Ever since I helped buy my friends GP its had a low speed misfire.
I have replaced the plugs/leads and ignition coil but it still persists.
Its has eventually passed two successive mots but only just on the emissions/misfire.
NO fault codes visible on dash or stored.
Admittedly its not been scanned with any FIAT specific tool but I have used a Snap on piece of kit ...

Any one got any ideas?...

You can find plenty of similar threads by searching this forum. For example:

https://www.fiatforum.com/grande-punto/450336-grande-punto-misfire.html

Good, you've changed to most obvious parts which probably needed to be changed anyway. Now compression test on the engine and fuel injector cleaning could be the next best thing to do. However Fiat specific diagnostic equipment can give more information but it's only useful to an expert. Sometimes it's worth the money.


The reason why there's no misfire fault codes stored is that the misfire doesn't reoccur a number of times required to set a code. So ECU thinks engine's good and can control the situation. Also if emission test shows no sign of fault like high HC, CO for example, it may not be too bad.


The Fiat engine typically has some broken exhaust manifold studs which cause a small leak when cold. Is the engine rattling and squeaking when cold and has poor throttle response but after a short time it returns to normal?


I assume it has had engine air filter changed? Also when there's a time for a timing belt change, have the valve clearances adjusted if it's SOHC engine without hydraulic valve adjustment.

nikrox said:

Ever since I helped buy my friends GP its had a low speed misfire.
I have replaced the plugs/leads and ignition coil but it still persists.
Its has eventually passed two successive mots but only just on the emissions/misfire.
NO fault codes visible on dash or stored.
Admittedly its not been scanned with any FIAT specific tool but I have used a Snap on piece of kit ...

Any one got any ideas?...

Click to expand...

What engine?
How many miles?
When was the cambelt changed?


Any leaks from the exhaust or the pipes connected to the inlet?

it's a 1.4,09 on 95k. Cambelt was done 2 years ago by a Fiat dealership.
No leaks.
The mot tester said it could be a faulty injector...BUT...its not mine ,so not here all the time and atm Im trying to sort a power steering issue on my car..joy in this heat..

Finally got a chance to take a look at the car today. It turns out that the plug nearest the belt wasnt screwed into the head fully....And Number 3 plug was black /sooty.
I spent a long time trying to clean the thread with a thread chaser,and finally got it to turn in more BUT still got about 10 mm to go. I bottled it at that point and put the cleaned,re-gapped (1mm) plugs back it.
The car is driving a lot better now,Im pleased to say.
Im torn between having another go at chasing the thread again or paying to get it done,or even a helicoil..

Booked the car in for a helicoil fix on Friday. ..I shall report back.Fingers crossed.

IMO it'a always better to (try to) re-tap (clean) an existing thread than helicoil it ... Re-tapening it would only remove carbon deposits and maybe very small particles of aluminium, while helicoiling would potentially get huge chips falling in the engine, still aluminium but much bigger !! That being said if the thread is gone it's the only solution ...

BRs, Bernie

I agree totally...I have tried to re-cut the thread and gone as far as I dare. Id hate to have a go and strip the thread and leave the car stranded on my drive.
So on that basis I booked it in whilst my car was being mot'd...

Im delighted to say that after 2 YEARS of chasing a mis-fire that the car is now officially running on all four cylinders.
Who would have thought that a plug not being screwed fully home was the answer?...Well you might have but I certainly didnt.
£50 well spent...

Next issue....

So It's the wee FIRE engine? In that case at least it's more or less accessible when compared to some of today's engines where the plugs "hide" buried deeply at the bottom of lengthy "tunnels" through the cam cover and head - like my wee 1.00 litre Ibiza!

On that note anyone remember the old Peugeot 505? One came in to us for service and I had to go and buy a special deep box spanner to get the plugs! At the time mostly i worked on the old BMC "A" and "B" series engines, couldn't believe the Peugeot! Not unusual these days though

Pugglt Auld Jock said:

So It's the wee FIRE engine? In that case at least it's more or less accessible when compared to some of today's engines where the plugs "hide" buried deeply at the bottom of lengthy "tunnels" through the cam cover and head - like my wee 1.00 litre Ibiza!

On that note anyone remember the old Peugeot 505? One came in to us for service and I had to go and buy a special deep box spanner to get the plugs! At the time mostly i worked on the old BMC "A" and "B" series engines, couldn't believe the Peugeot! Not unusual these days though

Click to expand...

I have one of those pugeout special box spark plug spanners!

There is a spark plug thread re cutting tool that passes through the thread into cylinder then expands and cuts thread from the bottom up . This avoids cross threading the recut.

Jack

Yes jack, I like the looks of those "reverse" cutting taps, but I think they are only for chasing existing threads which are perhaps a little damaged. - I could see it being very useful for the likes of when a plug has been screwed in a couple of turns cross threaded so only the first two or three threads are deformed?

https://www.amazon.co.uk/Sealey-MS0...V44GSDRMXR9&psc=1&refRID=WEHQKPFBMV44GSDRMXR9

Here's a video of a chap using one on his motor cycle:

https://www.youtube.com/watch?v=3UK8kEAnHVk

If it comes to the point where you have to install an insert I find a heavy coating of HMP grease on the tap/reamer and then never go more than two turns before unwinding the tap and cleaning the swarf trapped in the grease before recoating and cutting another turn or two then back out again, recoat with grease then back in and repeat. Quite slow and laborious - and a bit wasteful on grease - but you will catch nearly all of the swarf this way.

I was taught how to use Helicoils in college back in the '60's but one of the garages I worked in our boss bought a time sert kit for doing spark plugs. Previously we'd used Helicoils but, very infrequently, the insert would later come out with the plug during a service. (my "theory" is that this is caused by carbon build up on the threads at the combustion chamber end of the plug which then jam up and cause the insert to unscrew?) I became a complete convert to the Time Serts which never did this but, due to the cost, continued to use Helicoils at home on the very infrequent occasions I found it necessary (also my pal has quite a few Helicoil sizes which I can borrow!). Here's a video of the time sert system and how it's used:

https://www.youtube.com/watch?v=anjDQJtWFc8

Just lately I've become aware of these kits:

https://www.amazon.co.uk/14mm-Spark-Plug-Rethread-Cylinder/dp/B01D1HO356

They look like the Time Sert insert but are much cheaper to buy and look easy to use? Might give one a go if I ever run into a stripped plug hole again but I'm uncertain as to how the insert is installed and locked in place - I don't see anything like the Time Sert installation tool? Perhaps you locctite it in? I wouldn't be so keen on that.