Rough Idle, smell of gas in exhaust, less power than normal... water in gas seems wrong diagnosis

So here’s the back-story:

I filled up my car at the local gas station. It had been raining non-stop for the past three days and nights. 10-15 minutes later as I’m stopped at a light, the car starts shuddering, almost stalls a few times. Rough idle all the way home.

I limp home, guessing water in the fuel. Next day I go to Advance Auto and buy some fuel additives which are supposed to help with the water problem. The guy there mentions that he’s had a number of reports of similar issues from this particular gas station (it’s now out of business, go figure). Additives make things better but not fully.

2 tanks of gas later now and the rough idle is much better but not gone. Car seems to have less power and sounds a little bit raspy on acceleration (not noticeable unless driving near a wall which can bounce the sound back). My guess is that one of the cylinders is not firing, maybe needs a new spark plug?

The more worrisome thing is that as the car has started running more smoothly I’ve also noticed a smell of gas after startup (may still be there when running but I don’t notice it then) – coming from the exhaust. This seems to be in-line with the missing cylinder theory but I’m running blind here as I don’t have much experience on this topic.

The car’s a 1997 C280. 130K on it. It’s old but mechanically the thing has been a tank so far (Inline 6 I think) – very sturdy and responsive. I’m hoping that this whole ordeal hasn’t done permanent damage.

Any insights would be greatly appreciated.

When were the spark p;ugs and wires changed?

If there was water in the gas, you may not have it completely out yet. I’d consider having the tank dropped and flushed. Water will easily separate from the gas and collect at the very bottom, since water is more dense. It is probably underneath the fuel pick-up point, but still enough in there to continue to mess up the fresh fuel you keep adding. Gasoline will still absorb a certain amount of moisture, and it will dramatically affect the burn characteristics.

Once the tank is clean, consider a fuel system flush and a spark plug change. That should clear everything completely up for you.

Check engine light on? Codes?

Water in gas will be at the bottom of the tank. Easy enough for a shop to drain off some gas and examine it. If you’ve been fueling often at this station the fuel at the bottom of their tanks probably questionable and sounds like you got quite a bit.

By adding the fuel treatments you may have loosed stuff up in the tank and the fuel filters may be clogged or having less power, a fuel injector and sensors trying to compensate. But bad gas is bad gas and you got a full tank of who knows what plus water.

I would say the odds are that engine will run better over time without major work just get the fuel system cleaned get the bottom of the tank drained as mentioned above posts. “dropping” your tank will be very expensive… and only a last resort.

If you still have low power after two tanks, I’d suspect that your fuel filter is partially clogged from whatever junk was in the gas (besides any water). If you plan on keeping the car for a while, it might be a good idea to change the fuel filter, do a fuel pressure test, and change the plugs if they’re anywhere near due.

Thanks for all of the feedback guys. I’m bringing it in tomorrow.

@oldtimer 11, plugs and wires haven’t been changed for a while.

@mleich, light is on but I don’t have an OBDII adapter so I’ll see what the garage says.

I’ll update this thread with the outcome. In the meantime I take it that you guys think this is a relatively simple fix? Anyone want to wager on what it’ll turn out to be?

Update (sorry oldtimer, can’t figure out how to call you out here. Tried @oldtimer%2011, @oldtimer_11, quotes, everything.) Anyone know how this works?