Unstable fast idle
QIt’s been a while since I, or a member of my family, had an issue with a car where I was asking for help! The vehicle details first – it’s a
2014 Nissan Micra Tekna DIG-S with CVT transmission. The current mileage is 23,775.
This vehicle belongs to my sister, and is located around 22 miles away from me, so I don’t have instant access.
Short version of events, she took it to one of those places that only does MOTS, that she and her husband always use, and they failed it on emissions saying they couldn’t test it, as it wouldn’t hold an engine rpm of 2500. Over the weekend I got involved with my OBDII scan tool and found the revs could be up to 3000rpm for the test. Her husband has returned to the same MOT place today with the car, complained about their methods, they’ve re-tested it and issued a fresh MOT.
So, on the face of it, all is well. So why am I asking for help? Because there’s a bit more to it than the short version.
My sister has owned this Nissan Micra from new. She contacted me a few days ago in a text and was upset that the car had just failed the MOT. As outlined above, the Mot-only business she took it to had failed the car because they could not maintain a constant 2500rpm to conduct the emissions test, and had thus said it was a serious fault that needed immediate repair, even though on the face of it the car drove and functioned well.
As I own a Foxwell NT301 code reader, I offered to go and check the car for fault codes. There are no stored fault codes. Nothing. Not even any pending codes.
So, since the MOT business had said something to my sister like “Probably a dirty Lambda sensor”, I tried reading the live data off the car, without really understanding much of what I see.
Just for the record, I read off
‘O2B1S1(V) 2.250’-’O2B1S2(V) 0.760’.
While doing this I occasionally revved up the engine with the accelerator using my hand (since I was crouched near the footwell) and S1(V) didn’t vary much at all, but S2(V) sometimes dropped as low as
0.1 momentarily and occasionally went up to 0.8, but was mostly somewhere around 0.75-0.76. I assumed S1(V) was the pre-cat O2 sensor, and S2(V) the post-cat O2 sensor with cleaner gases, though I’m not sure what units the figures recorded are measured in.
During this process it did indeed become apparent that it was impossible to maintain a fast idle of around 2500rpm.
The rpm was stable below 2000rpm, and if I went up to near 3000rpm it was stable, at least when I tried it (my sister and brother-in-law today said it dropped back on them when approaching 3000rpm) but when I was there with my code reader, if I throttled back from 3000rpm to try to get to 2500rpm, it would suddenly fall to just above slow idle speed.
Today, (two days later), in my absence, my brother-in-law contacted another garage that repairs cars and does MOTS. They told him this kind of thing is a common problem. They told him they come across it a lot (though not on a Micra before) and it is usually impossible to find the fault, but if a speed anywhere in the test range could be held, they would test it and if emissions from the exhaust pipe were in range, issue an MOT.
My sister does not want to go to Nissan with this problem as she isn’t happy with previous treatment she has had at the dealer, and the prices they charge. Because of what my brother-in-law was told by the garage, and the fact he believed he could get a stable fast idle speed around 2800rpm, he went back to the original Mot-only garage – they put the Micra on the exhaust gas analyser again, and as a result they have now issued an MOT.
Though my sister sent me an image of the exhaust gas analysis (attached) and
I’m confused because for the fast idle HC and CO reading, and the natural idle CO reading, they’ve entered figures of ‘0’, and also not stated the actual rpm the engine was tested at.
I’m also confused because I myself have had these ‘SUN DGA5000 MOT’ print-outs, and have had a header that includes ‘Catalyst equipped vehicle with closed-loop control’ for one MOT and then ‘Basic