36 replies to this topic

[minimowta]

I've been chasing a misfire on my MY2000(X)Mpi for 18 months now.
It's worse when warm from tickover up to about 2k revs, doesn't seem so bad when cold. It may be there above 2k but not as bad. If I hold the revs at 1500rpm there is a regular misfire like it's not firing on all cylinders. When I take my foot off the revs drop to about 1200rpm, hold there for a second before dropping to tick over at about 800rpm. If I floor the throttle and quickly lift off (like a quick blip) the engine coughs and splutters before it picks up then the revs drop quickly and it nearly conks out before recovering to tickover.
It's been with the garage several times and here is what has been done so far.....
Engine rebuild after fixing gearbox problem including rebore/new pistons/cleaned and ported head/SW5 cam. It drives lovely now above 2krevs but still has the low revs misfire that was there before!
All sensors have been checked.
Injectors have been cleaned.
Pressure booster valve fitted.
Various connections in loom and to sensors have been cleaned.
Earth on bulkhead behind ecu has been cleaned.
New thermostat.
New plugs.
Tappets checked.
Some other observations....
I had the air filter off and put my finger over the hole to the idle valve and nearly got sucked into the manifold so I don't think there is a leak there. I took the plugs out and nos. 1&4 were noticeably more sooty than 2&3.

I hope that someone recognises these symptoms and knows where to look next. Any advise would be gratefully received!

Cheers.

[mykweb]

Firstly that is some list of things.

I would like to think that the engine strip down and ebuild would have sorted it. Providing this has been put back together correctly it would point me to being an electrical related issue.

When you say all the sensors have been checked, How? has it been diagnosed with a code reader?

I had an almost identical issue after installing my stage 3 head. It was purely tempature related. When it was cold everything was sweet as a nut. As soon as I hit 50 degrees on the water gauge it went to pot.

I swapped the water temp sensor first - no result
Then swapped the air inlet temp sensor - this did cure the issue.

Have you suck on some HT testers between the plugs and the leads? you should be able to see if any plug is getting a weaker spark.

Mike

[minimowta]

Firstly that is some list of things.

I would like to think that the engine strip down and ebuild would have sorted it. Providing this has been put back together correctly it would point me to being an electrical related issue.

When you say all the sensors have been checked, How? has it been diagnosed with a code reader?

I had an almost identical issue after installing my stage 3 head. It was purely tempature related. When it was cold everything was sweet as a nut. As soon as I hit 50 degrees on the water gauge it went to pot.

I swapped the water temp sensor first - no result
Then swapped the air inlet temp sensor - this did cure the issue.

Have you suck on some HT testers between the plugs and the leads? you should be able to see if any plug is getting a weaker spark.

Mike

Thanks for the reply.
I was hoping the rebuild would fix it as well! I'm thinking it's an earthing problem, that is why I cleaned the earth connection behind the ecu. I'm wondering if there is a fault in the wiring in the ignition circuit from the ecu to earth when firing the spark. Cylinders 1 & 2 derive their spark from the same circuit I believe.
The sensors were checked with a proper reader and the last time the garage had it they were trying to fool the sensors by introducing extra resistances to change the conditions to enable them to pinpoint the fault. I'm grateful for, and confident about their efforts and I'm confident that the engine has been put back together properly this problem was there before they worked on it...... another reason for me thinking it's an electrical issue.
It has had a replacement water sensor and (second hand) air inlet sensor and the plug to the water sensor replaced.
I didn't know HT testers were available - I'll look into that. I remember posting yonks ago that I had pulled the leads off while the engine was running and noticed a difference in the spark strength for different leads (but can't remember which) I was told off for doing that and darn't do it again! Could an earth problem on the primary side of the coil pack cause a weak spark? And would a weak spark cause the misfire?
Cheers

[tmsmini]

What about the purge valve?

[bluedragon]

And the spark plug wires (leads?) One of the few things not listed as having been replaced.

[mykweb]

HT lead testers are avalibe from most good motor factor stores or halfrauds. £10

Remove ht leads from plugs, placetesters in place and refit leads. Crank engine and watch teh lights

don't take them off when running as modern day sparks can cause serious injury.

Does sound like an earth issue.

Check the earth point behind the dash as well as the one behind the expansion tank.

Mike

[minimowta]

Thanks for the input guys.
Sorry! I'd forgotten about the leads. They were one of the first things I changed 18 months ago - didn't make a difference and should have been on my list at the top of this thread. Sorry about that!

I'm intrigued by the purge valve suggestion. According to my Haynes manual this thing seems to come into play at the point my misfire at constant revs shows up ie warm engine/1500rpm but I'm not sure if the coughing and spluttering on a quick blip of the throttle can be attributed to the purge valve or not, what do you think?

I'll definitely get one of these plug lead testers though - sounds like a useful bit of kit.

Thanks again for the info... There's always something to learn - I'm finding this experience as fascinating as much as it is frustrating! It'll be FANTASTIC to sort it out though!!

Cheers

[minimowta]

I don't think it's the purge valve.

I took the vacuum pipe from the purge valve off the metal tube that runs along the back of the head to the inlet manifold and plugged off the end of the metal tube to stop air being drawn into the inlet manifold.

I sucked the end of the vacuum pipe which connects to the purge valve and got a vacuum (I was rather hoping that would be open 'cause that would suggest a faulty valve). Mind you, it wouldn't hold the vacuum for long if I put my tongue on the end.

I left the vacuum pipe from the purge valve disconnected and took the car out for a spin and it's still the same. Shame - I was hoping that was it!

Any more ideas?

Cheers

[peterf123uk]

Hi

I had exactly the same symptoms , i have an unmodified W reg Cooper Sport, Mine happened when i went through a large puddle when it chucked it down months ago, I managed to limp home and instantly checked HT leads and plugs, found plugs had got water on them so the HT leads were shorting out. I know its a long shot and youve probably already changed them, I replaced all plugs and brand new HT leads, plugged in the code reader cleared the faults. run it up the road checked the reader and it was fine. So far 3 months down the line and its running ok!!

Again you probably done it by its just a thought!!!

Pete

Edited by peterf123uk, 26 May 2010 - 06:10 PM.

[minimowta]

Quick update before I start the day job.....

Picked up some HT lead checkers last night and tried them out. I'm getting regular flashing through the revs, didn't notice any holes especially when the 1500rpm misfire was evident so all looks well there! I was hoping the lights would match the spark strength (ie low spark=low light) but I suspect they are just on or off.

Checked the plugs again - 1&4 are definitley more sooty than 3&4. I cleaned them all and although they have only done 1000 miles or so the sooty ones are stained black as well. No sign of unburnt fuel and 3&4 look perfect! I checked the resistance of the leads using my Maplin multimeter (ohms full scale deflection 2000kohms!) - the longer ones (1&4) were 3kohms and the shorter ones (2&3) were 2kohms.

Took the car out for another spin and the misfire is still there Drives great above 2k revs the misfire seems to only be below this and when I blip the throttle hard from tick over. Is there anything in the electronics that only have an effect under these conditions which may need a good coat of looking at? Any pointers would be gratefully received!

Cheers

[miniboy1971]

Has the coil pack been checked over ? Mine was replaced a few years ago (seem to recall it might have had a hairline crack in it) and this solved my mis-fire.

I'm not clever enough to know how you'd check it and it's not partcularly cheap to swap just for the hell of it.

Hopefully someone far cleverer than me will post a method of testing it ?

All the best

[minimowta]

Has the coil pack been checked over ? Mine was replaced a few years ago (seem to recall it might have had a hairline crack in it) and this solved my mis-fire.

I'm not clever enough to know how you'd check it and it's not partcularly cheap to swap just for the hell of it.

Hopefully someone far cleverer than me will post a method of testing it ?

All the best

Thanks miniboy1971. I'm not sure how to check the coil pack either but I do know that the bottom two outlets feed cylinders 1 & 4 and the top two feed cylinders 2 & 3. Each cylinder has two sparks per 4-stroke cycle - one is called a "wasted" spark. I am suspicious of the coil pack because of the difference in colour on the plugs as mentioned previously, but I can't see why the engine should feel so sweet above 2k revs. Surely a dodgy coil pack would be dodgy through the rev band . I'm trying to resist changing things on a "suck it and see" basis and hoping someone recognises the symptoms and can point me in the right direction!

Cheers

[minimowta]

I'm still chasing this misfire

I took the coilpack off last weekend and cleaned up all the connections. I also checked the resistances.

On the primary side I am getting 0.8ohms between the spades from the brown/pink feed (this is the live feed I think) and each of the two return spades from the white/black and white/grey wires. I'm also getting 1.6ohms between the two return spades which I would expect as presumably the feed connects to each primary coil and the ecu earths the respective coil to fire the spark.

On the secondary side I get 7800ohms between HT lead outputs to cyl 1 & 4 and 7900ohms between 2 & 3. That confused me a bit 'cause I was expecting to read a resistance between the HT outlet and the earth points on the four bolt holes but I got nothing.

I also took the ecu out and checked the earth feeds. These all showed zero resistance between the plug and the bodywork except the shield earth to the lambda/crankshaft/camshaft sensors this registered a very high resistance of megaohms but I would of expected this to be of infinite resistance, surely there should be no direct connection between the ecu plug end of the shield and the bodywork?

Put it all back together took it for a spin and.........it's still the same

As mentioned previously all the sensors have been checked but I understand that they could be giving a reading but as it is not possible to drive and see what the sensors are doing under driving conditions it could be the wrong reading but not register a fault?

I'm running out of things to check and unless anyone out there has any ideas of what to look at next I'm going to have to start swapping sensors!

What do you think would be the best order to start swapping given the symptoms I've got?

Thanks for your help,

Cheers

[minimat]

Have you done a compression check?

[minimowta]

Have you done a compression check?

The engine was rebuilt 1000 miles ago. Runs great above 2000 rpm - love it! The misfire only appears to be below 2000 rpm and when I blip the throttle.