Purge valves and carbon canister
Discussion
Hi,
Been experiencing some rough running again on my '97 4.2. There could be a number of reasons for this, and I'm considering all things (will be ordering some iridium plugs soon
).
My question is about the valves and hoses that make up the purge system. The little hoses that connect the fuel breather, inlet manifolds and drivers side airbox via numerous connectors and the carbon canister thing near the fuelpump in the passenger side rear wheelarch.
What does all this stuff do, and if I took the inlet manifold hoses out of the equation (by running a hose between the two manifolds only), would this be bad? I suspect I'll just try it and see what happens, but curious if anyone (mikesr?) had any opinions?
This purge system is meant to be blocked off during engine setup, which leads me to think its not a fundamental thing. Possibly a nod to emissions?
Been experiencing some rough running again on my '97 4.2. There could be a number of reasons for this, and I'm considering all things (will be ordering some iridium plugs soon
).My question is about the valves and hoses that make up the purge system. The little hoses that connect the fuel breather, inlet manifolds and drivers side airbox via numerous connectors and the carbon canister thing near the fuelpump in the passenger side rear wheelarch.
What does all this stuff do, and if I took the inlet manifold hoses out of the equation (by running a hose between the two manifolds only), would this be bad? I suspect I'll just try it and see what happens, but curious if anyone (mikesr?) had any opinions?
This purge system is meant to be blocked off during engine setup, which leads me to think its not a fundamental thing. Possibly a nod to emissions?
First the disclaimer 
I'm no expert on the Cerbera. I'm a software developer by profession (hence the work I did with Trackcar to crack the MBE ECU) Most of the useful info below is from discussions with Trackcar and my subsequent experimentation on the car. Hopefully, one of the real experts will along to give us the official story but here goes anyway.
The setup as I understand it is: -
Thee are two inlets to the carbon cannister.
One is supposed to be open all the time. This has the breather pipe from the fuel tank connected to it via the rollover valve.
This allows air into the fuel tank such as when fuel is being used. It also allows air out of the tank when needed (eg the fuel tank getting warm in the sun and the air expanding). It vents through the fuel carbon cannister so that the fuel vapour isn't allowed to vent into the amsphere. However, the rollover valve can get blocked and the fuel tank doesn't vent properly which can lead to poor running. (Documented in my infamous "Sucking and Blowing" thread.
The vented vapours are burned off when the engine is running as described at wikipedia
Its the second hose to the carbon can that allows the fuel vapours to be burned off. This hose connects to the carbon can via the purge valve and is only open when the ECU applies power to it. This hose runs to the 4 way connector that sits under above the coil packs.
2 of the remaining 3 connections go one to each inlet manifold. The fourth goes to the off side air box catch can.
Why is the 4th hose to the air box needed?
We already have connections into the inlet manifolds. Surely they will be sufficient to draw air in through the carbon can and pull any fuel vapours into the inlets to be burned?
The biggest problem with the hose to the air box is that its only on one side of the engine and must upset the air flow at near closed throttle running.
Trackcar does a mod to remove this hose to the air box and block off the inlet and replace the 4 way with a 3 way.
This plus a decent rollover valve should reduce any rough running due to fuel tank not venting and only one side of the engine getting air via the carbon can. It is still (I think) MOT legal with respect to emissions compliance. Although technically still a modification with respect to insurance.
The other mod (which I haven't got around to yet despite having a catch can for nearly a year) is to vent the camcase breather (the big bore pipe) into an oil catch can, rather than into the air box, and vent the catch can to atmosphere via a filter.
This is not emissions compliant and would (should) fail an MOT. But it stops the oil sludge getting into the airbox which can only be good for near closed throttle running.
Another tip from Trackcar. Don't bother blocking the purge pipe when balancing the throttles. Do it with the car in running order. ie with airboxes and the like connected.
Do a static balance of the airflow for each inlet using a synchrometer and drilled screws in the butterflies and then balance the airflow of the two banks. Then put the airboxes back on and use the technique described in Faisals Cerbera manual which is Trackcar's (nee Joospeeds) way of balancing the throttle pots.
I wouldn't dispense with the carbon can connection to the inlet manifold hoses unless it does improve running. I think it would likely produce a fair bit of smell from fuel vapour in the carbon can.
HTH
mikesr
I'm no expert on the Cerbera. I'm a software developer by profession (hence the work I did with Trackcar to crack the MBE ECU) Most of the useful info below is from discussions with Trackcar and my subsequent experimentation on the car. Hopefully, one of the real experts will along to give us the official story but here goes anyway.The setup as I understand it is: -
Thee are two inlets to the carbon cannister.
One is supposed to be open all the time. This has the breather pipe from the fuel tank connected to it via the rollover valve.
This allows air into the fuel tank such as when fuel is being used. It also allows air out of the tank when needed (eg the fuel tank getting warm in the sun and the air expanding). It vents through the fuel carbon cannister so that the fuel vapour isn't allowed to vent into the amsphere. However, the rollover valve can get blocked and the fuel tank doesn't vent properly which can lead to poor running. (Documented in my infamous "Sucking and Blowing" thread.
The vented vapours are burned off when the engine is running as described at wikipedia
Its the second hose to the carbon can that allows the fuel vapours to be burned off. This hose connects to the carbon can via the purge valve and is only open when the ECU applies power to it. This hose runs to the 4 way connector that sits under above the coil packs.
2 of the remaining 3 connections go one to each inlet manifold. The fourth goes to the off side air box catch can.
Why is the 4th hose to the air box needed?
We already have connections into the inlet manifolds. Surely they will be sufficient to draw air in through the carbon can and pull any fuel vapours into the inlets to be burned?
The biggest problem with the hose to the air box is that its only on one side of the engine and must upset the air flow at near closed throttle running.
Trackcar does a mod to remove this hose to the air box and block off the inlet and replace the 4 way with a 3 way.
This plus a decent rollover valve should reduce any rough running due to fuel tank not venting and only one side of the engine getting air via the carbon can. It is still (I think) MOT legal with respect to emissions compliance. Although technically still a modification with respect to insurance.
The other mod (which I haven't got around to yet despite having a catch can for nearly a year) is to vent the camcase breather (the big bore pipe) into an oil catch can, rather than into the air box, and vent the catch can to atmosphere via a filter.
This is not emissions compliant and would (should) fail an MOT. But it stops the oil sludge getting into the airbox which can only be good for near closed throttle running.
Another tip from Trackcar. Don't bother blocking the purge pipe when balancing the throttles. Do it with the car in running order. ie with airboxes and the like connected.
Do a static balance of the airflow for each inlet using a synchrometer and drilled screws in the butterflies and then balance the airflow of the two banks. Then put the airboxes back on and use the technique described in Faisals Cerbera manual which is Trackcar's (nee Joospeeds) way of balancing the throttle pots.
I wouldn't dispense with the carbon can connection to the inlet manifold hoses unless it does improve running. I think it would likely produce a fair bit of smell from fuel vapour in the carbon can.
HTH
mikesr
Thanks both for your replies, Mike, I'm also in software for an American company
Good explanation of the system, although I may adopt Windsor's more simplistic approach and see where that gets me to start with. I'll leave the rollover valve and all that alone though, so there should be no safety concerns.
Have a good weekend!
Good explanation of the system, although I may adopt Windsor's more simplistic approach and see where that gets me to start with. I'll leave the rollover valve and all that alone though, so there should be no safety concerns.
Have a good weekend!
SimonSparrow said:
Thanks both for your replies, Mike, I'm also in software for an American company
Good explanation of the system, although I may adopt Windsor's more simplistic approach and see where that gets me to start with. I'll leave the rollover valve and all that alone though, so there should be no safety concerns.
Have a good weekend!
Absolutely. The KISS principle is always a good one to follow whatever the type of engineering Good explanation of the system, although I may adopt Windsor's more simplistic approach and see where that gets me to start with. I'll leave the rollover valve and all that alone though, so there should be no safety concerns.
Have a good weekend!
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