markandorra

Hi all

Merc 190e 2.3 16v

When at idle or in drive, I can hear a rhythmic thud or kind of pressure release noise coming from what seems to be inside my fuel tank. It is very rhythmic every 5 seconds or so, and makes no difference if the car is moving or stationary.

I had it down at the local mechanic and he informed me that he has no way to check as there is no access to look into the tank.

Related or not, I also have a slight irregular idle problem once the car is at working temperature. It kind of occilates between 900 and 1100 revs, and only happens sometimes and is temperamental. Furthermore, if I don't keep-up a bit of a light rev when putting it into reverse, the car can cut out easiliy. Same goes for when I rev it high, and then allow the revs to settle, sometimes they get too low and before they can recover back to 1000rpm the car cuts out. To start it again, can sometimes take quite a few turns of the starter (even though it always starts first time when cold).

When the car is cold it starts first time without any issues.

Had the plugs checked, Idle valve cleaned, relay and fuse checked, and now I am running a quality fuel cleaner through the system to see if that helps.

So, I'm not sure if the weird pressure release type noise in the fuel tank has anything to do with my idle problem, or if they are in fact related.

Any thoughts appreciated.

MJ50

moved to W201 forum...

BenzBopper

I've heard that some MB fuel tanks have some type of supports that keep the tank firm. Those supports come loose and make weird noises. It may not be your issue but it may be a place to start.

dolucasi

I have taken out a fuel tank at the JY one time. Other than the 4-5 bolts that secure it to the trunk bottom and firewall to the rear seat there is no other support structure. The tank has a large rubber gasket where it rests on the trunk and of course the filler neck has a gasket.

I'm guessing the issue here is that the tank pressure system is stressed here because there is some issue with excessive pressure in the tank and that is being released by the fuel filler cap.
Wild guess here as this has never happened to me in 33 years. I would first check the low hanging fruit. I assume you already checked if the hissing comes from the filler area.
Have you taken the cap off to see if it changes the hissing sound. Also does that change your idle?

Also the idle of 900-1000 is a bit high for M103/M102 engines. But yours is a 16V so might be different. If the idle behaves differently/strangely when warm, that means there is an issue in the fuel delivery system in closed loop operation. Could be many things but first give us feedback on what happens when you run with the gas cap off.

I suspect the fuel cleaner will have no effect on this.

Also, always state the mileage and condition of your car, it matters.

- Cheers!

190Efan

It does sound like it's some type of fuel issue. The gas tank is pressurized but it's also designed to alleviate pressure in hot driving conditions. Make sure you're only using premium level fuel. It's also possible that you got a bad tank of fuel that's causing your fuel system to clog which is working its way through your entire car, hence the idol issue. It's also possible that there's some carbon build-up going on given the age of your car.. Another easy thing you can do is take your car out for a good highway run periodically stomping on the accelerator pedal and blow out the carbon build up. I used to watch my engineer father do this on a pretty regular basis with our cars when I was growing up to deal with carbon build-up issues. You might also inspect your engine wiring to be sure no wires are frayed and no connections are dirty or loose. If you do these things and still have a problem, then it'll be time to look deeper. I was always taught to start with the really easy stuff first and then keep working your way through until the issue either reveals itself or is found in further diagnostic work. .

markandorra

HI, thanks for taking the time to respond.

Firstly the car is in very good condition for its age (1987). It has covered 205k km (127k miles).
I only just bought it a few months back and with little use so far. It has been restored to a large degree so I can only assume good wiring and fuel lines.

I have taken off the fuel cap while idling, however the pressure release noise does not change in any way. Neither does this affect the idle.
The sound is similar to a hydraulic valve when releasing pressure, but muted slightly (probably because it is happening inside the tank) and is definitely coming from the fuel tank area. The mechanic said that the fuel pump and filter seem to be fine.

I can only assume some type of back pressure in the fuel system that is accumulating in the tank, then being released. But released where? because there is no obvious sign of air or fuel leaking from anywhere.

I did read somewhere about an additional carbon fuel filter on these cars up near (behind) the front wheel arch, however I do not know if this could be related if indeed this was blocked for example.

Thanks

dolucasi

If you removed the cap and there is still hissing sounds coming from the tank area, that can not be coming from the tank IMHO. With the gas cap removed the tank is at atmospheric pressure

There are elements that are under pressure in the plumbing near the fuel pumps/filter/accumulator. Maybe pressure is release somewhere there.

The carbon canister under the driver's side front fender appears to be pretty benign. I have never replaced mine and the car has 205K miles.

There are various check valves in the fuel system and one or more of those could be failing you. One is on the driver fender near the charcoal canister. Others in the fuel pump plumbing.

markandorra

After looking at a schematic of the fuel system, it could be the vent valve (Breather Valve) located just under the fuel tank. After all it is a valve, and subject to pressure one would assume. I'm wondering if this can malfunction and discharge pressure...

Or there is pressure coming more from downstream in the fuel system that is causing a back pressure that manifests itself near the tank area.

I notice that there is a "Siphon Breaker" inside the tank. However, I assume that access to anything physically inside the tank is impossible? Do these tanks come sealed and pre-manufactured parts inside?

dolucasi

There is one final check valve on one of the hoses going into the fuel tank. However, since you removed the filler cap and nothing changed (meaning the hissing and the pressure release still happens) it is hard for me to imagine pressure is building up in the tank itself.
The tank does not have anything built in, everything is removable from outside (filler cap mechanism, strainer, fuel level sender are all removable)

Also, since the car is relatively new to you, do not overlook the possibility of someone inserting one of these check valves backwards by mistake.
I have an extra set of plumbing (for under the car), I can take some pictures to see how it compares to yours.

But I believe you will need to jack up the rear and have a look near the pumps as the car is running. I would recommend a ramp, instead of jack stands for this. It would be safer.
Your local auto parts stores have lightweight ones these days, they are very reasonably priced.

- Cheers!

markandorra

thanks
You could be right about the check valve being installed incorrectly. The car was re-painted and to a certain degree re-built. It was not done by mercedes.
I was thinking to take it down to another mechanic I know and to ask him to look around while the car is up on the proper ramps.
Will report back with findings.
thx