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Throttle Response Issue *Resolved*

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Wascally Wabbit1

Drag Racing Champion
Well the more I look into this the more I feel the evap system is in fact the root cause of my throttle issues. All the symptoms seem to point to this. For example, the purge valve only opens once the engine is up to operating temp. I only encounter throttle issues after the car has warmed up. The issues are worse when it's hot out. Capping the system the issue seems to go away but you cannot cap this system very long or you are essentially capping the vent to your gas tank once the car is warm. I am not sure what the long term effect of capping the system would be but it doesn't sound smart. I have dropped the hose size from the evap system (N80 valve) to the throttle body and the symptoms are greatly reduced.

My theory is I am having a similar issue with the evap system that I had trying to add a pcv valve to my current setup. I am simply pulling too much vacuum through the system when first applying the accelerator which in turn is pulling too much fuel vapor into the intake and causing issues with the MAP sensor trying to adjust fuel flow.

I was under the impression changing the intake manifold wouldn't change the amount of vacuum through the system as cylinder size doesn't change but where it does change is on the amount of air entering the system coming off idle which causes a sudden spike then sudden drop in manifold vacuum. This is what was causing me to pull so much oil through the valve cover when trying to fit a pcv. When I put clear tubing on the pcv and hit the gas and watched the vacuum gauge there would be a big spike before the pressure dropped and oil would gulp out of the pcv like it was being pumped out. I think I am having the same effect now on the evap system where right off idle I am getting a sudden spike in pressure, and with it fuel vapor, then a sudden drop in pressure hence the oscillating rpm.

So the goal is to offset the increased vacuum through the evap system when coming off idle with a smaller diameter hose. I fitted a new N80 valve today and have reduced the vacuum line down to 3/16". So far it seems to be working well. When it is hot out, like today was in the 80's, I am still getting a slight rev coming off idle at times (generally after sitting at a light idling for a length of time) but the rev is much smaller. And so far the car has played somewhat nice now for several days. I am going to try 1/8" hose as I have some lyin around and see what that does.
 
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W

Wascally Wabbit1

Drag Racing Champion
Well I reduced the vacuum line down to 1/8" and so far no codes. The little N80 valve was clattering away when I checked it so it's still doing it's job. It was almost 90 here today so if it was going to act up today would be the day and it did a little but still much more tame then yesterday and yesterday was still tame as well but it was on the fence as far as driveability. I feel pretty strongly the evap system is the root cause and if this doesn't get a handle on it I don't think anything will. I think I will likely go back to the stock valve cover as well and run the PCV. The car definitely runs better with the PCV hooked up. No question about that. I have a brand new valve cover in the garage sitting around anyway. Thanks for all the help folks.
 
W

Wascally Wabbit1

Drag Racing Champion
So interesting in that the issue is still there. But reducing the size of the evap vacuum line has made the throttle less touchy so it is not so hard to drive. The throttle now seems more like a normal throttle and I actually have to give it some gas to go. So now instead of only having the top 1/3 of the pedal travel as usuable (ie after 1/3 down there was no change on acceleration. You were floored) now I have about 3/4 of the pedal travel. So floored is almost floored now. This has made low speed driving and feathering the clutch a dream. But if I am sitting idle and it is hot out I will still get an initial rev to 1500 rpm what when starting in first.

Long story short I think I have 2 issues here. One is the initial rev issue and the other is the fluctuating rpm issue. I no longer seem to get the fluctuating rpm. I thought before they were related but it seems perhaps they are not. The rev issue I think may be from a built in function I think VW has where it automatically revs to keep the car from stalling. I was playing around and I can get the car to rev without touching the gas just by letting out the clutch. I have read about this on the GTI and Audi so I would assume that same feature is on the golf. There is no fix other than changing the driving style to give a little more gas when using the clutch. And maybe it's a symptom of the lower end torque loss. Don't know but it's hard for me to drive this way. I am used to being able to start off with no gas...which is how it should be. So I don't know there is a solution. And why only when it is hot out...who the #$*! knows. But I am going to see about maybe bumping the rpm at idle up 100 rpm or so and see if that make a difference. Maybe then there won't be such a fine line. Maybe then I will have a less bitchy car.
 
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M

MLue1

Drag Racing Champion
Wascally Wabbit said:
So interesting in that the issue is still there. But reducing the size of the evap vacuum line has made the throttle less touchy so it is not so hard to drive. The throttle now seems more like a normal throttle and I actually have to give it some gas to go. So now instead of only having the top 1/3 of the pedal travel as usuable (ie after 1/3 down there was no change on acceleration. You were floored) now I have about 3/4 of the pedal travel. So floored is almost floored now. This has made low speed driving and feathering the clutch a dream. But if I am sitting idle and it is hot out I will still get an initial rev to 1500 rpm what when starting in first...
Click to expand...
Playing Devil's Advocate here, it sounds like you have a open line letting in extra air into the system down stream of the N80 valve, by reducing the line to the TB to 1/8 you are effectively metering the opening. Assuming everything besides the opening is correct, it would seem when ever the N80 valve switches it would cause an over compensation.

If you can find a friend with another Golf 2.5, you can use a vacuum gage to pull a vacuum (with the car off) on the lines to and from the N80 valve.
 
W

Wascally Wabbit1

Drag Racing Champion
MLue1 said:
Playing Devil's Advocate here, it sounds like you have a open line letting in extra air into the system down stream of the N80 valve, by reducing the line to the TB to 1/8 you are effectively metering the opening. Assuming everything besides the opening is correct, it would seem when ever the N80 valve switches it would cause an over compensation.

If you can find a friend with another Golf 2.5, you can use a vacuum gage to pull a vacuum (with the car off) on the lines to and from the N80 valve.
Click to expand...

Actually. I have a vacuum pump. If I create vacuum on the line pre-N80 and it doesn't hold I know there is a leak. However I would think I would have thrown a code. Also, the N80 controls the flow through the line and measures vacuum which is how it detects system health. When I capped it threw a code pretty quickly.
 
W

Wascally Wabbit1

Drag Racing Champion
Well I am putting this one to bed. I have decided the rev issue is in fact related to driving style. If I get the rpm's up a bit before the clutch hits a certain point no rev. Problem was if I did this before the car would rev anyway just from touching the gas and I would get terrible clutch chatter. Before I could not get started in first without clutch chatter. Now I cannot get the clutch to chatter. So I can actually give it a little gas without an issue when starting out. It will take a little getting used to but I think with the throttle being so much less touchy it is easily doable.

On to my next challenge. OEM emblem back up camera install.
 
W

Wascally Wabbit1

Drag Racing Champion
Final word. Almost 100 degrees out today and the car is running great. All I have to do is blip the throttle a little if I have been sitting at a light for a while. Pretty happy that this issue is finally behind me.
 
W

Wascally Wabbit1

Drag Racing Champion
Final, final word. I ended up replacing the ECU as the issue would come back eventually no matter what I did. I am fairly certain this is why the previous owner sold the car. So far so good.
 
G

GroovyVW

Passed Driver's Ed
I will be doing an OEM backup camera install as well soon. So updates please [emoji1]


Sent from my iPhone using Tapatalk Pro
 
V

vermontmk6

Passed Driver's Ed
I have 2013 Manual trans mk6 and I get the oscillation, most of the time it's when I come to a stop and push in the clutch (RSR built) and come to a stop. Revs up to 1800, down to 1200, up to 1600, then evens out at 900 or whatever. I have keyless start, trying to figure out if this is worth figuring out how to do with my setup, thanks.
 
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