Bad Misfire

[davemar 16]

Currently my 205 Mi16 is running with the Jetronic setup with the dizzy converter and all that stuff. After doing some tinkering, I've now got a misfire, it feels like the car is constipated and weak and clearly not running on all the cylinders. I put new spark plugs in (as part of the tinkering process), and after just a mile or two of running with this misfire I've removed them for inspection. I've attached a photo of them (holding them in the same order in the engine). As you can see, the two on the left are black, the third is black, but with some white deposits, and the one on the right is clean as a whistle. So something very uneven is going on here. Any suggestions to what it is? All the HT leads seem fine, and the car was running smoothly before my tinkering, although maybe running a tad rich.

 

All I've really done in the tinkering process was to remove the distrubutor, its converter, HT coil (which I've replaced with a square one from the old barrel type). So apart from the HT coil nothing has changed. The main reason for changing the coil is I'm planning on changing to the Motronic ML4.1 setup so I was trying a few things out in preparation for that.

[DamirGTI 342]

Do a coolant test for combustion gases (with an block tester ..) and compression/leak down test ...

 

.. looks to me like the head gasket is on it's last legs between 1-st and 2-nd cylinder ...

 

Damir

[davemar 16]

Do a coolant test for combustion gases (with an block tester ..) and compression/leak down test ...

 

.. looks to me like the head gasket is on it's last legs between 1-st and 2-nd cylinder ...

 

Damir

 

I haven't got a block tester to hand, though there's no other evidence of coolant leakage. I did a quick compression test and all the cylinders were the same.

 

Even if the head gasket is getting past its best, I can't imagine it would suddenly get worse while the engine is out of use. These sort of failures (having been witness to about43 HG failures in various cars over the years) tend to happen while the engine is running.

 

I wonder whether the different type of coil has a different ignition timing to it, so whether it is a timing issue? As that is the only real thing that's changed between when it worked fine before and now.

[DamirGTI 342]

Double check the spark plug leads layout , you might criss connected them ..

 

Coils don't differ with regards to ignition timing , it's down to the dizzy or the ECU based on the management type .. but , the ignition amp could do a variety of misfiring acts if you've messed with it ...

 

Still more into engine mechanical issue by looking @ those spark plugs ... try the block tester on coolant to isolate that suspicion (borrow it at least if you don't have one ...)

 

Damir

[davemar 16]

Given the crappy weather and limited time and resources at the moment, I decided to clean the plugs and use a new coil (still square type) and try running it again. I ran it for about 20 seconds and it was still rough, and when I took the plugs out for inspection, the first 3 were carbonised and the 4th was clean, as before. I did notice when I took the first plug out some fumes came out of the hole. Not sure whether it was steam, oil or petrol based fumes though.

 

If anyone in the East Grinstead/Crawley area has a block tester they're willing to lend let me know.

[davemar 16]

I think I've got to the bottom of this one. When I was messing around trying to test the Motronic setup (and failing) I think the ECU had a headf**k and basically turned on the injectors for ages chucking loads of petrol into the cylinders. This explains why the plugs were struggling to fire. Today I decided to leave the bores open so the fuel could evaporate before trying it again. It now runs much better, so hopefully that was the problem. Haven't had a chance to drive it as my driveway is rather ice-bound now!

 

Also to explain why the 4th plug wasn't carbonised - the connector for that injector on the Motronic loom was missing a clip so it probably wasn't connected when testing, so that cylinder didn't get filled with petrol.

[davemar 16]

Oh well, not as fixed as I hoped. Took her out for a little spin and still running rough. I checked the plugs were firing by fitting them to my spark plug jig and spinning the distributor and they were all sparking. I'll try checking the injectors next.

[davemar 16]

It was one of the injectors not firing, the one on the right which corresponded to the clean spark plug. All it was was the contacts in the connector and come loose and retracted into the body like a sumo wrestler's nads on an arctic expedition. It must have happened when I was swapping looms around.

 

Well I hope that's fixed, just need to charge my poor old battery now so I can start the damn thing.