Mystery misfire

MartinClan · #1 20th May 2014, 07:30

Well, for the first time in nearly forty-odd years of motoring, yesterday I broke down. After queuing through some road works for about 15 minutes the Marlin started to misfire. It would idle OK - but as soon as I tried to apply the power it would misfire and grind to a halt. Eventually I pulled in to the verge and called the RAC as I didn't fancy diagnosing the problem at the side of a very busy and narrow A road.

The symptoms were quite odd. It would start and idle fine but as soon as I tried to pull away it would cough and die. Although I had been in a traffic queue the engine temperature had barely rose above normal. Of course by the time the RAC turned up the fault had completely disappeared and the RAC man was as baffled as I was.

Any clues anyone? I wondered about a vapor lock in the fuel system. Is that possible with fuel injection? Or perhaps the air flow meter?

All suggestions gratefully received!

Cheers, Robin

peterux · #2 20th May 2014, 18:56

I once had a problem with similar symptoms (but before setting off...) and it turned out to be the connector on the AFM was a bit corroded and dry. A bit of contact cleaner/lubricant and it was fine.
If it's not that I think I'd be looking for a camshaft or crankshaft sensor that is going open circuit when it gets a bit warm but difficult to diagnose until it fails completely.

...peter

morris · #3 20th May 2014, 19:53

The engine should run fine without the AFM plugged in or at least this is true with MAF based engines. It will just sit on a default worst case map and not perform as well.

It sounds more like a lack of fuel getting through, probably the high pressure pump on its way out. When you open the throttle plate to pull away it's just taken a big gulp of air and died but there was enough to keep it idling. Unlikely to be cam/crank sensors I'd have thought as it wouldn't start properly if the DME can't work out the engine position. Modern cars will refuse to even try and start under this condition but things may have been different with the M20. I've had coils go before and the car idles fine but when you try to pull away it rattles you to bits but doesn't stall. I think your M20 has a distributor anyway so...can't be that.

Thinking about it this sounds just like my mate's 330 that refused to pull away the other day. It idled fine but as soon as you lifted the clutch it died. He's fallen foul of the common problem on BMWs where the siphon between the two sides of the tank stops working and you can't let the tank drop below a 1/4 without getting stranded. It's especially bad on a hill start where the fuel runs to the back of the tank.

Patrick · #4 23rd May 2014, 19:04

Also check the TPS is adjusted correctly, but that shouldn't cause a misfire.

MartinClan · #5 18th September 2014, 17:26

Well I replaced the hp fuel pump and all seemed ok - until today when the same problem re-occoured. I am now wondering about the fuel pressure regulator. Does anyone have a spare I could borrow to try?

Also - can anyone suggest a source for a fault code reader. I still have the diagnostic socket so it might be worth trying that.

cheers, Robin

Mike · #6 19th September 2014, 13:15

Hi Robin

Have you checked all your engine electrical connections really carefully?

I don't know the M20 engine, but does it have an Idle Control Valve like the M50? If it idles OK the ICV is working, but when you accelerate you then rely on the MAF and TPS to tell the ECU what fuel to add. These, or the connection to these are therefore the likely culprits?

I had the same problem as yourself with my M50 going to Stoneleigh this year - I'd driven 80-90 miles, half of it on the motorway, then when I got onto some A roads it cut out above idle. I fiddled with the TPS and thought I'd cured it only for the problem to reappear 2 miles later.
Mine turned out to be a connection pushing through on the MAF connector (I'd obviously touched the MAF wires while fiddling with the TPS) - I know Ian says it should make no difference, but I am certain that was my issue as it has not reappeared since finding the problem and making a proper connection: a female in the MAF connector socket pushed out when the male was pushed in creating a tip to tip dodgy intermittent connection fault.

Out of interest I've read that a failed TPS should not cause the issue for the same reason Ian describes - it should revert back to closed loop and only result in less efficient fuel consumption but not a misfire.

Whatever your problem turns out to be, my money is on a dodgy electrical connection/component/wire somewhere.

I've just read the previous posts and I've obviously experienced the same problem as Peter having a dodgy MAF connection causing the problem - you might want to start there checking each wire very carefully - even for a broken wire?

MartinClan · #7 19th September 2014, 15:08

Hi Mike

I've been over the wiring very carefully - I don't think it's that.
I think the clue is that it seems to happen once the engine is warm, and then when it has been idling for several minutes. (The engine temperature is rock steady at about 90 according to the gauge.)

I would still like to track down a tool for the diagnostic port to see if that tells me anything. I have also ordered a cheepo fuel pressure gauge. If the pressure regulator is on the blink that should show something.

The only other thing I can think of is the low pressure pump - but that is by the tank so I don't see why that would be affected by a period of idling.

Cheers, Robin

Mike · #8 19th September 2014, 16:02

Do you have any components which can be getting heat soaked?

- £5 says its electrical!

peterux · #9 20th September 2014, 08:01

Quote:

Originally Posted by MartinClan

I think the clue is that it seems to happen once the engine is warm, and then when it has been idling for several minutes. (The engine temperature is rock steady at about 90 according to the gauge.)

I would still like to track down a tool for the diagnostic port to see if that tells me anything. I have also ordered a cheepo fuel pressure gauge. If the pressure regulator is on the blink that should show something.

The only other thing I can think of is the low pressure pump - but that is by the tank so I don't see why that would be affected by a period of idling.

Cheers, Robin

Hi Robin,
I've been pondering your engine problem and the symptoms do suggest 'heat soak' causing an intermittent connection or component failure, which 'fixes' itself when it cools down.
I agree the coil could be a candidate as it sits behind the radiator in the Sportster. (I know I had a spare but I haven't seen it since moving house so it may be in my lock up?)
Other possibilities are the crankshaft position sensor (I think I sold my spare, but I'll have a look?), Ignition lead sensor(I've definitely got one of those that you can have), Fuel pump relay?
You could try the old 'hairdryer' trick on the smaller components to try and induce the fault or maybe a can of freezer spray once you've created the fault.(This might be worth trying on the coil).

On fault code readers, the only one I am aware of is by Peake Research. But the early Motronic DME's had a very limited list of fault codes and even the manufacturers warn that the fault codes can be misleading.

I'm probably going over to my lock-up sometime next week so i'll see if I can find anything useful.

...peter

Patrick · #10 20th September 2014, 16:09

You could try heat wrap on pipes and other components. I have some around the brakes & over the exhaust.

I have a spare icv if you want it.

MartinClan · #11 25th September 2014, 07:17

I fitted a valve to prevent the fuel constantly being recirculated through the high pressure pump. Now it has to return to the tank. It runs fine, at full temperature, without any sign of misfire - in the garage that is. Tomorrow afternoon I will take it for a run and give it a proper test.

One slightly worrying other development. I noticed that the rubber O-ring that acts as a seal on the fuel filler cap seems to have swollen making it a pain to refit the cap after filling. It's a recent problem - it's been fine up until now. I wonder if the petrol companies have been fiddling with the recipe...

Cheers, Robin

jeremy · #12 25th September 2014, 07:34

hi robin, I have a spare fpr you are welcome to have if it suits, it is a 3 bar pressure regulator for a 2.5 m50 engine, part number 0280160503. I have a thread on this forum about something very similar happened to me ....I replaced maf idle control valve tps, fuel pressure regulator, injectors cleaned... the problem was in the end so simple, a faulty connector on the alternator, dropping voltage to ecu, and causing every fault code in the book, but essentially gud idle and spluttering under fueling was intermittent and frustrating.

MartinClan · #13 25th September 2014, 08:53

Quote:

Originally Posted by jeremy

hi robin, I have a spare fpr you are welcome to have if it suits, it is a 3 bar pressure regulator for a 2.5 m50 engine, part number 0280160503. I have a thread on this forum about something very similar happened to me ....I replaced maf idle control valve tps, fuel pressure regulator, injectors cleaned... the problem was in the end so simple, a faulty connector on the alternator, dropping voltage to ecu, and causing every fault code in the book, but essentially gud idle and spluttering under fueling was intermittent and frustrating.

Thanks Jeremy. FPR seems to be the same part as for the M20. I will see what my "test" tomorrow brings and, if I still have the problem, I would like to try your spare.

Like you I have already replaced a few bits. My first thought was the HP pump as that was also making a strange noise. They ain't cheep and it didn't solve the problem Grrr. I can see now why some people still prefer carbs!

Cheers, Robin

MartinClan · #14 29th September 2014, 12:33

I managed to borrow a fault code reader and... there weren't any.
And I have now added a one way valve to make sure the fuel goes back to the tank after its been through the HP pump. Previously it could circulate through the HP pump and swirl pot giving it some chance of overheating.

It's now a question of wait and see....

Cheers, Robin

MartinClan · #15 4th May 2015, 19:32

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

MartinClan · #16 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 · #17 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 · #18 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 · #19 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 · #20 14th May 2015, 08:29

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