Skoda Rapid diesel, doesn't slow down much with foot off the pedal - is this normal?

[mattytowel]

Hello

 

This is our first diesel car, so what I'm about to describe might be normal, but my wife and I find it unsettling.

 

When we are out and about, say doing 30mph in 3rd or 4th, we can take our foot off the throttle and the car continues at 30mph.  I haven't yet worked out if it is related to being warmed up if it's the same in each gear or what.  My wife actually reckons that when she takes her foot off the pedal it speeds up a little and then slows down slowly.  I've read that engine braking isn't as significant with diesels but I'd have expected some, but changing gear or going into neutral helps.

 

The car has done about 130000miles, apart from the above it drives alright.  We've had it two months, it has been regularly serviced at garages, and we bought it from friends who had it from new.  I went from Manchester to Portsmouth and back with no issues and got 13miles per litre, which I was pleased about.  The previous owners had averaged 20 to 25 thousand miles a year (motorway work trips) till the start of Covid, then under 10 thousand a year (mosty urban)  since then.  It idles at under 1000 rpm, I can check exactly when I next use it.

 

I wonder if:

the TPS or a pedal sensor is an on-off switch that isn't recognising that the throttle is closed / foot off the pedal

the throttle body or something else is all gummed up and isn't closing properly

something has been tweaked in the engine management settings

 

I look forward to getting some pointers, thanks in advance.

 

 

[J.R.]

There is no throttle on a diesel engine.

 

Well there is actually a butterfly called a throttle body but it isn't, its purpose is to create a partial vacuum under certain conditions to draw regenerated exhaust gasses (EGR) and to prevent running on on shutdown.

 

In any gear it should slow down to and then maintain the idle rpm, if you were in 6th gear that would be around 30mph but much less in 3rd or 4th.

 

Change your MPG display to instantaneous instead of average, when you come off the throttle it should initially jump to 199mpg and then to zero, if it shows something between 80mpg and 150mpg (my guess) then the fuelling is not being stopped because the ECU is not recognising that the accelerator pedal is not depressed.

 

If the vehicle is doing a regen the idle RPM will be higher and the off throttle speeds in each gear will be higher but what you describe is not right, it sounds like it is still fuelling.

[mattytowel]

Thanks JR, I'll check that when I'm out next,but I think it does go to 199mpg.

 

Wouldn't the butterfly also close, creating the partial vacuum and making it harder for the engine to run and so it slows down?

 

[J.R.]

I'm not sure of the conditions for the partial closure for EGR but a diesel engine does not need any help in slowing down, the engine compression does that very well all on its own.

 

In terms of emissions and especially oil consumption it is not desirable to create a vacuum on over-run in the inlet tract of a diesel engine.

[KenONeill]

37 minutes ago, mattytowel said:

Wouldn't the butterfly also close

No, because despite what @J.R. has implied, there is no actual butterfly on a diesel engine, just a venturi.

[mattytowel]

Thanks for your thoughts, here's what I've found on a flat empty road when the car had warmed up.

 

Trundling along at 30mph in 4th gear, take foot off the pedal, it slows to between 20 and 25mph and continues at that.

Trundling along at 15mph in 4th gear, take foot off the pedal, it speeds up to between 20 and 25mph and continues at that.

Trundling along at 20mph in (it was either 2nd or 3rd) gear, take foot off the pedal, it slows to about 15mph and continues at that.

 

When I take the foot off, then the instantaneous mpg use jumps to 200 mpg, then '----' then levels off at 60 to 70 mpg or so, so it is still deliberately fuelling

 

Both my wife and I are sometimes dropping down a gear to aid slowing without applying brakes.

 

I think tomorrow (if it's dry out) I will look to see what sort of accelerator pedal position sensor it has, and what the outputs are.

 

 

 

[varooom]

It sounds like you are just describing anti-stall, where the car will adjust power to keep you moving (within reason)

 

Pay attention to the revs in the above gears/speeds you mentioned above, and see how the car behaves when on an incline.

Do note however, you really shouldn't stress (labour) the engine this way.  Do the "experiment" and then make sure in future to change gears accordingly.  If you are on a flat, the engine won't be labouring too hard at low speed, at faster speeds the drag will make engine labour.

[KenONeill]

31 minutes ago, mattytowel said:

30mph in 4th gear, take foot off the pedal, it slows to between 20 and 25mph and continues at that.

What rpm does this, and your other examples, correspond to? I'm guessing somewhere between idle and 1_000 rpm.

[J.R.]

On 19/12/2022 at 19:44, mattytowel said:

say doing 30mph in 3rd or 4th, we can take our foot off the throttle and the car continues at 30mph. 

 

3 hours ago, mattytowel said:

Trundling along at 30mph in 4th gear, take foot off the pedal, it slows to between 20 and 25mph and continues at that.

 

A bit of a difference between the two reports!

 

It is indeed the idle stabiliser (anti-stall) system, normal idle RPM will be around 850rpm, during a DPF regen it will be higher at around 1000 rpm, as has been said you really should not be driving the car off throttle at such low revs in the higher gears, in some ways its a shame it has the anti-stall system because believe me if it were to stall at 20 mph it would be very traumatic for you and you would instantly change your driving style to make sure it would never happen again.

 

When a diesel engine stalls it is always a horrible experience for anyone with mechanical sympathy, it happens to all of us once and nobody would want to repeat the experience.

[mattytowel]

On 01/01/2023 at 00:35, J.R. said:

A bit of a difference between the two reports!

 

That's the difference between what I thought happened, and what I observed when deliberately paying attention.

 

It idles at around 700 and 800 rpm, I've tried resetting the ECU by unplugging its fuse for some hours but it hasn't changed anything that I am aware of.

 

It was worth considering, we will learn to live with how it behaves.   Thank you all for your input.