Intermittent Engine hunting & p0420

[MGF Anthracite]

Hi, I’m looking for some help -
I have a 2001 MGF 1.8i which has intermittent issues with the revs when coming to idle, it will hunt around and nearly stall but then recover and sit at the right level. I’ve changed all the regular suspects, but still no joy, any ideas? I’ve changed the following -
New air filter
New coolant temp sensor
New Throttle Position Sensor
New 52mm throttle body
New spark plugs, pencil coils and ignition leads
Second hand MAP sensor which the garage said the original was at fault, but also it had oil in the sensor - not sure if that’s normal?
Most of the above was as part of a big service, so needed doing anyway, but it’s still not quite right. I’ve done the TPS reset a few times, and it happily sits at idle until I start driving when the issue come back.

Also, I previously had p0420, would putting a new Cat and O2 sensors on it help as the car might think it’s overwhelming, or is this not related?

Not sure what to check next?

[Rob Bell]

Do you have an OBD reader that can provide live data? What you are interested in is (1) whether the lambda is actually working and (2) whether it thinks your engine is running lean or rich.

The cause on mine recently was actually a vacuum leak from a detached vacuum hose, but I also had a failed Lambda sensor. As my car is pre-EOBD compliant, there were no codes, but the diagnostic data gave me the clues I needed

The symptoms I had were similar to yours: ran fine on full-throttle, but would hunt badly at constant throttle, because the ECU wouldn't know how much fuel to throw into the cylinders to maintain efficient stoichiometry - the usual pattern of fuelling is to constantly increase/decrease fuel and rely on the lambda to tell you when the fuelling is "just right".

[MGF Anthracite]

Thanks Rob, I do have a basic OBD code reader so I’ll take a look at the live data to see if that helps. I’ll also see if I can see any air leaks as well, got to be worth a look. It’s such a good car when it’s driving, just an annoying little issue, cars eh….

[Rob Bell]

Yup... pesky cars. Always something!

[MGF Anthracite]

So I’ve looked at the live data & I think I can see what’s happening, but can you confirm -

1) O2S11(V) = 0.060 to around 0.725 - my understanding is that this fluctuation is correct for the pre-cat sensor?
2) O2S12(V) = 0.360 to around 0.870 - my understanding is that this is high for the post cat sensor which should be below 0.5, it vary rarely went down to 0.360. I’m guessing that the sensor is failing?

A visual inspection seems to show the sensor is quite old, but the cat was only replaced about 18 months ago so I’m thinking this is ok.

Would I be correct in my above assumptions and that a new sensor would fix the issues?

Thanks again all.

[Rob Bell]

Looks as though the pre-cat lambda is reading high - detecting a rich fuel setting. It should be oscillating around 0.5V when up to temperature. Old and failing lambda sensors can show a similar oscillation - usually worse when cold and then improving with temperature.

Given the abnormal readings from the pre-cat Lambda, the post-cat lambda is reading a near normal value - so to me, this exonerates the catalyst, which appears to be working normally

A genuine Bosch Lambda sensor (best to use OEM) only costs about 30 quid, so I would opt for that.

[Mike63]

Well that explains one point. I got a MIL and freeze frame captured P042, which my OBD App wrongly identified as Stuck EGR valve. As mine is also a 1.8i 2001, it's Euro-3, which applied 2000-2005. So it won't have EGR, and it almost certainly won't have a post cat lambda sensor. It sounds like Motorola AIEG / Rover chose not to apply the US CARB OBD codes. Freeze Frame also captured Engine Coolant at 136'C, which sounds like a much more common and potential HGF fatal air lock. I put a new cat on about 5 years ago, but maybe it's the pre-cat lambda sensor. Jobs for the weekend ...

[Chris Tideswell]

P0042 HO2S Heater Control Circuit (Bank 1 Sensor 3)

Any MEMS 3 car should have a post cat sensor, although the pre-heater isn't used in this sensor.

[Geoff.F]

Post Cat sensor fitted on the 2000 “Mk 2” F.

[Mike63]

Curious. Two votes for post-cat Lambda sensor, so I fully accept I may be wrong. It wasn't required for Euro-3, and I work for an OEM so I know how they think: not required = not fitted. Sometimes, there are credits for early standards adoption, and early adoption can get you advance info on the issues. I'll plug in tomorrow, and I'll bet you a pint of your perferred tipple that I don't have a post-cat lambda sensor. Any takers?

[Mike63]

OK Chris, Geoff, you're right, there is a post-cat lambda. I didn't see it from under the car, but it's there in the flash picture.

Based upon the Freeze Frame data ECT = 136'C, I figured I'd start by bleeing the coolant. Turns out it didn't need it, which seems odd. I recall the ECT is in the thermostat housing? Except I have a PRT up by the front radiator and so no thermostat on the engine, so I can't picture how the ECT ended up in a steam airlock, unless the car had lost half its coolant, which it had not.
So I cleared codes and went for a 15 minute buzz. No overheating. Idling on return, the car reported it was in closed loop fuel control, pre-cat Lambda as expected, post cat either stable 0.2V or stable 0.7V: neither being the right answer. 0.7V indicates it's up to functional temperature.

P0042 didn't re-appear as pending, but I got P0304 Misfire Cyl 4'. As I'd left the cover off, I went to the effort of a compression check, 170-150-150-165psi 4>1, and then a leak down as #4 was the highest compression, 25-25-25-35% 4>1. Leak down for #1 was hissing at oil filler indicating scored bores or broken ring. All the plugs looked the same. The ignition coils had rattled their bolts loose because one bolt on each had stripped, so I put in a couple of helicoils. As this is wasted spark, these are inductive coils and I doubt the aluminium cam cover has any impact on the coil laminate iron core gapped magnetic path. So maybe fuel, but otherwise no obvious cause. Better than HGF but worse than finding some positive cause to fix.

@Chris Tideswell P0042 HO2S Heater Control Circuit (Bank 1 Sensor 2)?

[Rob Bell]

That data all looks reassuringly normal. The cylinder compressions look okay to me (but I am not the world expert in these things). Interesting to read your leak down tests and the potential gas leak on cylinder 1? Might be worth performing a boroscope to look at the bores, but another thought that comes to mind is a cracked liner. Unfortunately, this is quite difficult to diagnose, and I gather the best approach is to remove the head and get the liners crack tested, so not necessarily the first thing you want to be getting into, but seems to be a possibility?

[Mike63]

Thanks Rob. I have a boroscope, very cheap these days. Unfortunately, checking the bores needs the mirror adapter at 45', and it's plastic and not a good enough mirror. Could also be a cracked ring and bore scoring. I'm Engine hunting I think is due to the MEMS ramping fuel too lean, 0.2V could be 16 or 17:1, not enough to support idle. So I think I do need a new CMS, but more bothered by the suspension at this point.

[Rob Bell]

It could indeed, you're right Mike - a failed ring and bore scoring may be the issue. I have a hunch that you will end up lifting the cylinder head to really get to grips with what is going on here - and of course, once into the engine, you will be in a good position to make the suitable repairs.