10/04/2026
Ryan’s Cupra in this week!
Another very interesting one this… came on the back of a truck severely misfiring on idle, burbling on decel, smoking extensively out of the exhaust.
lots of various faults for lambda, misfires, Hyde cylinder and rich codes - it’s been to a lot of garages with no joy on a solution, so sleeves where rolled up!
Was quite testing this one!
Firstly, TMAP readings where everywhere - so a replacement TMAP was fitted which then stabilised, diagnostics points out a stretched timing chain, so we started there - as you can see his tensioner had extended extensively, so new timing chains tensioners and guides sorted. Somewhat to be expected as the vehicle had hit 94k miles
Spark plugs 1&4 were heavily sooted up indicating a rich condition, to which we removed the injectors to have a look - not in a great condition! So new injectors fitted, whilst we where there we walnut blasted the soiled intake valves back to like new
All back together… idling almost perfect now, almost…. However!
Rich on idle fault code still present, along with lambda sensor 1 negative lead open circuit. It was clear a new lambda had been fitted previously, but we have plenty here at the unit so a replacement was installed an all was then well
So! Off to Unicorn for tuning as he originally wanted a stage 1 tune at another garage but received it back still misfiring, so it was flashed back to stock… or was it??
GTI performance software had been flashed on the ECU, which will run - but it is quite different to Cupra software! That doesn’t help
Rick flashed a stock Cupra 280 file on and began work tuning, and all was going fine until intake temps rose above 40c and then “BING” management light and a rich condition fault code, along with a negative lead open circuit lambda fault
So tuning was halted, and back to the unit
Sure enough, lambda was sooted up as was cylinder 1 & 4 plug - was it wiring to the lambda? This was checked previously at the plug and connector, and the loom was untouched.. so what was occurring?
Looking at the fault logs present and previous there was a smoking gun, 41c! As soon as the intake temps hit 41c the faults would occur and the lambda negative would short out. This was obviously because the lambda was contaminating until it no longer gave the ECU is 0.2-0.9v signal
So what happen to air as it warms? It becomes less dense, less oxygen rich meaning less fuel is needed to target its AFR - so something wasn’t either being fed back to the ECU correctly or something wasn’t performing as the ECU was asking…
Hmm…. I’d previously checked the oil for signs of fuel contamination as a leaking high pressure pump will cause rich condition codes, but oil was fine, but let’s just look at the regulator duty
Sure enough, as soon as it hit 41c on intake temps that regulator pegged at 89.99% so wiring was then checked.. and there I found the 5v TMAP signal wire with a corroded break - but still some continuity to actually feedback something! 🙈
Wiring repaired, replacement high pressure pump for good measure.. lambda cleaned up
Perfect. Good idling for over an hour, logging all good and a good kicking up the road and it’s now moving like it should!
Ryan can finally enjoy his car again 😎👌🏻
Thank god it’s Friday…..
