Runs rough after changing valve cover gaskets
 

I changed valve cover gaskets on my 1991 560SEL the other day and now the car runs poorly. Idle seems ok but acceleration is very rough.

I'm wondering if one of two things I did may have caused this. First, I had to disconnect the two fuel lines that pass over the valve cover, the main line and the one going to the pressure regulator.

Second, the vacuum line going from the pressure regulator to the valve cover breather hose broke some time ago. I had plugged the opening on the breather hose to prevent oily gas from discharging but I never did anything with the the end coming from the pressure regulator. While replacing the valve cover gaskets I decided to plug that too.

Is there any reason either of these things could cause sudden poor performance? I did check all of the spark plug wires and they seem ok.

Of course, I don't expect a diagnosis and I'm no mechanic, I'm just wondering if there are any common issues here.

fjpish, the 'vacuum line' you reference is actually a vent line for the fuel pressure regulator. If the regulator fails leaking gas is sprayed into the breather hose instead of around the engine compartment, possibly causing a fire.

So I guess it wouldn't affect the idle if it was plugged temporarily.

Is there any reason that disconnecting and reconnecting the fuel line at the pressure regulator would cause it to operate incorrectly?

Air in the line or something?

Check your spark plug wires. I would suspect that you crossed a couple.

Valve cover gaskets provide an internal engine vacuum seal.

Pull your dipstick sometime when the engine is running and it will probably die.

fjpish, plugging the fuel pressure regulator vent line won't affect engine performance. Verifying the spark plug wires are routed properly sounds like a good idea.

I don't know if this is causing the problem, however I would fix this. It sounds like the Positive Crankcase Ventilation system, that is designed to suck the blow-by gasses to the intake and burn them. Blow-by gasses are created when some of the ignited air-fuel mixture gets around the piston rings and into the cranccase rather than the exhaust. If you plug the holes, the blow-by gasses will come out elsewhere, which could be the intake where fresh air is supposed to enter (breather element or air filter on some cars) or may blow a seal, like the crankshaft's front or rear seal, and exit that way, creating an oil leak and a much more costly repair. Not to mention the pollution it produces instead of being burned in the engine.

Thanks, Andras. The PVC hose isn't plugged. The hose has a small branch that goes to the fuel pressure regulator in case of fuel overflow. See Ferdman's first reply on this thread.

Nice of MB to wrap up the ignition wires so I can't easily trace them individually from the distributor to the plugs... I guess I have to check them from both ends with a continuity tester.

I usually mark the ignition wires before removal with white nail polish.

I wish I had checked first. I know that the little 4-wire clip on the passenger side valve cover had them in the correct order. I didn't pay attention to the driver side.

The vent outlet from the pressure regulator - if sealed - will prevent the regulator from working properly. It mechanically compares fuel pressure to air pressure, using the vent hole. The reason for connecting to the breather tube is to prevent fire danger if the diaphragm ruptures. It must see some pressure close to 1 atm or it will not control properly.

Steve