Hello all, apologies if I have not found posts that already answered this but didn’t find exact answers!
We have a 1991 1.3 M2F digi in the family that has developed some issues. It seems to be running rich (plugs black and sooty), and doesn’t respond very well to full throttle when cruising or under load, hesitates then gets jerky on light throttle. Also when it’s warm, the idle will be ok initially then start the drop until almost stalls, then self corrects, then drops again after 30 secs or so and this cycle continues. 
So far I have checked for vacuum leaks everywhere, re sealed the inlet elbow to throttle body and the air inlet temp sensor just in case, checked all connectors, and just replaced the blue temp sender. It has also had new air filter and dizzy cap/rotor. Problems seems to remain.

I just read on another post that maybe I should reset the ecu after replacing the blue temp sensor by unplugging while running? Do I need to do this to get the ecu to acknowledge the new sensor?

Any other ideas I should try? Lamda sensor perhaps? 
Runs fine when cold and starts on the button so ruled out HT Leads for example.

 

hope you guys can help!
 

 

try unlpugging the o2 sensor and see if the problem goes away

 

was the blue temp sender a vw genuine part?  non genuine can be terrible , you can check the readings with a basic multimeter and idealy check the 2 wires for continuity all the way back to the ecu socket , repair pins are avail , it needs to be right 

 

the ecu see's the sensor straight away , although if you want you could buy a vcds lite lead and software and scan it for codes , it would also show live data to prove if the 02 and coolant sensor are okay

 

prob not the fault but also worth checking all the earths (brown wires ) are clean and tight  

Thanks Steveo, looks like the problem is the O2 sensor, unplugged it and 3 seconds later the idle came up and was perfect. Will change this.

Re the blue temp sensor, it was a Meyle part. I also tried unplugging this before I tried the o2 and the idle dropped even more, I read on another post here that the revs should rise when unplugged?

yeah revs should raise , might indicate other issues 

 

i wouldnt trust a meyle part for that , the whole running of the car is based around that sensor and its reading , so if its wrong then everything is bad 

 

get a bosch 02 on it ...then when funds allow id go genuine blue sender and buy a vcds lite kit ...you  can drive it around with the o2 unplugged if you wish 

Interesting view re Meyle, my partner works for VW heritage and this brand is what they sell and supposed to be OE quality. I wonder if the o2 sensor failing and telling the ecu to pump more fuel in than it needs was messing with the blue temp sensor which when disconnected would also tell the ecu to pump more fuel in and hence bog down. Anyway think it’s clear the o2 needs to be replaced so will do that then test again, thanks for the help!

no it cant mess with the blue sensor ...its just a thermosistor and the wires to the ecu , can all be checked with a £5 meter

 

most of the informed enthusiasts with these insist on using a genuine part , i know myself ive had laughable quality stuff before , dont just assume its good because they sold it to you , the car will never be right if its a faulty sensor