Hello everyone, last weekend my 2002 S (R53) with 90k miles started misfiring cylinder 3. It happened while I was driving it, was only 3 miles from home so I brought it back and have been diagnosing all week and am a little stuck.

Only code is Cylinder 3 Misfire. It's only running on 3 cylinders and I confirmed it's cylinder 3 by removing spark plug to #3 and engine roughness remains the same.

1) Checked the obvious Coil/Spark Plug/Wire - all good and sparking.
2) Swapped Fuel Injectors 3 and 4, cleared code, and threw Cyl 3 misfire again (assumed fuel injector is good). If code moved to Cyl 4 would assume fuel injector bad.
3) Measured voltage to fuel injector 3 and was identical to all other cyl's fuel injectors.
4) At this point I assume Cyl 3 has spark and fuel (and I did not have compression tester) so removed valve cover and inspected valves and cam shaft, everything appeared to moving correctly, rocker arms, valves, springs, etc). Put valve cover back on.
5) bought and ran compression tester - results:
Cylinder 1: 150 PSI
Cylinder 2: 145 PSI
Cylinder 3: 150 PSI
Cylinder 4: 150 PSI

Am at a total loss now, if it has fuel + spark + compression how can it possibly be misfiring? Any help greatly appreciated.

 

Hate these sorts of problems... Could be something flaky - like antisieze on the spark plug causing bad ground.

I would swap plugs and wires (like you did for the injector) to confirm it doesn't follow a bad one - wire may be worse in one position but ok when you move it for testing.

 

Just tried it and still stays on #3.

 

Let's take a step back - have you done any work on the car recently? Any new parts or mods?

What code reader/software are you using? Can you see any live data?

 

No, it's a stock R53 for 14 years. Only thing new was a new tank of gas (yes premium). Dropped cylinder 3 miles after a fill-up while I was driving specifically after accelerating after a red light in first gear.

My friend's actron AutoScanner. I can read live data but nothing in there seems very useful. it's generic info like engine RPM, speed, coolant temp...

Only codes is cylinder 3 misfire.

 

I'm assuming that is a P0303 code. If you know the specific code, it helps to post it. These are all the possible sources:
-plug
-wire
-coil
-injector
-oxygen sensor
-cat
-exhaust valve
-low fuel
-poor compression
-main computer

I think you have ruled out most of the choices, except for the oxygen sensor/cat and the ECU. Those are not cheap, so I wouldn't start throwing parts at it yet.

If you can find someone with a scanner capable of reading MINI-specific codes (or a laptop with INPA/DIS) you can read the oxygen sensor values and see if they are within range, and also look for other MINI specific codes (more than the generic codes the Actron will read) that might help narrow it down.

I don't have any other ideas, sorry.

 

Since it was a fresh tank of gas, perhaps that is a clue. You could have gotten some junk from that, or they could have made a mistake at the station, even if it was a good brand. Since the tank is full, it is hard to try other gas, but something to think about...

 

Yes it is the P0303 code.

I will be bringing to the shop with full diagnostics Monday :-/

 

I originally suspected the gas also, but by now I would have thought it would affect all cylinders, it is clearly only #3 misfiring. I have also put in a bottle of octane boost.

 

Seems like you are willing to do a lot of DIY. It is worth getting more capable software and connector (Autoenguinty or the the DIS/INPA software from ebay) if you want to be able to really understand your MINI.

 

Will look into better software...

But going back to the sources you mentioned which I have not ruled out (oxygen sensor, cat, and ECU), how could any of those cause a misfire. I have compression, fuel, and spark ? I just cannot get over how you can have all 3 of those and not get combustion. I'm thinking now it's something much simpler than all of this. But please explain why you think it could be ECU, O2 sensor, or cat causing it to misfire.

 

You can have a situation where the computer decides you have a misfire and deactivates an injector or spark, but you really don't. Your troubleshooting would be enough if there was no computer involved. It might be adjusting the engine trying to "fix" a situation that is really a bad sensor input.

 

Was there ever any resolution here? My new to me mini also has a misfire and was hoping to read how this one wound up...

 

BMW does this thing where it shuts off a cylinder if it detects a misfire in order to protect the catalyst. You could have a lean miss on #3 from a leaking intake gasket, that would explain all of your test results. Try a spray bottle with water around where the head and manifold meet at #3 and listen for a change in idle or even a sucking sound. Hope this helps. G