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Full Version: D-Jet part-load issue goes away when TPS disconnected
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fslick
I have a D-Jet question ('73 1.7).

Symptom is poor running/stumbling when transitioning from idle to part-load. It happens when the engine is both cold and completely warm, but seems to be worse the warmer it is (i.e. full engine temp + hot summer day). Starts and idles just fine hot or cold.

Interesting thing is that it's pretty much not noticeable if you VERY SLOWLY go from idle to part-load either with your tippy toes on the gas pedal or just moving the throttle with your hand on the throttle body. Of course you can't drive that way, so if you go from idle to part load normally or quickly, the response is poor and it stumbles but will catch up after about a second (or less).

Getting to my question, I took it to a [reputable] shop and they were able to figure out that it runs ok (at least doesn't have this problem) with the TPS disconnected.

I am going to take the car back to the shop to have them finish the repair (when they are free) but the engineer in me wants to know why this is a 'fix,' (albeit not the complete FI system). Also does it mean that the TPS is actually the problem? Is this a symptom of it being maladjusted?

(All the other usual suspects were checked and seem to be ok (i.e., new vac lines, new fuel lines, good compression and fuel pressure, fuel pump relocated, valves adjusted, checked dizzy, points, swapped ECUs for testing, A/F mix adjusted, replaced MPS, CHT, etc etc).
EDIT: I forgot to mention that replacing the TPS (with a used/known good one) has already been done as well.

Thanks for reading.
rjames
Verify that the TPS is calibrated correctly. Search here or Pelican for the procedure. If that doesn't work, clean the tracks on the TPS board. A pencil eraser worked for me.
If they are too badly worn, new boards are available.

Easily done at home. Don't pay a shop to do it unless you like throwing money away.
fslick
QUOTE(rjames @ Aug 17 2019, 10:10 PM) *

Verify that the TPS is calibrated correctly. Search here or Pelican for the procedure. If that doesn't work, clean the tracks on the TPS board. A pencil eraser worked for me.
If they are too badly worn, new boards are available.


Sorry, I pulled a cruel trick accidentally and this might not have been included when you sent your reply:

EDIT: I forgot to mention that replacing the TPS (with a used/known good one) was already done as well.
JeffBowlsby
Is it possible that your advance weights are sticking? They need to be lubed annually. 1 drop of 30 wt in the center of the dizzy shaft is what the factory manual recommends.
fslick
Thanks, Jeff, good to know and I'll add that to the annual maintenance list.

I guess it's possible but like I mentioned, the car runs pretty well with the TPS disconnected. I would imagine in a mechanical part like the dizzy that the sticking would still be noticeable even with the [electronic] TPS disconnected.

This does cut one of the canons of my question though -- how does the rest of the D-Jet system function without the TPS? Does disconnecting it to solve a problem usually point to that as the culprit?
rjames
QUOTE(fslick @ Aug 17 2019, 07:24 PM) *

QUOTE(rjames @ Aug 17 2019, 10:10 PM) *

Verify that the TPS is calibrated correctly. Search here or Pelican for the procedure. If that doesn't work, clean the tracks on the TPS board. A pencil eraser worked for me.
If they are too badly worn, new boards are available.


Sorry, I pulled a cruel trick accidentally and this might not have been included when you sent your reply:

EDIT: I forgot to mention that replacing the TPS (with a used/known good one) was already done as well.


Did you calibrate it correctly? If disconnecting it removes the stumble, then it seems likely that the TPS is still the issue.
fslick
QUOTE(rjames @ Aug 18 2019, 01:21 AM) *

Did you calibrate it correctly? If disconnecting it removes the stumble, then it seems likely that the TPS is still the issue.


Installing the TPS was a part of a much bigger list of stuff that the shop did. I will certainly double check with them this week and keep going down this route. Thanks.

QUOTE(fslick @ Aug 17 2019, 10:02 PM) *

...the engineer in me wants to know why this is a 'fix,' (albeit not the complete FI system).


Was still hoping one of the D-Jet virtuosos could shed some light on this question too, i.e. how does the FI system still work pretty well without the TPS.
BeatNavy
QUOTE(fslick @ Aug 18 2019, 01:57 PM) *

Was still hoping one of the D-Jet virtuosos could shed some light on this question too, i.e. how does the FI system still work pretty well without the TPS.

Not a virtuoso by any means, but here's my understanding:

The MPS is the primary input to the ECU for determining mixture (based on injector pulse width). (The CHT sensor also provides major input to ECU, but ignore that for this discussion.) With the TPS unplugged, you are still getting changes in manifold vacuum when you open or close the throttle. When you open the throttle to accelerate, the manifold pressure decreases, and the MPS "tells" the ECU and the ECU modifies injector pulses to richen the mixture.

The TPS is primarily for idle as well as better throttle response (when opening throttle/accelerating) and efficiency (when closing throttle/decelerating).

When adjusted correctly, the ECU "knows" when the throttle is closed and car is in an idle position. With idle circuit engaged the mixture is controlled primarily by the ECU knob (for those cars/ECU so equipped).

In addition to idle, the "arms" on the TPS that sweep across the tracks in the circuit board tell the ECU approximate throttle position. Two other arms can tell the ECU if the throttle is opening or closing. Again, I believe it improves acceleration/throttle response (and perhaps efficiency when throttle closing) over simple MPS input alone, which may be slower in terms of input.

In your case, if unplugging the TPS helps, I would bet it's still the issue. Is it "bucking?" That's a classic symptom of TPS wear (or, in general, the engine going very lean on and off rapidly for some reason). Of course you may have more than one thing going on.

There are variety of threads here describing how to inspect and clean the "tracks" on the circuit board as well as, if necessary, replacing it. A couple of examples to help understand how the TPS works:

http://www.914world.com/bbs2/index.php?sho...ottle++position

http://www.914world.com/bbs2/index.php?sho...p;#entry1717092

https://members.rennlist.com/pbanders/DJetParts.htm#Throttle Switch
fslick
Thanks for the succinct answer and resources, Rob. Much appreciated.

I've checked out the [amazing] pbanders site before and this symptom really nails it:
"Engine bogs from idle (poor idle to part-load transition)", with the causes being listed as timing/idle mixture. The shop undoubtedly has set both of these.

QUOTE(BeatNavy @ Aug 18 2019, 04:06 PM) *


In your case, if unplugging the TPS helps, I would bet it's still the issue. Is it "bucking?" That's a classic symptom of TPS wear (or, in general, the engine going very lean on and off rapidly for some reason). Of course you may have more than one thing going on.



No bucking, but at the same time I haven't driven the car at a constant speed for very long with the TPS plugged in because the part-load transition problem makes it tough to try (I live in the city). I'm going to stay on this path and see how it goes (try a new switch and make sure it's calibrated).


rjames
QUOTE(fslick @ Aug 19 2019, 09:29 PM) *

Thanks for the succinct answer and resources, Rob. Much appreciated.

I've checked out the [amazing] pbanders site before and this symptom really nails it:
"Engine bogs from idle (poor idle to part-load transition)", with the causes being listed as timing/idle mixture. The shop undoubtedly has set both of these.



Undoubtedly? Maybe, maybe not. In your first post you stated that the MPS was replaced. If it was, the replacement MPS will need to be calibrated. How did they set the mixture? It can only really be done correctly if the MPS epoxy has been removed to make all 3 adjustment screws accessible.
I'm betting the mixture is too lean, which is causing the stumbling.
Does the shop that's doing the work for you know how to work on DJet?

Checking the TPS for wear is easy. Calibrating it is just as easy- maybe easier since you don't have to remove it from the car.

If the shop doesn't know what they are doing, I'd recommend getting the car back, saving some $ and troubleshooting it yourself. There are plenty of people here who can help from afar, or maybe even in person.
Bleyseng
Timing first could be the problem so lube the centrifugal advance weights or hell rebuild the dizzy. Next is too lean a calibration of the MPS so uou have really poor transition to part load. A mis adjusted TPS results in bucking at 2800-3000 rpms.
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