We have a 2003 MCS. I thought my wife was crazy, but our rough idle and running is happening when the tank is less than half full. The noise and vibration reminds me of a go-kart or a Harley. Like half the engine isnt firing.

Fill the tank up and she runs smooth and fast (albeit I notice a single miss on heavy acceleration).

Codes are always random misfire about 2200 rpm and 24 mph.

Recently replaced bypass valve, spark plugs and coil pack.

Any ideas or thoughts, beside keep the gas tank full...lol

 

There are two fuel tanks - perhaps there's an issue with the fuel pump/tank 'equalizer'? (can't remember how that mechanism works)

 

So the story continues...

Stopped by the parts store and got a reader this is what I got the first time

P0300 random misfire
P0302 Cylinder 2 misfire
P0313 Misfire detected with low fuel


Fuel sys 1 ol-drive
Calc load 100
Ect 138
Stft b1 0
Ltft b1 5.4
Map 39.8
Rpm 2301
Speed 22

A few days later just the P0302 code twice.

So I tried starting it with the #2 plug wire removed, no difference still rough running. Inspected #2 plug, wet with gas, swapped with #1 plug, no difference, still rough and no difference with cable removed.

Metered the cables, all within 10% each other ~3.6kohm-4.4kohm.

Metered the Ignition Coil, same on both...
1-4 ~13.75kohm
2-3 ~ 13.75kohm

New plugs and coil about 6 mos ago, have not replaced plugs but meter shows they are about the same. Going to do a compression check, but I expect that to be normal, based on some residual gas in the cylinders. The #2 plug was very black on the threads, #1 plug was more white with a slight pinkish tint.

Not sure where to go. I feel the fuel pressure code is a false positive as it only showed once, and if it was a fuel problem #2 plug would not be affected. Was running smooth until the morning after a fill up on gas, but it seems to be specific to the #2 cylinder. Don't suspect a injector problem as the plug was wet. Computer Glitch?

Stumped.

 

Has this car always had good 91+ octane gas...some cars get a hole burnt in a valve....the fact you feel it more on a half tank might just be due to the being lighter.
I guess you could check the fuel preasure, or just do a filter...but I would expect somewhat random misfires....the compression test will tell you where to go...when you do the compression, I have read that if you do it "wet" and dry (with/without adding a bit of oil) it can yield some useful clues.
Does the car have any mods? Any history....
Does it do it when either of you have put gas in it?? One of you might top it off with good gas....then the other tops it off with 87...a bottle of injector cleaner wouldn't hurt, along with a top off of 93.

 

Another thing you might try, is move injector 2, and see if the misfire code follows the move....if you are getting mostly misfire 2, as opposed to random, this would rules out the injector, but not the harness/connections...but just look at them closely when you do the swap....besure to use new o-rings too when you do the swap. Might be a good time to clean/swap out the micro fuel filter and preasure regulator....good preventive mantance, but not likly the cause though.

 

So we have swapped spark plugs, cables etc...

Coil is rather new and the resistance looks good

when i swapped the fuel injector the code swapped to cyl#1

replace the fuel injector with new, no good same problem.

i gave up and brought it to a mechanic...

mechanic found a blown head gasket.

So Mini just shut cylinders off when you have coolant in the engine, so you never see white smoke in the exhaust.

The issue I believe with half a tank never actually had to do with the gas tank level but I believe low coolant. i think when it ran rough the coolant was low and did not fowl the ignition. When I topped it off (likely around a half tank) it ran rough as coolant was refilled.

so just a new head gasket was all that is needed. (sigh)