325i completely undrivable misfire

Post Thu Mar 26, 2015 5:02 pm

Hi

Car is a 325i cab. Just rebuilt the engine.

First of all the idle is perfect. When I first struck it up everything was fine.

It went for MOT and driving up there I noticed it was lumpy. Then it failed on emissions. Driving it back home it is fine top end and fine at idle but between 1.5 3k revs its misfiring or being starved. Or too much air. Whatver it's not right.

So the next day I pulled all the spark plugs and found one of them was very small gap. Sorted it. Put them all back in and it's awseome !! The car is pulling like mad. It's defintely firing on all cylinders. Perfect!

Then an hour later I start it up and it's back to the same! But worse. Now it stalls as soon as you push the accelerator. You have to really trickle it to get the revs. But again once it's reving high it seems ok.

I've just changed all the plugs.. No better

I've just changed the fuel filter. No better
I've changed the distributor cap and rotor arm... No better
I've listened to each injector clicking away.
I've checked resistance over the coil. It's fine

I've pulled the HT leads and i can see them arking so there is power to each plug for sure.

I'm rwally at a loss now of what else I can do.

As I said the car for a few hours was running absolutely perfectly.

But now it's back to misfiring or whatever this thing is

Please help!!

Post Thu Mar 26, 2015 7:29 pm

Bad when cold and ok when up to temperature? Blue plug?

Post Thu Mar 26, 2015 7:33 pm

Jona1971 wrote:Bad when cold and ok when up to temperature? Blue plug?

OR plug and socket for the injector loom under the inlet manifold (if it's a facelift loom.)

Post Thu Mar 26, 2015 7:34 pm

is there a way to check the blue plug with multimeter ?

I have had it running fine just now but and it was warm.. ill find out for sure in the morning whether its like it when its cold...

Post Thu Mar 26, 2015 7:37 pm

Yes the cold and hot resistance values are in the wiki

Post Thu Mar 26, 2015 7:39 pm

Brianmoooore wrote:
OR plug and socket for the injector loom under the inlet manifold (if it's a facelift loom.)

what would this be causing? do you mean a loose connection? or...

Post Thu Mar 26, 2015 7:43 pm

Measure the resistance values at the ECU plug. Do not go anywhere near the sensor itself or the plug/socket until you have.
Disconnect the ECU connector, identify the pins and mark them with a pen or permanent marker, measure the cold resistance, reconnect the ECU, start the engine and bring it up to full temperature, then switch off and immediately disconnect the plug and measure the resistance again.
Don't push the meter probes right into the buckets of the ECU plug or you will damage them.

Post Thu Mar 26, 2015 8:07 pm

thanks brian!

So you are saying to test the actual ECU itself or the wires that lead up to the ECU?

And do you have any idea which wires i must test (or where can i find out this info)

cheers

Post Thu Mar 26, 2015 8:08 pm

jimbom30cab wrote:Yes the cold and hot resistance values are in the wiki

Post Thu Mar 26, 2015 8:08 pm

Test the wires to the ECU. Pin details, and resistance values are in the wiki article.

Post Thu Mar 26, 2015 8:11 pm

ok great! ill check it all out.. have you heard of this problem before?

Post Thu Mar 26, 2015 8:16 pm

Samytee wrote:ok great! ill check it all out.. have you heard of this problem before?

It's quite a common issue for the plug under the inglet to corrode and give false readings to the ECU from the blue temp sensor.

Post Thu Mar 26, 2015 9:30 pm

maxfield wrote:
Samytee wrote:ok great! ill check it all out.. have you heard of this problem before?
It's quite a common issue for the plug under the inglet to corrode and give false readings to the ECU from the blue temp sensor.

Note that it's not the pins and buckets of the plug and socket themselves that are the main problem - it's corrosion of the wires under the rubber boot of the lower half, as a result of a design fault allowing water to collect there and be trapped, that's the real cause of problems.

Post Thu Mar 26, 2015 10:03 pm

Yes I know exactly where that is. It sits under the engine "upright" so I can imagine that it does collect water.

I have rebuilt the whole engine and had the engine on a bench. That circular electric plug seemed ok. The blue plug however I didn't remove from the thermostat as it was freaking tight!!

I have found that wiki page and will follow it regarding testing the ECU plugs and then the blue plug itself.

I was just wondering if this random misfiring was commonly down to this problem. As you can see I seem to have checked most other things. I really hope this works!

Post Thu Mar 26, 2015 10:16 pm

Samytee wrote: The blue plug however I didn't remove from the thermostat as it was freaking tight!!

It's conceivable that applying excessive torque to the body of this sensor can crack the NTC resistor element inside. It shouldn't be particularly tight, but if it is, and you've leant on it to try to unscrew it, you may have broken it.
The tests at the ECU plug will reveal all.