Water In Gas?
 

I have heavily modded 90 Vert. The car has run great in the past but I have been fighting an erratic performance issue. The car acts as though it is not getting spark or fuel under a load. Some days it will run great but the next time it is driven, which may be several weeks, it will stumble and run as though only 5 plugs are firing. It throws no codes. I have sent the ECM out for inspection and I have purchased another ECM with no change in performance. I removed the MSD and replaced the plug wires with no change. I have put a direct reading fuel pressure gage on the fuel rail and all worked as advertised. I am next heading to collect a fuel sample to see if it contains any water. Has anyone had experience with water in their fuel? This situation is driving me nuts.

90Indy

Seems quite possible if car has been sitting

If it was water it would never have the day it runs great. Put 2 bottles of Heet in. It can't hurt. Use the regular Heet not the Heet that also cleans injectors. I once got bad gas in the mountains of Colorado (not corvette) and it took 4 bottles of Heet to get it to run like normal. Dan

Have you changed the fuel filter?

Have you put a quality FP gauge on it while driving taped to the windshield so you can see what happens when it falls down?

Have you physically looked at the fuel sending unit?

Do you have original injectors?

FP Regulator?

What do your plugs look like. Rich, lean, wet, oil? If a car runs like crap, often the plugs will give you a hint.

If you really think the fuel is bad, don't fill it up. Suck it all out or run it empty and then open it up to see if there is anything in the tank. Right now, I'd check the pressure to see if it holds pressure and if it doesn't, maybe the pulsator in the tank needs to be replaced?

Like someone said, maybe injectors? Have you had them tested lately? What about fuel pressure? Is it delivering when it comes to WOT?

Distributor cap and rotor are wear items that can cause intermittent rough running. It won't throw a code either. It can foul plugs too.

If you think you have water in the tank , clean out the small drain tube in the area below the fuel cap... I always blow this tube out with an air hose in the spring just to make sure it works.. If this tube clogs up, water will fill up the area around the fuel cap and get in the tank...WW

Bingo.

The number one cause of water in the fuel is right here :iagree:

I think a side effect of my blocking the evap canister is that the tank always has positive pressure which may keep water out.

Doubtful. Especially if its cold.
In the event it starts raining, then water fills up the area around the fuel cap and inevitably ends up leaking into the filler neck through the vented cap or around the filler neck oring, or both. Your fuel cap must be vented allowing air in the tank but not out.

Good point