Fiat 500 twin air intermittent misfire when cold

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Fiat 500 twin air intermittent misfire when cold

Postby ncynotts » Sat Sep 23, 2017 8:40 pm

Hi guys, need a little help with this one...
Fiat 500 twin air 2011, 30 seconds after cold start has what feels like a single cylinder misfire for 10 seconds. Once cleared will be fine for next 24 hours.

I’ve scoped injectors and ignition primary, all ok. Decided to check running compression with wps, low, really really low, to the point I thought I’d got something wrong. So out with the old compression gauge, didn’t even register. Both cylinders same.

Before I got involved, a fiat guy said it would be twin air system at fault, but I want to prove it.

Has anyone done running compression on one of these twin air systems? I have a basic understanding of the system, and think the compression is being held down by the inlet valves not opening, but I’m stubborn, and need to prove it.

Hanks for reading, and I’m open to any suggestions.

Nigel
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Re: Fiat 500 twin air intermittent misfire when cold

Postby Technician » Sun Sep 24, 2017 12:05 am

Hi,

You may be interested in;

MultiAir's case, Fiat implemented an electro-hydraulic valve actuation system (inspired from the Common Rail technology), which uses a high pressure chamber, fitted in between the cam and engine intake valve. The volume of the oil inside the chamber can by adjusted through an on-off solenoid valve controlled via its own electronic unit.

By doing so, Fiat's system is capable of controlling the air mass and its pressure in each cylinder and at every stroke of the pistons.

You may also like; https://www.youtube.com/watch?v=Td9Gz_h7Qpg

Have you completed any scope tests!!
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Re: Fiat 500 twin air intermittent misfire when cold

Postby STC » Sun Sep 24, 2017 9:56 pm

Nigel

I love basics - I do

So out with the old compression gauge, didn’t even register


That is your start point ! If compression does not register on the gauge then the engine will not fire up or run.


Could we have 2 faults here - the Gauge and the Engine ??
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Re: Fiat 500 twin air intermittent misfire when cold

Postby Technician » Mon Sep 25, 2017 12:46 pm

Have you completed any scope tests!!


Could we have 2 faults here - the Gauge and the Engine ??


That is why I asked if any scope tests had been completed. Many years ago I had a Renault with low compression and blamed my very old gauge, reading 50 PSI, so I bought a brand new compression test kit and the gauge read 130 PSI per cylinder. Turned out the brand new gauge was faulty and the old gauge reading very low was correct.

Better to scope it with the WPS 500 :)
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Re: Fiat 500 twin air intermittent misfire when cold

Postby ncynotts » Mon Sep 25, 2017 8:29 pm

As I said in the original post, I first checked with the wps, then rechecked with the gauge, both agree with each other.

The car starts and runs fine, other than the intermittent misfire. The compression is, I suspect, being held low by the inlet valves.

I need someone who has dealt with this particular engine before, due to its unique nature.
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Re: Fiat 500 twin air intermittent misfire when cold

Postby Dcunning35 » Mon Sep 25, 2017 9:55 pm

Hi take a look at rym,s alfa mito post in this section. Theres a link to multiair info it may help.
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Re: Fiat 500 twin air intermittent misfire when cold

Postby RYM6746 » Tue Sep 26, 2017 12:29 am

Definetly sounds like a the same symptoms as the one I had. Try to get the map sensor voltage coupled with the VVL solenoid current and any fault codes. See if the detail in the waveforms match the ones I posted. Pretty sure the ECU runs some sort of diagnostic strategy on a cold engine to generate a fault code the low compression misfire is just the effect and not the cause (educated guess)
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Re: Fiat 500 twin air intermittent misfire when cold

Postby ncynotts » Sat Oct 07, 2017 6:18 pm

Sorry about the lack of updates on this, it’s been put on the back burner for a while.


Set it up today to capture solenoid current Monday morning, but typically behaved itself and didn’t misfire at all. I grabbed a quick screenshot with my phone, but had to dash as the wife was hassling me to leave.
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18DC0BD7-9AA2-471B-98EE-ED810AC9F09C.jpeg
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Re: Fiat 500 twin air intermittent misfire when cold

Postby RYM6746 » Mon Oct 09, 2017 7:16 pm

You are going to need to AC couple the MAP sensor voltage and bring the voltage level right down. So you can see each individual cylinders effect on the pressure.

It is difficult to see for sure, but there maybe some discrepancies in the current waveform (green trace). In particular you need to look for the type of difference in the lower picture.

It would also appear in the picture that the "hold" current on the green trace is "thicker" than the red trace. Is the suspect cylinder the green trace?
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Multi Air 2.jpg
Multi Air 1.jpg
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Re: Fiat 500 twin air intermittent misfire when cold

Postby ncynotts » Sun Oct 22, 2017 12:42 pm

Sorry guys, I think this will has to go as unfixed.

It is now back with customer as it no longer appears to have a fault. It appears to have fixed itself.

Thanks for all the help.
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