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?

[914_teener]

Fuel pressure.

[mepstein]

I had a bucking issue. sometimes bad, sometimes not. All new fuel and vacuum lines, cleaned injectors, new seals, ect. Checked mps, tps and fuel pressure. Plugs, wires, and rebuilt dizzy. Even installed a different engine wire harness. Finally went away with a different (used) coil. Maybe the coil was the fix, maybe not but it hasn't bucked since.

[boxsterfan]

My car has the "idle hunt" problem on warmup. But once warmed up, the idle is perfect at 900-950RPM.

I thought the "idle hunt" during warm-up was related to bad CHT sensor? $16 part.

[watsonrx13]

I have a '74 2.0 D-jet also. My runs very well but also has a similar bucking/hesitation at 2900-3100 RPM. When I have the gas pedal set at the 3000 RPM range, then slightly reduce, it bucks/hesitate. Once past 2900 it evens out.

I have replaced the TPS circuit board from Dave, new vacuum and fuel hoses, professionally rebuilt FI. Tested TPS, MPS, new fuel pump, plugs and wires. Newly rebuilt 2056 with Raby cam kit for FI engine.

My sentiments exactly: "Every once in a while the car would buck, most notably at highway speeds...it wasn't bad, just annoying."

I too am looking for a solution to this annoying problem.

--- Rob

[saigon71]

My car has the "idle hunt" problem on warmup. But once warmed up, the idle is perfect at 900-950RPM.

I thought the "idle hunt" during warm-up was related to bad CHT sensor? $16 part.

I always related idle hunt to the CHT as well, so I installed a new CHT and it made no difference. The new TPS board fixed the problem for me.

[DRPHIL914]

I have a '74 2.0 D-jet also. My runs very well but also has a similar bucking/hesitation at 2900-3100 RPM. When I have the gas pedal set at the 3000 RPM range, then slightly reduce, it bucks/hesitate. Once past 2900 it evens out.

I have replaced the TPS circuit board from Dave, new vacuum and fuel hoses, professionally rebuilt FI. Tested TPS, MPS, new fuel pump, plugs and wires. Newly rebuilt 2056 with Raby cam kit for FI engine.

My sentiments exactly: "Every once in a while the car would buck, most notably at highway speeds...it wasn't bad, just annoying."

I too am looking for a solution to this annoying problem.

--- Rob

try disconnecting the air intake temp sensor, see if that makes a difference.- I too had this at one time, but once i changed out the TPS circuit board, it was gone, but recently someone else had stated that in addition to a surging problem the bucking was improved by disconnecting the air temp sensor.
Logically the most obvious causes would be TPS, but if warming up partly improves it then maybe its an issue with the CHT, - just to play with this a bit, i put a potentiometer in line with the cht, and did see this clear up when i hit the right amount of resistance, also got rid of the lean running that causes idle hunting- but the real problem is either the CHT or the MPS which are not set correctly to have the proper A/F mix. ---

[larss]

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.

....

I also got this cycling after putting 1911cc together, worst during warm up. I found that if I put an extra 50 to 100 Ohms in serial with the 270 Ohm ballast resistor for the CHT sensor, the cycling was almost eliminated. The extra resistance makes the engine run somewhat richer (the CPU Believes the Engine is colder than what it is).

/Lars S

[DRPHIL914]

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.

....

I also got this cycling after putting 1911cc together, worst during warm up. I found that if I put an extra 50 to 100 Ohms in serial with the 270 Ohm ballast resistor for the CHT sensor, the cycling was almost eliminated. The extra resistance makes the engine run somewhat richer (the CPU Believes the Engine is colder than what it is).

/Lars S

- (IMG:style_emoticons/default/agree.gif) -exactly, i used the pot. to get the resistance that worked and then removed it and put the fixed resistance in like Lars did..

[r_towle]

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?

Clean your distributor ever?

[plas76targa]

Bob,
i get the same bucking issues. It's usually the worst if i try to take off without a good warm-up period. My car also sometimes cuts completely out at cruising speed. Most annoying!
I just installed a new coil per mepsteins suggestion which didn't improve the problem. The four wires going to the Dizzy are all soft insulation and look poor in quality and supplier support. I plan on replacing those too.

Good luck and hope you find the solution soon!

[jsayre914]

Bob,

I fixed this issue by removing the trigger points from the dizzy and cleaning them. Give it a try. I am not sure if they are available new (I couldnt find any) but if you find a new set, Get one for me too. (IMG:style_emoticons/default/beerchug.gif)


Good Luck!

[brant]

I have a '74 2.0 D-jet also. My runs very well but also has a similar bucking/hesitation at 2900-3100 RPM. When I have the gas pedal set at the 3000 RPM range, then slightly reduce, it bucks/hesitate. Once past 2900 it evens out.

I have replaced the TPS circuit board from Dave, new vacuum and fuel hoses, professionally rebuilt FI. Tested TPS, MPS, new fuel pump, plugs and wires. Newly rebuilt 2056 with Raby cam kit for FI engine.

My sentiments exactly: "Every once in a while the car would buck, most notably at highway speeds...it wasn't bad, just annoying."

I too am looking for a solution to this annoying problem.

--- Rob

try disconnecting the air intake temp sensor, see if that makes a difference.- I too had this at one time, but once i changed out the TPS circuit board, it was gone, but recently someone else had stated that in addition to a surging problem the bucking was improved by disconnecting the air temp sensor.
Logically the most obvious causes would be TPS, but if warming up partly improves it then maybe its an issue with the CHT, - just to play with this a bit, i put a potentiometer in line with the cht, and did see this clear up when i hit the right amount of resistance, also got rid of the lean running that causes idle hunting- but the real problem is either the CHT or the MPS which are not set correctly to have the proper A/F mix. ---

To both of you who richened the mixture by enrich ending the cht. Did either of you verify your mixtures with a wide band? The reason I ask a that my mps is already adjusted with a wide band and I'm not going lean on the mixture. Yet I too have the exact occasional 3000-ish highway sporadic buck.

I'll try the coil and trigger points next. Not sure what is causing this

[larss]

To both of you who richened the mixture by enrich ending the cht. Did either of you verify your mixtures with a wide band? The reason I ask a that my mps is already adjusted with a wide band and I'm not going lean on the mixture. Yet I too have the exact occasional 3000-ish highway sporadic buck.

I'll try the coil and trigger points next. Not sure what is causing this

I only have a narrow band AF sensor on mine (yes I know I can't at all compare it with a wide band).
Before adding the extra resistor to the CHT the reading cycled at Optimal, after adding the Ohms it was more steady at Rich but not bottomed out. I did the adjustment during warm up since it was there the big problem was, have not yet fully checked what the reading is when fully warmed up.
I did not use the gauge to adjust - I adjusted the resistance in small steps until the cyling in rpm (almost) went out.


/Lars S



/Lars S

[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.

[DRPHIL914]

I have a new wide band installed and will be testing this soon.
As for the suggestion about the harness, I agree and replaced this with a new one from Jeff B. Also have a new tps board from you, Dave. I would suggest also cleaning or replacing the injectortriggers. I did this too. It will be interesting to get the car off jackstands so I can test the A/F meter and see where the mixture is with those adjustments. ?

I have a '74 2.0 D-jet also. My runs very well but also has a similar bucking/hesitation at 2900-3100 RPM. When I have the gas pedal set at the 3000 RPM range, then slightly reduce, it bucks/hesitate. Once past 2900 it evens out.

I have replaced the TPS circuit board from Dave, new vacuum and fuel hoses, professionally rebuilt FI. Tested TPS, MPS, new fuel pump, plugs and wires. Newly rebuilt 2056 with Raby cam kit for FI engine.

My sentiments exactly: "Every once in a while the car would buck, most notably at highway speeds...it wasn't bad, just annoying."

I too am looking for a solution to this annoying problem.

--- Rob

try disconnecting the air intake temp sensor, see if that makes a difference.- I too had this at one time, but once i changed out the TPS circuit board, it was gone, but recently someone else had stated that in addition to a surging problem the bucking was improved by disconnecting the air temp sensor.
Logically the most obvious causes would be TPS, but if warming up partly improves it then maybe its an issue with the CHT, - just to play with this a bit, i put a potentiometer in line with the cht, and did see this clear up when i hit the right amount of resistance, also got rid of the lean running that causes idle hunting- but the real problem is either the CHT or the MPS which are not set correctly to have the proper A/F mix. ---

To both of you who richened the mixture by enrich ending the cht. Did either of you verify your mixtures with a wide band? The reason I ask a that my mps is already adjusted with a wide band and I'm not going lean on the mixture. Yet I too have the exact occasional 3000-ish highway sporadic buck.

I'll try the coil and trigger points next. Not sure what is causing this

[saigon71]

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.

Thanks for all the info Dave.

It started to rain early today so I couldn't get out to test the new coil I installed last night.

I appreciate all the suggestions on what to check next. I will post what the fix is as it seems a number of us have this problem.

The distributor has never been cleaned.

[watsonrx13]

Thanks for all of the suggestions. I'm going to remove the air intake temp sensor wire first and test this weekend. What is the recommended coil, the regular Bosch blue or a Petronix Flame-Thrower? BTW I have points in mine. New points, condenser and gapped correctly. Timing and valves recently adjusted.

Also, I've already cleaned the trigger points.

--- Rob

[watsonrx13]

I finally purchased and installed a new Bosch 'Blue' coil on the car and tested it. It ran the same, i.e. I still have the hesitation after holding the throttle at 3000 RPM, then easing off.

I also removed the wire from the air temp sensor and drove the car. With the wire disconnected the car wouldn't idle and it continued to hesitate at 3000 rpm. Here's the wire I disconnected and the temp sensor. If this is not the correct sensor, can someone upload a pic showing the correct one?



--- Rob

[914itis]

Bob,
Some may remember my long thread after installing my 2.0 djet bucking at 3300 rpm.
After months and tons if money changing injectors ,tps ecu, vaccum lines trotle body and gasket, the issue was a $100 fix.

It was the vacuum advanced on the dizzy getting stuck. I replaced it and that was it.

I know it could be a number of things as stated in your thread, but you can do a simple test by sucking air out of the tubes and see if the arm moves.

Give it a try
Here is the thread Buckling at 3300