Mystery misfire

jeremy · #1 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 · #2 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 · #3 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

AlanHogg · #4 29th September 2014, 20:13

I had similar a while back and it was poor connection to relay holder for the injectors. I had looked at all manner of things prior and really only found fault by chance . The cable at fault into the relay holder hadn't 'locked' into place so installing the relay pushed the female in the holder back making for intermittent contact.
I have at one time replaced the HP pump as it was making a hell of a racket and on its last legs but the engine still ran ok. so doubt its fuel related especially as it ran OK on its original set up.

MartinClan · #5 16th April 2015, 07:37

Well - the first sign of warm weather yesterday and the misfiring has re-appeared. So far I have:

Checked all the connections
Changed the fuel filter (I had a spare so that was free)
Changed the HP fuel pump (£££ doh but at least I have a spare now)
Changed the fuel delivery so that fuel is always returned to the tank after it has been through the fuel pressure regulator (no chance that it can re-circulate and overheat).
Improved the tank venting so there is no chance of any restriction.

None of these has had any effect whatsoever except on my wallet.

My current thinking is if it were the coil or other spark problem then there would be unburnt fuel exiting from the exhaust which you would smell (you can't) and also would cause a backfire (it doesn't).

So I am now thinking it's a sensor problem.
The TPS is a pretty simple, mainly mechanical, device and I can't imagine that being much affected by temperature.
The oxygen sensor is only used to fine tune the mixture (as I understand it) so I can't imagine it being that either.
So that only leaves the engine (camshaft) position sensor.- I have ordered a new one from GSF. They seem to be on special offer at the moment so the price wasn't too bad (££).

I am beggining to hate fuel injection...

Now I just have to wait and see....
As usual - any constructive suggestions welcome!

Cheers, Robin

morris · #6 16th April 2015, 21:43

Hi Robin,

Have you gone right back to basics on this e.g.

- compression test on each cylinder
- spraying carb cleaner for air leaks as a lean burn would cause this
- each plug out in turn still connected to the HT lead but grounded and turned the engine over to make sure you're getting a good spark on all cylinders (fuel pump fuse removed).
- pulled the lead on each plug in turn with the engine running to check it bogs on all cylinders (not sure this is a good plan H&S wise with HT leads mind you)
- just pulled each plug to check whether any are wet with fuel, oily or damaged etc
- checked each injector is firing (screw driver on each with your ear against the end)

All my previous misfire issues on various cars have come down to a bad pencil coil. In all cases the car would idle fine and even pull away gently but as soon as you tried to accelerate the misfire would start. I've always had an OBD2 scanner to hand with makes life easier on modern engines as it will tell you which cylinder and often that it is actually a coil but the symptoms seem very familiar.

You won't necessarily get back fires or smell petrol either if just one cylinder is not firing 100%, just a serious shudder.

I wouldn't suspect the cam sensor as that would either prevent the car starting or see it revert to a batch injection mode which would just be less efficient (not sure if m20 was fully sequential though anyway). Most other sensors such as coolant, air and O2 should all have a similar effect and you'd notice a lack of power and high fuel consumption rather than complete breakdown.

my money's on either an air leak or something in the ignition chain between the coil and the plugs

MartinClan · #7 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