D-jet troubleshooting, Only one item left on my list! Options

[saigon71]

My 1974 2.0 D-jet is running very strong, but there were two lingering issues with the FI that I wanted to address over the winter:

1. A severe idle "hunt." The engine would rush up to about 1200 RPM, then nearly stall...repeat cycle.

2. Every once in a while the car would buck, most notably at highway speeds...it wasn't bad, just annoying.

Last week I installed a new circuit board in the TPS purchased from Dave Sprinkle a few years back. It was calibrated according to the Pelican tech article using an ohmmeter.

I took a 210 mile road trip over the weekend to test things out. The car now idles perfectly through the entire warm-up sequence. Thanks Dave! (IMG:style_emoticons/default/beerchug.gif)

The problem is that I still have the same bucking issue. All my research points to the TPS. Any suggestions on what to check next?

[davesprinkle]

The interdigitated traces (love that word, I found it on pbanders site, every djet owner should read it: http://members.rennlist.com/pbanders/) on the TPS board are designed to act as little triggers for for an extra 'shot' of fuel. Similar intent to the accelerator pump on a holley.

The TPS wiper scrubs across these serrated traces and shorts alternately between two contacts. If this action doesn't happen, in other words, if the throttle moves WITHOUT generating these alternating connections, then the engine may momentarily go lean on a slight throttle tip-in and can result in the famous djet 'buck'. (Other problems can result in this behavior, but in most cases this is due to a problem with the TPS.)

I need to be careful to state that this should be viewed as a 'system' problem, and not as a TPS board problem. The TPS board is one part in the system, but there are others: the wiper where it contacts the boards, the leaf-contacts in the wiper stem, the connector contacts, the wiring harness, the contacts at the ECU. All of these things need to be healthy or you can get this problem: throttle movement without accel trigger.

In many cases, as djet guys know, the replacement board resolves the issue. This is fortunate, because the board seems to be the most prominent failure point in this accel enrichment system. It's also fairly cheap and easy to replace. But if simple board replacement still leaves you with the problem, then you'll need to confirm that ALL of the accel system still works.

The easiest way to do this is to switch on the ignition (don't start the car), and while being very quiet, listen for injector clicks as you slowly open the throttle. (Lean into the open engine bay for this. Put your ear as close to an injector as possible. And turn off the Motorhead, for cryin out loud, Lemmy's not gonna help you hear the damn thing.)

Another way to do this is to probe between the appropriate contacts AT THE ECU with a continuity buzzer or an LED that you rig up. You need to confirm the alternating connection as you move the throttle or you've got another problem than just the board.

By the way, all these wiring harnesses are unreliable. They've spent their life (40 years now) exposed to heat and hydrocarbon vapor and vibration. They're junk. Get a new one.