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?
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
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.
Yep just recently give it a new set of plugs and spark leads - still lumpy
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 &
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
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
Pne still need to get the plg leads in the correct order though.
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?
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.
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.
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!
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