Mk 2.5 1.8 vvt running rich

I have had my 5 for a year now. When first purchase the car I replaced the air filter for a colbalt and k&n kit. All was good. In January this year the back box had rusted so badly it completely separated from the pipe. I replaced with a stainless cat back. All ok for a week then CEL P0172 rich on bank 1. Replaced MAF sensor. Now runs really rich all the time. Idle bogs out then catches up again. Jerky drive when engine warm to hot. I have replaced plugs, leads, cleaned throttle body, had 02 sensors checked, replaced pcv valve, crank position sensor, coolant temp sensor, fuel regulator My oil is nice and clean (I did a service in October). Does anyone have ideas? The 3 mechanics I’ve been to don’t have a clue. Of course now lockdown means I need to solve this myself.
Many thanks in advance.

Possibly a MAF sensor issue.

I have replaced MAF sensor and the readI ha are good. I don’t have a CEL but a strong smell of fuel from the exhaust and I can shoot out black soot.

3 pin sensor under coil .rear of engine. Really common fault/issue

Thank you for your replies. Can you please explain what you mean by under the coil? I have 2 coil packs running from a 4 pin connection on the loom.

Rear top of engine below coils. Lift coils up and you will see 3 pin sensor there

I hope this may be of use as I had a very similar issue to this on my Mk2.5 1.6. I too replaced the MAF and it fixed the original problem I had of the car stalling when stopping at traffic lights, but the car never seemed right afterwards- revs hunting, jerky and surges of power (especially when cold) and fuel consumption was high. After a lot of searching and changing a few other possible candidates, I discovered that despite what many well know auto parts stores will tell you (including some who really should know better), the MAF on Mk2s are not all the same. Mk2s from 98 to 00 have an FP39 MAF, whereas 2001-2005 have an FS1E MAF. Many shops will sell you, and did to me, a MAF that is not compatible with your model. The FP39 will fit a Mk2.5, but the car runs poorly. I’d suggest double checking that you have the right MAF and a quick search on ebay will find you the right one if not.


I bought one used from eBay and it is FS1E. I think I will buy another as it’s still possible the new one is faulty.

So I had to bite the bullet and purchased another MAF sensor. This time brand new. It works. Problem solved. The lesson learnt- for a few more quid buy new not used.

1 Like