E30 325i lumpy on cold startup?

[fivesixseven8]

My E30 is really lumpy for a few minutes when it's started from cold. It seems like it isn't running on all cylinders, but I confess, I haven't tried to diagnose the problem at all yet. Any ideas on what it could be or how to go about figuring it out?

Cheers.

[Diesel Meister]

Not a mechanic (perhaps try one ) but mine did this exactly. Tried all sorts including swapping plugs, checking wiring and resetting the ECU (disco battery for 10 min, reconnect, idle for 10 min then drive normally for a bit) no dice.

In the end the biggest improvement came after swapping in a scrap-yard Crank Position Sensor. Maybe pull yours and check it's not gunked up, perhaps checking all connections under the bonnet (plugs, leads etc.)?

Good luck!

[fivesixseven8]

Diesel Meister said:

Not a mechanic (perhaps try one ) but mine did this exactly. Tried all sorts including swapping plugs, checking wiring and resetting the ECU (disco battery for 10 min, reconnect, idle for 10 min then drive normally for a bit) no dice.

In the end the biggest improvement came after swapping in a scrap-yard Crank Position Sensor. Maybe pull yours and check it's not gunked up, perhaps checking all connections under the bonnet (plugs, leads etc.)?

Good luck!

Cool, will give it a go! Not the most pressing of problems as it's perfectly fine after a few mins.

[phelix]

Check the temp sensor that the ECU uses?

When it's running lumpy what happens when you rapidly pump the brake pedal (while stopped!)? This lets more air into the engine via the brake booster. Lower engine speed means the car's running lean, higher idle speed means it's running rich.

[fivesixseven8]

phelix said:

Check the temp sensor that the ECU uses?

When it's running lumpy what happens when you rapidly pump the brake pedal (while stopped!)? This lets more air into the engine via the brake booster. Lower engine speed means the car's running lean, higher idle speed means it's running rich.

Will try that in the morning, it idles fine though, just lumpy when I apply any throttle. I think it's running fairly rich as it gets through petrol rather quickly.

[Oi_Oi_Savaloy]

Is your car using losing/using water at all but there's no discernible leak on the floor?

Sometimes a lumpy start up can mean that the engine is clearing water from the cylinder head.

That could either mean it's a head gasket issue (on it's way out) or the cylinder head is fked (corrosion leading to an improper seal between waterjacket and firing ring).

Classic signs are your oil dipstick level is rising (as water fills the sump), or excessive steam out of the exhausts (water escaping out of the exhaust), your engine is really lumpy for about 30 secs or so on start up, you seem to be losing coolant at excessive rates of knots but you've had the coolant system pressure tested and all is well and there's no discernible leak either. Oh and there will be mayo under the oil filler cap and sometimes in the dipstick too.

Obviously this is a worst case scenario but i'm in the middle of exactly this problem and it all started with a crap idle on start up.

I sincerely hope yours is ok.

[Diesel Meister]

Good points re. water ingress - can happen, esp if your HG is toast. The temp sensors (brown/blue?) are probably some of the most replaced parts on the M2B25 - I went through 3 trying to suss it. Good thing they were free!

I also tried replacement ECU. Best to start with the basic mechanics then look at the electrics though.

[Oi_Oi_Savaloy]

Yep - definitely start with the basics and work your way up.

If you do have to change your ecu - it's a straightforward to swap one bought on ebay for yours in the car. It's a 20 min job and that includes you dropping the spanner into the driver's footwell 30 times......

[E30M3SE]

Cold start sensor is blue, brown is the temp guage sensor.

It's the first thing I would try for £18 odd.


ETA, the sensor can be tested, should have resistence of around 3k ohms when (stone)cold and around 200 ohms were hot.

Edited by E30M3SE on Thursday 17th December 18:26