

Two sources of such a leak are easily overlooked: 1) intake manifold not tight or leaking gasket, 2) PCV valve does not close at idle. Please don't just start replacing sensors -- it's not that hard to diagnose them accurately with a decent manual and a multimeter (but just about impossible to do from here!).

Cameron says: I have a couple other thoughts; the trouble is that there are a few different things that can cause bucking in general. Assuming the ignition system has been safely ruled out (ignition points in good shape?), I would check the following, in this order:

1. The throttle position switch. This is the small square black box mounted on the outboard side of the forward end of the intake manifold. The cover is removable (early ones were secured with screws and later ones simply snap in place). Remove with care: the insides are delicate. (Also, don't unscrew the adjustment screws (upper and lower, there are only 2) at this point.)

   What can happen here is that as the contacts slide over the printed "circuit," carbon deposits can interrupt the signal to the ECU and result in bucking. Look for black arcs that follow the path of the 3 moving contacts. A pencil eraser can clean these off, but if the tracking is deep enough you'll need a new one. This switch goes for about $35. Easiest way to determine if this is the cause is to simply unplug the wiring harness from it and drive the car. It won't accelerate as well and you'll be running in "limp home" mode but if the bucking goes away, you will have immediately isolated the cause.

2. The base points. These are the points that are in the bottom of the distributor, held in place by two screws. These generally last for decades. Again, look for carbon between the contacts which open and close. Use a business card and slide it back and forth between the points to clean them. If they're pitted too much, they may not be salvageable. These run about $65.

3. Wiring to the injectors. I've had almost all of these fail over the last 4 years. Gently pull the rubber boots away from the connectors that plug into the tops of the injectors and look down into the white plastic housing that contains the wires and their terminals. There's a lot of temperature change in this location and the wires can give out. In my case, I had 2 or 3 strands of wire remaining: this was enough to signal the injectors at idle or at high revs but the vibration of the engine was just right between 3000 and 3500 rpms that I experienced SEVERE bucking.

4. Ground wires for the injectors. These are the wires attached to the rear of the intake manifold by a single screw. A poor connection or corrosion here can lead to symptoms described in (3) above.

Problem resolved -- it was the throttle switch.

More info:

The fact that a worn throttle position switch is causing bucking indicates that it's worn in one spot, but not ruined (you'd get an open circuit and no "accelerator pump" action at all if it was totally shot). The wiper has simply worn a groove which is deep enough that the "button" on the end of the wiper is no longer making contact in that area. Clean the surface, rotate the wiper on its mounting post slightly so that it's now contacting in a new arc, but still within the wavy area of the printed circuit board, and apply some dielectric grease to all surfaces on the printed circuit board where the wipers travel. Problem is fixed, probably forever! Just clean and reapply new grease every tune up. Dielectric grease is available at any auto parts store.