Random hard jolt when going from engine braking to accelerating

[Chris Boddum]

Hi!

I recently had most of my issues fixed, but the last one seems to persist. If I engine brake and then press the accelerator, the engine will sometimes not smoothly bring on the power but instead bring it on hard causing what feels like a hard jolt in the cabin. It'll happen randomly and the amount of jolt is is from barely noticeable to quite a bit. There are no sounds associated with the jolt. The best way to describe it is that it feels like driveline slack, so when you go from engine braking to accelerating, the engine has to make the slack go in the opposite direction.

What can this be? It seems to happen mostly between 2000-3000 RPM and only really on the motorway (so in higher gears).

The car is a 2015 Aygo with the 5-speed manual transmission, done around 37k miles.

[ian rooke]

Hi What car and gearbox type please 

[Chris Boddum]

9 minutes ago, ian rooke said:

Hi What car and gearbox type please 

Ah, I totally forgot that, I'll make sure to edit it into the original post. It's a late 2015 Aygo with the manual 5-speed transmission. Is the power delivery when going from engine braking and onto acceleration supposed to be 100% smooth, or is some random "driveline slack"-like feel just supposed to happen in this class of car? 

It's important to note that it happens completely randomly, but mostly when the engine is hot. It sometimes happens, it sometimes does not.

[Chris Boddum]

15 hours ago, Chris Boddum said:

Ah, I totally forgot that, I'll make sure to edit it into the original post. It's a late 2015 Aygo with the manual 5-speed transmission. Is the power delivery when going from engine braking and onto acceleration supposed to be 100% smooth, or is some random "driveline slack"-like feel just supposed to happen in this class of car? 

It's important to note that it happens completely randomly, but mostly when the engine is hot. It sometimes happens, it sometimes does not.

I seemingly cannot edit my post anymore, so I'm sorry for the extra post. However, I just remembered that I fitted larger tires to the car than the factory recommends, with them being 175 75 15. Would a slightly larger tire, other than causing a the speedometer to be a little further off, cause any issues like the ones I am describing?

[Wiz201]

yes, I'd try tyres at the normal factory size, and see how you go. its a major difference from 165 60 15

[Chris Boddum]

24 minutes ago, Wiz201 said:

yes, I'd try tyres at the normal factory size, and see how you go. its a major difference from 165 60 15

I think you might be right. I added them, as they were the only winter tyres I had laying around. I'll make sure to buy some proper summer tyres for it when spring comes around. Thanks!

[Gerg]

I've only driven an Aygo as a courtesy car, so I can't remember what it was like to drive much.

But this sounds like a continuation of the last problem you reported.  I think the drivetrain slack is much, much magnified by a slightly hesitant engine - so your original problem, and whatever was behind it,  might be behind this too.  Did you get to do all the repairs you were thinking of before?

I would not anticipate your different tyres making a difference to this problem, but you will have made a difference in plenty of other areas.  Your tyres have an 11% larger circumference.  You must have the softest riding Aygo in existance.  And your speedo and odometer will mis-read as well, which can have legal implications if someone were to pick up on this. 

You have turned your 5th gear into an 'overdrive' 6th now - a real motorway gear, if there are any wherever you are!  This is an interesting experiment to see how the car drives, but, sadly, your insurance company might not be best pleased if you haven't told them about this change.

Annoying but true.

This link is very illuminating:

https://tire-calc.com/comparison/165-60-r15-and-175-75-r15/

[Chris Boddum]

1 hour ago, Gerg said:

I've only driven an Aygo as a courtesy car, so I can't remember what it was like to drive much.

But this sounds like a continuation of the last problem you reported.  I think the drivetrain slack is much, much magnified by a slightly hesitant engine - so your original problem, and whatever was behind it,  might be behind this too.  Did you get to do all the repairs you were thinking of before?

I would not anticipate your different tyres making a difference to this problem, but you will have made a difference in plenty of other areas.  Your tyres have an 11% larger circumference.  You must have the softest riding Aygo in existance.  And your speedo and odometer will mis-read as well, which can have legal implications if someone were to pick up on this. 

You have turned your 5th gear into an 'overdrive' 6th now - a real motorway gear, if there are any wherever you are!  This is an interesting experiment to see how the car drives, but, sadly, your insurance company might not be best pleased if you haven't told them about this change.

Annoying but true.

This link is very illuminating:

https://tire-calc.com/comparison/165-60-r15-and-175-75-r15/

Thank you for the interesting answer! I made sure to check on the legislation before fitting, and it is within 2% of the law, so barely legal - but still legal. It's VERY smooth riding, especially on the motorway, and the RPM seem to be around 2800 when travelling at around 65-70 mph. 

I did fix most of the previous issues by having the chain serviced. However, I still imagine it needs the more correct oil of 0w20 in it (compared to the 5w30 that is has right now). Crossing fingers that it may solve the problem! Else I'll just live with it, it's really no big deal compared to how well it drives.

[Gerg]

1 hour ago, Chris Boddum said:

travelling at around 65-70 mph. 

I'm sure you are working off a 'corrected speed' now, aren't you? So your 65-70 mph (was that indicated?) is now 71-77 mph (actual).  You might be getting better fuel economy too, but you need to add an 11% improvement on that to get an accurate mpg figure, as I'm sure you know.

I think the difference you'll find between the 0w/20 oil and 5w/30 will be quite subtle.  I knew someone who thought that they could tell the difference when a garage put 10w/40 in his car at a service instead of the 5w/30 it should have had, but he wasn't certain. And that was someone who was very meticulous about every aspect of his car.  In that case he was correct, confirmed when he asked at the garage some days later, and his mpg had gone down a little as well.

In the grand scheme of things I think such small changes in oil are all about fuel economy improvements rather than engine longevity, although in the extreme cold there would be a benefit in the 0w/20 flowing a little quicker, and the engine will also turn over a little more easily when starting.

Arguably, this thinner oil drive started waaaay back in the late 1970s with BP VF7, the actual viscosity of which was.......I can't remember exactly.  Probably a 10w/30.  That was when almost every car ran on a 20w/50 oil.  As per this ancient video:-

 

[Wiz201]

8 hours ago, Chris Boddum said:

I think you might be right. I added them, as they were the only winter tyres I had laying around. I'll make sure to buy some proper summer tyres for it when spring comes around. Thanks!

I run my Aygo on all seasons, consider getting a set of those. Much better for the winter in this country, when we may only get three days of snow.

[FROSTYBALLS]

Using a tyre calculation site, substituting 175/75/15 tyres for the standard 165/60/15 tyres, the speedo will under read by 11.5%  which means at a true speed of 66.9mph your speedo will show 60mph. 

Any speedometer showing less than the true speed is illegal and an under read of 11.5% is way off legally. 

A larger tyre will only increase the under read.

[Gerg]

7 minutes ago, FROSTYBALLS said:

at a true speed of 66.9mph your speedo will show 60mph. 

Yes, you're right, I should forget any attempt at multi-tasking ever again!