Intermittent Rough Idle and slow start.

Hey all.

I'm having issues with a slight rough idle. The rpm's always Pulses a little bit at idle and sometimes feel more like a miss or a stumble. It has a brand new timing belt, the stumble was present before replacing it. Not all the time but sometimes it starts a bit slow even when warm.

I'm going to try adding some Diesel Purge see if maybe I can clean the injectors out. And also I'm considering trying bumping up the camshaft timing/ torsion value see if that resolves anything, but I'm not sure if I should be bothering with that cause the car feels great other than at idle. I don't know what the torsion value was before replacing the timing belt because my Camshaft Position Sensor wasn't giving me a reading.

I just replaced the fuel filter and air filter so those are both good, and the camshaft position sensor.

Any advice on where to look for the issue would be great.


IQ: fluctuates from 4.4 to 4.7
Torsion Value: 0.5

11:43:53 Group 001: Injection Quantity
903 /min Engine Speed (G28)
4.4 mg/str Injection Quantity
6.6∞KW Injection Duration (specified)
74.7∞C Coolant Temperature (G62)


11:43:53 Group 013: Idle Stabilization (Injection Quantity Deviation)
0.14 mg/str Injection Quantity Deviation Cyl. 1
0.05 mg/str Injection Quantity Deviation Cyl. 2
-0.02 mg/str Injection Quantity Deviation Cyl. 3
-0.14 mg/str Injection Quantity Deviation Cyl. 4


11:47:09 Group 004: Pump-Jet Valves
903 /min Engine Speed (G28)
4.0 ∞BTDC Injection Start (specified)
6.6∞KW Injection Duration (specified)
0.5∞KW Torsion Value

Well apparently the original torsion value might be written on the timing belt cover, but I really don't think it's the timing.

but here's what I'm going to start looking into:

1. bad alternator pulley.
2. dirty injectors.
3. Vacuum leak
4. water in fuel

Possibly the in tank fuel pump? I just found out mine isn't working and ordered a new motor for it. I also have a bit of a rough idle, but just at random times. I'm hoping that'll fix it

Out of curiosity, how does the cam and hydraulic lifters look?
Is the chamfered edge still on the lobes?
Are any of the lifters dished?

Voluble said:

Possibly the in tank fuel pump? I just found out mine isn't working and ordered a new motor for it. I also have a bit of a rough idle, but just at random times. I'm hoping that'll fix it

Click to expand...

Thanks I'll keep that one in mind.

csstevej said:

Out of curiosity, how does the cam and hydraulic lifters look?
Is the chamfered edge still on the lobes?
Are any of the lifters dished?

Click to expand...

I haven't taken a look at them yet. But I'll definitely check them out if this continues. At the moment i believe I'm seeing excessive movement in the serpentine tensioner (could be a bad tensioner?),also the Alternator pulley works properly but seems a little rough so I might start with the pulley, and maybe a new tensioner. not sure what the lifespan is on the tensioners, the cars got 266,000km.

I was getting a camshaft position sensor error previously, and the timing belt was due so i was convinced it was either timing or the sensor, but neither solved the problem.

Check alternator pulley for the one way clutch to see if it is free. Also are you on OEM DMF. I had a similar experience with my car. Thought the pulley was bad and it was. Once replaced, you could hear the DMF chattering. After it was replaced, it worked like it should and didn't have the miss any more.

joep1234 said:

Check alternator pulley for the one way clutch to see if it is free. Also are you on OEM DMF. I had a similar experience with my car. Thought the pulley was bad and it was. Once replaced, you could hear the DMF chattering. After it was replaced, it worked like it should and didn't have the miss any more.

Click to expand...

Thanks, yeah I checked the pulley. It does spin one way as it should but It’s not very smooth so I may just swap it out.

The DMF was replaced about 60000km ago.