Technical Punto 75 uneven running and accelerating

[Demolite]

I have a '98 Punto 75 (1.2 8v 180k km) and it seems to have developed an annoying jerking when driving. No injector light on dash. May still have error code in memory?

Happens usually when accelerating but also at steady speed (60km/h+). Position of throttle doesn't matter when this "Intermittent Power Reduction™" happens. Feels like flatspot when pressing gas during. Stops or can't feel it after 4500 rpm. Usually goes on for entire acceleration, but stops after a while at steady speed. Not all power is lost, just reduced noticeably.

Does this when hot or cold, but usually works for first few kilometers; works well for longer when ambient is very cold (-15C). Maybe related to air temp in engine bay? Lambda working nicely, idle and when driving. Idles ok. Headgasket changed 20k ago. No other HG symptoms.

This weirdness is also quite precisely timed at about 2 second intervals when it happens. Interval becomes longer when accelerating, but still consistent time between.

I have one spare coil pack I have switched around without help. Tried different leads too. Plugs normal. Seems to have a too consistent an interval to be a mechanical problem. Feels softer than loss of spark.

Map sensor hose intact, disconnecting causes stalling.

Any help?

Also is it normal for spark and injection to shutdown for 1-2 seconds when revs go to about 6000? Limiter? Rev gauge jumps to 0 and then back. It's very aggressive and I thought the engine seized up when it happened the first time.

 

[Mixter]

I think it's best to read the error codes from the ecu. Because it's a very strang problem. And go from there. I also think it's someting with the injectors. Maybe 1 or 2 injectors can't produce the normal amount of fuel. Hope this helps!

 

[blu73]

My feeling would be lack of fuel. Either because the fuel filter need changing (i would just change it if you don't know when it was last changed, if ever), or the the fuel pump is on the way out, or an injector(s) is failing. None of these i would expect to log anything in the ecu and so the EML would not be coming on.

Could also be crank sensor on way out, this rarely throws an EML and symptoms normally start with a slight misfire that gets progressively worse as the sensor deteriorates further.

 

[Demolite]

I'll look into those, thanks.

At first I suspected fuel pump/filter issues, but its strange as it works well when cold (most fuel needed on cold acceleration) and at higher rpm. But it is true electrical parts may work well cold, but become intermittent when they warm up. I need to investigate more.

Does anyone know about the rev stalling thing I mentioned, maybe it has something to do with this? Awfully harsh for a rev limiter. An idea came to me: it might be that the crank sensor is not working past that rpm(ecu gets confused), so possibly it's working poorly at lower rpm...

Meanwhile I'll get my hands on an oscilloscope and test crank signal, fuel pump current and injector signal during the anomaly.

 

[flisko]

I have a '98 Punto 75 (1.2 8v 180k km) and it seems to have developed an annoying jerking when driving. No injector light on dash. May still have error code in memory?

Happens usually when accelerating but also at steady speed (60km/h+). Position of throttle doesn't matter when this "Intermittent Power Reduction™" happens. Feels like flatspot when pressing gas during. Stops or can't feel it after 4500 rpm. Usually goes on for entire acceleration, but stops after a while at steady speed. Not all power is lost, just reduced noticeably.

Does this when hot or cold, but usually works for first few kilometers; works well for longer when ambient is very cold (-15C). Maybe related to air temp in engine bay? Lambda working nicely, idle and when driving. Idles ok. Headgasket changed 20k ago. No other HG symptoms.

This weirdness is also quite precisely timed at about 2 second intervals when it happens. Interval becomes longer when accelerating, but still consistent time between.

I have one spare coil pack I have switched around without help. Tried different leads too. Plugs normal. Seems to have a too consistent an interval to be a mechanical problem. Feels softer than loss of spark.

Map sensor hose intact, disconnecting causes stalling.

Any help?

Also is it normal for spark and injection to shutdown for 1-2 seconds when revs go to about 6000? Limiter? Rev gauge jumps to 0 and then back. It's very aggressive and I thought the engine seized up when it happened the first time.

had that jerking on my car and changing coil pack pair fixed it, also remove the plastic above the coil packs, also no light ever came on

 

[fingers99]

Also is it normal for spark and injection to shutdown for 1-2 seconds when revs go to about 6000? Limiter? Rev gauge jumps to 0 and then back. It's very aggressive and I thought the engine seized up when it happened the first time.

Rev limiter, I think, doesn't come in till 6,500, maybe more.

Change fuel filter anyway, I think it's an electrical fault of some description. Look on wiring diagram for power source to rev counter, trace back (strip harness, etc.)

Hard rev cut might just be TDC sensor (it's a known symptom, but not one I've ever seen on a FIRE engine). What a faulty TDC sensor will do is signal to the ECU that the engine isn't running, so, injectors don't fire, plugs don't fire.

 

[blu73]

TDC sensors (aka crank sensor) do tend to start 'misbehaving' when they warm up when they start playing up - will eventually just stop working altogether from my experience, usually after a long journey and your far from home. Can just get a couple of sensors for a few quid each from scrappies to just test the theory - if it solves it then buy a new one.

 

[Demolite]

I'll try a sensor from the breakers, measuring while driving is too difficult. Shouldn't a sensor from a 55 Punto fit? The 75 is quite rare here. Theres a snow blizzard going on now, so testing is delayed a bit.

As I was diagnosing today, I noticed the idle has a slight pop now and then, might be misfire... Can't be felt in the cabin, and the engine is so silent there.

Haynes says there isn't a fuel filter on the '98 model, its just a mesh in tank. I'll check it out. Also if anyone knows, can the heater core be changed without opening any air conditioning connections? Filling it up ain't cheap, and its just been serviced last fall.

 

[fingers99]

P55 crank sensor should fit fine. They're all much of a muchness in FIATs although some have different length cables.

 

[blu73]

Aye, P55/60/75 or even the 16v sporting (85). Also cinquecento or seicento - the cables are alot longer on the cento's but it will do the job fine just to test if it makes difference

 

[Demolite]

Wasn't the crank sensor. I hooked up an oscilloscope today.

So far i can rule out:

-Crank sensor (works normally when measured)
-TPS (measured engine off)
-MAP (measured during acceleration)
-Coil signals (measured during acceleration) (Coils/leads/plugs can still be faulty)
-Lambda (goes lean during low power)
-Injector signal (no big change during low power = should be roughly same amount of fuel)


All is pointing toward ecu and sensors working as intended. Engine is leaning out or fuel is not burning which causes the low power pulses.

Will need to get fuel pressure gauge and test under load.

 

[fingers99]

The pumps are pretty reliable by any standards. Fuel filter, however, is the most neglected service item.

 

[flisko]

P55 crank sensor should fit fine. They're all much of a muchness in FIATs although some have different length cables.

will crank sensor from mk2 fit?

 

[fingers99]

Never tried. They'll be the same type, but, again, wiring length may differ.

 

[flisko]

just looked and it appears to be same lenght

 

[fingers99]

Go for it.

The only minus point about using s/h TDC sensors is they can develop intermittent faults which won't always show on a static test. This is why folk using aftermarket ECUs often switch to more reliable hall effect types.