Hello! Im a mechanic and am hoping for some wisdom regarding puzzling stalling on the following vehicle:
Fiat grande Punto 1.4 8 valve, 2006, Marelli 5SF3.M2 control system
Symptoms: Stalling when warm / hot at idle, difficult re start and repeated stalling - no fault codes.
Checking data the only possible issue is lean condition - worse at low rpm / idle - but zero vacuum leaks.
So far the following have been replaced:
Crank sensor (twice, and phonic learn ok)
Throttle body
Coil pack and leads
ECU (and clocks, body module, engine bay fuse box, key, etc)
Engine wiring loom
Temperature sensor
Disconnecting either Crank sensor or Temperature sensor will allow the engine to run without stalling but obviously not able to drive normally like that.
The O2 sensors are switching ok and swapping pre and post cat sensors makes no difference.
I never like 'firing the parts canon at the problem' but was able to source the above cheaply and after previous bad ECU intermittent fault on previous Marelli system thought best next step but sadly no.
Unplugging / capping off the following makes no change: IAT, MAP, cam sensor, VVT, knock sensor, servo hose, (other than EML on)
All earths & power feeds good.
Im using Snap on & EOBD, some data seems iffy (ignition timing does not seem to correlate to observed using strobe light?) The following listed in data:
Injection control module: programmed
Universal code: not received
Key status: off (with KOEO??)
engine status: parking
Motorised throttle (recovery): not active
Accel pedal (recovery) : 16

PLEASE help

Hi!
Fuel pressure? should be 3.5 Bar, pump with integrated filter is in the tank.
BRs. Kjeld

Real car mechanic or "wanna-be"?
Zero vacuum leaks. Sure? PCV system fully operational? What's the MAP reading?
Crank sensor, twice. What about camshaft sensor (there is VVT)?
Leads and coils. What about spark-plugs?
Throttle body. What for? What about fuel pressure and injectors?
ECU. Why? What happened? Maybe it's "shot" too (something else is killing ECUs).
Temperature sensor. Which one exactly?
Unplugging...makes no change. Change in what: engine stalling, starting, presence of error codes?
Earths and power good. Are you sure? Maybe alternator (or AC) is loading (mechanically) the engine.
Ignition timing does not... So what about timing belt (and VVT, plus valve clearances)?

Fuel pressure good and maintains pressure during stall

Er yes 'real' mechanic - 20+ years diagnostic level technician, if that matters, so to respond:
Yes zero vacuum leaks (smoke check and vacuum drop checks carried out) MAP sensor tests good using mightyvac and reading data (1012 mbar KOEO, approx 350mbar idle)
Camshaft sensor signal scopes out ok - no cam/crank correlation codes during normal operation and stalling condition - code triggered when VVT or cam sensor unplugged but no change to fault (it still stalls)
As to why I replaced the whole set of management components listed - because they were available for less than £100 and known good - I have personally had to replace a Marrelli ECU previously that was faulty with no fault codes but tested good by BBA. All other components that could lead to ECU failure renewed as stated.
Spark plugs changed at same time as Coil pack & leads.
I did not believe there is an underlying mechanical issue as forcing it into operate without crank sensor allows steady idle at 800rpm with no stalling (crank sensor replaced twice as previous technician replaced with unbranded part so wanted to rule out by fitting genuine part (no gap adjustment on these later ones and phonic learn ok)
I have been unwilling to remove rocker cover for internal inspection / valve clearance check as i do not have the special tool for re alignment on refitting which i believe is required on this engine (similar to 500 - its the 8v VVT 1.4 head) and do not have timing set - i am not too familiar with the Marrelli system but would think if there was an issue with cam timing, etc it would trigger a correlation code???
Temperature sensor replaced was the Coolant temp sensor - data for IAT good.
No AC and no excessive alternator load, earths & power supplies defo good. Injector spray patterns good.

Hi (who ?),

two things here: temperature and time, problem appears only at warm/hot temp, so obiously after a certain time …

I'd plot the temp sensor(s) curves in real time and record the fuel pressure as-well during this session. If found correct re-check for air-leaks (I gently spray petrol on suspected areas...), specially around the vapor recycler.

Not too much left after all you did and those tests, maybe valve clearance ? BTW does it have VVT ??

BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

1. Idle intake pressure should be (probably) closer to 300 mbar (290-320 for example). Confirm PCV system operation.

2. You don't need special tool for cover, that's BS to stop people servicing their own cars, DIY.
Car guy doesn't need a guide from hobbyist, but it's there, take a look: Grande Punto Guides: Valve clearances.

3. Same with timing belt: special tools are required only when there is suspicion of belt skipped a tooth and/or VVT is replaced or disassembled (hydraulic actuator wheel on the camshaft). Standard servicing still can be done classic way, DIY (by making marks - yes, professional mechanic shouldn't do that nowadays, but technically it's still possible). And "special" tools are NOT so special, easy to copy (with basic metalworking skills).

4. VVT is sensitive to oil pressure/flow. You can replace oil filter (yes, extra filter only change - standard practice in some industries, but method unknown to most car guys) and clean the VVT solenoid valve. No "long-life" BS in this engine (change oil every 10000 km, max., if you care).

5. The 8V FIRE family engines are also more sensitive (since adding VVT) to valve clearance, lash (old traditional tolerances like ±0,05 mm are now too coarse, cut that in half, which will be imperial one "thou", that's your target). What's the car mileage? My bet - valves never been adjusted.

6. As "Bernie" suggested, some sensors should be re-checked, live data, long-run. Sensors, timing, VVT, entire fuel system (maybe there is something wrong with venting, one-way valves, vapour collecting, return lines, or whatever there is - try running the car with fuel tank cap open).

Many thanks for the replies Bernie & Punto grandePL - I am well aware that Im probably not following the completely 'correct' diagnostic process on this one but it came in as a 'spend up to three hundred quid then scrap it' deal. I dont like being beaten by faults so pushed on a bit to see if I could stop it going to the crusher.
I have ruled out any fuel supply issue with gauge at rail (pressure & volume spot on) and double checked PCV/tank/system for leaks, operation, etc. (smoke checks and spraying around related areas with brake cleaner, blanking EVAP, etc) Also introducing an air leak shows o2 sensor going lean, and rich when spraying brake clean into throttle.
Graphing out temp sensors shows all ok there. (live data)
Good to know I can take a look at valve clearances, etc without worrying the cam sensor clearance as was led to believe, will pop cover off and have a look. Is there a way to check static valve / VVT timing using basic tools? (valve lift at TDC, etc) On the Ford Kas with same lump there are at least factory paint marks to see if its jumped a tooth but not in this one...
Still confused that it can idle quite happily with no stalling if Crank sensor / etc disconnected...pointing away from mechanical fault I assumed (always dangerous to assume isnt it!..) Thanks for the replys tho

Waow, You did all I could suggest ! FYI the crank sensor only tell the ECU when the 4 cylinders are @ mid stroke (all at same place), THEN the cam sensor tells the ECU which cylinder is in which stroke...

BR's , Bernie

Yeah, but "crank sensor" is often called "TDC sensor" for a reason. Maybe not in this car (worth checking).

sussexmechanic said:

Still confused that it can idle quite happily with no stalling if Crank sensor / etc disconnected...pointing away from mechanical fault I assumed...

Click to expand...

So next thing to (re)check, electronics (with oscilloscope, a real one). Swapping and/or disconnecting sensors has no effect, because they are not operational (or there is a wiring issue, ECU does not see the signals).

The crank sensor is also called TDC (Top Death Center) sensor because that's what it is !

The trick is that it detects missing teeth on the sonic wheel BEFORE the engine reaches its TDC position, then count the number of consecutive teeth (converted in degrees) to determine when the engine is actually at TDC and ,in combination with the cam sensor, which cylinder is @ end of compression stroke ...

That's interesting case study. :chin: Any Update?

Hey You (sussexmechanic)!
What's the status of the car: Fixed, Sold, Totaled?