Mystery misfire - Page 2

MartinClan · #21 17th April 2015, 09:29

The problem only occurs when the engine is thouroughly warm. After 10 minutes rest the fault disapears until the next time.

The m20 only has the single sensor for engine position which I assume is used to determine when to fire the injectors and spark. Without a signal the injection will stop as its the only indication that its running. Or am I wrong?

There does seem to be some history (google) of hall effect sensors becoming fa ulty when they get hot -which is why I decided to change it. I think the sensor has some simple electronics as well as the hall effect device. A single transistor I understand which can be affected by heat.

Anyway we will see... (Assuming last wednesday isnt the only warm day of the year.....)

Cheers, Robin

morris · #22 17th April 2015, 18:33

Hi Robin,

no I think you're right. If the engine only has a crank position sensor rather than cam too, it would almost certainly stop totally (and not start) if it was completely dead as it's the only thing available to time injection (with a few other things such as temp thrown in). I guess the distributor is attached directly to the cam??? so that gives you your spark timing. But i think you said it idles with the fault but won't rev? I would expect a bad crank sensor to not even manage an idle. It sounds like the M20 will be using wasted spark and batch injection as normal so ignore what I said earlier about it reverting to that.

If the TPS were bad you'd definitely have drive-ability problems especially on wide open throttle as you'd not get the necessary enrichment, but the engine should still run without too many problems under normal driving. It is basically just a potentiometer but I've had 2 die on much younger engines. In that case the car just kept going into limp mode every few days, but still worked fine the rest of the time. Had several occasions where I had to quickly drop the clutch, stop and restart the engine on the motorway or going through bends at speed which isn't fun. On modern cars with drive by wire, the TPS has 3 tracks and the outputs are compared by the ECU for consistency. As a safety feature, if one starts acting up it will put the car in limp mode to prevent a dangerous condition where the throttle is left wide open by a fault that it then can't detect in the feedback loop. On a cable driven throttle this isn't necessary as the springs will always return the throttle plate so it may only have a single track. I can see how you'd be skeptical on how it would fail but the tracks can burn out. I don't think replacement TPS of this simple type are normally that expensive so it may be worth swapping that next if the CPS doesn't sort the problem. On a drive by wire car it normally means a replacement throttle body so £££.

After that I think you need to start looking at more general issues you'd get with EFI or Carbs.

[EDIT] the other thing to say is I'm pretty sure the M20 uses a VR sensor rather than Hall effect. Even the early non-Vanos M50s were still using VR.

MartinClan · #23 20th April 2015, 07:30

Well the sensor has arrived so I can't put the job off any longer. It has a 3 wire connection and so I think it is probably hall effect.

I was looking at the job last night. The sensor is easy to get to but the wiring is a nightmare as it passes through a purpose made trunk in front of the engine and behind one of the pulleys. I will have to take the cowl off and the radiator out to access it which is a bit of a pain. It think it is routed this way on purpose to avoid proximity to the HT circuits so there is no cheating taking an easier route unfortunately. It's about time I changed the coolant anyway....

Cheers, Robin

MartinClan · #24 21st April 2015, 07:37

Well I managed to change the sensor last night without pulling the whole car to bits. I have fitted it temporarily with the wiring running over the top of the engine. I left the old sensor in place just tied up out of harms way. If that proves to have done the trick I'll do it properly later on in the year.

Now all I need is a few warm days to test it....

Cheers, Robin

MartinClan · #25 4th May 2015, 19:08

Well... it's not the crank position sensor! The problem occured again today. Now, as soon as the engine is up to temperature, it starts to missfire gradually getting worse until it stops.
The "good" news is that it is getting worse so at least it should be easier to find!
I havenow changed the coil. The saga continues......

Robin

MartinClan · #26 4th May 2015, 19:32

Ps. While changing the coil I slso pulled all the plugs. They were all text book perfect..

MartinClan · #27 13th May 2015, 07:32

An update!
Following a suggestion from Brian Moooore of E30Zone fame I checked the connector, in the engine loom, for the injectors. Apparently, due to the location, these tend to suffer from corrosion due to electrolytic action. This is inconviniently sited under the inlet manifold. A real pig to get to on an E30 I would imagine but on the Sportster it just meant removing the bonnet side and the AFM. Of course it was 100% perfect....

But - I did find evidence of a water leak from the pipes that heat the manifold that could well have been dripping on it. I sorted this out and went for a 40 mile run. It was perfect for the first 35 miles and then the problem re-occoured albeit not so bad as it has been before.

So - I now suspect the problem may be an accumulation of several small issues. Next job is to move the coil which is currently sited just behind the rad and gets cooked both by the air through the rad and the close proximity to the exhaust manifold. But where to put it? Can any other of the Sportster builders remember where they put theirs? I had thought of moving it right back to the top of the bulkhead but this would mean a new, longer, HT lead.

Cheers, Robin

PS. Does anywhere know where I can get a new, orginal spec, coil at a reasonable price? Bosch or Beru both seem to be an arm and a leg...

8 Valve Ed · #28 13th May 2015, 09:13

I put my coil on the bulkhead for that reason, it was either next to the dizzy, right next to the rad, or the bulkhead with a longer HT lead, given the HT lead I had was ample long enough and had been shortened by tying knots in it (by the PO I hasten to add)...

To try the solution would be very cheap, a long HT lead, perhaps a couple of quid, and some tie wraps to hold things in place to test it? It it works fine, easy to try. May not NEED a new coil in the short term. The coil I'm using must be best part of 40 years old, it runs the V8 faultlessly.

Patrick · #29 13th May 2015, 19:00

This is where my coil is:

The injector loom plug we ended up completely changing as the connectors were a bit rotten.

MartinClan · #30 14th May 2015, 07:18

Thanks guys.

8 Valve Ed - good advice!

Patrick. Intersting that your connector was rotten. That's exactly the problem described by Brian Moore. Good location for the coil. Unfortunately my air filter is there! I think it's going to end up on the bulkhead.

Cheers, Robin

8 Valve Ed · #31 14th May 2015, 08:29

Keep us posted, will be good to hear when it's sorted and what actually fixed it.

phil.coyle · #32 17th May 2015, 09:39

Just a thought but have you checked the "tea bag" pick up filter in the tank.
If this is blocked you will get the same sort of fault. It will tick over all day but once the pump gets overheated through the extra effort it will stop, wait 10 minutes and off you go again. This was gained through experience.
My apologies for the interjection.

MartinClan · #33 17th May 2015, 17:55

Thanks for the input Phil. There is no filter in the tank - its external :-). But there is always fuel in the system so I dont think its a filter problem.

This weekend I moved the coil to a cooler location but too soon yet to see if this has made any imrprovement. To be honest I dont think this is the problem but it's another thing eliminated from the equation.

Cheers, Robin

phil.coyle · #34 17th May 2015, 18:21

I'm surprised that there is no filter on the pick up pipe in the tank, is the external filter before the pump?
The only other thing I can think of is the fuel regulator or the fuel return line being blocked or faulty, but it's an outside chance.
Phil

MartinClan · #35 18th May 2015, 07:30

The Marlin fuel tank is pretty crude. It doesn't even have any baffles. But there is a filter before the low pressure pump and another (actual BMW) one before the high pressure pump so the fuel is well filtered.

I wondered about the fuel pressure regulator but its a simple device and I would imagine it fits into the catagory of either its working or its not. I can't see how you could get an intermittant fault. Fuel return line is a straight run back to the tank and is made from (new) 8mm cunifer so unlikely that it is blocked.

Nevertheless all ideas welcome! I just need to wait for some warmer and dryer weather again now to see if my previous changes have made any difference.

Cheers, Robin

phil.coyle · #36 18th May 2015, 09:48

How about the fuel pump relay opening and closing when it gets warm, or the possibility that the spade connectors have pushed out the bottom of the relay socket. Do you have anyway of checking fuel pressure when it immediately conks out? Have you blown any fuses on the fuel side of things lately.
How about resetting the throttle position sensor, because the TPS values are programmed in the ECU sometimes after a lot of use the TPS goes off a bit due to wear. If you turn the ignition on (do not start engine) and press the throttle pedal fully up and down about 10 times, then turn the ignition off, finish drinking your tea and after a couple of minutes start the engine. All this does is give the ECU a re-valuation of the TPS.
I'll keep thinking!

Phil

MartinClan · #37 7th June 2015, 15:55

Did a round trip of 70 miles today. Ok for the first 30 and then a breif reocurence of the problem and then it went away. However on the way home it came back big time and at one point I didnt think we would make it....

I have fitted a fuel pressure gauge and that told me the fuel was fine. However I now have another clue. This time I watched the rev counter. Each time the misfire appeared the rev counter started moving erratically. (Over reading....) The rev counter is driven dire tly from the ECU as I recall. So next thing to do is change the cpu i think. I have a spare somewher purchased just in case.

The saga countinues.....

Cheers, Robin

phil.coyle · #38 7th June 2015, 16:31

I'm sure you must be aware that electronic ignition coils and points type coils are different. It was earlier in you posts that you mentioned your V8 ran well on the coil that you are now using on the troublesome engine. An ordinary coil will run an electronic ignition set up but very poorly. The electronic coil stays "charged" all the time, where as an ordinary type builds up to the charge and releases when points open. You can get a Lucas electronic coil for about £20 I've got one on my EFi, but if you do a lot of google searches about coils you will get to know what I'm going on about.
Phil.

MartinClan · #39 7th June 2015, 17:34

Well the spare ecu I have is a wrong un. I'll have to look again. In case anyone has one lying about the part number I need is 0261 200 173.

I did have one other thought. I would be intersted to know what the forum thinks.... These problems all started after I replaced the alternator (over 12 months ago now). I wonder if there is any chance of a dodgy alternator interfering with the ecu. Certainly must be a theoretical possibility....

Cheers, Robin

Ps. I am using acoil designed for electronic ignition!

morris · #40 7th June 2015, 17:54

I suppose if the alternator isn't working at its best and failing to charge the battery properly, the engine will be running the battery down slowly perhaps to the point where there's not enough energy for a good spark. If you don't have a voltage gauge on your dash I guess you could run the car with some fly leads to a multimeter in the cockpit and see what's happening over the period up to when the problem occurs.

It may be worth sticking your head lamps on as well when the problem happens to see if the extra drain makes it worse.