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.
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.
Last edited: