Punto spark plugs and leads

Today I went out the the wifes Fiat punto MK1 1242cc 8valve 1999 to have a look as to why it is idling roughly/chuggy. Whilst the engine was Idling with a pair of pliers I took off spark plug number 3 lead and the idling
actually got smoother rather than rougher! - very strange it seems to be working back to front! - Does anyone know why its more chuggy when I put spark plug lead back onto cylinder number 3?
A few weeks back I pulled out spark plug 3 and it was jet black tip whereas all the other plugs were correct colour (gray/brown)
So it seems like the engine is idling better with spark plug lead on cylinder number 3 removed (but then as you rev it up it goes lumpy because its only running on 3 cylinders instead of 4) Any Ideas anyone please?
***
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sounds like over fuelling, the two middle plugs always seem to run richer than the rest. Either that or you have a more serious problem. Does it run ok, not lumpy, with a fresh set of plugs? Is it used only for the 1 mile school run, one mile to the supermarket, and similar driving pattern, where the engine never gets hot? What did the last MOT emission printout say about emissions? It could be running on 3 because one plug is bridged out with a sooty carbon deposit, and not firing, or mis-firing under some conditions. Modern engines have several sensors, failure of any one of which can cause over fuelling. There is air inlet temp, and engine temp, if either erroneously tell the ECU that the engine is cold, it in terms of the old days, pulls the choke out, and over fuels, causing sooty plugs. Then there is the MAP sensor which controls how much fuel the engine needs for a given load and throttle position, failure can also cause over fuelling. There are other possibilities, Fiat now seem to use HT modules, and wasted spark, so if you pull one plug lead you upset another cylinder. I assume you've checked distributor cap and leads, a tracking distributor cap can stop one plug firing, all or part, of the time, and may only play up first thing of a morning after it has absorbed moisture. If it is nothing obvious, and gets worse, you may need to go into diagnosis mode, compression check, to see if it's head gasket, slipped a cog timing belt, or similar problem. I mention that because for a 1999 engine it's worth inspecting the timing belt.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks for your reply.

Yep just recently give it a new set of plugs and spark leads - still lumpy afterwards.

No, its been on longer runs.

We just got it second-hand and its next MOT is a year away (NCT/MOT every 2 years on cars here in Ireland) smells a bit 'petrol rich' out of the exhaust. But no I haven't put it on a machine to check emissions.

Its only a single point fuel injection engine and not a multipoint if that has any baring and only 1 plug (cylinder number 3) was black and sooty. If it was over rich problem I would have thought all 4 plugs would be black & sooty?

Its all electronic. Has ECU. Don't have a dist cap. - I have checked and sparks are getting to each plug so Coil Packs seem to be working OK.
Not losing water and no oil in rad expansion tank and no mayo in oil cap so maybe head gasket is OK. - Timing belt might have sipped I will have to check. If timing belt slipped a cog how would the car perform at speed, because at speed its very responsive and runs OK it just when its idling its chugging.
---------------------------------------------------------------------------- ---------------------------------------------------------------------------- ------
***
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Have you got the leads in the right firing order? If not could be delivering the sparks in the wrong order.
Nick /////
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks, yes I have checked with Haynes Manual. - strangely if I swap Plug 2 and Plug 3 on that coil pack the engine still seems to start up and run! - does that seem right?
***
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
This would be what they call a "Wasted Spark" setup. Reduces the complexity/cost of the system as only two ignition coils are required and plugs are fired in parallel. One spark to to the compression stroked cylinger and the other to the cylinder on the exhaust stroke and is thus wasted.
Pne still need to get the plg leads in the correct order though.
Nick /////

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The plugs fire in pairs, so if 1 is running fine it would most likely be a problem with 3 cylinder. Try getting the compression checked on all cylinders. If 3 is unusually low then that may be where to look. Valve not seating well, or similar? David.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks, About a month ago the wife took it into a garage and said the compression levels were OK - I feel like though if compression testers don't cost too much (maybe get one from Halfords or similar) then I would like to buy one and check for certain myself.
***
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Make sure the compression tester is mechanically compatible, there are several sorts, one is where you just push a taper rubber bung in the plug hole and has a tyre valve inner to retain the max reading, not very good, and no good for the arse about face engines with the exhaust at the front and the plugs on the firewall side. Then there is the screw in one by Gunsons, with adapters for different plug sizes, and ok if you have enough clearance. You may find Gunson ones on eBay.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Could well be the sparkplug leads. They seem to wear out after about five or six years of use. I've had it on my 2000 Punto Sporting 1.2 16V and also my brother in law on his 1999 Brava 1.2 16V. Original Fiat replacement cables are not cheap though! Ciao, E-Cie.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Brand new leads have already been fitted, and all new spark plugs so it not that.
***
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.