Facelift Purge Valve

I have a 1996 Jaguar XJS with the AJ16 engine.

Still working on woosh valve, I think the Rochester Valve was backwards.

I’m having potential trouble with the purge valve? The one by the charcoal container…when is it supposed to open? From the looks of the electrical diagram, when the key is on?

Mine seems to be bench testing fine when I apply 12v to it, but when I put it on the car, with the key on, it remains closed? I put a multimeter on the harness, with the key on, and I got 11.77v.

So, it seems to be getting power, and it seems to work when I apply 12v of power, but I can’t get it to work when the car is giving it 12v.

Check the harness voltage with an old fashion test lamp instead of DVM. Lamp will put a load on the circuit, whereas the DVM will not. If the lamp lights brightly you should be good to go.

I think I found what I needed. According to a service manual:

The fuel tank can be filled to approximately 90% of its capacity. The additional 10% of volume allows for expansion of the fuel without escape to the atmosphere.

When the engine is switched off, the fuel tank pressure is maintained at a positive pressure of 1.0 to 1.33 psi by the pressure control valve (aka Rochester valve). Pressure above 1.33 psi is released by the valve to the charcoal canister.

When the engine is running, manifold vacuum acts on the pressure control valve, which opens the vent line from the fuel tank to the charcoal canister. Air enters the charcoal canister and flows to the tank to replace the fuel delivered to the engine and maintain atmospheric pressure in the tank.

If the pressure control valve fails, the fuel tank cap will vent the fuel tank pressure at 2.0 to 2.5 psi.

When the canister purge is enabled, the ECM meters purge flow to the intake manifold through the normally closed evaporative emission control (purge) valve (EVAPP). Canister purge is enabled by the ECM based on engine coolant temperature only when closed loop fuel metering control is operational.

Purge Flow Monitoring for OBDII
The ECM detects purge flow in two ways: if closed loop fuel metering correction indicates a large movement toward lean when purging is enabled, or if the idle control valve corrects for increased air flow when purging is enabled, the ECM has confirmation that purging is taking place.

Evaporative Emission Control Valve (EVAPP)
The new, redesigned evaporative emission control (purge) valve is a normally closed pulse width modulated valve. The amount of valve opening (and canister purge flow) is determined by the ECM drive signal allowing the ECM to accurately control purge flow for the prevailing engine operating conditions.

What I think is happening, is that the Rochester valve I have installed is not releasing pressure below the threshold of the fuel tank cap, so the tank is being exhausted into my trunk, hence the fuel smell. The whoosh I hear is the 2.0-2.5psi thats still trapped in the tank.

When I pulled the Rochester valve, I turned it around because it was indeed much easier to blow out of the CAN side than it was from the TANK side. I reattached the vacuum hose. The problem could be that if the vacuum fails, I may starve the fuel tank of replacement air.

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This is why i replaced mine with electric valve (runs off left o2 sensor 12v). My new Rochester valve failed after only 2 years. One day while driving got a nice pop from tank (cavitation). Luckily it did not bend the metal inwards.

I also have a 3psi relief valve in parallel, for when car sits. 3psi is a good value, it would take parking in the hot sun, imo, to open valve. With valve closed, fumes stay in tank as it cools down.

Search archives, i posted photos and such when i did this a year or two ago.

I did just that. I bought the same valve as you as well. I’m copying everything but the 3psi relief valve since the tank cap has a 2-2.5psi relief valve on it….although it’s not clear whether it’s a positive pressure relief only?

Anyways, the electric valve should free up any negative pressure built up by driving?

Thanks for your response! And the excellent write up!

My 1988 did not come with relief cap. Interesting.

Yes, you should get no negative pressure, which is bad for tank and fuel pump.

Positive pressure isn’t that bad really, and unless you’re parking outside in the middle of Australia in the summertime, shouldn’t have a problem.

And i misspoke, the electric valve closes when engine is off, so no gas smell in garage, even without 3psi valve.

I guess i could have forgone 3psi relief valve too, if i bought relief cap. But i like my old fashioned looking cap.

The only question I still have is why doesn’t the purge valve actuate with 11.49v running to it?

I plugged in the replacement valve you used (which has the same pigtail) and it also doesn’t actuate! It would be very convenient to tap into that wire, since it’s right there!

I pulled a pair of wire into the engine bay. Will tap into the 02 heater wire this weekend. My wiring chart shows that it does get 12v with the key on, but then varies from 0v-6v “swing with engine running”?

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You sure you tapped into correct o2 wire? I’m using the one for o2 heater, which is always 12v. The 02 sensor wire will report 0-5V to the ecu?

I haven’t tapped that wire yet.

Please take a look at the attached wiring chart narrative.

Page 63.pdf (340.1 KB)

Page 64.pdf (838.0 KB)

Yours is a facelift, correct? Do you have four o2 sensors? Totally different from mine. I would maybe find another 12V source.

Yes, and I do have four sensors.

I found a white switched wire going to the my headlight relay in the engine compartment! Perfect?

Page 4.pdf (424.5 KB)

I also found a White with Red Stripe that says B+ when Active, but nothing when inactive. Description of circuit is simply, “Ignition Power Supply” to the ABS Control Module.

ABS Diagrams.pdf (455.6 KB)

Thanks for your continued help.

Your car is quite different. Just make sure it’s 12V with ignition on. My headlights can be turned on with ignition off, so not a good choice for me.