Technical Fiat cut out and won't idle

[RexNoctis]

Hi all,

I'm back and have some interesting issues with my wifes Punto Mk2 1.2 FIRE engine.

She had been having issues getting it to start, espcially in cold weather. I (mistakenly) assumed the battery was good, it's less than 18 months old, reads 12.4v normally and ~14v when the engine is running. I bought a cheap battery jump starter to get her out of trouble, that died fairly quickly, then she managed to melt some jump leads when at work. At this point, I suspected the starter motor was trying to pull too much current so I replaced that and had the same issue. I finally went and did what I should have done at the start and borrowed another battery and the car started immediately with no problem.

This leads me to the current fault, while the car was idling, with no-one touching it, it suddenly cut out. Now it will start instantly without any hesitation but will only stay running with my foot on the throttle and the battery stays light on. If I take my foot off the throttle it stops immediately. When running with foot on the throttle, it runs perfectly, no choppiness or variation in revs.

I've put my scanner on it, that is reporting no faults. I've also checked the earth straps, crank sensor (connections only, I suppose the sensor itself could have gone bad), throttle cable looks fine and moves easily. All fuses are fine but I haven't checked all the relays. I've also tried disconnecting the battery again and leaving the car for an hour or so based on this post: Rough Idle

I'm suspecting electrical, mainly due to the suddeness when it cut out and the fact that is what I've been messing with but going by the battery debarcle, I don't trust my judgement any more!

Any ideas on what to check next? Is it possible to check/test any sensors before replacing them?

Cheers!

 

[RexNoctis]

OK, I have some improvement, I an a moron and had forgotten to connect the alternator cable to the back of the starter when I replaced it.

The car will idle now but at very low RPM if I let of the throttle gently. If I rev it, the revs will go up and run perfectly smoothly but it will stall as soon as I release the throttle.

I'll work through the rest of the troubleshooting for rough idling but it idled perfectly before I replaced the starter motor so I really hope it's not suddenly leaking air gaskets or something!

 

[jackwhoo]

You may have killed alternator by running engine without alternator lead / s attached.
Measure battery voltage with engine running and engine off

 

[RexNoctis]

I was afraid of that but I think it's OK;

Engine off: 12.1v
Engine idling: 12.5-13v
Engine ~800rpm: 14v

I have to give it a bit of throttle to idle cleanly at 800rpm but when I do, the voltage looks OK.

 

[jackwhoo]

Investigate your idle air control valve - a part mounted in throttle body.
It controls idle speed from signal from ecu.
Have any electrical wires been disconnected from throttle body?

 

[RexNoctis]

Hey Jackhoo, thanks for the advice!

I've been nowhere near the throttle body, yesterday I popped the air filter off and checked all the connections in case I knocked/pulled something while fitting the starter. All seem good and there is no air leakage, stalls instantly if I cover the intake.

Your comments about the alternator got me thinking, obvously running without the cable connected is not good for it. I've just temporarily removed the auxillary belt and the car idles perfectly. I was expecting the alternator to be hard to turn by hand whereas is actually virtually freewheeling so not sure how that could cause the car to bog down at idle.

 

[akyttala]

My Punto had similar idling problem last summer, it didn't idle, but worked normally when I keep foot on gas pedal. Driving was good, but if I forgot to run it ~1000rpm for example on traffic lights, it cut off. With foot on pedal it started again. After a while it died completely and as Punto is our second car, I left it in garage for few months. Month ago I had time to examine it and almost right away I realized that crankshaft position sensor was faulty. After changing it Punto worked fine. Cranck sensor is quite cheap part to buy for test.
Of course my problem with Punto had nothing to do with alternator, but years ago I wondered some other vehicle (can't say a car as I have noticed that cars are only made in Italy) that had short circuit inside battery and alternator took so much load that it could only idle without auxiliary belt. But if you have already changed battery, it is quite unlikely that both batteries are short circuited.