M40B18 - Slight misfire & running rich - SOLVED

[DelaneyE30]

Hi all,

Just carried out a refresh on my M40 1.8. The car ran smooth and wasn't noisy before I stripped it. Returned good MPG too.

The cam was good as it had been replaced less than 50k miles ago. I put new hydraulic lifters and valve stem seals in it as preventative maintenance. All breather and vacuum rubber hoses on the intake system are new genuine items. Car has a new timing belt etc and the timing is perfect. Checked multiple times. New gaskets everywhere. New plugs (not fouled up as a result of misfire). Injectors pressure tested (all good) and new o-rings. Fuel rail checked and is clear. All filters new. No cracks in big pipe to throttle body. New blue temp sensor fitted.

Engine starts easily without issue but has a very slight misfire and mildly lumpy on idle. The misfire occurs right across the rev range and as you slowly increase the throttle you can hear the small intermittent flat spot as revs increase. The car does not cut out or die but the misfire is there. It is running rich as fuel consumption is down and exhaust smells bad. This occurs when the engine is both cold and hot.

Where should I start with diagnosing the issue? My thoughts are leaning to AFM, CPS and TPS sensors but would like some knowledgeable guidance.

I will update as I check and rule out the various sensors and suspicions.

[williambarker]

Hi all,

Just carried out a refresh on my M40 1.8. The car ran smooth and wasn't noisy before I stripped it. Returned good MPG too.

The cam was good as it had been replaced less than 50k miles ago. I put new hydraulic lifters and valve stem seals in it as preventative maintenance. All breather and vacuum rubber hoses on the intake system are new genuine items. Car has a new timing belt etc and the timing is perfect. Checked multiple times. New gaskets everywhere. New plugs (not fouled up as a result of misfire). Injectors pressure tested (all good) and new o-rings. Fuel rail checked and is clear. All filters new. No cracks in big pipe to throttle body. New blue temp sensor fitted.

Engine starts easily without issue but has a very slight misfire and mildly lumpy on idle. The misfire occurs right across the rev range and as you slowly increase the throttle you can hear the small intermittent flat spot as revs increase. The car does not cut out or die but the misfire is there. It is running rich as fuel consumption is down and exhaust smells bad. This occurs when the engine is both cold and hot.

Where should I start with diagnosing the issue? My thoughts are leaning to AFM, CPS and TPS sensors but would like some knowledgeable guidance.

I will update as I check and rule out the various sensors and suspicions.

Hi,
i cant help with the misfire but i had the same lumpy idle issue with my m40. The first thing i did was replace all pipes and found a vacuum leak so repaired that too. Then i also found that when on idle the throttle mechanism on the engine wasnt fully closing. If you shut the car down and open and close the throttle from the engine bay you should hear a very small click as it fully shuts for idle (i think its a sensor for fuel-air ratio which was basically out of sync with the engine)
Hope this makes sense, im not an expert on this but that seemed to resolve my poor idle!
Bill

[Brianmoooore]

An exhaust smelling of fuel does not necessarily indicate a rich mixture. With a weak mixture, some fuel will fail to ignite, and pass straight out of the exhaust.
I suspect williambarker's post above is on the right trail. It's common for cars of this age to have passed through the hands of owners who's approach to a problem is to bodge/mask it, rather than solve the actual problem, so you can't assume that any of the factory preset adjustments that should never be touched haven't been well and truly 'got at'.
Look carefully at the throttle stop and AFM adjustment for any signs of them being tweaked.
You don't say the age of your car, but your sig. says brilliant red, suggesting a late one. Does it have a cat. and other emissions equipment?

[DelaneyE30]

Hi,
i cant help with the misfire but i had the same lumpy idle issue with my m40. The first thing i did was replace all pipes and found a vacuum leak so repaired that too. Then i also found that when on idle the throttle mechanism on the engine wasnt fully closing. If you shut the car down and open and close the throttle from the engine bay you should hear a very small click as it fully shuts for idle (i think its a sensor for fuel-air ratio which was basically out of sync with the engine)
Hope this makes sense, im not an expert on this but that seemed to resolve my poor idle!
Bill

Hi Bill,

Thanks for the help. I will check the throttle position sensor and report back tomorrow. I think we may be on the right track with that suggestion. Thank you

[DelaneyE30]

An exhaust smelling of fuel does not necessarily indicate a rich mixture. With a weak mixture, some fuel will fail to ignite, and pass straight out of the exhaust.
I suspect williambarker's post above is on the right trail. It's common for cars of this age to have passed through the hands of owners who's approach to a problem is to bodge/mask it, rather than solve the actual problem, so you can't assume that any of the factory preset adjustments that should never be touched haven't been well and truly 'got at'.
Look carefully at the throttle stop and AFM adjustment for any signs of them being tweaked.
You don't say the age of your car, but your sig. says brilliant red, suggesting a late one. Does it have a cat. and other emissions equipment?

Hi Brian,

Thanks for the suggestions and help. the sealed cover on the AFM does not look to be broken however the little cap on the adjuster screw is missing suggesting its been tampered with before. As above I will check the TPS and go from there.

Shes a late '89 and does not have a cat or o2 sensor.

[DelaneyE30]

Checked and setup TPS correctly, checked and cleaned AFM and checked all intake pipes for leaks. Pressure tested injectors again and checked for leaks. Everything is good. Yet the misfire still remains. It sounds and feels suspiciously like the timing is retarded slightly. What would cause this? Surely a damaged or faulty crank position sensor would result in the car not starting or cutting out? Going to check the sensor tonight and check if I overtightened the timing belt too causing a slight retardation.

[Brianmoooore]

Timing belt doesn't affect ignition timing and the tension on it will only affect valve timing to a minuscule amount. Ignition timing is controlled entirely by software, using the pulses from the crank position sensor as a reference.
The only fault I know of that can disrupt this is a rare one where the outside of the crank pulley rotates slightly around its hub as a result of a failing rubber piece between the two.
To check the valve timing, refit a crank locking pin in the flywheel and then check (with a straight edge) that the top of the square section on the camshaft is exactly parallel to the head face.
Are you sure everything is as it should be inside the distributor cap?

[arrisbmw]

and like to add , what plugs are you using . I,m not 100 % sure what is best . I know the ngk have significantly less ohms reading in the centre electrode.
1.7 compared to 4.7 K ohms on bosch . And someone said the multi point plugs are not good in m40 . What is Brian's take on plugs , did they leave the factory with bosh , even though i read quite a few people say NGK ,S WORK BEST IN m40 engine. but I do find my m40 defo runs better following a good clean inside the d izzy cap and claen the rotor to see nice shiny metal . i,ll be looking here regularly to see the fix , as i have a very slight mis fire .
and like you have tried numerous thing. its defo got better but occasionally splutters

[Contours]

Left the factory with Bosch F8LCR plugs which can still be got. I have used the NGK's but they do not seem to make much difference. Runs great on either.

[DelaneyE30]

Timing belt doesn't affect ignition timing and the tension on it will only affect valve timing to a minuscule amount. Ignition timing is controlled entirely by software, using the pulses from the crank position sensor as a reference.
The only fault I know of that can disrupt this is a rare one where the outside of the crank pulley rotates slightly around its hub as a result of a failing rubber piece between the two.
To check the valve timing, refit a crank locking pin in the flywheel and then check (with a straight edge) that the top of the square section on the camshaft is exactly parallel to the head face.
Are you sure everything is as it should be inside the distributor cap?

and like to add , what plugs are you using . I,m not 100 % sure what is best . I know the ngk have significantly less ohms reading in the centre electrode.
1.7 compared to 4.7 K ohms on bosch . And someone said the multi point plugs are not good in m40 . What is Brian's take on plugs , did they leave the factory with bosh , even though i read quite a few people say NGK ,S WORK BEST IN m40 engine. but I do find my m40 defo runs better following a good clean inside the d izzy cap and claen the rotor to see nice shiny metal . i,ll be looking here regularly to see the fix , as i have a very slight mis fire .
and like you have tried numerous thing. its defo got better but occasionally splutters

Thanks for the input, folks. I'm delighted to say I got the issue sorted.

I rang an engine builder here in Ireland that has tuned and built engines for us in the past. Described the issue and he said, check the valves, sounds like one might be sticking open.

So pulled the cover off and sure enough, one of the little rocker arm seats had shifted slightly causing the valve on number 3 to stick open ever so slightly but enough to upset it and cause the smallest of misfires.

Re-assembled everything and the engine sings now. The relief is great as I was losing my mind going round in circles replacing parts.

Thanks all for the help.

[arrisbmw]

good glad you got it sorted. I guess that could not happen ordinarily , its just because you rebuilt the head yes ?
as i remember that rocker arm has the lifter one side and the valve he other, dome shape head where the cam touches it and thats the bit that wears especially if your spray bar gets blocked in the 8 holes.

[DelaneyE30]

good glad you got it sorted. I guess that could not happen ordinarily , its just because you rebuilt the head yes ?
as i remember that rocker arm has the lifter one side and the valve he other, dome shape head where the cam touches it and thats the bit that wears especially if your spray bar gets blocked in the 8 holes.

Yes delighted and yes, if the head hasn't been touched it's unlikely they would cause any issue. This was on the valve side, the little circular shaped seat above the collets had shifted slightly. Likely happened when rebuilding the head. Yes, M40 heads are known to be weak, especially for that reason. The cam wears very easily.

[williambarker]

Glad you got it sorted. You done some very good engine maintenance in getting there though!!

[intuitivejohn]

Thanks for this.

I bought a 91 that has this problem....starts and runs fine...but has this little "skip" or "miss"
like timing is bad. Feels like it's just one cylinder not firing or working right.

Wondered if it could be a bad sensor..but typically they are an either work/don't work.

And...
1) It doesn't happen all the time
2) Once the car warms up it goes away.
3) The car runs like a top.

So stuck valve makes the most sense of everything I've seen. Was getting ready
to do a timing belt..and valve adjustment...and will make sure to pay attention
to your instructions. (only 102k)

It's not a big deal at all...and since car runs so well it's not demanding my attention..Can
live with it.

Once again, makes the most sense. And fits the description
Thanks, John