Fixing the fuel tank vacuum issue
Discussion
Fixing the fuel tank vacuum issue
I bought my Tuscan mk1 around a year and a bit ago now and it came with a hole drilled in the fuel filler cap. Suspicious I thought but not a big issue. I bought a new fuel filler cap and after driving a few miles got the characteristic vacuum problem that a lot of TVRs report where when you release the fuel cap you hear a hiss of air being drawn into the tank and a boom as the fuel tank walls 'pop' back into their original shape. Not great.
So to troubleshooting...
Starting at the carbon canister I wanted to make sure each connection was doing what it should. I used a brake bleeding tool (e.g. Laser 8018) just to test whether various pipe connections where going to atmosphere or against a stop/blockage where a vacuum could be generated.
The carbon canister located in the front driver's side wing hatch has 3 connections as shown in the diagram. The purge valve is normally closed and only opens under certain conditions when the engine is up to temp etc and is controlled by the ECU (I don't know the exact conditions but they can be found elsewhere for the MBE ECU) and is pulse width modulated.
Connecting the brake bleeder to the vent line with the purge valve fitted gave me a vacuum with and without the filler cap fitted. I should have not been able to pull a vacuum with the filler cap removed. So I knew I had a problem further toward the fuel tank.
Next step was to locate the roll over valve which is tie wrapped to the passenger side B pillar. I removed that and found it was blocked. Rust came out of it when I tapped it against a hard surface. I tested the lines to the fuel tank and to the carbon canister with some low pressure air (bike pump) and both were clear. I then replaced the roll over valve with a Mocal TRL7 making sure the arrow pointed upwards as per the instructions. This now meant that the line was clear right through from the fuel tank to the carbon canister middle input connection.
After that I then removed the purge valve solenoid and tested that with a 12V battery. Unfortunately that was blocked too and wouldn't operate. It looked as though it was blocked with black carbon from the canister over the years. I replaced this with a used a purge valve from a Ford Fiesta C1B19G866AA which came with an attached plug lead. I used an alternative valve as I couldn't easily find an exact replacement unit. The Fiesta valve bore is slightly larger as compared to the original (8mm vs 6mm I think). This replacement has been mentioned elsewhere and the only hassle I found was having to change over the connecting plug. I changed the old rubber hoses in this area to accommodate the slightly larger purge valve tubes, no real issue. I checked the wiring to the plug was OK and tested the valve operated fine when 12V was applied. All sorted. I also checked the line to the manifold was clear too.
Finally, went for a decent drive, got back home and released the fuel filler cap and no hiss! Job done. The only thing I haven't checked is that the ECU pulse width modulation is present. Any thoughts? Should I be able to feel the valve vibrate as the valve opens/closes quickly when the car is up to temp with a few revs applied? Or best attach a scope?
Anyway, no petrol smells in the boot anymore, less petrol smells in the garage on the whole and peace of mind that another job is ticked off. Hope this helps others.
I bought my Tuscan mk1 around a year and a bit ago now and it came with a hole drilled in the fuel filler cap. Suspicious I thought but not a big issue. I bought a new fuel filler cap and after driving a few miles got the characteristic vacuum problem that a lot of TVRs report where when you release the fuel cap you hear a hiss of air being drawn into the tank and a boom as the fuel tank walls 'pop' back into their original shape. Not great.
So to troubleshooting...
Starting at the carbon canister I wanted to make sure each connection was doing what it should. I used a brake bleeding tool (e.g. Laser 8018) just to test whether various pipe connections where going to atmosphere or against a stop/blockage where a vacuum could be generated.
The carbon canister located in the front driver's side wing hatch has 3 connections as shown in the diagram. The purge valve is normally closed and only opens under certain conditions when the engine is up to temp etc and is controlled by the ECU (I don't know the exact conditions but they can be found elsewhere for the MBE ECU) and is pulse width modulated.
Connecting the brake bleeder to the vent line with the purge valve fitted gave me a vacuum with and without the filler cap fitted. I should have not been able to pull a vacuum with the filler cap removed. So I knew I had a problem further toward the fuel tank.
Next step was to locate the roll over valve which is tie wrapped to the passenger side B pillar. I removed that and found it was blocked. Rust came out of it when I tapped it against a hard surface. I tested the lines to the fuel tank and to the carbon canister with some low pressure air (bike pump) and both were clear. I then replaced the roll over valve with a Mocal TRL7 making sure the arrow pointed upwards as per the instructions. This now meant that the line was clear right through from the fuel tank to the carbon canister middle input connection.
After that I then removed the purge valve solenoid and tested that with a 12V battery. Unfortunately that was blocked too and wouldn't operate. It looked as though it was blocked with black carbon from the canister over the years. I replaced this with a used a purge valve from a Ford Fiesta C1B19G866AA which came with an attached plug lead. I used an alternative valve as I couldn't easily find an exact replacement unit. The Fiesta valve bore is slightly larger as compared to the original (8mm vs 6mm I think). This replacement has been mentioned elsewhere and the only hassle I found was having to change over the connecting plug. I changed the old rubber hoses in this area to accommodate the slightly larger purge valve tubes, no real issue. I checked the wiring to the plug was OK and tested the valve operated fine when 12V was applied. All sorted. I also checked the line to the manifold was clear too.
Finally, went for a decent drive, got back home and released the fuel filler cap and no hiss! Job done. The only thing I haven't checked is that the ECU pulse width modulation is present. Any thoughts? Should I be able to feel the valve vibrate as the valve opens/closes quickly when the car is up to temp with a few revs applied? Or best attach a scope?
Anyway, no petrol smells in the boot anymore, less petrol smells in the garage on the whole and peace of mind that another job is ticked off. Hope this helps others.
Interested in the purge cable solenoid - do you have any pictures / description as to where this lives? I'm guessing it's down by the headlights / bulb change access panel?
You also mentioned changing it to an alternative valve - would you have a link / part number to that?
Would it be the Vauxhall one? Part 55566514:
You also mentioned changing it to an alternative valve - would you have a link / part number to that?
Would it be the Vauxhall one? Part 55566514:
Edited by Malcster on Wednesday 14th August 10:08
Yes it's by the headlights / bulb change access panel on the driver's side. Very cramped area, I did find I could unclip the cannister to gain better access to the hoses but could never manage to unclip the line that went to the roll over valve. Too scared to break it!
I went with the Ford Fiesta C1B19G866AA one as the original valve had a different connector that I found hard to find. I did see a valve with what I thought was the correct connector for the loom side but it had a right angle for the input/output rather than going straight through. So I found a second hand valve that came with the loom side connector attached. That way if/when I need to change the purge valve in the future I know it's only going to be cheap and there will be plenty around.
The valve you found "looks" the same but I can't say anything about the connector. There are lots of similar but different valve connectors between the different manufacturers. If you have that part to hand then it might be worth finding the correct plug connector and trying it out.
I went with the Ford Fiesta C1B19G866AA one as the original valve had a different connector that I found hard to find. I did see a valve with what I thought was the correct connector for the loom side but it had a right angle for the input/output rather than going straight through. So I found a second hand valve that came with the loom side connector attached. That way if/when I need to change the purge valve in the future I know it's only going to be cheap and there will be plenty around.
The valve you found "looks" the same but I can't say anything about the connector. There are lots of similar but different valve connectors between the different manufacturers. If you have that part to hand then it might be worth finding the correct plug connector and trying it out.
Gassing Station | Tuscan | Top of Page | What's New | My Stuff