8 replies to this topic

[whereswally?]

Hi everybody, first time ever on a forum ( and what a fantastic forum I might say) so please excuse any obvious mistakes.

I have a year 2000 Cooper sportpack, standard other than a K&N induction kit and cat-back exhaust system. The car runs well above 2500rpm but on light constant throttle (below 2500) or steady cruising the car misses quite badly and can be heard to backfire through the filter when you come off the throttle quickly. Accelerating hard the misfire clears completely and it pulls well and cleanly. It also pops away at idle although the idle speed is steady.
So far the head gasket has been replaced (comp test revealed leak between cylinders 2 and 3), plug leads, plugs, valve clearances adjusted, a new TPS fitted and coolant and air temp sensors. Oh and all the earths cleaned that have been mentioned in other topics (see, I found the "search" facility!) together with resistance checks on the coil pack and other sensors; all check out okay.
Fault codes only revealed a coolant sensor fault.

Any ideas you have would be appreciated massively, the cars fantastic but the misfire is getting me down!

Oh and it will be going on the dyno in a couple of weeks but i'd love to sort it before then.

Thanks for any and all help -Steve

[Dylan8660]

A good list of things checked and replaced, I'm going to have a guess at back pressure. What exhuast and manifold have you got on it? have you checked the all exhaust joints for leaks?

[whereswally?]

A good list of things checked and replaced, I'm going to have a guess at back pressure. What exhuast and manifold have you got on it? have you checked the all exhaust joints for leaks?

Manifold is (as far as i'm aware) standard. The system is stainless with (if memeory serves correct) a small plate with "MSM" die stamped in it; twin tailpipes, one silencer, exits to the side and approx 50mm diameter. It's also obscenely loud, motorway driving is an experience! Joints are fine with no leaks, but strangely there was no gasket between cat and downpipe, is this correct?
The misfire is definitely not getting worse and has been there since acquiring the car so back pressure sounds plausible. What systems have people used on the mpi with success?

[v21ash]

Joints are fine with no leaks, but strangely there was no gasket between cat and downpipe, is this correct?
What systems have people used on the mpi with success?

There should be steel shim gaskets on the cat flanges, at one time you could only get them from Rover but Minispares ect should stock them by now, (normal motor factors don't normally list them, let alone stock them).

I had a Playmini full stainless system and an LCB on my Mpi and had no problems with it.

My Mpi developed a miss fire shortly after it having a service, i found one of the new spark plug tips had melted away after fitting a new set she was fine. But as you seem to of checked or replaced most of the obvious stuff (like plugs ect) i guess that doesn't help

[whereswally?]

Okay, thanks, i'll see if I can pick up a gasket for the front of the cat in that case.

Mulling over the back pressure and i'll see if I can beg, borrow or steal a standard system locally to check, it's probably going to save my sanity by being quieter anyway.

Talking to a local "expert" today and the possibility of a non-standard cam was also raised (don't ask: the car has an "interesting" history, supposed to be nearly stock but a previous owner is now confessing to some changes), presumably the easiest way to check would be with a DTI on the end of a push rod and measure the lift? Anyone know the standard lift on an mpi cam or are all performance mini cams just altered duration with stock lift??!

During the day I get to work on bikes; there so much simpler......

[v21ash]

There's only one uprated cam that can be used on the Mpi without ECU mods, and it's very mild, it's either minispares or minisport that make it (they both make one for the Spi but only one of them does an Mpi cam). Don't know the lift of this or a standard cam unfortunately.

I've got one in mine and i was told it should be setup the same as a standard cam, which i did and it caused it to idle erratically. Luckily i have a vernier belt drive so after a little adjustment it was fine.

If it has had a different cam fitted that is not suitable for the Mpi or the ECU has not been chipped to suit then that could definitely be the cause of the problem.

[whereswally?]

Thanks for all the info so far guys, been out in the sun working on the old girl today: net result a sheared off top engine mounting on the bulkhead. Are these things just made of tin foil and spit?!
Exhaust is off and as soon as the motor mounts repaired i'll let you know how I got on with the backpressure/cam issue.

[Dylan8660]

As your saying that the engine has had mods try and find out as much as you can from the previous owner as it could be important. I'm thinking that there may be flywheel and or camshaft sensor problem giving you sparks when you dont want them.

[whereswally?]

As your saying that the engine has had mods try and find out as much as you can from the previous owner as it could be important. I'm thinking that there may be flywheel and or camshaft sensor problem giving you sparks when you dont want them.

Agreed, this sounds like the route to go; swapping the exhaust back to standard made no difference (after I'd welded up the engine mount, replaced the bushes, burnt my arm on the downpipes....you get the idea).
Next check (after a phone call or two) will be the lift on the cam and i'll check with Piper/Swift etc. on the what the standard lift should be. I'll also pinch the scope from work and see whats going on with the CKP, I figure I should be able to see if the reluctor ring has moved by the degree positions of the missing poles? its strange as it pulls so well after 2500rpm.
A bike engine beckons.......