318i stalls after startup and won't hold revs (M40)

[E30318iM40]

Hi all, I have owned many BMWs, including an E21 with the M10b18, but am new to E30 and the M40b18 engine.

My problem is tickover is very rough and when the engine is cold the cars starts and stalls out immediately. The car starts perfectly and sounds like it is idling very smoothly for a second before revs drop and the engine dies. If i hold the throttle open then the revs increase but if held at a constant revs after a couple of seconds the revs drop and the engine dies. When warm, the car will hold an idle, but it seems low (about 800rpm) and is rough, the car shakes quite a lot and there seems to be a faint misfire.

I have covered nearly 300 miles in the car since owning it and it seems reliable and drives well, keeps up with traffic and does not stall out when stationary, though this is only once the engine has warmed up. I also notice that when coming off the gas somewhat and holding the car at lower revs, say when traffic is slowing on the highway, the engine begins to stutter somewhat and applying more throttle will smooth it out again.

I have tested and checked the AFM, TPS, cold start temp sensor (blue plug) etc, both at the plug and at the ECU and all appear to be working well. The AFM gives what appear to be a correct set of readings as the mechanism is moved up and down. Vacuum leaks in the induction system are present I am sure, but only very minor and not enough to cause the issue. As for timing, the car runs on all four cylinders and revs ok, power is a bit down on what I would expect but she still goes. It could of course be a tooth or two out on the belt, but seems timed well when on song.

After all this, I deduce the problem is fuel related and think it could be the in-tank fuel pump or possibly the pressure regulator at the rail, I'm leaning towards the pump, giving enough fuel to start the engine and keep it running while the fuel/air mixture is leaner, but when the car requires a richer mixture the problems occur.

Has anyone else with the M40b18 had a similar problem which turned out to be fuel related? These M40s seem very complicated and have many idle issues I will admit. I'd love to get this sorted so I can use the car, it has MOT etc, and give some time to the bodywork and get the car nice again. Sadly, with a few other issues to fix I may end up breakeing her for spares.

Many thanks in advance, J

[BenHar]

First thing is to get rid of all leaks on the intake side. Even a tiny leak will mess up the idle.

Ben

[Tineca]

If someone can keep it running spray around the engine with carb cleaner. Then if it revs up you have isolated an air leak. Also clean inside the MAF.

[bmw9818]

that or a can of lynx body spray lol

[E30318iM40]

that or a can of lynx body spray lol

haha, joking aside I managed to get an E39 530d started that had a dodgy camshaft sensor using deodorant into the inlet, kr kr kr kr and she went

forgetting about vacuum leaks on the E30 for the moment, I doubt they're causing the car to die after starting up or misfire, though no doubt they contribute to a rough or hunting idle and I will be looking into such things when I can get the car to stay running when cold so I don't have to rev the life out of it to warm it up to get it to idle.

in my vast experience with M57x diesel units, the stuttering on part throttle has usually been bad fuel or a failing pump, though i admit this doesn't show itself so much with petrols. Just thinking the old M40 seems to be getting all the air it needs in the correct manner so thought it would be fuel related. Couldn't hurt to try a new pump I guess at ~£30, but shelling another £45 plus for an FPR means its starting to cost, especially if the issue remains.

Does M40 top N42 as the most fragile and tempestuous BMW engine? (ignoring the V10!)

[bmw9818]

nah m40s are pretty good i was running one for a few years til i swapped it out, if youve ruled out any air leaks and everythings tight and secure, only two other things as you said, fuel you can get an inline pressure gauge and see if thats where fault if pumps no good, or go through the ignition system,

when i got mine it was sitting for a good 7/8 years so rather than doing the odd part replacement i just replaced most parts as i could get them cheap at the time so just refreshed it

my fuel pump did fail however it was gradual and mainly the symptoms were starting issues, had to lift the back seat and kick the top panel to get it going , eventually changed and it was okay

[Brianmoooore]

M40 is probably the most reliable engine they've ever made.
Take out all the spark plugs and check the colours of the tips.
Buy yourself a £20 fuel pressure test kit from ebay, of the type that includes a tee piece. This will check the fuel pump, FPR, and the injectors.

[martauto]

that or a can of lynx body spray lol

Does this "really" work instead of carb cleaner ??

Mart.

[Tineca]

Only the africa one the java one isn't as good

[E30318iM40]

that or a can of lynx body spray lol

Does this "really" work instead of carb cleaner ??

Mart.

Carb cleaner, brake cleaner, electrical cleaner... body spray, hairspray... anything with a high alcohol or solvent content will rev up a leaky intake or help start an engine struggling to get fuel....
What it won’t do is help with this shonky E30 I’ve bought... guess I will be checking over the intake for leaks again then, but with such a dramatic leak to be occurring after the AFM and no signs of a leak between it and the TB, where is it coming from?? Injector seals maybe?? Will have to do the spray test

[Brianmoooore]

Since the outlawing of CFCs, the propellant in aerosol cans is usually a pressurised hydrocarbon, which a spark ignition engine will happily run on. I do it all the time with my road cars.

[Bmwliam14]

it seems to me this is the same problem i have with my mb40 i found a crack in the pipe that connects from the afm underneath the big black pipe there is a small pipe that fits into that like a L shape that pipe was cracked i sealed it with some tape for a while till a find a second hand pipe that i can fit on to there but its still rich in fuel i can smell it from the exhaust pipe very lean on fuel

[E30318iM40]

Well I’ve checked and re checked all air leaks on the inlet side and the problem remains. In fact, it’s only running worse than it has been haha. It doesn’t even sound right under acceleration any more, like there is a misfire even up the rev range. There is also a squawk noise from the belt area, but having tightened the aux belt the sound remains so I guess the timing belt is over tightened or wasn’t set properly.
My dad has a skip coming on Monday so I’m now over to checking the ignition side of things and anything to get her to run the 3 miles back to my house as well as last week or she will be getting towed sadly!
When I have time I will be stripping the old M40 right down and will get to the bottom of whatever this is. My guess is... bad timing.

[Brianmoooore]

M40 is probably the most reliable engine they've ever made.
Take out all the spark plugs and check the colours of the tips.
Buy yourself a £20 fuel pressure test kit from ebay, of the type that includes a tee piece. This will check the fuel pump, FPR, and the injectors.

[E30318iM40]

So, bit of an update. Checked plugs etc, they seem ok and the engine seems to be getting plenty of fuel though not had time to pressure test yet. I got it to run so I could move it to my house, but not very well, as follows.

I notice that my ICV is the 2-wire type and it is working correctly, though when the TPS idle switch is pressed the DME is not ignoring the reading from the AFM as I was led to believe, but is in fact still serving fuel accordingly. I opened up the AFM to find two deep grooves in the carbon track over the lower third of the throttle, you can see the orange of the pcb showing through. Testing with a multimeter on the AFM pins while operating the brass arm by hand I notice a steady set of readings above the work patch and in the patch a set of garbled readings. It appears that the car fires up fine, reg’s itself enough to open the AFM flap past the worn patch, where the car ticks over fine, but as the flap closes and the brass arm falls back into the rough patch the engine wobbles and shuts down.

I adjusted the arm on the AFM to start just above the worn patch and hey presto, car idles at 1k revs quite happily, albeit receiving too much fuel through the injectors. Still, the car drive back to my house, it was a bit of a lively drive.

So it looks like the AFM is shot here. I have investigated moving the brass arm further out to a non worn section of track but it doesn’t really adjust in this way and I am looking for a replacement unit from a lower miles car.

So there you go, but if progress finally.

[Brianmoooore]

TPS doesn't instruct the ECU to ignore the AFM - it switches to another software map. There shouldn't be much output from the AFM at idle though, because the airflow is barely enough to move the door.
If your AFM track is worn that much, it probably is time to start searching for a replacement, although it should be possible to bend the wiper arm to bring the contact slightly inwards. Worth a try as a short term fix.
If your car doesn't have a cat., make sure you get the earlier type of AFM with a CO adjustment pot. If you do have a cat. it doesn't matter if it has a CO pot or not.
Correct way to check a AFM is to connect a 6 or 9 volt batter across the pot. track, and measure the voltage on the wiper as you manually open the door.
Should always increase. Any drops, however brief, can form a feedback loop and oscillation.

[E30318iM40]

Slight side note Brian. While removing and refitting the main HT lead yesterday I notice that the universal ‘sports coil’ the previous owner has fitted does not correctly fit the HT lead. Looking at pics of OEM ones I notice they have a stud in the center of the plug for the lead to fit snugly on. This coil does not and just has a ~10mm copper ring that the lead sits in. I am amazed any contact is made at all really, there is a thin wire spring round the end of the terminal in the lead, but even that must be barely touching. I would imagine this contributes to the M40s messiness, even if it doesn’t cure the lot, so let’s see when I have time to repair or replace it with an OEM one, likely the latter.

[E30318iM40]

So, another update on this one... after fitting my new coil along with new Bosch plugs and improving the running of this M40, I notice the misfire and poor idle, stalling when cold, still happens. So I take off the lower inlet manifold to have a look at the injectors and access the brown plug temp sensor and I find this... cyl 1 gasket pretty much non existent, cyl 2 not a great deal better. Cyl 1 had also folded over three times so was obviously causing a pretty big gap. So there we go... that’l be my air leak!
As for the brown plug, I notice I have a second blue plug temp sensor put in there, I guess the person who worked on it just hoped it would work! So I’ve ordered a new gasket, £7 and a new sensor £7.50. Let’s get this lot in and see where I’m at then!

[Brianmoooore]

The aerosol spraying test should have shown this up easily, and your spark plugs tips should have been showing a weak mixture.
Manifold gasket failure like this is fairly common on a M20, but I don't think I've seen it on a M40. There's always a first time!