Sunday, 24 October 2021

E30/E28: Faulty AFM (Air Flow Meter) repairs, worn carbon-track quick fix


The AFM or Air Flow Meter uses a metal flap that is opened air entering the engine to move a brass arm up and down a carbon-track. The position of the arm along the carbon track alters the electrical resistance through the circuit and this lets the ECU know how much air is coming in, so it can deliver the right amount of fuel to the injectors. The E30 uses a BOSCH AFM unit, also used in many other BMWs with Motronic or Jetronic injection systems.

The analog electrics in the BOSCH AFMs rarely ever fail, but there are two areas that are failure prone after higher mileage - a worn carbon track and a faulty air-temperature sensor.

Saturday, 23 October 2021

E46 318i Touring: New front drop-link for MOT

The 318i Touring was up for MOT this week and failed only on the O/S front anti roll-bar link. Normally I would replace these as a pair, which would have been more cost effective as you can get a set of two from eBay for under £15. Time was an issue and I wanted to get it back for a re-test so had to buy one from Euro Car Parts for the same price, around £15. Annoyingly, the eBay pair were Lemforder and the single piece from ECP is Starline, but needs must.

The car has only just passed 104k miles, so this could well have been the original anti-roll bar linkage and boy was it seized on. I only got a couple of turns on the top nut before the 5mm Allen-key in the stud stripped and rounded, so had to cut the rubber boot off to get a 17mm spanner on to the flats at the back. Even so, progress was painstakingly slow. Couldn’t even get a grinder in there as things are surprisingly tightly packed on E46. This made an otherwise quick and easy job last a bit longer, but it is nearly 20 years old.

Tuesday, 5 October 2021

E30/E28: Adjusting AFM (Air-Flow Meter) - change air/fuel rich/lean mixture


This is for BOSCH AFM units, fitted to most E30 and other BMWs fitted with Bosch Motronic or Jetronic injection systems.

The AFM, or Air Flow Meter, is a metal flap that is drawn open by air entering the engine and the position of the flap tells the ECU how much air is coming in so it can inject the right amount of fuel. The stock parameters for the AFM are set at the factory and unique to each car and BMW recommend that these settings should NEVER be adjusted... so do it at your own risk!!

Why would the AFM need adjusting??

  • You have done a ‘gas-test’ and the results show your car is running rich (too much fuel for amount of air) or lean (too much air for the fuel being supplied) and wish to adjust for emissions / fine-tuning purposes.
  • You have an air leak after the AFM, so more air is getting in than the ECU is supplying fuel for and the engine won’t run, especially when cold.
  • You have an over-fuelling problem where too much fuel is being supplied for the amount of air and flooding the engine.

Adjusting the AFM in the second two instances is a quick and dirty way to get your car running well enough to drive it, but is not a long-term fix for an underlying problem, so bear this in mind before messing with the AFM.

Wednesday, 29 September 2021

BMW E21 323i Baur wins HERO-ERA Summer Trial

I've been interested in HERO (The Historic Endurance Rallying Organisation) since owning my red E21 and had intended to enter that car in one of the smaller events, as they host a few throughout the year. The big one is the Summer Trial in June and it was nice to see another E21 taking first place this year. A Baur cabriolet no less. Better late than never to throw up a pic of Angus McQueen and Mike Cochrane's HERO winning 323i...

Pic: Classic & Sports Car Magazine, Aug 2021

HERO EVENTS & ERA EVENTS - The Original Classic Rally -

Sunday, 26 September 2021

E30 318i: Replacement Speedometer/ Odometer

Had to bite the bullet and buy a replacement speedo / odometer for project E30 this week as after many different attempts to fix the original one [THIS POST], I had to declare it totally junked. These are not cheap and sit around the £65-70 region, but I got a deal on one for £59... still an unwanted cost but at least I no longer have to tape an old iPhone to the dash with a speedo app!

Now the question is, do I swap in the number row from my old odometer or leave the ones in from this one, some 100k miles less.... Hmmm.

As per the pics, the areas of concern on my old speedo are not problem on the replacement, with resistors, capacitors and other components all looking mint. There is also no charring to the back of the PCB, so no sign of this unit overheating in the past and the odometer gearing is perfect. This unit has still covered 154k miles over the last 35 years, but has clearly had an easier life than the one from my E30!

Showing the healthy resistor on the newer speedo on the left and the burnt out one to the right.

E30 318i: Speedometer / Odometer faults + check list

If your E30 speedometer is broken, your odometer isn't turning or both, then here is a troubleshoot check list to find and fix the fault. Doing the work yourself is a bit fiddly, but totally doable if you are handy with a soldering-iron.

** A more detailed guide / thread all about E30 speedometer / odometer faults by 03IRM3 on is here -, which was a great help. **

NOTE: If your speedometer is not reading, before starting to diagnose the instrument the first thing to check should be if there is a pulse being generated by the speed-sensor in the differential, see E30Zone Wiki here.

Broken Odometer drive gear.
This is the most common problem when the odometer has stopped turning. The drive gear from the stepper motor that turns the larger cogs tends to split on higher mileage cars causing it to skip teeth and not turn the odometer gearing. Replacement gear sets can be bought from eBay for a few quid.

Burnt out resistor.
This is the main power resistor on the 12v + input to the PCB and it's never usually failure of this component that causes it to burn out. Instead, failure of a component further upstream is usually the culprit by drawing way too much current, causing the resistor to work double-time and overheat. This one was still working amazingly, given the heat it had been producing that charred everything around it, but I threw a new one in anyway. The value was about 150 ohms, so that's what I went with.

Blown capacitor.
Sudden failure of both speedo and odometer can often be down to the main power capacitor blowing. This is visible in the above pic to the right of (and next in sequence to) the burnt out resistor. It's a basic 220uF electrolytic capacitor and is very simple to replace with only two pins needing de-soldering, so should be the first step in troubleshooting a dead unit, even if the cap itself doesn't appear blown. Mine had been scorched by the overheating resistor, but seemed ok. I replaced it anyway, but it did not help with mine.

Snapped wire on Odometer stepper-motor coil.
This is rare, but does happen as 03IRM3 has seen one and mine had this too so I guess it does happen, where one of the single strands of copper coil for the stepper-motor snaps off its pin on the PCB. It’s possible that this happens due to time/age and vibration in the vehicle, but could likely be due to manhandling the unit while it is out for other repair work. Either way, it’s an easy fix to just solder it back in place and will be stronger than it was before at least.

UAF chip blown.
If none of the above fixes have worked and both speedo and odo are still not working, the likely culprit is that the UAF I/C chip has malfunctioned and is not processing inputs and outputs correctly, which leads to the resistor burning out, caps blowing and the odometer coil overheating. These UAF2115 chips are common, used in many speedo/odo units, taxi-meters, tachographs etc., so replacements can be found on eBay or electronic component sites for a few £/$, but this is probably the most advanced fix needed for these units and the question at this point is whether it's worthwhile. 03IRM3 has replaced chips as a last resort only to find the unit still won't work, so I decided that with all other fixes not working and my odometer still needing new drive gears that it was time to junk it and seek a replacement unit... [THIS POST].

Wednesday, 15 September 2021

E30 318i: Replaced AFM (Air-Flow Meter)

I’d been messing with my AFM in order to get the car running ok when cold and when I finally fixed the massive air leak on the inlet manifold I then struggled to reset the AFM back to standard. 
In order to get the M40 to idle ok while the engine was cold I adjusted the mixture setting to run richer. Obviously this was providing enough fuel to stop the engine cutting out while the plenum was filling with as much air as the cylinders could gulp through the gap in the manifold gasket. It runs and drives ok in this setting, but idles quite high (about 1100rpm) and you can smell the stink of neat petrol out of the exhaust. Ok for short trips shunting the project car around but will need addressing to enjoy it before summer ends!
The real problem began when I had repaired the gasket, got the engine running smoothly and then tried to reset the AFM back to a stock, or near stock value. To begin with I had been putting marks and taking photos to recall where the settings were before I started messing, but you know how it is, once you get immersed into a several day long tweaking session (take that as you will!) the tendency is to get lost along the way.
I gave up trying in the end and decided the only way to dial this engine in and be sure I was not fighting with a dodgy AFM was to buy one that had not been messed with. This is in my interests anyway, as I got one from a car with far lower miles on it so the carbon-track is less worn and I know the air-temp sensor is OK too, which was a doubt with the original one.