Alfa low throttle jerks - Alfa Romeo Forum
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(Post Link) post #1 of 17 Old 3 Weeks Ago Thread Starter
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Alfa low throttle jerks

Hi, i have noticed that my Alfa 147 1.9 JTDm jerks when i'm pressing the pedal gently, maybe 20-30%. I have a diagnostic tool so i have been driving around with it and monitoring various parameters. Everything seems fine except for the fuel pressure. Sometimes when i accelerate and release throttle, fuel pressure sticks around 1000 Bar and starts displaying random values. Sometimes the actual consumption peaks high when driving slow, like it shows 2 times more than usual(around 18l and it looks like it happens at the same time when the fuel pressure is displaying wrong values but not sure). It feels like a loss of power or engine breaking, then it jerks and after i pass that pedal range it pulls straight away(Pressure values are correct at higher load). I have checked the pedal potentiometer and it displays correct values. What i was thinking is that one of the components is giving wrong pressure values and because of that the pump is underfueling(Maybe ?). Everything else looks fine and the car performs excellent other than that. Do you guys have an idea what could it be ? Also, what are the correct injector fuel correction values for JTDm engines ?
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(Post Link) post #2 of 17 Old 3 Weeks Ago
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Correction values should be as low as possible. I think 25% logs a code and illuminates the MCSF.

I think the fuel pressure sensor is by the oil cap at the flywheel end of the fuel rail. I think the other end of the fuel rail has the fuel pressure regulator. It could be either of these which has the fault. Fuel pressure should be quite steady but likely to increase with engine speed to some degree.
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Well it doesn't show any error codes, but what i ment by the percentage is that the car jerks when the throttle pedal is pressed 25-30 % and kept in that position.
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Correction values are around 1 on all four cylinders when driving.
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True, fuel pressure should be stable but when acellerating and releasing the throttle, values should be around 350-400 Bar, but instead the value gets up to 1000 Bar and oscillates around that number for some time and after it drops back to normal. Also at higher loads it shows correct values.
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Other than the fuel pressure, what you said sounds good.
Does it only do it whilst driving, or can it fluctuate during fast idle (car stopped, engine, say 2500rpm)?
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Haven't thought of that, let me check 😁
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It seems fine at the first look, haven't been able to reproduce it ? I'm really running out of ideas what it could be... Car hesitates on low throttle but runs completely fine after that point. It seems like jerks are more pronounced when the car is hot.
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Rpm is stable on fast idle
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I have calculated the consumption by the odometer and it is completely normal. I'm stumped by this
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Mine was maff with similar symptoms
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Try resetting the throttle. That solves the jerking on petrols
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Is your car a Euro 4 (with inlet swirl flaps)?
If so, can you diagnostically read if the swirl flaps are open or closed or even better, perform actuator tests to see if they work?

Other than that, I suspect injectors. A poor spray pattern could produce these effects, I think bit I'd want to know fuel pressure before taking injectors out for testing.
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Yes my car is Euro 4 but i haven't seen the swirl flap actuator anywhere nor it is available as an option in the diagnostics, i have jtdm 8v engine. When you had a bad MAF what else have you noticed along with jerking(side effects ?).
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The fuel pressure is correct at high demand, it goes all the way up to 1600 bar and the car runs excellent. Also jerking appears only when the car is hot (working temperature around 80-90C)and pressing the pedal gently. What is the easiest way to reset the throttle, as diesels dont have TPS, only pedal potentiometer.

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Throttle reset is for petrol engines only.
Fuel pressure seems ok. Even if it fluctuates by 100 bar or so it should be ok.
How does the car idle?
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The car idles stable, all the time at 850 rpm, the pressure is not fluctuating at all, only sometimes it kinda gets stuck at 750 bar when i release the throttle and then raises to 1000 bar for some time, after that it drops back to idle pressure. What is actually weird is that the car demands for that pressure with pedal released.. ? I have noticed that because in the diagnostics i can see the actual pressure and requested one by the ecu. Have no idea what causes pressure sticking on 1000 bar while off load.
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