Help to diagnose a fault

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Adam
Joined: Sat 21 Jul, 2007 18:43
Posts: 337

  Z3 roadster 2.8
Location: Newport
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Help to diagnose a fault

Post by Adam »

I'm trying to work out what this annoying fault is on my car. I have used diagnostic software (carsoft) and check for codes and sorted anything out that came up. Now its clear but im still getting the problem.

1998 Z3 2.8 m52

I have changed both Cam sensors (by the vanos and the one down the back/side of the engine) as I know they cause issues and I had a spare set of them. Just to rule that one out.
The diagnostic only pulled up a dodgy mass air flow sensor so I replaced that and cleared the code, I'm still getting the fault but no codes now. I have also cleaned out the idle control valve.

The fault is:

Randomly the car appears/feels to cut down on a cylinder and runs lumpy. I can put my foot down and it gurgles then clears. It can happen at idle and while driving. There is no pattern to when it does it. The car doesnt stall. It can run fine for days then driving down the M4 motorway about an hour into a journey it cuts and feels like you suddenly have lifted off the throttle and back on. it can last only a second while moving or gurgle for about 5 seconds, clear and carry on. Sometimes when you pull off from lights etc and it decides to play up you almost bunny hop away as it feels like it clears then fluffs up...clears. By applying the throttle appears to clear it. Its a pain in traffic as it makes me look a right tool when it fluffs up and im revving it to clear, I get the look from some people thinking "idiot revving"

Its frustrating, I have tried a fuel line and injector cleaner, fuel from different stations but to no avail. Im now starting to scratch my head a little.

I would think that if an injector was going down it would flag up something regarding an error code. The spark plugs are my next possible thing, they are just over a year/3000 miles old and genuine BMW plugs from the dealer. The fuel consumption has also dropped by about 5-8mpg then normal for the driving conditions.

any suggestions??
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Southernboy
Joined: Thu 07 Oct, 2010 12:39
Posts: 6436

  Z3 roadster 3.0i
Location: Johannesburg

Re: Help to diagnose a fault

Post by Southernboy »

Have you checked the coils on the plugs ? You may have a coil which has a connective fault, and the "revving" simply vibrates it back into a connection, but which can as easily vibrate to a disconnection. ie. may be the connection point between the coil and the plug, or the connection between the coil and the car.
As you suggest, it may be an injector too. You can hear the injectors opening and closing... remove the plastic fuel rail cover and use a small plastic / rubber hose as a stethescope at each injector, and listen for the "regular" click of each one in turn...if you can do it whilst the motor is going through one of it's "hiccups" and it is an injector, you may hear the missed clickand possibly identify which cylinder is failing to ignite.
"Normal is overrated"
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Adam
Joined: Sat 21 Jul, 2007 18:43
Posts: 337

  Z3 roadster 2.8
Location: Newport
Contact:

Re: Help to diagnose a fault

Post by Adam »

I will look at that this weekend. Thanks for the feedback
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