If the freedom of speech is taken away then dumb and silent we may be led, like sheep to the slaughter.

- George Washington

Wednesday, 26 October 2011

Technical conundrum

Technical issue, and car-related, so if that's not your, er, bag, man, please move on.

Ford Mundaneo, 2 litre TDCi. A while ago, the reversing lights stopped working. I traced the fault to the switch on top of the gearbox. Getting at it necessitates removal of the air cleaner and associated gubbins. I removed enough stuff to determine what tools I would need (yup, got 'em) and then put it all back. As it was summer, I then forgot about it. Recently, the reversing lights have started working again, just in time for the dark mornings and evenings. Thank you, whoever you are.


At around the same time (impossible to say for sure), I started getting issues with the car. One was the 'engine malfunction' light, which came on at start-up and stayed on permanently. The other was poor cold starting - the car would start OK, but for the first 10 seconds would run very rough and pour smoke out of the exhaust. After that, it seemed to clear and ran fine thereafter. Performance up to maximum speed was completely normal, fuel consumption normal too. I took these two together and came up with a faulty glow-plug or two - I've had exactly the same symptoms as before on a Land Rover diesel where a glow-plug went bad. As the car was running fine otherwise, I wasn't too concerned. It passed an MoT (with max revs emissions test) in this condition.

When the nice man at Enfield Auto Recovery in St Neots drained the nasty old petrol I had mistakenly put in the car at the weekend, he told me that the air-flow sensor had been disconnected when he looked at the car. Impossible, said I, as the car is running fine: easy cruising at 100 mph and averaging 50 mpg (not at the same time, obviously). Also, the connector for the AFS is on top of the engine, easily visible, and I see it every time I check the oil. The connector is about 12" from the oil filler.

Can I really have missed it every week for six months? What he said makes sense, as I can remember disconnecting it to remove the air cleaner, but a) he was convinced the car would barely run with it in that condition and it was fine, and b) it was replaced for the return journey and the EML stayed on, with no change in performance. Oh, and c) I am pretty rigorous about checking everything is in place after I have finished a job. Working on bikes makes you a bit particular like that.


Yesterday I started the car again for the first time since I returned. It started perfectly, with no rough 10 seconds of throat-clearance. And the EML light is off. I've just tried it again now. Perfect. I am wondering if the garage really have cured the problem and it has taken a couple of heat cycles for the fault to clear in the ECU. I am delighted, of course. But I have a couple of nagging questions:
  • Did I forget such an obvious thing when I took the air cleaner off in the summer?
  • Can I really have missed such an obvious thing for six months?
  • Would the car really have run this well with the AFM disconnected, and even passed an MoT?
I am thinking 'not very likely' to each of the above.

I think it will remain a mystery, but this will not spoil my enjoyment of having a car which starts well and runs without error codes again.

Mind you, passenger and load-carrying duties are over for a while now, so it probably won't be used again for a week or three. Back to two wheels, thank Heavens.


  1. The label on the engine cover seems to say TDCi, which I think is the first of the proper Common Rail diesel engines that Ford released into the wild.

    I'm more familiar with VAG TDi engines, but the mass air flow sensor that you found disconnected is more associated with the emissions side of things (EGR) than performance and tuning. You will possibly find that this sensor also reads ambient air temperature or ambient air pressure as well as MAF.

    If the ECU can't read the ambient temperature, it may overfuel slightly until the engine temperature comes up (and becomes most significant).

    Although it's rare now, if your car was struggling to meet emissions, a friendly MOT tester might pop the connector off the MAF in order to temporarily fool the ECU into running "better" for long enough to pass the test. In practice, once the ECU has worked out which way is up, it would go back to levels that would fail. Used to work a treat with the EEC IV and EEC V injected petrol Ford engines I used to run....

    If you have a problem with a MAF, take it apart and clean the (very fragile) sensor wire (brake or carb cleaner). It works by using electrical current to maintain a temperature in the wire - the more current required, the greater the air flow. Over time (especially if you fit a reuseable performance air filter) it will be progressively covered by a thin layer of sh1t. This acts as a thermal insulator, giving a false reading. On the IZZ engined Toyota my wife drives, this problem is reported as a lambda sensor fault because that is where the mis-match is noticed!

    You are also correct that some faults will automatically clear the MIL (management information light) when they are corrected, or a few starts after they are corrected. There will probably still be a fault logged on the ECU if you have a code reader, but it will be shown as "intermittent".

    Hope that helps

  2. I would answer yes to all three questions.

    With the sensor disconnected there will still be airflow into the engine but not at the required amount which would explain your starting problems.

    I pulled the air flow gear apart on my Seat the other week and forgot to reconnect the pipe to the air cleaner box. Duh!

    When I cold started the car, the revs kept jumping up then dropping down, then back up again. Mine is a carb so the massive fluctuations in revs was the carb trying to compensate for the improper air / fuel mixture during the cold start.

    That's as I see it anyway.

  3. I am more familiar with things like mass airflow meters on Land Rover V8s - petrol, of course - and until now all my diesels have ben relatively simple beasts. My understanding is that the MAF sensor measures the airflow into the manifold, and allows the ECU to match it with the right amount of fuel. That's what the hot-wire meters do on LRs. But it seems that on the TDCi that work is done by the T-MAP sensor, so I am not really sure what the MAF sensor actually does. My Haynes manual is no help.

    It may be that the MAF sensor doesn't do much, hence the negligible effect on performance.

  4. Sorry, Mick, your comment got spamtrapped and I have only just seen it. Thanks for the very detailed reply.

    As I said above, I am familiar with the Lucas (?) hot wire sensors used by Land Rover on the V8 petrols, but I have no idea about this in relation to diesels. I suspect you are right, as the effect on performance was minimal, but possibly the emissions were affected. Even so, it passed an MoT emissions test like this. The effect was exactly like running on two cylinders for the first ten seconds - bad misfiring and a lot of smoke - hence my assumption of faulty glow plugs.

    Are they still using the hot wire system? That's very old-tech now.


  5. Yes, hot wires are still out there, with no moving parts. They largely gave up on the old flap meters you found on '80s Rovers and Fords because they were not terribly linear, suffered from pulsing and had a habit of sticking. If you have a better idea, the Patents Office is available on-line!

    Don't ignore the glow plugs (the old-fashioned test is still valid), but with modern diesel engines some of the things you think are happening might not be....

    EGR is often dependant on the MAF sensor, but the valve is only usually open while on light load/light throttle, or sometimes as part of a turbo boost control system. When at either end of the power scale the EGR valve will be closed anyway, so a both a high revving or an idle test could still be passed. The idea of EGR is that it increases the combustion chamber temperature which reduces the NOx levels coming from the exhaust. I don't think they measure NOx for the MOT.

  6. To Bucko - diesel engines are not like petrol, you don't have to hit a specific mixture to make them work.

    As long as there is enough air to burn the diesel, that's fine. If there is too much fuel for the air, it's blown out of the tailpipe as black smoke (or white if you're a really long way out).

    If you put in too much air (by jacking up the boost) you just have a really clean burn and an unnecessarily short life for your turbo.

    Diesel fuel injection timing, however, is critical.

    Your problem with the Seat may have been that the carb was missing the damping effect of the air box and filter. Or, of course, the driver sitting in it ☺

  7. Interesting reading from your knowledgeable commenters.

    Thank goodness my car's a Company Car!

  8. As Mick Anderson says diesels are very different from petrol engines. In an older carburettor fed engine the ratio of air to fuel is determined by the jetting, and with the exception of Stromberg and SU carbs, is not compensated for with regard to varying air pressures. Go up a high mountain pass in something with a Webber or Solex carb and it will run rich.

    The airflow sensor measures the actual volume (allowing for density) of air going in, and the injection system provides an appropriate amount of fuel.

    With a diesel there is normally no throttle valve, and the sole means of control is how much fuel is injected. As Mick said so long as there is at least as much air as is needed for complete combustion the main thing a sensor would do is provide information for emissions etc.

    I suspect (but am happy to be proved wrong) that the other purpose is to limit fuel delivery when the the turbo isn't spinning very quickly. Otherwise you would get puffs of black smoke every time you booted it from idle.

    Probably unrelated to this, but possibly to the reversing light issue is earthing. If Ford have their wiring systems designed by Fiat then have a good look for lots of black wires all congregating at a strip connector conveniently located where it gets covered by sh!t thrown up from the road. Add poor quality push-on spade terminals and you have a perfect recipe for interesting and hard to isolate electrical malfunctions...

  9. Richard (at work)26 October 2011 at 23:42

    Great to read so much knowledgeable comments, and thanks to you all. The Mundaneo has always kicked out a lot of almost-smoke when you boot it, but it's only visible at night in a following car's headlights. In daylight, it's clean as a whistle.

    I'm pretty sure the rev light issue is the switch. I have traced the wiring with a miltimeter all the way back from the light units, and (until the other day) there was no current after the switch but 12V reaching it. Ford electrics are usually reliable, I think. ICBW, of course.

  10. Not a techie but I have exactly the same problem " the reversing lights stopped working. I traced the fault to the switch on top of the gearbox ". Matey the Mechanic tells me I have worn away the metal bit between the sensor and the switch. ie New Gearbox required except that his mate is "machining" (wow!) a new bit which he will "weld" (wow!) in to get me through the MOT.

  11. Richard (at work)27 October 2011 at 03:06

    Don't forget that non-functioning reversing lights are not a fail point on the MoT - I asked last time as they were not working then. Guy laughed at me.

    I am unable to weld (project for next year?) or 'machine' in the proper sense, although I am a master bodger who can make odd bits do all sorts of useful things. My best one was making a hair grip function as a headlight bulb retaining clip, saving about £300 on the alternative, which was a new light unit from Honda. It was still working when I sold the bike. Three weeks later, I found the original clip, which had sprung over my head and into a pile of leaves. But yeah, 'wow' just about does it.

  12. To Joe Public - I haven't had company cars for a couple of decades, but I didn't leave them alone when I did....

    To Microdave - On a diesel, the MAF isn't used to check up on the turbo. That's what the MAP (manifold absolute pressure) sensor is for. Your comment about the air sensors job is only really true for NASP petrol injection.

    All turbo diesel engines kick out some smoke while they are coming on boost, even zero mileage vehicles. You need fuel to provide combustion for the expanding gas to drive the turbo, so while off-boost the fuel inevitably comes first. In daylight you can barely see it, unless someone has done a bit of performance tuning, or the engine is a bit tired.

    As for completely clean exhausts on diesels, that what DPFs (diesel particulate filters) are for, and you really don't want to start that discussion (or to buy a car with a DPF).

    Regarding the reversing lights, it's probably the switch. Have you taken the connector off and shorted the two wires together? If the lights come on, it's something to do with the switch. Pull the switch out, plug it back on to the loom and see if it works by hand. If it does, it's probably wear, but the switch plunger has probably worn as well as anything in the gearbox. Change the switch, even if you only go to a factors for a cheap part. If it still doesn't work, "adjust" it - a new component is nice and clean and generally easier to bugger about with.

  13. Mick, thanks for the info about smoke as the turbo kicks in. I didn't know that, and it is reassuring.

    Yes, I have bypassed the switch and everything works, so I know that's where the fault is. What I intend to do is simply replace the switch (unless it's about £50 when I have a look). Getting to it is a bit of a faff, necessitating removal of all the air intakery and other stuff, so if I deal with it it's got to be done in one shot. I don't get enough fiddling time as it is, and I'd rather spend that time on the bikes :)

    As I said in the original post, the car is an appliance, and I will do with it what I would do with a defective part in the dishwasher - replace the part and forget about it.

    Thanks again - helpful post.

  14. @ Mick - thanks for the correction. My car still has a carburettor so I admit that I'm not fully up to speed on all the latest stuff (nor do I really want to be...

    "And you really don't want to start that discussion (or to buy a car with a DPF)."

    Richard will love me for disregarding that, but a while back I popped round to see a mechanic friend, and he had the PF removed from a Citroen people carrier, and was trying to blow the crap out of it with an airline. The warning light had come on despite there being an automatic cleaning system supposedly good for 80k miles, way more than this vehicle had done.

    As a replacement was some £ hundreds, and he hadn't much to loose, I suggested pouring some hot water through it in reverse. Two buckets later and the water ran clean. Worked a treat!

  15. Love that old-skool mechanic! If it works, it works ...

    There's a common theme here, if anyone is following things like the compulsory ABS for bikes argument.

    1. We don't need complexity, although the dealer network loves it.

    2. A slight loss of notional efficiency, if it leads to greater fixability, is a net gain for the owner and the planet.

  16. To Microdave: Here is the idiots guite to DPFs.

    It's a fine filter in the exhaust to catch soot (black diesel smoke) and stop/start city driving clogs them rather quickly.

    On cars (lorries are generally different) there is a regeneration cycle which chucks some unburnt diesel through the engine into the filter, starting a fire that turns the soot to ash. Eventually there is no room for the ash and the filter has to be changed, although vehicle manufacturers will tell you that this is a 100k mile item.

    What they don't tell you is that you have to do lots of mixed driving to keep the filter clear. To clear the filter you need to drive for 20-30 minutes at a steady speed (50-70 mph). This gives the regeneration cycle time to work. There is no dashboard indicator to tell you what's going on unless there is a warning light that has come on (in extremis), in which case it will go out. Regeneration will start even when the light is not on, so this is not a good guide.

    They also don't tell you that a car can lose up to 10% of is fuel economy because of the diesel used in regeneration.

    This is not an economy system, it is an emissions control system. It reduces emissions in town and moves them out to the motorways.

    Don't just remove the filter unless you can be sure that the regeneration system has been nobbled. If regen starts without a filter, [insert your own dire warning here].

    Told you that you didn't want to ask....

  17. Thanks for the extra info - I'll pass it on. IIRC this Citroen had a separate tank of some special fluid to run the regeneration cycle. This was nearly full, and there was a light to indicate when it needed replenishing.

    The driver concerned lived out in the sticks, so I assume he did a fair bit of steady driving up and down "A" class roads, rather than being stuck in town.


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