Fuel tank pressurized

[RenaissanceMan]

Running an S52 w OBDII conversion in my E30. Whenever the fuel tank gets close to empty, the tank gets pressurized enough to let out about a 7 second continuous burst of air when I unscrew the fuel cap to refuel. Car is running flawlessly. It has been so long since I completed the conversion that I can't remember if a relief valve was part of the system. Where should I start in terms of diagnosis? TIA-

[roguetoaster]

On a standard E30 it should vent via the expansion tank and then through to the charcoal canister in the engine bay, although this also connects to the throttle body. On E36s the vent line connects to the same type of valve, but exits at the throttle body before the throttle plate.

[RenaissanceMan]

On a standard E30 it should vent via the expansion tank and then through to the charcoal canister in the engine bay, although this also connects to the throttle body. On E36s the vent line connects to the same type of valve, but exits at the throttle body before the throttle plate.

Thanks for the reply! So is the valve on the E36 just a simple one way valve, or is it electronically actuated?

[roguetoaster]

They're both actuated based on some sort of criteria.

[jbontke]

Not sure what year yours is, but my 36 325e has a line form the fuel tank to the evap canister, then a one way check valve from the evap canister to the intake manifold. No electronics. I would check and see if your line is clogged.

[RenaissanceMan]

Thanks for the replies. Took a while for work to slow down enough to pursue further. So when I did my swap I kept the motronic controlled pressure valve in the circuit. The later e30s didn't have a one way valve, there is a 2 lead wire that opens and closes the valve. The Bentley says I should be getting battery voltage across these pins with the ignition on but I'm only getting 5 volts. Tested the valve and it does open with 12 volts applied. So I'm assuming my issue is the valve not opening and subsequently allowing pressure to build. Now need to figure out why the voltage is so low. Any other feedback is welcome!

[roguetoaster]

Later E30s definitely have a valve.

[RenaissanceMan]

Here is a pic of the valve type I have:

Can anyone confirm I should be getting 12v to this solenoid instead of 5v? Thanks-

[Northern]

Valve looks more or less right.

ETM makes it look like it should have 12V from the main relay and have a switched GND at ECU Pin 68 (1998 E36 ETM pages 92/93)

should have 12V whenever the car is on, and can check the GND side between the connector/ECU connector

[RenaissanceMan]

Just want to make sure I'm understanding circuit correctly. With key on i am actually getting 12v from the red/white wire pin if I ground to the chassis. So I am assuming the issue is the ground to the brown wire pin. According to the ETM the brown wire goes to pin 68 at the ECU. So where does pin 68 get ground from? Just the mounting point of the ECU? If so then the problem would lie somewhere along the path of the brown wire, correct?

[e30austin]

if you have the purge valve hooked directly from the fuel tank, to the inlet manifold, without the charcoal canister in between, that is 100% your problem. the manifold vacuum will put pressure on the fuel tank, which is not good.

i usually just let the fuel tank vent to atmosphere, in the engjne bay, with a small, in-line filter (think air cooled VW or the like) at the end of the line.

[RenaissanceMan]

E30austin, I have the vent line from the tank going to the valve and then to the canister and from the canister to the inlet on the manifold. My issue is that the purge valve is not getting 12v and subsequently not opening when it actuated. Trying to figure out why it is only getting 5v. My swap is obd2 and I would love for that part of the system to work properly.

[Northern]

any chance the connector is swapped with something else?
Not sure of keying on connectors, but it's just a two pin JPT like the IAT (which is 0-5V) and some other stuff under the intake?

[RenaissanceMan]

any chance the connector is swapped with something else?
Not sure of keying on connectors, but it's just a two pin JPT like the IAT (which is 0-5V) and some other stuff under the intake?

That's a great thing to check but wouldn't the car run erratically if that was the case? I would have also thought something like that would have resulted in throwing an OBDII code... I labeled everything pretty comprehensively when I did the swap, but I will certainly double-check. However, isn't the fact that I get 12v when I ground the plug to chassis an indication that 12v is intended for the plug? Still trying to make sure I understand the behavior of the switch.

[Northern]

Just want to make sure I'm understanding circuit correctly. With key on i am actually getting 12v from the red/white wire pin if I ground to the chassis. So I am assuming the issue is the ground to the brown wire pin. According to the ETM the brown wire goes to pin 68 at the ECU. So where does pin 68 get ground from? Just the mounting point of the ECU? If so then the problem would lie somewhere along the path of the brown wire, correct?

The ECU switches pin 68 to GND to activate the valve, I don't know what the logic is for the ecu to turn the valve on/off and I wouldn't bother trying to check for ground at the pin, just check resistance between the GND pin and pin 68 of the ECU connector.

If the plug is correct, and the continuity check is good, I'd look for blockages in the lines either visually or by popping the gas cap off and trying to blow air down the lines to the tank