Hi guys, first post on the forum as a proud, new MX-5 owner.
My model of MX-5 is: 2007 2.0 NC GT PRHT
The car is in very good condition aside from bushes in the front wishbones needing doing to pass the NCT. The emissions are slightly out as well and I was hoping you guys would be able to help me.
Engine/Oil Temperature: 97C Low Idle (760 RPM) CO 0.50 vol% (0.30% maximum)
HC 34ppm High Idle (2828 RPM) Lambda .98
CO 0.83 vol% (0.20% maximum)
HC 22ppm (200ppm maximum)
The car is stock engine and exhaust wise apart from a cobra sports backbox. It is probably overdue a service by inspecting the oil and air filter, I have yet to check spark plugs/coils/MAF for fouling.
Got the car warm and at idle, the AFR is reading 14.3 with the downstream lambda sensor reading .9V
My first thought is obviously a fluids and filter change, and to check spark plugs + clean MAF. But it seems likely to me that the upstream lambda sensor is failing, would I be right in thinking this? There is a slight bit of smoke from the exhaust when you blip the accelerator at a standstill, and you can smell the petrol from the exhaust so it’s definitely running rich, however the car idles and drives well with loads of power and no hunting from the engine. It also seems to get up to and maintain operating temperature just fine.
This will be my first petrol car so please go easy on me and thanks in advance for any help!
Where does the coolant temp marker read on the gauge, should be just short of midway?
I ask as the stat can fail at your age of NC leading to the engine no operating at the correct temp.
I was just consideri g the emissions test this morning.
I have the cobalt box, the left pipe gets dirtier quicker than the right.
Visualising where the probe goes at my local mot station, he always pops it in yhe right hand side.
I wondered if this could affect emmissions if say most of the gasses are coming out of the other pipe.
Your lamda sensor I believe should read more like .45v not .9 so it does support your upstream lambda sensor theory although the upstream sensor is a pukka AF sensor not a basic Lambda so of course is much more expensive.
I had very similar issues to this a couple of years ago, even down to the low idle speed and it turned out that a new MAF and AF sensor did the trick. Got some Denso items from Opie oils
Yeah, I had a feeling from the research I had done that it is the MAF + upstream Lambda sensor.
Not the worst thing to do, couple of hundred euro all in. Will do a full service, clean the MAF and get a new Lambda sensor and she should be perfect.
The car was given a service with oil (5w20 fully synthetic) and filters. Unfortunately it turned out very little oil was in it and I have no idea how long it was run with that amount of oil in it, but the engine still ticks over like a mouse’s heart. I hope that won’t result in any rod knock or slipped bearings in the near future and I will be dilligently servicing it in my ownership to come. The MAF was also given a good clean, and I opted not to do the upstream lambda sensor just yet as upon inspection, it almost looked brand new (externally) and I could tell work had been done to it as there were fresh zip ties holding the wiring for it in place. Neither the service or the MAF being cleaned helped with the 14.3 AFR or the rich smelling exhaust, so I kept looking.
After some research and looking over the car some more to get to know it, I discovered the battery connections were loose on both terminals with the screw-locks tightened all the way down, and the battery itself was zip tied in place. After creating a homemade solution to fix it properly in place using the OEM clamp I found hiding under the seat and some threaded bar, and fitting some copper sleeves around the battery terminals using 3/4" tubing we had lying in the shed, the battery is now connected up properly and sitting tight.
To my surprise, once I started the car up it idled better, throttle response felt better and my OBD tool and the torque app showed an idle at 14.7 AFR, with the rich smell from the exhaust gone. At this stage I hope the cat hasn’t burn through, but I should be in a position now where I can throw in a bottle of redex/dipetane and send it through the NCT (after getting the front bushes done).
Who would’ve expected that to be the problem? Not me anyway!
Most vehicle ECM components work on a 5v regulated supply from within the ECM itself. Some are better than others at regulating this from the nominal 12v supplied from the battery. Once the regulated voltage drops below the 5v threshold due to a drop in the battery voltage all sorts of odd things can happen with the various sensors that send information back to the ECM.
Looks like this may have been the problem with your car. Presumably it started ok and everything else seemed to work?