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I'm trying to do some basic diags for p0304 misfire on cylinder 4. R53 2006 with about 67k miles. I swapped between cyls 3 and 4 - plugs, wires, injectors. Cleared code each time - its still on cyl 4. After reassembling everything i now also have p0313 - misfire detected - low fuel level. P0313 was not a one time code - it also comes back each time now after clearing codes. I'm using the car scanner app with a vpeak bluetooth obdii dongle.
Next i want to do compression tests. I've read this is the procedure. Am i going to be doing it right? Helpful tips are greatly appreciated!
Run engine to operating temp. Turn off and cool for 15 mins.
Remove fuse for fuel pump and remove coil wires. Relieve fuel pressure at fuel rail schrader valve.
Hold throttle butterfly open (will helper flooring gas pedal while cranking accomplish this?)
Check compression on each cylinder while cranking.
I'd recommend a leak down test instead. It will tell what is causing low compression (valve, rings, head gasket, etc.) and often catches issues that a compression test can't.
But before going down that road, you should purposefully mis-wire the plugs. Connect the output of the coil pack for cyl 1 to the plug on cyl 4 and the output for cyl 4 to the plug on cyl 1. Clear the codes and start the engine (believe it or not the engine will run this way). If the new error is for cyl 1 misfire then you have a faulty coil pack.
Although I agree with @mrbean , a leak down test is not something most DIYers can do themselves because of the need for an air compressor.
A long time ago I found this pdf file describing how to do a compression test (or rather 3) and how you should interpret the results. It has worked for me all those years so it might help you also.
Apparently I can't upload pdf files so here is a jpg instead.
I'd recommend a leak down test instead. It will tell what is causing low compression (valve, rings, head gasket, etc.) and often catches issues that a compression test can't.
But before going down that road, you should purposefully mis-wire the plugs. Connect the output of the coil pack for cyl 1 to the plug on cyl 4 and the output for cyl 4 to the plug on cyl 1. Clear the codes and start the engine (believe it or not the engine will run this way). If the new error is for cyl 1 misfire then you have a faulty coil pack.
I swapped the wires so coil output #1 was feeding cylinder #4 and vice versa. Code came back on #4, so i guess the coil is not bad.
I did a compression test after warming engine, pulling fuse #37 for the fuel pump and removing the plugs, then reconnecting plugs to the wires and setting them aside with rubber around the plug ends to avoid sparking against anything. My wife cranked the ignition each time with accelerator floored.
PSI readings were 160 - 157 -160 - 169. Interesting that the cylinder with the misfire code was highest. I'm pretty much at my limit. I'm probably going to try my luck with Bimmerworks in West Chester PA.
Although I agree with @mrbean , a leak down test is not something most DIYers can do themselves because of the need for an air compressor.
A long time ago I found this pdf file describing how to do a compression test (or rather 3) and how you should interpret the results. It has worked for me all those years so it might help you also.
Apparently I can't upload pdf files so here is a jpg instead.
Thank you for this! I didn't see it until after i was done for today. Definitely good to go by the next time i do this!
One more thing you can check before handing it off to a pro. Pull the valve cover and check the valve lash. Collapsed lifters are known to happen on these engines and it might not reveal itself from a compression test.
