I've had my 159 1.9JTDm almost a year and there has always been an issue regarding low rpm juddering.
On the flat the revs can be increased from idle, while in gear and as they rise to about 1150rpm the engine starts to become prone to juddering. It is a violent judder, like you might expect of the car was about to stall, but it comes in at speeds well above idle speed.
It runs at about 2-4Hz and accelerating only sometimes lets you drive through it. Ultimately I need to drop down a gear.
I accept it isn't healthy to drive at such low revs but occasionally in traffic you just naturally slow with everyone else, and if you try to apply throttle when in the juddering risk speeds it usually produces juddering.
My EGR is working well
The problem is either hardware or software related (
) . If it is a software issue then nothing can be done (unless it is remaped)
That leaves hardware possibilities. My car has the later swirl valves and actuator, which move very freely and quickly. Potentially they could be oscillating quickly enough.
When the swirl valves close quickly the intake manifold pressure measured by the MAP sensor jumps instantly. I just wondered if the excess in pressure causes a slight increase in boost, itself producing a sudden lurch, which makes the control system pick another control point for the swirl valves..and the thing kangaroos merrilly.
It was just an idea anyway. So how to test it ?
I tested the wiring for the combined temperature/MAP sensor (Bosch call them T-MAP sensors)
Purple = Gnd
white/orange = temperature sensor output
white/red = MAP sensor o/p
I connected a 0.47uF capacitor between Gnd and MAP o/p with the idea of smoothing the output from the MAP sensor a little bit. It made an instantly noticeable improvement, but there was still juddering in 5th gear (high loads) and 6th
I changed the cap for 10uF and now the only juddering is in 6th gear, where if you let the revs come down to 1200rpm and then try to gently accelerate, then the juddering starts at about 1250 and goes on upto about 1400rpm. (Additionally the engine can feel very flat and unlikley to accelrate)
However, for the rest of the gears, even under high load conditions, I can now let the revs die away to 1100rpm and then I'm able to accelerate smoothly out from the lower rev range.
In all but 6th gear, at about 1300rpm the torque starts to build strongly and the car is nicely driveable. (Previously I only had useful torque from 1400rpm, thanks to the juddering)
Just wondered if anyone has any experience with this ? I wonder if the reaction of the swirl valves in the ECU is programmed for the original swirl valves which I think operate more slowly, and so using the later type means a feedback loop has been set up.
My manifold is the black plastic intake manifold (not the early version alloy one).
If your car judders too, is it an alloy manifold or a plastic one ? Just wondering.
I'm going to monitor this and perhaps drop the value of the capacitor a little because I don't want to tempt overboost by damping the rate of change of the MAP o/p signal too much, but I feel there is enough headroom in the system to cope. Remaped cars have much more aggressive turbo control so I think this could be a low risk fix.
Has anyone else had a problem with incurable juddering at lower revs ?