Motronic double fuelling - go back to K-Jet?

[fulani 4]

Hi all - a few years ago, bored of the constant issues with K-Jetronic and chasing a positive ground in the electrics somewhere, I had my 1.9 converted to Motronic ignition.

 

Since then, it has bared scraped past a number of MOTs on emissions, and fuel consumption has been markedly worse. It does start and idle beautifully, not hunting around for revs, and solved the problem I had where it would die once it began to warm up (but was fine once actually warm). It now can't pass an MOT.

 

For the last year my car has been with a classic garage near me, who work on a lot of different stuff, but have experience with 80s and 90s ignition systems. They have determined that the ECU is telling the injectors to fire fuel for 5 milliseconds instead of 2.5, hence unburnt fuel in the exhaust, and why it can't pass an MOT. They've sent the Motronic ECU to a specialist, who confirmed it was working as it should. We've tried another one, same thing. Seems that it just isn't compatible with the standard 205 8v engine (cam profile?), so he wants to put it back on K-Jetronic. I thought I had all the parts, but the boxes I provided to him don't seem to have the actual ECU or wiring harness.

 

Just wondering if anyone else has had a similar issue having done this conversion (from a ZX Volcane), and if there's an easier way to fix this than going back to the old K-Jet and distributor etc.

Edited December 5, 2025 by fulani

[petert 630]

No mate. That is rubbish. The 8V Motronic 1.3 is very similar to the Mi16 version. I have run it with much bigger cams than the D6B version. Have you checked the fault codes? That will reveal all. It’s a very clever ecu. Report back with the codes.

[welshpug 1,710]

never seen KJet on a peugeot !

 

vw golfs certainly, a very mechanical system, using a metering head and no electric injectors, just jets much like a carb.

 

peugeot used Jetronic LE2 on the 128 bhp 1.9.

 

on a 205 motronic system the CO potentiometer in the AFM can fail, but also is a part you could get from RS components and replace,   

 

I successfully did so on the CO pot of the ZX MP3.1.    and you also have the coolant temperature sensor.

[SRDT 226]

The only 205 with a K-Jetronic is the Turbo 16.

Peugeot used the K-Jet on the 604 and 504 V6 and also the 505 but mainly for export.

[SRDT 226]

5 hours ago, fulani said:

Seems that it just isn't compatible with the standard 205 8v engine (cam profile?)

(...)

conversion (from a ZX Volcane)

The ZX Volcane non cat engine (D6E) has the same cam than the Jetronic engine (D6B), the only change is the use of the cat (DKZ) lower compression pistons. The ZX Volcane cat engine is the exact same DKZ than the 205 GTI with AFM and distributor on a ML1.3 Motronic ECU.

[DamirGTI 354]

Which Motronic is it ? 1.3 with the AFM or the MAP 3.1 ?

 

I converted mine on 3.1 , MAP ,  with lambda along with 605 ECU ..

Engine with high CR , ported head and some other stuff - it runs sweet , and very much the bennefit over Jetronic system .

 

No issues with mine , only a bit of a cold start glitch .

During winter time , cold start injection pulse is trimmed higher which makes the engine running rough for a while till it clears aka heats up , the colder the ambient temp the longer and more rough  .

I added trim pot resistor to fool the ECU during the cold start to avoid that .

 

Tried a few different ECU's  XM , 405 , 605 and it was the same on all of them , this cold start enrichment , unsuitable for 1.9 engine and think it was the same with ZX ECU too .

 

One other thing i found , these old Motronic systems really do not like Zirconia O2 sensors ! only Titania .

 

D

 

Edited December 5, 2025 by DamirGTI

[fulani 4]

1 hour ago, SRDT said:

The ZX Volcane non cat engine (D6E) has the same cam than the Jetronic engine (D6B), the only change is the use of the cat (DKZ) lower compression pistons. The ZX Volcane cat engine is the exact same DKZ than the 205 GTI with AFM and distributor on a ML1.3 Motronic ECU.

Thanks. Any idea why it would be fuelling for exactly twice the duration it should be?

[petert 630]

3 hours ago, fulani said:

Thanks. Any idea why it would be fuelling for exactly twice the duration it should be?

It must be constantly throwing up mixture fault codes because it runs in closed loop mode 99% of the time, using the O2 sensor. Does it have one? Without it wouldn’t properly.

 

Does it have a diagnostic light?

Edited December 5, 2025 by petert

[welshpug 1,710]

if its a zx mp3.1 it doesnt use a lambda probe, just a Potentiometer.

[DamirGTI 354]

It's down to the cold start enrichment mapping within the ECU id say , why as much even for what seems to be the same 1.9 engine i really do not know .

 

Motronic 3.1 also uses green ECU temp. sensor , little bit different than Jetronic blue but not by much .. 

 

With or without O2 sensor makes no difference really on 3.1 , it does lit up the engine check light without or unplugged but if anything it goes much better with it unplugged , just slightly increased fuel consumption still way better than on Jetronic .

 

If it holds that injection pulse for longer aka post cold start whilst warm , id check the MAP pipe might be partially blocked or kinked somewhere ..

 

D

[petert 630]

My bad, I wrongly assumed it was M1.3.

[soundguy 14]

On 12/5/2025 at 9:02 AM, fulani said:

....fuel consumption has been markedly worse...

Have you tried a known good/new coolant temp sensor? Maybe its failed and the ECU is permanently in cold start/rich mode?

[fulani 4]

On 12/6/2025 at 7:58 PM, soundguy said:

Have you tried a known good/new coolant temp sensor? Maybe its failed and the ECU is permanently in cold start/rich mode?

Yes, just heard from the garage. They've double checked it's the right sensor, it's outputting the correct levels when cold and hot. They've tried bridging it, and ECU is still overfuelling.

[SRDT 226]

Then it could be the CO pot.

Also did they test the sensor values from the ECU connector? Maybe there is a problem with the wiring loom itself.

[fulani 4]

Where is the CO potentiometer? Is it part of the AFM?

[SRDT 226]

There is no AFM on a Motronic MP3.1, on a 405 the potentiometer (Bosch 0 280 101 012) is near the battery.

[welshpug 1,710]

on a zx from memory its bolted to the ecu cover.

[PhilNW 63]

Have you sorted this yet?

 Just been re-reading this topic, the motronic 1.3 kit came from a ZX volcane?

 

Is the ecu the right one to match your car? From what I understand the motronics system was used on lots of cars uncluding BMW. Going back to basics was it the right ecu for the volcane in the first place? Does the ECU have a model number on it?

 

The double injector timing (5ms instead of 2.5ms) could suggest it is matched to a bigger capacity engine. Where did the 5 and 2.5 ms injector timing info come from?

Edited December 16, 2025 by PhilNW

[fulani 4]

I don't remember exactly where it was sourced from, Miles did it all for me more than ten years ago now.

 

Will try and grab the model number. The chap who's had it at his workshop for over a year now wants to put it all back to the original Jetronic. I've found all the bits Miles gave me back, other than the actual ECU. It feels like a backwards step to me, but at this point I just want it back on the road!

 

The other option he's mentioned is a completely aftermarket ECU system, but they seem pricey...

 

ETA: Going back through my PMs, it was sourced from Cybernck - who said it was MP3.1. Miles then had to create a whole new wiring loom for it, and said it was missing a swirl pot. He also had to source a flywheel (and I definitely don't have the old one), so going back to Jetronic is starting to look less and less like a good idea.

Edited January 14 by fulani

[welshpug 1,710]

why dont you just get the car to miles?

 

4 hours ago, fulani said:

 

The other option he's mentioned is a completely aftermarket ECU system, but they seem pricey...

 

 

as miles has already made a loom, and the MP3.1 used a crank sensor, a proper TPS potentiometer and a wasted spark coilpack,  the only thing you would need to do is change the ecu plug to make an aftermarket ecu run it.

 

 

4 hours ago, fulani said:

 

ETA: Going back through my PMs, it was sourced from Cybernck - who said it was MP3.1. Miles then had to create a whole new wiring loom for it, and said it was missing a swirl pot. He also had to source a flywheel (and I definitely don't have the old one), so going back to Jetronic is starting to look less and less like a good idea.

 

 

 

 

I guess you mean the CO pot, not SWIRL pot.        as in the fuel correcting potentiometer I mentioned before, if one wasn't fitted then the fuelling will be way out.

[SRDT 226]

The Motronic flywheel can stay but you need to check if the thermostat housing was changed or modified to remove the distributor.

[pug_ham 245 3 Cars]

Not sure if this might help but I watched Kitch's video on his Up'N'Down youtube channel which had an over fueling |Mi16 caused by a faulty fuel pressure regulator.

 

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