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-   -   Occasional RPM fluctuation : Getting to the root cause (https://www.team-bhp.com/forum/technical-stuff/135586-occasional-rpm-fluctuation-getting-root-cause.html)

My safari when brand new had rpm fluctuation which used to be quite common. Idle would swing between 600-900. Problem used to be more when it was "cold"
Tata did not know what was wrong, so they changed everything. Including half the engine block. So at 30K I had a new top engine. Common rail, fuel pump, head, everything was new.

Now the vehicle is almost a 100000, and sometimes, rpm starts fluctuating. Happens a lot when cold, and on hot engines only sometimes.
I thought this was special, till I saw another safari being started at around 5 degree C, and rpm fluctuating wildly.

So I guess, many owners ignore this.
I want to get to the bottom.
All evidence points to Fuel temperature sensor, or Idle control valve, or an O2 sensor.
I am trying to get OBD II to connect, but Tata, being Tata, can't even get OBD II right.

But still, I could somehow get RPM and Fuel pressure(Bar) graphs.

And guess what, the rpm fluctuation is mirrored by a fluctuation in fuel pressure. Throttle position is 0.0. which means no issues with the electronic throttle.

However fluctuation from 260-310bar in idle fuel pressure which should be at 280 bar for a Safari class engine.

I do not want to go on a part replacing spree, so is there any way I can dump what is the output of the O2 sensors and the Fuel temp sensor. I am unable to get a reading on fuel temp sensor.

Is there any BT software for windows(if not android) which will communicate with Tata ECU?

Is there a place I can buy their silly cable?

Anyways, here is the RPM and the Fuel pressure Graph. What could cause fuel pressure to fluctuate? IS it in the Cause area of fluctuation or effect area of fluctuation?

You have mentioned that this happens during cold weather. Does it happen during summers/hot weather also? If this does not happen during hot weather then I have a theory about what may be the cause of this.

I think the ECM is over compensating for high density air during winters and in cold places. But this compensation is causing the O2 sensor to go nuts and that in turn is asking the ECM to cut off fuel. The turbo charger is amplifying this effect.

I faced something similar with my Swift in Sarchu. The night I was there the temperature had dropped to 4 degrees C. In the morning when I started the car on the stock ECM, the RPM was fluctuating by about 100 - 150 RPM even though the ambient temperature was around 10 - 12 degrees C.

Well like I said earlier, this is only a theory. I don't have any data to back this theory up.

Where is the O2 sensor in Safari located ?

Which model Safari is this (BS --) ?

Spike

The problem is more at high altitudes, and/or cold weather, esp in the hills, but it happens even at 35+ ambient in Delhi
Its BS-III. I think BS-IV versions have O2 sensor, but BS-III may not, right?

^^ Right, that is why I asked. I was surprised after reading the post above, hence asked. There can also be BS-IV versions without an O2 sensor, but that depends on the manufacturer.

Spike

My 3.0L DICOR has this problem the rpm although does'nt fluctuates but it changes on its own between 600-7900 RPM. I am told that its either the rail pressure sensor gone kaput or fuel pump has the problem. Anyway, I ignore it as it still gives me 16-17 Kmpl and occasional 19 kmpl (very very rare!). So, don't fix it, if it ain't broke is the key with such problems.

My Old 1999 Indica which lasted for some 2.3L Kms started giving varying RPM problem, it was like the old trucks/buses which had governer, the only difference was the fluctuation was more rapid in my Indica. The mechanic told that some balance/governer plate in the pump was culprit. Although I had no problems with that it still gave me a nice 20kmpl.

Could it be a problem with the Idle air control valve? Does safari have one?

If the idle air valve had a problem then it would act up at all times like for example even during normal city driving when you have stopped at a red light.

I'm surprised to learn that BSIII diesels didn't have an O2 sensor.

Hi guys,

I just got the 75,000 km service done in my Jetta AT, along with the replacement of the brake pads, the intercooler had a small hole so the service advisor suggested to get it replaced as well.

After taking delivery, since yesterday I've been experiencing a fluctuation in the Rpm while idling.
This result is a slight jerk each time it fluctuates. The minute the car is in motion again it's not felt.

I took the car back to the service centre but nothing is showing on the scanners. Could you guys shed some light on this?

Please mention the Model year and Engine type if its old Jetta(PD or CR).

Do you see the fluctuation clearly in the RPM meter? Or is it just the a weird rattling sound? Since your car has done 75k, and it does not show up in the scan and there is no warning light, my guess is it the worn out Dual Mass Flywheel.

If the Workshop is not able to diagnose it, I'd recommend you visit a mechanic/workshop familiar with VW/Skoda cars, if it is the flywheel any experienced technician will be able to diagnose it just by the sound of it.

Hi Anshuman, it's a 2012 Volkswagen Jetta AT TDI

Yes the fluctuation is clearly visible on the Rpm meter. It fluctuates between 700 - 900 with the air conditioner on and between 500 - 700 with it switched off. Thanks, I will get the flywheel checked in the meantime.

How fast or exactly is the fluctuation? Are you sure its not the DPF in action?