Technical Fiat Punto mk2/2b 1.2 8v (2004) - Rough idle, mostly on electric load

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Technical Fiat Punto mk2/2b 1.2 8v (2004) - Rough idle, mostly on electric load

mikeb1

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Hi,

I have a Fiat Punto 1.2 8v (2004) which sometimes has a really bad, rough idle.

Other than that the car runs fine, there`s no check engine light no excessive fuel consumption, starts quick, drives smooth.

The only problem is when the car is in idle and it`s RPM starts fluctuating noticably. Specially if I put some electric load on the car. Turning on the lights, using the power window.

I already tried to use various fixes with no luck. I went to two repair shops they didn`t find anything. I also went to an official Fiat Service Center. In the service senter they didn`t find the cause of the problem but reseted the ECU and said it fixed the problem. Well, it fixed for 5 minutes. After that it came back again. So I don`t really trust them anymore.

What I already tried:

- Checked the ECU for error codes with an ODB scanner (no errors)
- Reseted the ECU (by disconnecting the battery for an hour)
- Cleaned the throttle body
- Checked for vacuum leaks
- Replaced the idle air control valve (IACV) on the throttle body
- Replaced the whole throttle body form another car
- Replaced the battery
- Replaced the pre-cat lambda (O2) sensor
- Replaced the MAP sensor
- Replaced the spark plugs
- Replaced all ignition coils

I also bought an ODB scanner and checked the car with the Multiecuscan software but couldn`t find anything suspicious. I attach the Multiecuscan logs here. You can check it if you have the free Multiecuscan software. I mark the moment when the rough idle starts in the log. The image only shows the RPM but the log shows many other details as well.
View attachment Fiat Punto 1.2 8v 2004 - rough idle - multiscanecu log.csv.zip
Fiat Punto 1.2 8v 2004 - rough idle - multiscanecu log.jpg

Created a video which shows the whole problem. (turn on the captions on the video for details)


Do you have any suggestion what I should check? My next guess is the altenator because it happens more often under electric load.
 
Last edited:
I`ve just replaced all of the injectors from my other Punto, but the problem is still there. So it`s not the injectors.
 
Im glad somebody else has this issue....

I have it too... not always replicated on request

I found also heavy electrical load was causing the rough idle to get worse...

My logic was the alternator kicking up hard to generate more power which the engine wasnt predicting and nearly stalling.... hence the up and down bit on the revs

Aircon (on an 8v is rare) would probably amplify the issue too... (make it worse)

Check the condition of the auxilary belt....
I had to replace mine as id lost 2 of the ribs
U having ac will make it more money

Since i havent been able to replicate this since the change i cant confirm 100% it fixed mine

But i do wonder if the fuel pump is lacking pumping power and causes a rough idle? No idea tho

Im yet to see whata actually causing mine

Ziggy
 
Could you monitor the battery voltage while the engine is acting up? If should be over 13V if all is good. You can do this with a multimeter or should be able to check from the OBD2.

I would be inclined to check the voltage drop between the positive of the alternator and positive of the battery while the engine is running. Do the same for the negative. Should be a very small reading.

I had charging issues before and took me ages to figure out what it was. Turned out I was getting 1V drop between alternator negative and battery negative. Redoing the gearbox ground connection solved the issue. One of the symptoms I had was I used to start the car and it used to rev up and down as if you were pulseing the accelerator. I believe this is the ECU detecting that the system voltage had dropped to a critical point and increases the rpm to generate more charging power. This restores the voltage level. ECU drops the rpm to normal, the voltage immediately drops again and the cycle continues.
This differs from your sysptoms as mines never dropped below normal idle rpm.

If you hooked your battery up to another car while running as if you were trying to jump start and see does it still act up. If it doesn't this would confirm it is a charging problem.
 
Meanwile I replaced the alternator for a brand new Denso one. The original alternator was charging the battery with 13.95V on idle and did`t want to go more than 13.98V even when I evved the car around 3000RPM.

Now the new Denso alternator charges the battery with 14.05V and goes up a bit when I rev the engine.

Sow now the battery and alternators are brand new producing good values. I also changed the belt.

I thought with this fix I found the solution for the rough idle issue. But no. The rough idle still kicks in on electrical load...
 
I also replaced the camshaft sensor because it was cheap and easy to replace. But it has no effect on the rough idle. Now I`m just guessing what may cause is issue.
 
Check the condition of the auxilary belt....

But i do wonder if the fuel pump is lacking pumping power and causes a rough idle? No idea tho
Ziggy

Yes, I replaced the belt with the new alternator but no change.

I also think about the fuel pump which could be the next thing to inspect.
 
Could you monitor the battery voltage while the engine is acting up? If should be over 13V if all is good.

I would be inclined to check the voltage drop between the positive of the alternator and positive of the battery while the engine is running. Do the same for the negative. Should be a very small reading.

When the rough idle kicks in the battery fluctuates between 13.6V and 13.9V. So that looks normal when the engine slows down and speeds up by itself. I checked it with Multiecuscan.

I`m also going to check the voltage drop between the battery and alternator.
 

Attachments

  • rough idle - rpm and battery voltage.jpg
    rough idle - rpm and battery voltage.jpg
    75.6 KB · Views: 458
#does this engine have an idle control valve or is it a electric throttle. Sounds like a sticking ICV but I don't konow if this engine has one.

Robert G8RPI.

It has an ICV on the throttle body. I already cleaned and then replaced the ICV only, but that didn`t help. Then I replaced the whole throttle body with a used one (the used one also had the ICV and throttle pos. sensors) but no change.

Basically I already replaced/cleaned all the parts which were described in other topics (pre-cat O2 sensor, ICV, ignition coil, spark plugs, HT leads, battery, alternator, MAP sensor, etc...)
 
When I was searching the net for a possible fix about this strange rough idle issue, I found many similar videos of the MK2/2b Puntos. Which makes me think this is a common issue.

Unfortunately none of these videos describe if the problem was fixed at all.




 
I would be inclined to check the voltage drop between the positive of the alternator and positive of the battery while the engine is running. Do the same for the negative. Should be a very small reading.

I did the voltage drop test but didn`t find significant voltage drop between the battery and the alternator.
 
This is a very weird hunting issue indeed
Ive had 2 puntos and both have done this

The only think ive never touched was the fuel system
Since there is no filter why would i?
I found no correlation between fuel and when it does it

Only electrical.load - which my belt mostly cured.....
But i still get it from time to time...

Ziggy
 
Perhaps run 12V straight to the pump so it runs at max and see if that affects it? Faulty crank sensor or wiring issue? I'm only guessing at this stage. Next would probably be to replace the ECU and body computer.
 
Perhaps try removing the aux belt tempareraly and see how the engine runs. You could hook your car up to another car, as if you were jump starting, to keep your battery charged.
 
Faulty crank sensor or wiring issue? I'm only guessing at this stage. Next would probably be to replace the ECU and body computer.

I haven`t replaced the crank sensor yet. But that`s the next on my list. Fortunately I have another Punto 2 (not the 2b) so I can switch that part easily.

But I already replaced the camshaft sensor, but that didn`t change anything.
 
Are the ECUs and body computers interchangeable between the 2 and 2b do you know? The more I think about it, I'd be inclined the do that test I mentioned by disconnecting the aux belt. It would confirm Ziggy's theory that it is in fact the altenator kicking hard under load.
 
Perhaps try removing the aux belt tempareraly and see how the engine runs.

Actually I removed the alternator belt and the engine runs much better of course.

With the old alternator the engine was charging the battery at 13.8V.

Then I installed a new Denso alternator and belt, now it charges the battery with 14.5V on idle. Which is totally OK.

But the new alternator and belt didn`t fix the problem.

 
Are the ECUs and body computers interchangeable between the 2 and 2b do you know? ... do that test I mentioned by disconnecting the aux belt. It would confirm Ziggy's theory that it is in fact the altenator kicking hard under load.

That`s what I was thinking about also. But with the new alternator and belt even when I put on some electric load on idle the battery drops down to 13.8V for a few seconds and then back up to 14V and the rough idle sometimes kicks in.

These charging values look totally fine for me and also the rough idle problem is not permanent, it starts totally random. There are days when it does not come up at all.
 
When I replaced the O2 sensor I checked the old O2 sensor with Multiecuscan and it didn`t look OK. I did a before/after check with the new 02 sensor. And the new looked it definitely better. The throttle response became also better.

I thought OK, now I got it, this is the fix for the rough idle. But unfortunately the rough idle came back a day later...
 

Attachments

  • multiecuscan pre-cat o2 sensor before-after.jpg
    multiecuscan pre-cat o2 sensor before-after.jpg
    228.8 KB · Views: 147
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