Mechanics

HIGH RPM MISFIRE

1998 BMW Z3

Engine Performance problem
1998 BMW Z3 6 cyl Two Wheel Drive Manual 77,000 miles

2.8 L engine Misfire at approx 5800 RPM, sets CEL. Sometimes hear knocking after misfire.

Generic code reader does not read codes. Peake code reader (BMW specific) indicates misfire in various cyl, no pattern, but cyl 4 never has come up. No other codes detected.

Car runs and idles fine otherwise, even after code is reset. Plugs, coil connectors, fuel filter replaced. Coils OK. 1 can of Techtron put in tank. Fuel pressure OK, Mass Air Sensor, idle control valve, and DME (computer) OK. Air filter clean. No vacuum leaks detected.
Why only high RPM misfire? Also how is fuel volume checked?
Avatar
Jmai
November 6, 2009.



High RPM mean load.

It misfires erratically when it comes under load or accelerates. The engine is obviously misfiring under load but does not do it all the time. The most likely causes here would be a defective ignition coil, bad plug wires or worn out or fouled spark plugs.
I do not know what you mean by" coil ok".

Tiny
Mathiaso
Nov 7, 2009.
Engine has individual coil packs above each spark plug Spark plugs replaced and properly torqued. Another set of coils was installed to eliminate a defective coil. Also new coil rubber boots were installed.

Could it be a crank sensor or cam sensor slowly starting to go? Would this affect all engine RPMs or only show up at high RPMs? Car starts, runs and idles fine after misfire code is reset.

Also my generic OBD II reader detects no codes, but a BMW specific one, Peake, detects the misfire code.

Tiny
Jmai
Nov 7, 2009.
I would suspect the cam sensor.
it is the only one responsable for telling the computer wich cylinder is a top dead center and wich one is approching so the computer can energize the relay ,so the relay can energize the injector.
fuel volume test is good approach, it seem like the engine is starving.it start and runs rough.
Low fuel pressure creates a lean fuel condition that can cause lean misfire, hesitation, rough idle, hesitation and misfire on acceleration (high rpm)
.


http://www.2carpros.com/forum/automotive_pictures/46384_fuel_volume_test_1.gif



Have the injectors cleaned, walmart is the cheapest place so far I know.plug injectors causing bad spray pattern can cause misfire.
check fuel pump wiring for moisture.check the fuel filter and lines for obstructions and the operation of the fuel pressure regulator before you condemn the pump.

On european EFI systems, the OEMs recommend using a static pressure test with the engine and ignition off. This is done by bypassing the fuel pump relay and energizing the pump directly

Static Fuel Pressure Test
With the key on, engine off (or with the fuel pump energized), fuel pressure should come up quickly and hold steady at a fixed value, one once per second.

Tiny
Mathiaso
Nov 7, 2009.
Clogged catalytic converter

will need replacement

Tiny
Jmai
Nov 12, 2009.
Thanks for feeding that back to us.

Dr. Hagerty
Nov 12, 2009.