I noticed remarkable under pressure in fuel tank when opening tank cap when engine running. Something is probably blocked ? How should air exchange happen ? Not through cap ?

 

Hi and welcome to the forum.

You are not suppose to open the fuel tank cap while the motor is running. Will usually toss a CEL and could cause a vapor lock (just a guess).

 

Do you think there should be under pressure ? by the way what is CEL ?

 

CEL is Check Engine Light
and it's operating as it should, fuel systems have a pull AND return line
I'm not sure about volvos, but many manufacturers actually dump a little exhaust into the tank to displace oxygen

 

With my bad english I mean negative pressure. Original problem is CEL and can't find the reason.

 

negative pressure sounds like a clogged return line to me

 

Why dont you start by saying what kind of volvo;turbo or non turbo, mileage, if A CEL is on get the codes by the obd1 or if 95 and up obd2 in a local auto zone and start from there ;-)

 

When I had a cracked fuel line hose above the tank, whenever i'd turn the ignition to on (engine is still off), there'd be a huge whiff of fuel vapor if I had the windows down.

I assume the system needs pressure to operate as designed I guess.

 

An 850 should have a slight vacuum applied to the fuel tank by way of the charcoal vapor canister and the purge solenoid.
The purpose of the vacuum is to capture all fuel vapors into the canister and at higher RPMs the vent solenoid opens which creates a slight vacuum leak pulling stored vapors out of the canister and into the intake to be burned.

There are times when you shut down the engine and the underbody heat is enough that you may have a pressure in your tank but it is not the average condition you will encounter. What sets that CEL (Check Engine Light) is if you do remove the cap with the engine running as the computer thinks that you have a leak in the hydrocarbon recovery emissions system.

Stripes, Who is it that introduces exhaust into the fuel tank ??

 

Thanks for information, as I understand now, negative pressure is ok, when engine running. Is it possible that some other way this canister is malfunctioning and lights CEL (PO172 code)? (tank cap is closed, maf is new, o2 sensors are new, can't find any air leaks)

 

Is the car turbo?

 

PO172 code - engine running rich? Usually points to a bad MAF, or cracked vacuum hoses. You are saying the Maf is new so I am stumped.....

 

Every time I had that code I had a vac leak. You have to look real close to the elbows. IPd usually send silicone to people as a replacement for the rubber vac elbows.

 

No turbo. I can't find any leak, but of course there can be. After shutting down the light it comes again after 50-500 kilometers.

 

If you're looking for reasons for tripping an engine rich code you should also take a look at the fuel pressure regulator vacuum line. If the diaphragm inside the regulator ruptures then the vacuum line will draw raw fuel into the intake. Take the vacuum line off the regulator and see if you notice or smell gas. Also look at the nipple of the regulator where the vacuum hose attaches to see if it's wet. If you notice the sight or smell of any gas then the regulators diaphragm has failed and needs to be replaced.

It's one of those things that can be easily overlooked.

 

As long as I'm taking a shot in the dark, is the air filter in good shape too ??

 

Thanks, I check the regulator next. Air filter is good.

 

Seen that code today on a car and the intake tube was 1/2 off the intake. Gross leak. Seeing that your code comes back later, you likely have s small leak.

 

Regulator seems good, no leaks.

Before, the engine made slightly different sound after startup for a while. I think it has something to do with evap-system. Now I don't hear that sound. Does this thing in the picture have something to do with it or what is this ?