Running GREAT and then rough…..hhhhmmmm, engine runs rough at one place in the power band Options

[ctc911ctc]

-Update -

I removed the TPS, calibrated it as per instructions on the Bird. Here are some pictures.
I would like to be able to "RING" this out, not sure what I would be looking for. Though we know that one course is to install a new board and then calibrate. There is little else I could find to assure operation. Any experience with asserting these things are good.

Some pics



Any ideas would be great!

-End of Update-


I am a bit befuddled as to which way to go so I decided that the 914W team would like to hear about this puzzle.

Background
'74 2.0, garaged from '84 until 2018, 22K miles
Running since June.
Repaired or replaced:
Entire Fuel System - tank, lines, injectors, pump, pressure reg
Adjusted the valves
Removed and rang-out the engine harness,
Rebuilt the MPS
Rebuilt the Plenum and tested it for leaks - none observed
New gaskets at the plenum and manifold at the engine
Rebuilt Throttle Body, very minor leaks around the actuator (throttle), New circuit card
New Plugs, points, cap and inspected the distributor for all of the regular ailments - super tight.

Engine idles great, pulls like a monster - sounds like a German symphony

HOWEVER
Last night I drove it back from the paint shop on 93 heading south from NH to Boston (lots of beeps from newer Porsches (IMG:style_emoticons/default/smile.gif) (IMG:style_emoticons/default/wink.gif) ) and found that the car started to buck and the engine runs rough when just a small amount of throttle is applied.

More Detail

If you are doing 55 and want to maintain that speed, it is necessary to apply a small amount of gas to sustain the speed. This is when the cars starts to exhibit this "miss/run-rough" puzzle. If more throttle is applied the bucking stops, the engine is smooth and all is well. Conversely, while de-accelerating there is no bucking.

This problem is NOT velocity-dependent, sustaining at 30/40/50/60 will exhibit the same reaction.


WHERE WOULD YOU START TO LOOK IF YOU HAD THIS CHALLENGE?

[JeffBowlsby]

Looks like a lot of wear on those traces, to my eye.

Clean them with an eraser and spray the traces with DeOxit spray.

You can test the continuity between each circuit with an ohmmeter to verify that the TPS is doing its job.

[914_teener]

Looks like a lot of wear on those traces, to my eye.

Clean them with an eraser and spray the traces with DeOxit spray.

You can test the continuity between each circuit with an ohmmeter to verify that the TPS is doing its job.

+2 on that board. I'd probably replace it. Also +1 on the MPS... or kinked fuel line.

[ctc911ctc]

Though the card is new, I might get the 45-year-old card back and put it in (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) ..........curious how worn the new one looks, even though it only has 60+ miles on it - all of the wipers are scratching at the same rate - does not look too bad with the naked eye.

As to Fuel pressure - would this cause the bucking at maintaining speed but not when accelerating?

I will start the debug list - my next stop after verification of pressure will be ignition.

Thank you Teeners!

CTC911CTC

Looks like a lot of wear on those traces, to my eye.

Clean them with an eraser and spray the traces with DeOxit spray.

You can test the continuity between each circuit with an ohmmeter to verify that the TPS is doing its job.

+2 on that board. I'd probably replace it. Also +1 on the MPS... or kinked fuel line.

[Spoke]

Though the card is new, I might get the 45-year-old card back and put it in (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) ..........curious how worn the new one looks, even though it only has 60+ miles on it - all of the wipers are scratching at the same rate - does not look too bad with the naked eye.

You did a test which seemingly exonerates the TPS:

Disconnected the TPS
The hesitation was MUCH worse, the bucking was about the same.

A main function of the TPS is to give the injectors extra pulses for acceleration so hesitation would be expected.

But w/o the TPS in the circuit, the bucking was about the same. This would seem to eliminate the TPS from being the culprit.

[brant]

Though the card is new, I might get the 45-year-old card back and put it in (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) ..........curious how worn the new one looks, even though it only has 60+ miles on it - all of the wipers are scratching at the same rate - does not look too bad with the naked eye.

You did a test which seemingly exonerates the TPS:

Disconnected the TPS
The hesitation was MUCH worse, the bucking was about the same.

A main function of the TPS is to give the injectors extra pulses for acceleration so hesitation would be expected.

But w/o the TPS in the circuit, the bucking was about the same. This would seem to eliminate the TPS from being the culprit.

I agree.. seems like something else is causing the bucking
I have also always understood that unplugging the TPS would test the trace circuits
and I have experienced this test working on my car.

I would follow Chris' advice.
testing the MPS, the CHT, and wiring loom would be my next move....

was there any work done to the car right before the problem started?
I always like to start sleuthing with the last thing touched

[ctc911ctc]

What changed right before detecting the bucking? Really nothing.

The car had not been driven since 1986. We got it working but could not drive it since I did not have a clear title - after 6 months of working this, I was able to get the gov here in Mass to provide a title. The first stop was the paint shop, on the way there I took side roads, all stop and go. Since I was not sure as to the car's operation I did the shakedown on side roads, no faster than 35-40.

On the way back I took the Interstate and cruised between 55 and 65. The car ran great until I was trying to maintain a constant speed and then the bucking would start.

Will check next:
grounds
Fuel Pressure
Wiring to the ECU from the TPS (have done this before)
Trigger points and operation
Ignition system (complete)

Last, I found the Tech Tips 700 book very helpful - thank you George!

Thank you Brant!

Though the card is new, I might get the 45-year-old card back and put it in (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) (IMG:style_emoticons/default/dry.gif) ..........curious how worn the new one looks, even though it only has 60+ miles on it - all of the wipers are scratching at the same rate - does not look too bad with the naked eye.

You did a test which seemingly exonerates the TPS:

Disconnected the TPS
The hesitation was MUCH worse, the bucking was about the same.

A main function of the TPS is to give the injectors extra pulses for acceleration so hesitation would be expected.

But w/o the TPS in the circuit, the bucking was about the same. This would seem to eliminate the TPS from being the culprit.

I agree.. seems like something else is causing the bucking
I have also always understood that unplugging the TPS would test the trace circuits
and I have experienced this test working on my car.

I would follow Chris' advice.
testing the MPS, the CHT, and wiring loom would be my next move....

was there any work done to the car right before the problem started?
I always like to start sleuthing with the last thing touched

[Spoke]

Will check next:
grounds
Fuel Pressure
Wiring to the ECU from the TPS (have done this before)
Trigger points and operation
Ignition system (complete)

Also check the CHT sender.

For overall operation of DJET, you should check the AFR although this requires installing an oxygen sensor in the exhaust. I did this on my 1.8L DJET when I first got it running. Turns out it was running very lean and I had to adjust the MPS.