[stuffedgus]

Hi there, I am hoping someone could narrow down the possible cause(s) for the engine idle dropping low/car sputtering upon start-up. I've searched, but a lot of the results are specific to certain cylinders/different codes.

The codes I'm getting are:
P0300 - Random/Multiple Cylinder Misfire Detected
P1343 - Misfire cylinder 1 with fuel cut-off
P1353 - Misfire cylinder 6 with fuel cut-off

The car is a 2002 325i Touring/5 speed. The entire cooling system is only a year old and the DISA and hoses have all been recently replaced. Any help is much appreciated.

Thanks!

 

[NZ00Z3]

Well done in providing both the codes and their descriptions.

Misfires are caused by Electrical and Mixture problems:

Electrical (tends to be cylinder specific)

• Plugs and coils. Swap with another cylinder to see if fault follows
• Harness. Dirty connectors, damaged wires, faulty coil earthing resistor, missing coil earth straps (cylinders 1 and 6)
• DME. Inspect the 2 sets of 6 MOSFETS (Black boxes) around the outside of the DME PC Board for damage. Inspect for PC Board track damage. Any damage will be obvious.

Mixture (tends to be multiple cylinders)

• Too rich. Too much fuel, not enough air. Faulty MAF or Pre-Cat sensor fooling the DME into thinking 1 or the 2 prior causes is happening, blocked Cats, O2 pilot mod on CCV, if the problem is bank specific, a leaking injector.
• Too lean. Too much air, not enough fuel. Faulty MAF or Pre-Cat sensor fooling the DME into thinking 1 or the 2 prior causes is happening. Vacuum leaks
• The DME has a performance tune. The DME tables may not match the MAF or injectors fitted. The Air/Fuel tables have been incorrectly altered. Put the DME back to its original tune and see if the problem is fixed.

As your problem is with multiple cylinders, start with the mixture.

Mixture Diagnostic
With a hot engine at idle, look in the “Live Data” section of your scanner and record the 4 fuel trims, 2 short term and 2 long term. If any are >8% then you likely have a vacuum leak.

Rev the engine to 3,000 rpm for 30 seconds and watch the short term trims.

• If they reduce significantly, might even go negative, then it is confirmed that you have a vacuum leak to find. Smoke test the engine to find the vacuum leaks. Read: How to Smoke Test
• If they increase (larger positive numbers), then you have a fuel supply issue. Fuel pump or fuel filter.

What this test is doing
When you have a vacuum leak, there is air entering the engine without going through the MAF. This is un-metered air. The volume/ratio of un-metered compared to metered air is highest at idle and lower under high rev’s.

This test is using the 4 fuel trims to firstly identify the likelihood of there being a vacuum leak present (any fuel trims >8%). It then uses the short term fuel trims to see if the ratio of un-metered to metered air changes significantly under high rev’s to confirm the presence of a vacuum leak.

When you have a fuel supply problem, the short-term fuel trims get more positive as the engine starves of fuel and the DME has to put extra in so that the engine runs correctly.

 

[stuffedgus]

Well done in providing both the codes and their descriptions.

Misfires are caused by Electrical and Mixture problems:

Electrical (tends to be cylinder specific)

• Plugs and coils. Swap with another cylinder to see if fault follows
• Harness. Dirty connectors, damaged wires, faulty coil earthing resistor, missing coil earth straps (cylinders 1 and 6)
• DME. Inspect the 2 sets of 6 MOSFETS (Black boxes) around the outside of the DME PC Board for damage. Inspect for PC Board track damage. Any damage will be obvious.

Mixture (tends to be multiple cylinders)

• Too rich. Too much fuel, not enough air. Faulty MAF or Pre-Cat sensor fooling the DME into thinking 1 or the 2 prior causes is happening, blocked Cats, O2 pilot mod on CCV, if the problem is bank specific, a leaking injector.
• Too lean. Too much air, not enough fuel. Faulty MAF or Pre-Cat sensor fooling the DME into thinking 1 or the 2 prior causes is happening. Vacuum leaks (see first tip)
• The DME has a performance tune. The DME tables may not match the MAF or injectors fitted. The Air/Fuel tables have been incorrectly altered. Put the DME back to its original tune and see if the problem is fixed.

As your problem is with multiple cylinders, start with the mixture.

Mixture Diagnostic
With a hot engine at idle, look in the “Live Data” section of your scanner and record the 4 fuel trims, 2 short term and 2 long term. If any are >8% then you likely have a vacuum leak.

Rev the engine to 3,000 rpm for 30 seconds and watch the short term trims.

• If they reduce significantly, might even go negative, then it is confirmed that you have a vacuum leak to find. Smoke test the engine to find the vacuum leaks. Read: How to Smoke Test
• If they increase (larger positive numbers), then you have a fuel supply issue. Fuel pump or fuel filter.

What this test is doing
When you have a vacuum leak, there is air entering the engine without going through the MAF. This is un-metered air. The volume/ratio of un-metered compared to metered air is highest at idle and lower under high rev’s.

This test is using the 4 fuel trims to firstly identify the likelihood of there being a vacuum leak present (any fuel trims >8%). It then uses the short term fuel trims to see if the ratio of un-metered to metered air changes significantly under high rev’s to confirm the presence of a vacuum leak.

When you have a fuel supply problem, the short-term fuel trims get more positive as the engine starves of fuel and the DME has to put extra in so that the engine runs correctly.

Wow, thank you for the thorough breakdown of possible culprits--this is super helpful. I'll try the mixture diagnostic test, per your suggestion, and see if that helps narrow it down.

 

[stuffedgus]

Okay, just checked the short/long term fuel trims. Data as follows:
Short term fuel trim 1: 3.9
Short term fuel trim 2: 3.1
Long term fuel trim 1: 0.0
Long term fuel trim 2: 0.0

With RPM at 3000:
Short term fuel trim 1: 14.1
Short term fuel trim 2: 11.7

Based on your earlier post, these numbers would indicate a fuel supply issue. I hope to bring the car into my mechanic to get a final diagnosis, but if these numbers do indicate a fuel issue, do you think it is likely the pump? Fuel filter was replaced last year.

Thanks!

 

[NZ00Z3]

Yes, likely a pump.

 

[stuffedgus]

Quick update: took the car to the mechanic and they couldn't find any signs of the engine misfiring; the coil packs looked good and relatively new. They cleared the codes and the car has been running for about a week without throwing any codes again, but the tach still blips at idle and occasionally will dip below 500 and sputter momentarily.

I know I have a leaking valve cover gasket, but other than that do you think there is anything that could be contributing to the rough idle/erratic rpms (and not throw a code)? Thank you.

 

[NZ00Z3]

You will not know until you have done a smoke test.