2.1 MV Digifant stalls when hot

[Jono75]

1990 T3 2.1 MV engineed Caravelle has a problem in that it stalls when hot. If you let it cool down it works again.

If you disconnect the lambda sensor then it doesn't have this fault. I have replaced both of the water temp senders and the lambda sensor.

I am not sure where I read it but I believe disconnecting the lambda sensor leaves the ECU - Digifant I - running in the default map. Which means it is ignoring the temp readings from the engine and leaves it running rich.

Anyone else got experience of this

Ta

Jono

[Aidan]

Which lambda did you fit ? In my experience only the Bosch one  0 258 003 957

It's not ignoring the temp sender it just runs the base map without correction

If there are any air leaks on the exhaust side then the lambda will read lean and the ecu will try to richen the mix

Also I have come across a blocked spigot on the plenum chamber preventing the vacuum to the fuel regulator from acting, this is important as at idle this vacuum is strongest and makes the regulator open and pass more fuel back to the tank thereby reducing fuel pressure as the ECU map is expecting. But disconnecting the lambda didn't on it's own improve the running except temporarily, 1/2mile later it would stall at the next set of lights. I reamed out the spigot with a jewellers screwdriver and lo and behold all good.

[Jono75]

Which lambda did you fit ? In my experience only the Bosch one  0 258 003 957

It's not ignoring the temp sender it just runs the base map without correction

If there are any air leaks on the exhaust side then the lambda will read lean and the ecu will try to richen the mix

Also I have come across a blocked spigot on the plenum chamber preventing the vacuum to the fuel regulator from acting, this is important as at idle this vacuum is strongest and makes the regulator open and pass more fuel back to the tank thereby reducing fuel pressure as the ECU map is expecting. But disconnecting the lambda didn't on it's own improve the running except temporarily, 1/2mile later it would stall at the next set of lights. I reamed out the spigot with a jewellers screwdriver and lo and behold all good.

I don't think it was Bosch. I fitted the one Brickwerks supplied. The part number they show is 025 906 265 B. It used to have a CAT  which was so old it effectively was a pipe. I fitted a straight through pipe and bossed the new sensor in. There should not be any airleaks as the exhaust has been fixed up!

For me disconnecting the Lambda makes the problem go away. Just feels like it would be much better to have it working!
 

[Aidan]

025 906 265 B is the VW part number- if you have the packaging then check what you got from Brickwerks as Brickwerks supply either Bosch or Beru, I've never had long term success with the Beru one; the Bosch number I gave you is THE one in my experience, but before replacing anything check that the signal wire is completely insulated at the joint to the wiring loom, the voltage from the lambda is low so it doesn't take much to mess up the signal. You can interrogate the signal with a multimeter to see how it is reading, the voltage should go up and down as ECU micro changes the fuelling, if it goes to one extreme and stays there then ECU is not understanding the info from the lambda and is just enrichening the mix as per your experience

I don't know why the Bosch work and other brands seem not to, I think it must be to do with refresh rate, wave form, or hysteresis or something and the ECU being sensitive to the subtle difference. The ignition system was designed by Bosch in conjunction with VW, it's just a variant of Motronic

[Jono75]

025 906 265 B is the VW part number- if you have the packaging then check what you got from Brickwerks as Brickwerks supply either Bosch or Beru, I've never had long term success with the Beru one; the Bosch number I gave you is THE one in my experience, but before replacing anything check that the signal wire is completely insulated at the joint to the wiring loom, the voltage from the lambda is low so it doesn't take much to mess up the signal. You can interrogate the signal with a multimeter to see how it is reading, the voltage should go up and down as ECU micro changes the fuelling, if it goes to one extreme and stays there then ECU is not understanding the info from the lambda and is just enrichening the mix as per your experience

I don't know why the Bosch work and other brands seem not to, I think it must be to do with refresh rate, wave form, or hysteresis or something and the ECU being sensitive to the subtle difference. The ignition system was designed by Bosch in conjunction with VW, it's just a variant of Motronic

Thanks Adian, time to get technical with it I want a readout of the Lambda so I can check if it goes haywire when the vehicle stalls. I need to be careful I don't introduce another issue when measuring it. I will do it momentarilly to start with sitting on the drive with a multimeter. Will post back here for others with a conclusion!