Brief power losses under light load

[Hopkins]

I noticed while doing a couple of trips on the motorway this weekend that when I was maintaining a constant speed at about 4k revs, there was the occasional "tug" as if I had briefly come off the power. It didn't seem to happen when I floored it, but I did notice that power at the top was lacking - the car didn't want to get much above 8.2k revs in fourth.

When I filled up with petrol mid way, the fuel consumption (which I record habitually at fill up using an app) was lower than expected for the type of driving I'd been doing on the last tank.

At the end of the final journey of the weekend I amused myself with a couple of rapid take-offs from the lights, only to notice that my engine had stalled when I came to a stand still at the third set of lights. It restarted without hesitation, although the engine light was now on. Checking the ODB II codes when I pulled into my driveway a few minutes later I saw the following codes:

P0335: crankshaft position sensor A circuit
P2097: post catalyst fuel trim system too rich
P0172: system too rich bank 1

Over Christmas I had a load of work done, which included the following unexpected line:

"Crank Position Plate (second-hand) fitted (Required to eliminate engine fault code)"

My first question is: could the earlier symptoms I described, and the two "rich fuel" codes, be caused by the ECU not getting accurate positional data about the eccentric shaft or are we likely to have two independent issues here?

My second question is: is there any risk of further damage if I were to drive this the 40 miles or so back to the garage (The Performance Shop) for them to fix?

I should mention that, during the service over Christmas, the spark plugs were changed. Coils and leads were not - they have been used for about 20k miles since the last change.

[350matt]

sounds like a couple of issues here
I'd start by cleaning the MAF sensor and ensure its o-ring is in good nick

Also has the 20 brake pedal 'stomp' been carried out post crank sensor change ?

[Hopkins]

Thanks - I am unaware of the 20 brake pedal stomp! Is this a "special procedure" to reset something? I can ask this over the phone.

[350matt]

the brake pedal stomp , clears the ECU 'learned values' back to the factory default

It involves pumping the brake pedal 20+ times within 8 seconds with the ignition on and engine off
on an S1 car the oil pressure gauge should sweep from right to left

[warpc0il]

This is what is being referred to as the brake pedal stomp - https://www.rx8ownersclub.co.uk/forum/v ... 25&t=34703

However, is sounds to me like you have either a loose battery connection or a bad (loose/corroded) earth connection.

Either would cause interruptions in power and any number of random CEL codes, including those you've seen.

[Hopkins]

Thanks - I'll check connections and MAF and reset the ECU. Is there anything else which the ECU reset will "forget" which isn't mentioned in the linked post?

[Hopkins]

(For example, I had a minimap with a few tweaks including idle revs, quantity of oil injected, cold soft rev limited, etc. Does that all remain post ECU reset?)

[qwakers]

yes. hard stored info stays no matter what you do, only adaptations get reset.

[Hopkins]

Update. I checked battery connection and earth connection under the battery box, but no other earth connections yet. I performed the PCM reset procedure successfully. I also got the Bluetooth OBDII device running and the app OBD Auto Doctor up and running and, after watching a video about MAF sensors, was able to verify that the engine load was going as high as 100% - according to the video if there are issues with the MAF then it will show up as the engine reporting that it is not under full load.

I've subsequently got through a tank of petrol and I can confirm that there is definitely a fuelling issue: last MPG calculated as 12.5! And this was two runs up the A12 in moderate traffic, which would normally have resulted in 24-25MPG. The "brief power losses under light load" were still present and also occasionally then manifested more violently as judders under medium to heavy load which required coming off the throttle and reapplying to quell.

At the end of last night's journey the following codes had flagged up.

I'll book back in with The Performance Shop, but do these developments hint at a likely diagnosis?

[qwakers]

Maf, upstream o2, injectors.

you need to look at maf flow rate, injector duty cycle , lambda and short and longterm fuel trims. I'd bet at a overreporting maf making it think its lean, dumping loads of extra fuel in and then report Rich on front o2

[350matt]

Possibly faulty O2 sensor I'd say, brand new ones from Mazda are a fortune but I did find a Delphi equivalent for a lot less
let me see if I can find the part no

[350matt]

found it DOX 1449

https://www.autodoc.co.uk/denso/1665600 ... cwQAvD_BwE

[Hopkins]

Thanks for the suggestions. For the work I had done in the recent service the whole drive train came out, so it's possible that something failed during the process of disassembly/reassembly. This car has done 120k miles - of which I've done two thirds - and it's in great condition otherwise.

[Hopkins]

Oh, regarding the MAF sensor: if it is over-reporting, is this frequently solved by cleaning or does it tend to indicate a broken sensor? I've tended to assume that cleaning can be a fix for under-reporting only!

[qwakers]

failed sensor most likely.

i doubt its a failed front 02 sensor because both front and rear agree with each other.

you need to log all the parameters i mentioned above and post the log here for us to get a idea.