I apologise if this is in the wrong place but this is my first post.
Anyway, I recently bought a 2005 mk2.5 1.8 with immaculate bodywork but there was a CEL and an intermittent stalling issue. Mostly while idling the engine will run fine then drop to around 550-600RPM for a few seconds then return to normal, this happens every couple of minutes while idling and also happens much less frequently while driving at higher RPMs too.
I plugged in an OBD reader and saw the CEL was throwing code P0172 System Too Rich (Bank 1) so immediately thought O2 or MAF sensor so used the live view function and saw the MAF is working but showing values slightly higher than I would have expected (4.6-4.8g/s at idle) and the rear O2 shows 0.000 most of the time with occasional jumps to 0.005V however this may just be the inaccuracy of my cheap OBD reader. The LTFT shows -20.3 but i assume that’s just a result of whatever problem i have.
To solve this i cleaned the MAF but still no change. What would you suggest next? New MAF? New O2? (I also checked for vacuum leaks with a smoke test but couldn’t find anything)
P.S. Had new HT leads, Cam sensor, Crank sensor, sparkplugs and front O2 in January (the plugs need replacing again though as it’s been running rich and they are fouled)
Unfortunately it is a generic code for rich fueling and without another more specific code to identify a component, some work needs to be done to identify the cause(s).
Always a good idea to replace dubious parts with known working examples to take them out of the equation.
In an ideal world the Diagnostics would identify the correct part to replace but I am seeing lots of situations where diagnostics cause confusion.
You seem to be on the right track but I would suggest replacing the MAF, even if just borrowing a good one off someone to discount it as a possible causal factor until this issue is resolved.
Might also be worth talking to the previous owner - they my have some idea as to the problem.
I would add that the crank sensor seems to be responsible for more than its fair share of MK2 running issues and often does not produce a specific error code. There has been some unreliability with aftermarket new sensors.
Unfortunately the previous owner is not very helpful as she ‘knows very little about that sort of thing’ but I was going to try and find a replacement MAF sensor today or tomorrow. Is there a good way to test if my replacement crankshaft sensor is faulty or is that just a case of finding one that I know definitely works?
Here is a good video on testing a crankshaft sensor. The guy knows what he is talking about.
The other thing to check with the crank sensor is the toothed trigger wheel that operates off the crank pulley. There are 4 teeth that actuate the crank sensor and corrosion or damage can render these partially effective or ineffective.
After fitting a new MAF sensor today I have had no drop-outs, my MAF readings have gone back to normal and my O2 sensor readings which have also gone back to normal too so I assume that fixed my problem. Thanks for the advice anyway.