I've been trying to solve an issue that makes the driving rather unpleasant

I have a 2056 D-Jet that I daily drive. Recently I replaced the fuel filter and since then, the car has developed this behavior which I don't see why it would be related. When I put in first gear and get going, the car doesn't pick up and is bucking. As I progressively press on the pedal, it suddenly picks up and gets going normally. While it's trying to get going, I've noticed that my A/F gauge tends to go lean and out of range.

Things that came to my mind: faulty throttle switch which influences the fuel delivery, hence lean.

On the TPS, I checked the open/close circuit of the 2 traces with the 10 switches and everything is good. Idle switch kicks in also. I checked the resistance of the whole D-Jet harness on the 25 pin connector at the CPU as suggested by Anders and everything is within range.

Replaced the points and adjusted Dwell. Fuel pressure is set at 29psi (analog gauge inline).

I swapped the TPS, the CPU, the MPS, the coil without any improvement.

I'm running out of ideas...

Maybe someone has experienced something like this

Possible kinked fuel line or CHT wire is not fully seated. Good luck, this is not unusual; funny how it starts up after doing a service or repair....

I will look at the CHT wire but as far as the kinked line, seems to me the engine would be starving on heavy load (80mph) which would show on my A/F gauge, but I don't see unusual values there...

It was fine before you changed the fuel filter?
Is that all you did that day?

Go back, see if the filter is flipped or a kinked line.

Rich

Filter in correct orientation. No pinched lines.
I strapped a GoPro to the engine lid and filmed the fuel pressure gauge while accelerating. The needle is very steady at 29-30psi. Still puzzled...

If everything else is good, I suspect your CHT circuit.

Verify that the CHT is tight and seating properly in the cylinder head. If you have a compressor, pull the CHT and blast the hole out with compressed air. I chased my tail for a few months over the summer only to find out there was a piece of debris under the CHT causing an intermittent connection.

The CHT itself is easy to test on your kitchen stove using the PBanders site.


Let us know what you find out.

Resistance value of CHT at room temp is ~2200 ohm, with engine warmed up (head temp gauge at 325), resistance value is 53 ohm. All normal.

What year is the motor? If it is a 73 then the Ballast Resistor might be an issue. Try adding a 1000 ohm variable resistor to the CHT. Radio Shack volume control and see what varying the resistance starting at 270 ohm does.

Does it do this when cold and hot?

Does acceleration at higher RPMs seem ok?

Did you verify the TPS is working in the circuit?

Test the TPS with the following steps:

1) Remove the power (12V) to the coil. Make sure it doesn't touch anything.
2) Turn the ignition to ON but don't start the car.
3) Slowly open the throttle. You should hear the injectors fire as you open it.

Firing the injectors will not dump a bunch of fuel in the intakes as the fuel pump will not be running. They will bleed off some of the fuel as the fuel pressure drops.

I actually have a variable resistance with a knob in the cabin that I can adjust when needed as I read on my A/F ratio gauge. I only need to play with it when I climb a long steep highway hill where the ratio gets in the upper 13.

I though about playing with it to see if it would impact. It's a nice feature to have as you can adjust in real time.

The engine runs very nice except at very low RPM. I have been running this engine daily for over 2 years without this issue so it's not due to it's new configuration. I tested the TPS "accelerator pump" mode 10 switches with the multi meter. I like the idea of the injector testing. I will try it for sure.

I was thinking maybe I should try some injector cleaner fluid... do they work or is it BS?

I revisited my Dwell adjustment that ended up being at 50-51 which is at the limit and brought it down to 46-47 which is more in the middle of the range. It did help a little. Maybe fuel is not the right place to be looking at...

I appreciate all the help

Are you rechecking the timing after you change dwell? Incorrect timing can also be a cause of bucking.

You should dump the points. They are so neanderthal. Install electronic ignition of your choice and never worry about setting dwell again.

Have you tried removing the wire from the CHT to the variable control and just connect it directly ?

I recently had a wire abrade; it ran from the coil to the dash and shorted...

Will recheck timing and variable resistance wire.
As for electronic pickup, I've had 2 that burned... Yes they are nice but...
Will keep you posted.

I had a very similar problem a while back. Cutting out and bucking at low RPM's

I turned out to be the CHT ground. McMark...Mark now Mid Western transplant made a custom CHT ( not sure where he got the thermister) with a separate ground. I installed it and it has been performing well since, about a year or two ago.

My theory is that due to heat cycling and the metals involved it eventually builds up resistance at the ground contact on the head and starts causing problems.

Testing in situ means checking the resistance to a known ground with your trusty DVM and then is tests out fine.

I suggest removing and replacing or cleaning it as some others have suggested and then see what happens.

Hi, might be a good idea to check it out by replacing both the CHT and your variable one with a fixed value resistor.

Maybe about 330 Ohms at your 2.0 (60 Ohms plus the 270 Ohms of the additional ballast resistor)

I use a 180 Ohm, 0.25 W - but my car is liquid-cooled and this is the value for my engine when hot.

Regards
Norbert

Could be a vacuum leak. I had similar symptoms on my Scirocco (CIS Injection). Idol was moving around. Cranky when cold. Engine bucking until 3K rpm then smoothed out. Once I found the leak, all of my problems were solved. Made me a believer. Maybe you bumped a hose when you changed your filter?

Check your motor mounts. If they both break it'll do exactly this. Timing could just be a coincidence.
Glad to hear it's working well. I should make some more now that it's proven. Where'd I put that thermistor part number...