E30 318i M40b18: High / erratic idle issues NOW FIXED, stable at 800rpm + good throttle-response finally

After my cutting out when cold problem and no low-end throttle response from a busted lower inlet-manifold gasket (THIS POST) and high / pulsing idle issue from a poorly set throttle-stop screw (THIS POST), I have finally got the old E30 ticking over properly. It now sits at about 800rpm on idle and throttle response is smooth across the range. Pleasant.

E30 M40 316i / 318i: Adjusting Throttle-Body and Idle Stop-Screw

Yes, this is set at the factory and they say it should never be messed with, but as the youngest M40 engined BMWs are approaching 30 years old now, chances are the stop-screw and throttle-plate will have been adjusted at some point in its life. 

Here I will explain what happens when it is adjusted, the problems it may cause if it has been and how to reset it back to stock. 

WHY HAS IT BEEN ADJUSTED?

The stop-screw should have a paint mark on it from the factory to show if it has been messed with, but after so many years the mark may no longer be visible. If you feel yours has been adjusted in the past, the main reason this has been done is likely to falsely raise a low idle caused by another issue, say an induction air leak. If an over lean mixture is causing a low idle, the tendency is to tweak the idle stop-screw to get the car to tick over at higher revs and stop the engine stalling out or running lumpy. This is a great short-term workaround, but will cause other issues with the AFM / DME and fuel delivery further down the line, particularly when the underlying issue is worked out.

SYMPTOMS:

The issues you will get with a poorly adjusted throttle stop-screw are:

High idle.

Hunting / pulsing revs. 

Poor or no low-end throttle response.

Erratic idle (if throttle position switch TPS is not engaged).

Bogging at high revs (where wide-open throttle WOT switch is not engaged).

ADJUSTING:

There is only one way the throttle can be adjusted, via the stop-screw, though the throttle-cable itself can be adjusted to change throttle response somewhat, mainly with how the pedal / cable reacts to driver input and will not affect the fuel/air ratio and the engine idle.

** The intended job of the idle stop-screw is simply to stop the throttle-plate from jamming in the throttle-body and being difficult to open when the pedal is pressed and not to change the car’s idle characteristics. BMW recommend that this is never played with, so do so at your own risk. **

1. Back off the lock-but using an 8mm wrench.

2. Use a small flat screwdriver to wind the idle stop-screw in and out. 

* Clockwise will push the throttle-linkage further from its rest position, holding the throttle-plate open slightly and allowing more air to bypass it while the pedal is not pressed.

*Anti-clockwise will allow the throttle-linkage to close further and will reduce bypass air and choke the engine while the pedal is not pressed. 

STOCK SETTING:

You will need a set of A/F / imperial feeler-gauges as this is how the stock throttle-plate aperture is measured. 

** If you are only adjusting the throttle-plate and do not need to adjust the throttle position switch (TPS) then the throttle-body itself does not need removing and you can skip to step 7.**

Removing the Trottle-Body:

1. Remove the main air-inlet hose from the AFM to the throttle-body by loosening the jubilee-clip and easing it off.

2. Unclip the throttle-cable from the throttle-linkage and remove the two screws holding the throttle-cable mount from the top of the throttle-body housing using a 10mm wrench. Move the cable assembly to one side.

3. Remove the two water hoses and air vacuum-hose from either side of the throttle-body by undoing the jubilee clips and teasing the hoses off with a screwdriver.

4. Remove the six nuts from the upper inlet-manifold using an 11mm wrench and the two locating screws from the manifold using a 10mm wrench. Lift the upper inlet-manifold so that the throttle body can be fully accessed. 

5. Remove the throttle-body from the inlet manifold by undoing the four nuts using a 10mm wrench.

6. Turn the throttle-body upside down and remove the throttle-position-switch (TPS) by removing the wiring connecter and undoing the two cross head screws.

7. Back off the lock-nut of the stop-screw using an 8mm wrench. 

8. Use a small flat screwdriver to adjust the throttle stop-screw by the notch in the end of it. Obviously, clockwise will move the screw further out and make the throttle-butterfly rest in a more open position. Anticlockwise will allow the butterfly to close more.

9. For the OEM setting, the throttle-butterfly should be 0.377” from the housing, so use your feeler-gauge between the side of the tube and either side of the butterfly until you find a happy medium.

10. Tighten the lock-nut up again with an 8mm wrench.

11. Now take the throttle-position switch (TPS) and locate it back in the housing so that the switch is depressed while the throttle is in the fully closed position. With the two screws loosened, the TPS can be swivelled left and right as in pic below to adjust the point at which it engages. The ideal placement for the switch is to have it click closed while the throttle is open about 1mm. (You will hear the quiet click from the TPS as it opens and closes.) When you’re happy with the TPS placement tighten the screws to lock it in place.

12. Before refitting the throttle-body, check the operation of the butterfly and TPS and make sure the butterfly is not binding to the body while fully closed as this will ruin low-end throttle response.

13. Refitting is a reversal of steps 5-1.

E30 318i [M40]: Massive air leak SOLVED - lower inlet-manifold gasket

I had checked and re-checked the idle control valve [ICV], the air-flow meter [AFM], ignition system, tested for the fuel pressure, performed tests on the DME [ECU] to make sure that was working and even laboriously tested the wiring loom for continuity, but still the engine idles terribly when cold. It ticks over lumpy, shakes the engine side to side and cuts out at will, even when power is applied. 

I checked the timing and all was good, under the rocker-cover showed no defective valves and the cam-shaft doesn't look all that worn. I guess the only thing left to check is the injectors... right?? Now hold on a minute I hear the E30 and M40 engine gurus cry, the symptoms you've described point towards an obvious massive air leak in the induction system! 

This is true and is also the very first thing I looked for and tested. Initially I performed the 'spray' test using a can of brake-cleaner (though 'Easy-start', carb-cleaner or electrical-cleaner work too, anything with alcohol content). This involves running the engine and spraying brake-cleaner around the intake-manifold, ICV, vacuum hoses, throttle-body etc. If unwanted air is being sucked in, say through a split hose, the brake-cleaner will be sucked in and combusted by the engine causing it to rev higher. If you are spraying in one particular place and the engine revs increase accordingly then you have found your air-leak. Thing is, I did this test over and over again and it showed nothing. 

I’ve heard that air can be sucked in past the injector bodies when the rubber sealing rings have degraded over time and that the lower intake-manifold is also a main culprit, but spraying round this area made no difference to the engine revs or affected the poor idle. I did a compression test and the readings were a bit low, but not enough to cause any issues.

At this point I decided it to bite the bullet and begin stripping things down. The removal of the lower intake-manifold is quite complicated because it involves removing the fuel-lines from the rail, which is why I put it off, but clearly if you want to fix something then you need to get your hands dirty so off came the fuel lines and away came the lower manifold to reveal.... well, as you can see from the photo, a virtually non existent gasket. Sigh. 

Cylinder 1 has a massive gap where the gasket is no longer there, so whenever there is a vacuum in the plenum it’s just filling up with fresh air even with the throttle-body shut and this explains why the ICV appears to be doing nothing and leads people to look there first. Whoever was refitting the manifold was obviously in a rush because the broken gasket has folded over on itself, causing the air gap to be even bigger. I guess they just tightened it hoping to make a seal, but alas no. Cylinder 2 is not much better with a noticeable gap and the rest of the gasket squashed flat. Cylinders 3 and 4 are better, but still paper thin. It looks like this may be the original gasket and the past 35 years have not been kind to it (or to me for that matter).

So... looks like I have found the culprit of the rough idle. Finally! New gasket is ordered, a snip at £6.88! Amazing how such a simple thing can cause such a severe problem and goes to show how sensitive these ‘analog’ ‘80s cars are to air metering issues!

E30 318i M40: Misfire and self-revving - symptoms of major induction air-leak

This video shows the symptoms of an air-leak in the induction system, where unmetered are drawn in after the throttle-body causes a terribly poor idle, a permanent misfire and the engine to rev itself gently. The tendency is to look at the ICV (idle control-valve), ignition coil, AFM (air-flow meter) etc., but chances are it will be an air-leak on the induction. In this case it turned out to be a damaged and disintegrated gasket where the lower inlet-manifold mates to the cylinder-head. 

E46: New secondary air-pump - one last bash...

Thought I would have one ultimate stab at the E46 as it is such a nice car to drive and only let down by shabby bodywork, which I can do something about, and the super low idling problem, which apparently I can't.

I refitted the Vanos-actuator and checked for anything I'd disturbed under the inlet manifold, finding a loose wire connector which I clicked back in, though the CCV was all plumbed up fine. A replacement secondary air-pump and its hose / filter was £30 and the only thing now missing from the inlet arrangement. It took no time to fit, just two 13mm bolts holding it on and swapping the hoses over, but has still, alas, made no difference.

I've spent nearly £300 on parts to cure this engine of its fault, but nothing has done the trick so I guess I will have to call it a day and scrap her after all. Again a shame, as it has 9 months MOT and 4 months tax left on it. The running gear is sound, I sorted the brakes and the engine runs fine when underway, pulling well in all gears over 1500rpm. Handling is as sweet as any E46. The interior and exterior are in terrible nick, but thats nothing that couldn't be sorted with a day or two spent on it. That's only worth doing if the car is useable however, and it sadly isn't.

On top of the impossible low-idling, the handbrake-shoes have collapsed in some way and stick on completely in reverse with the handbrake off. This is probably from being used so much to stop the car while my right foot was still keeping the revs up, but is extra work regardless. A large M12 bolt holding one of the front shock-absorber struts broke while I was attempting to fit the coilovers months ago. I've been driving on it and the strut hasn't moved at all, but it too will need replacing at some point and removing the stud will be interesting. Inside the car, the fan for the heater/blower doesn't work and the driver's door card is missing, meaning no speaker or door handle. Exiting the car with dignity can only be done by lowering the window and reaching out for the other handle, but all these foibles I rather liked. Using it every day was a nightmare though and, with the 5-Series engine swap underway I just won't have the time and money to restore it, so I'm hoping I can get the £200 I paid for it back from a breaker. That means I'm down about £300 on the whole deal. I could make the £500 back and probably more by breaking the car, but I could really do with it out of the way quickly now so a loss it may have to be.