On my past few drives the car has been stumbling when I accelerate specifically when warm and in every gear except first, in my 2.0L D-Jet in a 75'

A few of things to note:
- Ground strap at transmission has been cleaned
- Installed a second ground at battery ground point to the engine fan
- New spark plugs Bosch W175T2
- Engine compression 134-145 all cylinders
- Fuel pressure set to 28 psi
- spark plug new wires
- 123 Ignition curve 2 with vacuum advance attached, 10 degrees advance at idle
- Injectors have been rebuilt by Mr. Injector
- All vacuum hoses have been replaced
- AAR is in working order and has been oiled

Not really sure where else to look, the engine idles < 1000 but is not smooth at times jumping +/- ~ 200 RPM

Any advice would be appreciated.

Clean the contact strips on the TPS...

Have you tried adjusting the fuel mixture with the knob on the ECU? Clock-wise a click or two might fix your hunting idle.

Stumble sounds like either your TPS as wndsrfr pointed out, or car is running too lean, which could also explain the hunting idle.

I had a similar problem. I found a new TPS & that did the trick.

Thank you everyone. I just removed the TPS, which I replaced a few years back (>5 I think).

I will check continuity, clean up with an eraser and then reinstall, adjust and will report back.


I started to clean up the contacts with the eraser...

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Looks like the board is worn out. 914 Rubber has new boards.Click to view attachment
Here is what a new board looks like .

DP

I saw the new board, looks great and will order one for a backup.

I cleaned mine, put some dielectric grease and checked continuity on all contacts throughout the range of the switch and it checked out.

There are no broken contacts and will install and adjust for idle and WOT.

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I reinstalled the TPS and it was still stumbling, I tried adjusting the TPS which made it slightly better but not good enough, I'm surprised because the TPS has < 6000 miles. I purchased it new in the box...

I ordered two boards last night.

I think this is where you need to look. Where is the stumble? Immediately coming out of idle? Or at all RPM's on acceleration?

Hunting idle is usually a sign of being too lean. Stumbling on acceleration can also be explained by going suddenly too lean (or, more precisely, more air as throttle opens not compensated fast enough by more fuel). If it happens right from idle, enriching your idle mixture could help. Stumbling can also be caused going the other direction (too rich) or in general too big a change in fuel mixture relative to the amount of air the engine is taking in. In your case, I'm betting on lean.

The "dirty / worn TPS" situation is often a bucking at steady cruise caused by poor contacts with the traces. If you look at your TPS, you'll see there are also two other contacts that tell the ECU if the change in throttle is acceleration or deceleration. Make sure those are clean as well.

You don't have an Air/Fuel gauge do you?

Hey Rob,

I think I may have overstated the hunting idle, I have adjusted the ECU and mixture, it is better then I have seen it in a while.

The problem I have is under acceleration in 2nd and above gears over ~ 1500 RPM. If I maintain speed there is no problem. The is bucking/stumbling when accelerating

I go the new boards on Friday, 914Rubber has some fast shipping. I redid both of my TPS's and installed one. While it is better I'm still having some stumbling in the 2500 to 3000 RPM range after many adjustments. I checked the timing and it is still at +10 static on the 123 distributor.

Any other suggestions would be appreciated.

Thanks so far.

Hi,

did you checked the trigger contacts which are located in the distributor?
If the contacts are worn or the whole assembly is not aligned correctly, you will get stumbling under acceleration.

Besides that, there are contacts located in the TPS, which signal that the gas pedal has been hit what should result in an enrichment of the mixture. These contacts need to be carefully aligned, to let the ecu differentiate between acceleration and coasting.
br

Benno

Hi Benno, I have a 123 distributor so no trigger points. I adjusted the TPS but had trouble using a Dvm checking resistance and used the continuity to the point that there was no continuity between contacts 17 and 12 fiddling back and forth because of the contact on the posts on the arm of the tps

Fuel pressure maybe too low, AFR could be too lean...

Hi,

I‘m refering to the trigger points for the ecu not for the ignition which can be found as well in the distributor.
here you can find a description how to check and adjust these trigger points ->
http://www.914world.com/bbs2/index.php?showtopic=271523

br
Benno

This evening I did several things in attempt to further debug my problem:

1) Adjusted the TPS again using continuity on my DVM, based on the German site translation of https://jetronic.org/index.php/de/autos/d-j...Leerlaufkontakt (NO RESOLUTION)

(2) Adjusted fuel pressure to 30 PSI (NO RESOLUTION)
(2.1) Adjusted fuel pressure back to 28 PSI

(3) Advanced the timing beyond 10 degrees static timing. The problem was minimized but the stumble was still present, but with added problem of a high idle ~1300 RPM
(3.1) set timing back to +10 degrees at idle

(4) Pulled the plug wires one at a time to see if there was an effect, and there was. The wires when grounded generated a nice spark. I had previously tightened the connectors, the were loose even though the wires are new

The engine is just not idling smoothly and I went back to the advice of this forum that there must be a vacuum leak - even though I had replaced all vacuum lines earlier this year.

(5) Checked all vacuum lines again and did not notice any problems.

(6) Sprayed brake cleaner near at throttle body (TB) gasket. I noticed a change and at one point got the engine to stall. I tighten down the TB but the idle was the same.

I also remember reading here that the plenum can be leaky. I sprayed brake cleaner under the TB mount on the plenum and think I found a leak between the plenum body and the TB, the engine stalled after a liberal amount of brake clean a few times.

Any thoughts/confirmation before I remove the plenum this weekend?

Order new intake runner hoses if you’re going to remove the plenum. Your old ones are likely shot if they haven’t been replaced in a while. Plus trying to remove them without destroying them isn’t worth the hassle.

i agree with all the input above andhaving been dealing with my d-jet for the past 10+ years, i'd say you are on the right track, seeing that idle change at these location sshows you have leaks that have to be addressed. with that said i would say that the TPS board you cleaned up still has an issue so glad you ordered a new one. Sometimes just cleaning it will do the job, but that looks like it was more than just oxidation but wear as well. I just had this happen again on mine. last time i did the TPS board was probably 5 years ago, and the car was doing exactly what you have happening with the bucking. Pulled my TPS with eraser and like a magic eraser, its back to normal. beatnavy and the others have said, those vac leaks will keep you from dialing this in, so just get all new seals- plenum, throttle body, intake runners etc.

once you get all that done you should be able to dial in that 123ignition distributor and set your timing and idle, and enjoy the car. Mine is now runnning like a new car, probably better than new with the 123 distributor.

I removed the plenum and located a crack on the top side and possibly the beginning of r one the other side. I cannot see any light through these cracks but suspect when the engine gets hot they open up.

Also the TB mount does not look even, I will also remove the power coat and sand the surface flat.

I started to clean them up and will weld them tomorrow and put it back together

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looks like your throttle body gasket isn't sealing,almost like the plenum is not flat in that area ?,that crack you have will fix up with a little JB Weld.
Other option is to goop it up will sealer run it and see ,in the mean time buy a used good plenum,good luck.

The crack exposed...

Click to view attachment

and hopefully repaired. In the process of waiting for the paint to dry and then will reinstall.

Click to view attachment

I would like to learn to TIG weld... but will keep practicing to MIG weld

Well done on the welding...metal isn't very thick in that area,Tig may have been too hot anyway.
Make sure the TB and gasket are well sealed on the plenum,please report back after test run.

Got everything installed new with new boots between the intake runners and the plenum. I added very little Loctite gasket maker underneath the TB gasket just incase it was not perfectly level

Adjusting the TPS was curious; I adjusted it on the bench measure resistance but it would always read a resistance until
(1) the middle wiper was past the gap and on the next trace on the lower part of the board
OR
(2) I got past the gap and would turn the TPS CCW

The contacts on the plastic arm are in contact with the post which will always measure a resistance.

I also tried the continuity adjustment measuring continuity until there is not and backing off 1 hash mark. This got be close but it was not until I adjusted the TPS by hand making sure that from idle I was able to mash the pedal with no bogging through the pedal range.

I guessing that we are all adjusting the TPS until the middle wiper is on the gap between the traces, I had to turn mine about 1 hash mark CW so the engine would rev smoothly when masking the pedal.

I set the timing to 10 degrees static timing at idle, measured the CHT 2.62kOhms at 75 degrees. The engine idles between 920 and 980 when HOT.

The drivability is night and day to what it was, I now see that for the most part most of the past 6 months debugging was chasing the vacuum leak at the plenum and the TB.

The car now accelerates smoothly although there is a very slight hesitation at 2800 & 3200 when I mash the gas pedal in 3rd and 4th but I can live with this as I generally do not drive like I stole the car.

I would be interest to know what advance curves on the 123 distributor you are all using particularly those with a 75-76 ECU.

Thank you all for your help

Excellent result,my experience with injection always tells me to look for air leaks before venturing into component faults first,the D-jet system is very sensitive to "false air " issues,I have no experience with this system but the principal is the same with any FI system.
The 123 distributor ?,I noticed someone else using 10 degrees static ,is that a requirement and do you dial back total advance to suit,just interested why such a high static advance, know little about these so just asking,cheers.

I followed up with Ed at 123ignitionusa.com and he recommended 10 degrees static advance with curve 2; I checked the advance at 3500 rpm which came in around 28 degrees total advance which is just about right.