[Stephan282]

I am chasing a engine stumble/no start problem (covered with another post). I bought a fuel pressure test kit but the kit dies not come with a suitable adapter to fit the threaded tap (Schrader valve) at the front of the fuel rail. Checked with my local Auto parts store to no avail. Can somebody provide a source or part number? Thanks

 

[TimeCapsule]

I am not sure if the v8 and v12 fuel pressure ports are the same. With the cheap HF tool set, I can easily connect and test pressure on mine.
Can you post a picture?

 

[Stephan282]

Here is a photo of the port. The cap is taken off (15mm wrench)

 

[TimeCapsule]

The port appears to be the same as on the v12.

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This is the set I've got and this is the hose I used:

 

[Stephan282]

I have the larger set from HF ($89). It does not contain this hose. And the largest fitting towards the fuel rail valve is a 14mm...

 

[Redrodent]

Anyone have trouble getting the cap off? Mine is frighteningly tight and there’s no place for a backup wrench. I’d much rather destroy the cap, then crack the fuel rail

 

[Stephan282]

OK- replaced the pressure tester with the one suggested above. Same piece of junk - weeping fuel from the gauge but at least I got the right connection hose. Used a different gauge, shows 50PSI with ignition on and holds it after ignition is turned off. However: This morning after a short trip (5miles) the engine stumbled after re-starting it. Turned off engine and removed the fuel cap with a big "woosh". After that the engine started up fine. I did the same every time before I started the engine this morning while running errants ("woosh" every time) and it fired up without hesitation. So I believe TimeCapsule is spot on with the fuel tank vent issue he suspected.How do I fix that? Where is the check valve located and is that the issue?

 

[TimeCapsule]

Sorry to hear that. Carbon canister on my coupe (and I believe all w140/c140) is in the rear drivers side fender well. Plastic liner/cover must come off.
The tank venting system components, besides the canister that can be easily checked and replaced are the purge valve in the engine bay and the little L-shaped one-way valve (don't remember the name) that is just outside the tank, on the line to the carbon canister.
Make sure the lines are not collapsed or plugged.

Good luck!

PS On the early cars at least, there is no monitoring of the EVAP system, while for the later, OBD-II compliant cars, there is a check procedure that involves applying slight pressure and monitoring the drop over a specified period of time. When the latter check fails, the infamous P044x code(s) is(are) showing up

 

[Stephan282]

Thanks a lot! So I started my investigation and here is where I stand so far:
1. Fuel pressure at rail with ignition on, engine not running: 50PSI, slightly higher with engine on idle.
However, turning off engine will see pressure drop immediately to 45PSI, then rising again to 55PSI and then stable
2. I have pressure in the fuel tank, not vacuum. This becomes obvious when unscrewing the fuel cap. Pressure is building in the tank only after a few minutes of engine running .
3. Checked purge valve: Disconnected incoming hose, no pressure ("Woosh"). Checked hose to canister connection with vacuum/pressure tester (engine off): open connection, no vacuum, no pressure. Checked purge valve function: OK

So I guess the issue is somewhere between the tank and the canister. Vent valve?

Any suggestions?

Thanks again

 

[Stephan282]

I accessed the charcoal container in the left rear wheel well. I disconnected the incoming line (from the gas tank) and checked with my vacuum/pressure tester. No built-up of either vacuum nor pressure (should I not see pressure building due to the vent valve? Looks like the valve is always open). Then I connected the tester to the inlet of the container - same result. And yet I have pressure in my gas tank. So as it stands I have open lines from the vent valve to the purge valve. What am I missing? Any suggestion?

 

[TimeCapsule]

A little pressure (don't know what MB allows, for Jaguar it is 2 psi) is normal. Vacuum or high pressure are not.
I think you have access to the diagrams, but this is what is at play:

If the tank is over-pressurized, that means that the venting is not working properly. But this should not have a connection to engine running and stumbling. My recollection (and the reason I brought the EVAP system up) is that applying vacuum to the tank is what causes the maladies.

I am assuming you've already checked - is the vacuum line from the intake manifold to the purge valve (Y58/1 on the diagram) in good condition? Attached and free of cracks?

 

[Stephan282]

Thanks! Yes that is what I thought: Vacuum in the tank creates an issue, not pressure. I checked the purge valve: With engine at normal operating temperature, I hear the ticking and at the inlet I feel a vacuum (suction). So I assume that valve is ok. I have not yet checked the vacuum line to the manifold but will do so today. In any case: Since I first wrote about the issue and investigated the cause, the problem did not show again. I probably have to wait for higher ambient temperatures. Can fuel vapor in the fuel rail be an issue?

 

[TimeCapsule]

Can fuel vapor in the fuel rail be an issue?

If your fuel pressure test shows that the system holds pressure, then, I think it is unlikely that fuel evaporates in the rail.
The problem is intermittent is my experience with those is that they are, 99% of the time, electrical.

Sorting out the EVAP will not hurt anything, but it appears that the problem is elsewhere.

Good discussion, I enjoyed it.

 

[Stephan282]

That's what I am afraid of.....Thanks and also thank you for your valuable input!