M20B25 - Hunting on startup, stalling when coming to a stop

Post Mon Jan 19, 2015 10:00 am

I've got a slight issue which I'm trying to get to the bottom of. The car's just had an Inspection 2 service carried out by Barry recently (500 miles ago), so service related fixes are out of the question I'm guessing.

1) On startup, whilst the engine is cold, the car would start hunting after about a minute of it being started. This would last for about 1 or 2 minutes whilst the car warmed up. Once the car warms up, the hunting stops. During the hunting, the revs fluctuate and the rev counter needle moves up and down.

2) When driving along and coming to a stop at traffic lights, when putting the clutch down just before coming to a stop, the revs dip and the car nearly stalls, but after a second it recovers and goes back to normal again. The rev counter needle drops to below 500 and the engine lights would flicker on as they do when you put the key into the ignition and turn to 1st position. Once or twice the car actually stalled as the revs when so low.

3) Generally the car feels like it is idling weak and low, and feel sluggish in the lower rev range.

I've checked for air leaks and have cleaned out the ICV. Barry has also adjusted the tappets to the correct setting for a car with a 6 branch manifold, which it has.

The Car is a 318i cab converted to 325i
The engine is an Early High Compression M20B25 with the Later Motronic 1.3 setup mated to it.

I'm thinking it could be the blue temp sensor, and/or the dizzy cap and rotor?

Any ideas?

Post Mon Jan 19, 2015 1:21 pm

My first guess would be the Throttle Position Switch either not working, or not set up properly. What are your hot and cold idle speeds?

Post Mon Jan 19, 2015 2:31 pm

The Throttle Position Switch clicks as it should, so I'm its most likely working correctly I'd imagine.

Also, regarding the idle speeds, they read at 800 when cold and idling high, and just over 500 when warm. Too low!

Post Mon Jan 19, 2015 2:44 pm

No click usually confirms that a TPS is not working, but a click is not sufficient proof that it is working.

Post Mon Jan 19, 2015 2:55 pm

"To test the TPS, remove its plug and connect a resistance meter to pins 1 and 2. You should get a short circuit ( 0 Ohms) with the throttle closed, going open circuit with the throttle just off its stop. Pins 1 and 3 should be open circuit until the throttle is 2/3 open, when it should go short circuit. If you are not getting these readings, remove and clean the TPS."

What does it mean by going open circuit, and going short circuit in terms of readings on my multimeter? I have a multimeter, and have successfully tested the blue sensor at the ECU before, as I knew what readings to look out for.

So for the TPS, set the multimeter to resistance and then observe what readings?

Post Mon Jan 19, 2015 2:59 pm

open circuit means infinite reading (or 1 on a cheap multimeter), and closed circuit is 0 ohms (or as close as possible).

Post Mon Jan 19, 2015 3:16 pm

OK, so:

Measure resistance across pins 1 & 2:-

Throttle closed - Closed circuit (0 ohms reading)
Throttle open - Open Circuit (Very Large ohms reading)

Measure resistance across pins 1 & 3 now:-

From throttle closed ---> 2/3 open - Open Circuit (Very Large ohms reading)
Past 2/3 - Short Circuit (0 ohms reading)

Is the above correct?

So at pins 1 & 3 it should have a very large reading right from closed throttle to 2/3 throttle?

Post Mon Jan 19, 2015 4:18 pm

German-Whips wrote:Is the above correct?

Yes, that suggests your TPS is reading correctly.

With that checked, and the Blue Temp Sensor checked (which you say you've read before) then I would expect an air leak to be the problem. These can be a right arseache to find - have you tried spraying carb cleaner around the engine bay at idle, to see if the revs rise?

Post Mon Jan 19, 2015 4:52 pm

Have tried the carb cleaner method, and couldn't get the revs to rise.

Post Mon Jan 19, 2015 10:11 pm

I had your fault #2.

Long shot but my fault was due to a faulty solder joint on the 'throttle closed' microswitch inside the TPS.

It would check out fine when testing the pins when on the car.

I had the TPS apart on the bench testing it with 12v and a bulb and happen to notice a small spark. One of the solder joints of the microswitch was dry. The contacts were touching but not properly soldered together, I assume giving an intermittent reading when driving due to vibes. Just re-soldered the joint.

I can't remeber but i think you have to prise the TPS open to check this an then re-glue it together.