Having Idle problems after replacing the head gasket on my mk1 Punto 75sx 1.2L 8v, (so Multi Point Injection). The car was off the road almost a year before I finally finished the job.

Build/Progress thread is here:
https://www.fiatforum.com/punto/338090-my-200-95-mk1-75sx-paddy-punto.html

Anyway, to the problem......

- Overheated over a year ago (October 2014) with suspected head gasket problem. Turned out to be stuck thermostat, which I replaced. The overheating had caused water to start leaking out of water pump, so this was replaced.

- Decided to replace head gasket, so had cylinder head skimmed, I re-lapped the valves and cleaned ports etc.

- Head gasket back on, had trouble with timing at first but then got it right, then timing belt teeth started to rip off (not tensioned tight enough/generally perished).

- New timing belt & tensioner, car started fine, but was now idling badly...

- When car is started – surges slowly up to 3000rpm and beyond until you touch the throttle, then it drops to 1500rpm, fluctuating up and down to about 2500 constantly (bruuum-bruuum-bruuum etc.)

- Had this problem a few years ago – replaced Lambda/Oxygen sensor which fixed the problem that time, but unplugging it now does not affect idle at all. Also replaced coil pack and fuel filter before this point.

- Trying to fix this problem recently: new Throttle Position Sensor, new Idle Control Valve, cleaned throttle body, ECU reset procedure, unplugged Lambda sensor, checked coil pack sparks, adjusted throttle cable.

- No luck with any of this, now thinking it could be the MAP sensor (can’t think of anything else). When the wiring to MAP sensor is unplugged, car won’t start for more than a few seconds.

Getting frustrating now as I've thrown parts at this problem and had no luck, that's even before I tax and MOT it....:bang:

Any help would be much appreciated.

well sounds intriguing.. don't have a definitive answer for you but what i would be looking at myself would be the following things at this point.

have you got an engine management light on?

do you know if the fuel filter has ever been changed, often overlooked service item.

have you checked condition of the spark plugs?

have you tried disconnecting each injector in turn, my thought being if one is failing it won't really affect the running but if it is it will make massive different.

have you checked condition of all vac lines, and pulled them off and blown through each one to check none are blocked?

double check the timing is right.

Have similar problem on my 60sx SPI.
Seems strange as my SPI is different then the MPI 75sx.
However, in my case I have a fix that seems to be repeatable.
The hunting happens to me very often from cold. Once the engine is warmed up to the normal operating temperature the idle will still hunt. However, if I switch the engine off for 2-3 seconds and then restart the idle will stop hunting and the car will drive normally until I stop and let the engine cool. Will be nice to know if this fix is applicable to you on your MPI engine.

thanks for the replies fellas, much appreciated.

Changed the fuel filter less than 2 years ago, solved an acceleration lag issue I had at the time, so I don't think that's the issue.

No engine management light on when running (the injector light), just the normal 3ish seconds when you first turn the key before starting the car.

Put in new spark plugs when I had the cylinder head on the bench, but haven't check them since so will give that a go.

Will try disconnecting each injector next, although my thinking is that it isn't a fueling issue, as the engine does run smooth enough....when it's slowly surging up the rev rage before dropping and starting to hunt.

Vac lines/leaks is something I haven't checked properly yet, and is what I'm now thinking could be the problem. I also have a feeling the gasket(s) between the throttle body and intake manifold may be damaged (or not even there!) which could be letting in air.
I was planning to make new ones out of gasket paper at the weekend, but i'll spray some brake cleaner around with the car running to look for leaks first.

My problem doesn't seem to be affected by temperature unfortunately, have left the car running in the driveway various times for over 30 mins with no changes, battery disconnected for a while, starting it again straight away after shutoff, no changes. I can't/shouldn't take it out on the road as it's been SORN for about a year and has no MOT, Tax or insurance.

I believe all this points towards something that is 'constant', i.e. a vacuum leak that's always there, or a sensor that's consistently out.

Thanks again for the help, will report back soon

leaks around the throttle body is defo something i'd check too actually - exactly the kind of thing that would cause this kinda problem. Not sure if you can buy them still but easy enough to get some gasket paper and cut some new ones out

(SOLVED) 75sx Idle Problem - Fluctuating/Surging/Hunting

all fixed! got to the bottom of it eventually, let me tell you a tale...

checked the vacuum lines and they were all fine, then i got on with making 2 new gaskets for the throttle body and pre-inlet manifold.
the first gasket within the TB was fairly tricky to cut and took a while, as it includes openings for the idle air control valve etc. The old gasket was broken in nearly 3 places, one section near the idle valve bit was completely separated, but held down in place when assembled. i vaguely recall noticing this months back, but thought nothing of it at the time (doh)

the second gasket was easier and is the 'middle' gasket on the inlet (the order going: TB top - gasket - TB bottom - gasket - spacer section - gasket - inlet manifold. This middle gasket was missing completely, i think because it was causing a high pitched whistle a while back due to only being secured down by 1 out of 2 throttle body nuts!
(long story, there's a thread for that here: https://www.fiatforum.com/punto/322879-sheared-bolt-through-throttle-body.html )

so put it all back together, and the car started up and ran fine (idle fairly high though) but the high picthed whistle/screech had returned! confirming to me that the gasket was causing it. so i removed the pesky gasket again, screech was gone, but idle still quite high (air still getting in post TB, but no gasket to vibrate and whistle).

then i discovered something odd. I assumed tightening the 1 nut that secures the TB would 'seal' and gaps more so less air would get in, lowering the idle. (over tightening this is what caused the stud to shear previously...) I did this with the car running, and the opposite actually happened and the idle went up. then loosening the nut, this lowered the idle speed, and i got to a point where i could lower the idle to a more decent point (near 1000rpm)

seemed a bit odd but my thinking behind it is that, by over tightening the TB on one stud to compensate for breaking the other one, it's actually been opening the leak on the other side more (like a see-saw).

what i might do is get some instant gasket instead, which will mould to the leak and not cause the whistling.

hopefully that wasn't too lengthy, it's a learning process all the time with this car haha. i was too busy to take photos but could get some next time i'm working on the car, hopefully an interesting read to someone!

Re: (SOLVED) 75sx Idle Problem - Fluctuating/Surging/Hunting

or you could replace the broken stud and have 2 studs holding the TB down.... whereabouts did the stud break, top, middle, right down near the manifold surface???

yeah had looked into that before, it sheared right at the top just below where the nut tightens. problem is the stud comes out from inside the inlet manifold with no apparent way of physically removing it, and i have a feeling it's riveted/welded in or something, after feeling around with my finger I don't think it's a hex bolt or anything like that.

not ideal I know, and have kept an eye out for an MPI inlet manifold but never seen one. the throttle bodies themselves for Mk1's seem rare as well

i am pretty sure they are just threaded on the end, and screwed into the manifold. Grab it with some mole-grips as tight as you can get them and try turning it. Whats the worst that can happen, it snaps - well its snapped already. Maybe if you take the mani off the car you can apply some heat with a blowtorch to help loosen it to.

Remember though fiat had to make said thing, and they would use the easiest and cheapest method and that would be to drill a hole in the mani and tap it and then just screw the stud in. It will have thread lock or the like on it so it will be bloody tight.

kazer92 said:

yeah had looked into that before, it sheared right at the top just below where the nut tightens. problem is the stud comes out from inside the inlet manifold with no apparent way of physically removing it, and i have a feeling it's riveted/welded in or something, after feeling around with my finger I don't think it's a hex bolt or anything like that.

not ideal I know, and have kept an eye out for an MPI inlet manifold but never seen one. the throttle bodies themselves for Mk1's seem rare as well

Click to expand...

not too familiar with the part yopu're describing..,
:idea:could you drill + tap a smaller thread / stud in the snapped off bit

I've done similar with such things before,

Charlie - Oxford

its like this Charlie


I believe he is saying one of them studs is snapped, like the bent one, which goes up through the TB spacer and TB and then 2 nuts tighted it down. Its never gonna seal properly with just one bolt, never ever ever

yeah that's right blu. I had thought about that in the past Charlie but didn't think it feasible at the time, might look into it again. I did try using JB weld to stick the sheared bit back on......that was an almighty flop i seem to remember haha

I hadn't even though of that blu you're probably right, can't exactly do any more harm.... will get out the mole grips and maybe put some rubber to grip the shaft (phrasing)

would be fairly easy to size/make another stud to fit i suppose, although i've never done any stud threading before

well if you get it out just buy some threaded bar the right size - will only be a few quid.

In my case I think I managed to fix it as well. Last weekend I had a thought about the problem and decided to replace the idle valve with a spare one from an other throttle body I have. After a week of usage I can confirm the problem is fixed. I have no more hunting and high idle. It looks like the old valve was sticking somehow causing the bad idle. The one I replaced was a new valve I bought off ebay cheap a couple of years ago. So they don't last well. The one I put back on now seem to be original Marrelli part made in Switzerland?

had a productive few hours on Saturday morning and the problem is fixed! :slayer:
working in the drizzle (see not-quite-waterproof parasol)


took the throttle body assembly off right down to the inlet manifold, reached in around where the studs go and immediately knew they were threaded in as Blu said they might be. grabbed 2 M6 nuts and put them on the end, had just enough thread to get them to lock together, and then twisted the stud out with relative ease.

nuts locked together on the stud (plastic bag to stop anything getting into inlet):

removed stud:

hole left in inlet manifold:


hopped in the Fiesta down to Wickes, picked up half a meter's worth of M6 threaded rod for £2.79

cut to length:


locked 2 nuts together on the end and screwed into the inlet manifold with a little spritz of WD40, then reassembled the throttle body etc.

a bit close for comfort with the length of the stud as the nut is only a couple of threads clear, I think I judged the length fairly well but the new threaded rod was able to screw deeper into the inlet manifold. not really an issue but would rather some more clearance.

The gasket I had previously removed from the inlet under the throttle body I put back in, since now it wouldn't cause the whistling/screeching noise as it would be properly clamped down.

Started the car, and it sounded like it should have for the last 2ish years since I sheared that bloody stud. nice steady idle at around 1000rpm.

I let it run for a good while, and noticed it did have a little stutter, as if the engine was on the verge of stalling at idle. this will probably go away if it gets a few miles behind it but not a massive issue. i had a go at adjusting the idle throttle screw, but left it at normal as it was hard to get only a little increase in revs, only a jump to 1500rpm.

so there we have it, after many hours of work, research and head-scratching, along with probably £50 quid on new parts that were OK to begin with (idle control valve stepper motor, throttle position sensor etc), it turned out to just be a air leak (probably a massive one too).

thanks for all your help everyone, now just the small matter of an MOT.....

slight stutter is nothing to worry about, they sometimes stutter when you reset the ecu and they are re-learning the idle parameters etc. If it continues or gets worse come back to it but i suspect it will clear up after a drive and a few heat cycles

kazer92 said:

I let it run for a good while, and noticed it did have a little stutter, as if the engine was on the verge of stalling at idle.

Click to expand...

What rpm was the engine running. If you already tempered with the idle screw it may be you can try to readjusted slightly and see if the stutter goes.

you should really ever need to adjust the idle screw ever - if you do there is something wrong with the car.

blu73 said:

you should really ever need to adjust the idle screw ever - if you do there is something wrong with the car.

Click to expand...

yeah I had heard this was the case, but I know i've fiddled with it before in an attempt to fix the idle problem. I assume the best thing to do is just loosen the screw as much as possible, then wind it up so it's only just 'taking the weight' of the butterfly valve?
when i did tighten it with a little allen key when the engine's running, nothing happens to the idle until a certain point, then it increases a fair bit up to maybe 1400-1500rpm (from memory)

erm i dunno really tbh as i never moved one but i think what i would do is back it off until it stutters a little and then go back the other way until it idles nice and around 800-850rpm. If its idling around there when at operating temp then just leave it i guess