Engine problem M42b18/318is

Post Mon Jun 30, 2014 12:44 am

This is going to be a bit of an essay unfortunately as don't want to miss anything that could give any clues away!

I have a 318is/ m42b18 engine

Back in march she was running a bit rich (but nothing major) so a friend wired in a potentiometer to act as an inlet air temp sensor? set at about 8 degrees...

Ran for a few months with no problems whatsover.
About 3 weeks ago, noticed i had fuel leak, inlet manifold came off, mended that with a bit of new hose, ran fine again for about a week.

Then whilst driving I noticed a sudden loss of power, she was struggling to idle and around town in 1st and 2nd I was really having to keep the revs up to keep her alive. Engine was sounding really raspy and throaty.

Pulled over and noticed smoke coming from potentiometer and quickly disconnected it.

Checked over all air intake hoses when i got home and noticed a slight split in a hose going from iacv. chopped the end off to make it good and reconnected and no difference.

All my intake hoses were in a bit of a state and had some very loose connections due to splits etc but they had been like this since I've owned the car. Anyway to elimate this I've put completely new intake hoses throughout.

Checked sparkplugs and they were a little worse for wear, and 2 of the chambers had oil in them so I replaced sparkplugs and resealed the cam cover to stop the oil getting in.

Put everything back together and still no better, worse if anything. Have gone back and checked screws of new spark plugs and they came out black so would appear to be over fuelling?

The engine is running and will idle without dying however is sounding horrendous. Its back firing very often and smoke has been seen twice coming out of the back of the inlet manifold. Its running extremely irregularly, hard to describe but if it was a human heart it sounds like is beating twice then dying twice.
Have heard a few strange knocks also, but only one or two occasionally, nothing regular.

At the back of the engine near the IACV where there is an earthing point from the head of the engine to the battery, the bracket looks as though something else should be connected which i could have missed perhaps whilst putting things back together?

Any ideas greatly appreciated, thanks in advance!

Post Mon Jun 30, 2014 6:13 am

To start, is this a 318iS or an M40B18? It can't be both, because the 318iS used an M42 engine.

Secondly, don't bodge things. Fitting a pot into the electrics... I don't know what to say to that.

Finally, TEST the Blue temp sensor and report back:

http://www.e30zone.net/e30zonewiki/index.php/Blue_Plug

Post Mon Jun 30, 2014 9:56 am

First port of call is definitely to test the blue sensor, since this has a massive influence on mixture strength. Unfortunately, it's well hidden on the M42, in the cylinder head, near the join with the block, and under the inlet manifold. Test it from the ECU plug,both stone cold, then at full running temperature.
There was no auxiliary earth strap fitted to an iS engine, so that's not an issue. Only engine to body strap is between one of the engine mounts and the chassis rail.
Smoke from the IAT sensor is a bit worrying, since that's a signal circuit and definitely not intended to carry the sort of currents that can cause smoke. Current source is the engine ECU, so that could have been damaged.

Post Tue Jul 01, 2014 8:25 pm

Okay I tested the ecu whilst cold and the reading was about 10m ohms... read on the wiki it should be about 3500..?
The engine wont run for long enough to get warm to be able to test that -

should i still find the actual sensor and test that or is it obvious its failed?

thankyou both for your help

Post Tue Jul 01, 2014 8:37 pm

You should still find the sensor, as the fault may be in the wiring. If the sensor reads 10 ohms, then it's duff and should be replaced. If it reads around 3500 ohms, you've got a wiring problem in the loom somewhere.

Post Tue Jul 01, 2014 9:12 pm

10M ohms sounds like the sensor is disconnected by some means or other. Could be a break in the wiring, a total internal break in the sensor, or possibly the wrong pins on the ECU.
Are you absolutely sure you measured at the right pins?

Post Tue Jul 01, 2014 9:41 pm

Brianmoooore wrote:Are you absolutely sure you measured at the right pins?

Good point Brian. The Wiki is biased towards Motronic 1.3, which will give the wrong pins for the OP's ECU. Correct pins are 78 and a ground such as 34 or 28.

http://www.e30zone.net/e30zonewiki/inde ... tronic_1.7

Post Tue Jul 01, 2014 10:24 pm

Pin 43 is listed as being the temp. sensor earth on that link for the iS ECU, but I can't confirm that from experience.
Don't forget that some of the earth pins in these lists are only earthed when the plug is connected to an ECU. They only connect to the actual earthed pins in the plug by internal links in the ECU.

Post Fri Jul 04, 2014 5:46 pm

Yep definitly measured pin 78 and 28. Will test the actual sensor soon -
thanks for your help both of you

Post Sat Jul 05, 2014 6:55 pm

You won't get a meaningful reading across 78 and 28 with the ECU unplugged. It has to be 78 and 43.
28 and 34 are connected to earth wires in the plug, and will give zero ohms if you measure between either of them and the car's body.
The wires from the sensor go to 78 and 43, and although 43 is earth when the ECU is connected, it isn't when it's unplugged.
This is done to avoid forming loops in the wiring which would act as a transformer secondary and be prone to picking up interference. A similar thing happens with audio systems, and results in a 50 Hz hum if you get the wiring wrong.

Post Sun Jul 13, 2014 7:28 pm

Okay Brian I've done the reading from 78 and 43.

2.819k ohms, that doesn't seem too badly out?

Post Sun Jul 13, 2014 9:08 pm

2.8k is a reasonable 'cold' reading with the weather as it is at the moment
What's it like hot? Needs to be measured ASAP after stopping the engine, as the sensor soon starts to cool off.

Post Sun Jul 27, 2014 4:35 pm

i can't measure hot as engine will only run for a couple seconds before dying :/ I've not done anything with the car for the last few weeks due to a deadline with work that i've had to finish, however that task is almost complete... and i've bought an m52 engine, and entire doner car cheap off a friend and with low mileage.

Decided to give up trying to mend and throw money on parts and bits on this current engine as its not what i want anyway. Feel a bit frustrated as would have liked to have got it going again but it seems in a extremely bad way now and i have always been planning on doing a swap...