5 replies to this topic

[Padwah]

I took my 1994 Mini Sidewalk SPI for its MOT recently and it failed its emissions test rather miserably as pretty much everything is over the required limits:

 

 emissions.png   18.79K   8 downloads

 

I recently had an issue with the car running rough and a strong smell of petrol from the exhaust, after reading a few other posts on here replaced the fuel trap and all vacuum hoses and things seemed much better and it drives very nicely with no hesitation, flat spots or bogging down. However, as its failed the emissions test there is obviously still something wrong.

I took it for a drive this morning for 20 minutes to make sure everything was up to temperature and then used the rovermems android app to see what was going on. A couple of things that I noted were that the lambda sensor did not enter closed loop mode and was measuring ~300mV, I think this means a lean mixture? I tried pinching the fuel return line, as mentioned here: https://memsfcr.co.u.../lambda-sensor/, to force a rich mixture and that had no effect on the lambda voltage reading, though I don't know if that test applies when its in an open loop situation. The throttle_pot_percentage was also reading ~11%, I don't know if that's normal or that it means my throttle is stuck on?

 

 mems.jpg   24.62K   11 downloads

 

I also tried measuring the temperature either side of the catalytic converter with a laser and it was 110C on the engine side and 100C on the exhaust side, but the car had been idling for quite a while at that point so also not sure if that indicates a dead cat or not!?

 

I'm planning on replacing the lambda sensor with a Bosch unit and possibly the catalytic converter as well, but I would appreciate it if someone could take a look at the attached copy of the reovermems output and tell me if there's something out of the ordinary that I might be overlooking? I usually work on old Volkswagens so the mini's and fuel injection are a bit of a mystery to me!

Attached Files

•  rover_mems_export 20.txt   134.07K   4 downloads

Edited by Padwah, 20 June 2026 - 05:12 PM.

[68+86auto]

So unfortunately you don't have the O2 value from the emissions test BUT based on the lambda, it is likely high. Lambda is calculated from multiple gasses. I'm no lambda expert though as that isn't used here.

 

There are a few facts we can establish:

- low HC so it isn't an ignition misfire issue.

- high CO so there is excessive fuel.

- high lambda which is likely caused by high O2.

 

To me it sounds like an exhaust/manifold leak before the O2 sensor.

 

Because the CO is so high, it is NOT the catalytic converter at fault. The cat may be damaged if it has been like this for long but replacing it will not fix the fault.

 

There is some good information about emissions here: https://crypton.co.z.../emissions.html

Edited by 68+86auto, 20 June 2026 - 10:58 PM.

[Padwah]

Thanks for taking a look, I was assuming high CO due to a faulty or stuck lambda sensor, hadn't thought about the sensor being fine and it actually is seeing too much O2! I'll try and figure out how I can do a leak test on the engine and exhaust manifold and the manifold to exhaust joints.

[NLinPEN]

The system not reaching closed loop is of course an important warning sign. I do see some other, unexpected, test results in that screen shot. Such as temperatures being at 200 or zero. Or values listed as 255, which usually means that it is higher than what the computer can measure (beyond its maximum value). Idle control valve position is listed as 75, whereas my understanding is that it should be around 30~35 when the engine is at operating temperatures and idling.

[Padwah]

Thanks for taking a look, I wasn't aware of the default value for the idle control valve and must admit that this was probably my fault! I think I remember adjusting the idle stop screw trying to adjust the idle speed when I had issues with the vacuum pipes! I've found the guide on setting it here: https://memsfcr.co.u...epper-motor/ so I will go through that this week. As far as I am aware the sensors that are reading 200/255 (ambient temp, fuel rail temp, lambda frequency, lambda duty cycle) aren't actually used in the SPI hence the default values. 

 

With regards to a potential leak in the exhaust manifold, is it straight forward to remove the manifold and replace the gaskets from above the engine bay or will I need to get it up on the axle stands?

[NLinPEN]

Thanks for taking a look, I wasn't aware of the default value for the idle control valve and must admit that this was probably my fault! I think I remember adjusting the idle stop screw trying to adjust the idle speed when I had issues with the vacuum pipes! I've found the guide on setting it here: https://memsfcr.co.u...epper-motor/ so I will go through that this week. As far as I am aware the sensors that are reading 200/255 (ambient temp, fuel rail temp, lambda frequency, lambda duty cycle) aren't actually used in the SPI hence the default values. 

"adjusting the idle stop screw trying to adjust the idle speed" is a misunderstanding that apparently happens often. You cannot adjust the idle rpm because it is fixed in the software of the MEMS. The most detailed explanation about this idle stop screw, and how to adjust it correctly, is this Facebook post:  https://www.facebook...341224044395839

Edited by NLinPEN, 22 June 2026 - 01:40 PM.