Robbie3015

On my E500, last January, the check engine light came on. Took it in and I was told it was a software update. Did it on the spot. No problem. About a month later, it came on again and the gas smell after fueling that had always somewhat lingered in the car, got consistently heavier, so there was something wrong. Dealer advised to replace the fuel sensor for ~$700.00. I wanted to check things myself first. Opened the thing up. Sensor head and small rubber hose were OK, but there was fuel in the recessed body area. Removed the sensor (you don't really need the special tool for the lock ring, just two strong hands and a good grip) and it turned out the gaskets were deteriorating. Went back to dealer, (in the meantime, you would hear the vacuum pump frantically trying to pull a vacuum and gascap warning lights were coming on). Dealer replaced both the seals, no more gas smell ($300.00).

Now after 1200 miles light comes back on again. I checked things myself first. Sensor is still good, no cracks. There is a tiny bit of fuel (looks old and oily) in the recess, but that might be from the repair, I do not smell anything in the car and I don't hear the vacuum pump anymore. Brought it to the dealer. It has been there now for more than 2 days. The service manager says they get the same fault codes, but they can't find anything, The system seems to be leak proof, car runs great. They are scratching their heads, I am scratching mine. Anybody seen something similar and maybe has some advice here?

paulv

The EVAP monitor runs every drive cycle providing all the prerequisite conditions are met (all cars are somewhat different), so you could realistically drive several hundred miles before the monitor runs. When the monitor fails (usually after 2 drive cycles), the CEL is set. You will most likely not have any driveability problems with a failed EVAP system (unless the purge valve is stuck open causing the air/fuel ratio to go very rich).

EVAP systems are complicated and many technicians dread trying to fix them. Considering valves, carbon cannister, wiring & connections, and all the vacuum tubing, the problem could be anywhere.

When you said that the "sensor is still good", how did you [electrically] test it?

Regards,
paul..

Robbie3015

What I meant by sensor is still good is that the sensor does not have a leak at the connector or is cracked anywhere else. Thanks for your info. Dealer just called me and they say that the fueltank is cracked where the cables are fed through the tankwall. He says a $2,200 repair, including labor. Not happy!