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Im currently trying to get to the bottom of an issue with High long term fuel trims (+20-25% give or take) which is showing my car appearing to be running extremely Lean, struggling with this as the A/F ratio looks ok and o2 looks ok...

Alfa Romeo Giulietta 1.4 Multiair 2014 TCT

Symptoms are:

Slight Rough idle Idle climbs by itself to 11-1200 rpm then settles momentarily then back up High LTFT on idle of 25% When revved at 1500, 2500 the LTFT trims go back to normal STFT seem ok No fault codes Oil level is fine Air filter is fine Spark plugs look good MAP is fine o2 sensor reading looks ok? Air fuel mix is ok? Feels stuttery to drive on low revs through the gears 1-3, wide open throttle seems normal

Ive undertaken a Vacuum test and the results are:

At idle

8-10 Hg fluttering between the 2

At 1500 rpm

Drops to 3 Hg, increases to 17Hg drops to 5 Hg then settles back to 8-10 Hg

At 2500 Rpm

Drops to 3 Hg, increases to 18 Hg, drops to 5 Hg then back to 8-10 when idling

3000 rpm is the same

The MAP correlates with 68-70 kPa

Smoke test has been carried out, no vacuum leaks found

Compression test done @ 235 psi (+/- 1%) across all 4 cylinders

PCV changed as it had a slight oil leak, no changes

Adaptations reset after each test

I know the MA unit on these can be temperamental but with a solid compression ratio across all 4, i would think to rule out any hydraulic actuator issues

Any advice or experience would be welcome

Im moving onto thinking around a timing issue or exhaust blockage somewhere?

Thanks in advance

Vac gauge

02

idle trims

1500 rpm trims

2500 rpm trims

MAP

  • Does the AFR sensor respond if you add fuel or induce a vacuum leak? No MAF? I can't comment on the vacuum readings as I'm not that familiar with the multiair system. – Ben Aug 3 '18 at 23:22
  • Havent checked by inducing a leak, obviously the AFR do respond through the rev range as above. MAP not MAF controlled, MAP seems to correspond with mechanical Vac gauge readings so assumed ok. I also put another MAP on as a check and showed the same readings. Thanks for the response – – james walker Aug 4 '18 at 9:04
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So, i now have a baseline to go from. I managed to ask another Giulietta owner to measure his vacuum readings amongst other parameters with the same Multiecuscan software as i use

Without the boring conversions

His vacuum at idle was : 19.2 Hg My vacuum at idle is: 8 Hg

So, my theory and feel free to shoot me down is:

Compression is likely higher than normal, but consistent across all 4 at 235 PSI ish Vacuum is extremely low at idle

Which is causing the fuel trims to be high at +25%, basically adding fuel to compensate for low vacuum as the MAP thinks the engine is under load at idle only, when truly under load the trims fall back in line when vacuum is less relevant, this is essentially a false lean condition as the 02 is reporting ok - the integrator value on MES is showing rich, which i believe means its adding fuel

With the Higher than normal compression and lower than normal vacuum - could this very well be a Cam timing issue by which its a bit retarded or late...

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Excessively High Idle Speed

IAC - Stick a pencil under the throttle linkage and disconnect the idle control, this allows the engine air without the IAC, It'll tell you if the IAC is causes it to jump

Vacuum leaks at the Intake Manifold or hoses, Take a propane torch and turn it on, but don't ignite the flame, Then go around your engine engine until your hear VROOM

Fuel Injection or engine control systems malfunction, meaning the TSS, TP, PCV, ISS, IAC, IAT, DTR, ECT, CMP, BPP, EVAP, EGR

Engine misses at idle speeds

Spark plus are faulty

Spark Plugs wires are faulty

Sticking EGR valve or PCV valve

Short circuit In the Ignition, coil 

vacuum leaks at the Intake Manifold or hoses, Take a propane torch and turn it on, but don't ignite the flame, Then go around your engine engine until your hear VROOM

Abbreviations

EVAP - Evaporative Emissions Control
PCV  - Positive Crankcase Ventilation
BPP  - Brake pedal Position switch
DTR  - Digital Transmission Range
CKP  - Crankshaft Position sensor
OSS  - Output Shaft Speed sensor
EGR  - Exhaust Gas Recirculation
TP   - Throttle Position Sensor
ISS  - Turbine Shaft Speed
CMP  - Camshaft Position
IAC  - Idle Air Control
MAF  - Mass Air Flow

When the high fuel trims are too rich

leaking fuel pressure regulator - Pressure gauge attacked remove the vacuum. did It jump, If not It's bad
leaking Injectors - can you smell fuel
O2 sensors - False positives being sent to to the mass air flow sensor 

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