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Mpi Transplant - Misfire Under Load


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#1 craig 1010cc

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Posted 16 March 2019 - 05:40 PM

Finally got my 85 city back on the road with the transplanted running gear from a 98 MPI. All untouched from when we removed it from the donor, other than a service.

I have made life a little more difficult in that I have retained the front subframe, side rad and I'm not running a servo. I'm also running the original centre clock and oil pressure and water temp gauges (I've got a smiths 80mm tacho which I disconnected as it wasn't reading right).

 

1st proper road test (after lots of idling to bleed out the cooling system) and I've got a bit of an issue. It defiantly feels a bit flat (I've had MPI's in the past) and under load there is a misfire, which if you apply full throttle from low RPM, it sounds like it is popping through the throttle body.

 

I've had the plugs out (they've done a couple hours idling and 30 mile drive) and they are like new with just a bit of white on the electrode.

 

Any thoughts? an air leak was the 1st suggestion (particularly where I blanked off the servo pipe), so I shall try squirting brake cleaner about the place to try and pick any up.



#2 Steve220

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Posted 16 March 2019 - 06:22 PM

Have you taken over the charcoal canister with it too?

 

Have you double checked all sensors?



#3 Wiggy

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Posted 16 March 2019 - 09:57 PM

Are you sure the misfire wasn't there when the engine was in the donor?

#4 craig 1010cc

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Posted 16 March 2019 - 10:11 PM

I deleted the carbon canister and have a simple vent pipe on the tank now

We only drove the donor on and off the trailer (it wasn’t roadworthy), so it could be a pre existing fault.

I have change the water temp sensor (the fan came on as soon as we fired it up, the rest are as we got it

#5 Wiggy

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Posted 16 March 2019 - 10:43 PM

Although I don't know the consequences of getting rid of the charcoal cannister; I'd do plug gaps, plugs, leads, then coil pack. In that order I reckon.

Coil packs do tend to gradually go, either when hot or when under load. I've replaced quite a few on different cars. Whereas individual coils seem to just die suddenly, in my experience anyway.

#6 craig 1010cc

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Posted 17 March 2019 - 09:46 AM

Plugs and leads are new, coil pack could be a good shout. An air leak is coming up as a likely cause, so going to check that first, then try and get the codes read before replacing parts

#7 craig 1010cc

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Posted 17 March 2019 - 05:20 PM

Well, some progress.
Tappets, all a bit lose (except 1&2 which were spot on) so reset them to 0.3mm
Fired up, squirting brake cleaner about, no difference so stuck the air filter back on and headed out.
Whilst the water temp was low, no miss and pulled well, as it got above 75 the power started to drop off and an occasional miss returned. So something is breaking down with heat which does sound coil pack related

#8 craig 1010cc

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Posted 20 March 2019 - 06:18 PM

Decided to head out after work and put a few more miles on and try a couple of things out. 

1st thing was on start up from cold, if you don't catch it with the throttle, then it fires up and stalls straight away. 1 blip of the throttle and it idles fine.
So headed out (satnav on board to check the speedo accuracy, over reads by 10% so about normal) and sure enough, when it hits 75deg then you start to get a mis. But it wasn't as bad as before. Did get 1 backfire through the throttle coming off a roundabout in 3rd at 20 and flooring it but it didn't feel as bad as before and better than Saturday.
So head to the local tescos to top up the tank with some fresh ultimate as the tank had 5ltrs of fresh super in it and the rest was about 1/2 tank from Sept and the rest who knows (was in the donor vehicle when we bought it). Managed to squeeze 10ltrs in (and the gauge was reading about 3/4's before I topped it, so not far out either). The drive back from tesco's and the mis seems to have gone, but it is still holding back

Decided to whip the plugs out, both the outer 2 are now a little sooty, the inner 2 are both still clean.






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