Excessive engine breaking/jerking when letting off throttle.

[shawnmos]

So this may just be a coincidence but I recently had the VVT replaced by the dealer and now I am noticing excessive engine braking when I let off the gas, to the point that the car feels like it's jerking.

Now, manual cars normally do this a bit, especially in the lower gears but it's now noticeable it in 3rd and 4th gear as well.

Does anyone have any ideas as to what would cause this? I'm thinking vacuum leak that may or may not have been caused by them disassembling a bunch of stuff, but I'm not sure. I'm hoping it doesn't have anything to do with them not adjusting the timing correctly when they replaced the chain.

Another possibility is that it's excessive drag caused by the AC compressor. I added some more R134a and overfilled it a bit. I let some back out, but it's possible it could still be overfilled. However, the problem should go away when I turn off the compressor but it does not.

Thoughts?

[Reds3]

How are your engine mounts?

Also if you had a vacuum leak you should be able to see a difference in your fuel trims on your AP, assuming you have an AP or some kind of data logging device hooked up.

[Jeff23spl]

just ac compressor turning on and off could make some drag but you can easily turn it off just to compare if it is that...

When i drove my skyactive 3, i switch AC off to pass a car and for a low power engine, it makes a difference....

btw have you added petrolum equivalent r134 or real r134 ? I'm curious because in here, we need a special licence to manipulate freon but not the small petrolum products replacement in can....And some specify to use just half or 40% of the freon weight needed....

[Fstrnyou]

Engine breaking is called ZZB.

[MSMS3]

Originally Posted by Fstrnyou Engine breaking is called ZZB.

Ha ha. OP probably does not even know why this is funny. Spelling must not be a high priority. I'm not the grammar and spelling police. We all make mistakes, but that was funny.

As to why his engine braking may be excessive, there are lots of possible explanations. I'd lean toward exploring engine and transmission mounts first. These cars are getting a lot of age and miles on them. Fixing one thing, especially if it increases or restores lost power, can reveal other problems that were latent. An abrupt movement of the engine and transmission when letting up on the gas may be causing this sensation. That points to one or more bad engine or transmission mounts.

Also, if the ac solenoid clutch is sticking so that the ac compressor is turning all the time, even when it is turned "off" this could, indeed, put a heavy load on the belts, pulleys and introduce a lot of drag to the engine. That would stress weak engine/transmission mounts. It will also burn out the compressor. OP should look and see if the compressor is turning when the ac is switched off. It should stop when off, and resume when turned on. That is visible.

[shawnmos]

Originally Posted by Fstrnyou Engine breaking is called ZZB.

Doh

Originally Posted by Jeff23spl btw have you added petrolum equivalent r134 or real r134 ? I'm curious because in here, we need a special licence to manipulate freon but not the small petrolum products replacement in can....And some specify to use just half or 40% of the freon weight needed....

I assumed it was real r134 but it came in a can so maybe not?

Originally Posted by MSMS3 As to why his engine braking may be excessive, there are lots of possible explanations. I'd lean toward exploring engine and transmission mounts first. These cars are getting a lot of age and miles on them. Fixing one thing, especially if it increases or restores lost power, can reveal other problems that were latent. An abrupt movement of the engine and transmission when letting up on the gas may be causing this sensation. That points to one or more bad engine or transmission mounts. Also, if the ac solenoid clutch is sticking so that the ac compressor is turning all the time, even when it is turned "off" this could, indeed, put a heavy load on the belts, pulleys and introduce a lot of drag to the engine. That would stress weak engine/transmission mounts. It will also burn out the compressor. OP should look and see if the compressor is turning when the ac is switched off. It should stop when off, and resume when turned on. That is visible.

Thanks, I'll check over the mounts, though the PMM was replaced not too long ago. I'll check the compressor too.

Originally Posted by Reds3 How are your engine mounts? Also if you had a vacuum leak you should be able to see a difference in your fuel trims on your AP, assuming you have an AP or some kind of data logging device hooked up.

I don't have an AP but I have Forscan on my laptop. I honestly don't know enough about what they should look like to know whether they are off.

[Jeff23spl]

STFT and LTFT combinned should remain inside a +/-10% with a proper maf tune (Cobb say 8%)

have you checked if it is different if you switch to neutral just to isolate motor/trans from the equation when you decelerate ? The car should almost ''fly'' in neutral or you may have a stikky brake or a bearing...

[shawnmos]

This was just a 5min drive around the block.

[shawnmos]

Originally Posted by Jeff23spl STFT and LTFT combinned should remain inside a +/-10% with a proper maf tune (Cobb say 8%) have you checked if it is different if you switch to neutral just to isolate motor/trans from the equation when you decelerate ? The car should almost ''fly'' in neutral or you may have a stikky brake or a bearing...

It does not do it in neutral. Only when the car is in gear.

I just replaced the RMM hoping that would fix the problem but it did not make much difference.

[shawnmos]

Attaching a few different drives/days. The more I've been reading the more it sounds like a fuel cutoff when I let go of the gas.

[Quazz236]

What RPM does it happen at? I've noticed that when I let the gas go and coast in gear the engine feels somewhat rough around 3K. Like lets say I'll go up to 5k RPM and let the gas go and roll (any gear btw) when the RPM's start dropping the engine feels smooth till about 3K then it gets rough between 3k-2.5ish.k and then under 2.5k RPM it feels ok again. IDK if it's something similar or not.

[shawnmos]

Originally Posted by Quazz236 What RPM does it happen at? I've noticed that when I let the gas go and coast in gear the engine feels somewhat rough around 3K. Like lets say I'll go up to 5k RPM and let the gas go and roll (any gear btw) when the RPM's start dropping the engine feels smooth till about 3K then it gets rough between 3k-2.5ish.k and then under 2.5k RPM it feels ok again. IDK if it's something similar or not.

It seems to happen at any RPM. I've actually noticed since replacing the RMM it no longer jerks when I let off the throttle. So maybe that was part of the problem.

However, it still jerks violently when I first start to accelerate. It doesn't seem to matter the RPM, though it's worse the lower the gear. It's still somewhat noticeable in 5th or 6th gear though. Just feathering the pedal a bit and it jerks a lot.

[Fstrnyou]

I think I have noticed a similar phenomenon with very little throttle input. Off throttle, the fuel is cut off. But at very light throttle where I'm still slowing down but not as much as off throttle, it feels a little jerky. I just assumed it was the nature of having a 3.5" intake and maybe the MAFcal was slightly off in that area. Or maybe the throttle tables needed to be refined in that area. It didn't bother me enough dig into it.