I'm running an old tune that I have been running for years without a problem. Had a problem with my fuel tank, so I replaced it, which involved significant re-plumbing. First fill on new tank, and the idle now oscillates. Once its fully warmed up, the idle becomes steady, but is more prone to stall under load change (forward to reverse shift). 5 to 10 % throttle runs terrible. Cuts in and out. Once above ~10% throttle it runs OK, but sound lean. BLM under low throttle is around 112 or so. Above 10% BLM is around 90! Previously before tank replacement and this fill up, BLM ranged from ~120 to 135, not perfect, but reasonable. Even though it sounds and acts lean, throttle response it fairly normal, no bog. A long hard acceleration does not run out of fuel, so I think the flow is OK. Can't measure fuel pressure at the moment, because I am on a trip, about 200 miles from home and have no resources. One other symptom is that it took the engine forever to warm up. Temp finally made it to 195 and stayed there, but it seemed to take about 5 times longer than normal to get up to temp.

So I guess the real question is, any ideas what is going on? I don't know if this could be bad gas, fuel pressure/flow issue, or some other ECM malfunction. Unfortunately, if it is bad gas, I won't know for another 400 miles.

how much gas is in it?

have you checked fuel pressure?

idk man, if it took longer to warm up then i would suspect the opposite of running lean. If it was running lean it would run hotter. Do you have any codes? Try resetting your ecm by disconnecting battery cable (negative) for a few minutes. Run some fuel injector cleaner through it too. It could also be the weather effecting how it is running. It is a computer controlled vehicle.

Thanks, I did drive it back home. Put gas in it on the way, and it still ran the same. The first chance I will have to look at it will be tomorrow - crazy busy season. I do have evidence of high fuel pressure. The return line blew holes in it on 2 different occasions. The first time I blamed it on 28 year old rubber fuel lines, but now I think those old lines were aggravated by too much pressure.

If it does turn out to be my return line is plugged, I still think the ECM is over compensating. This thing has headers and glasspacks, so you can easily hear how it runs - very uneven crack to the sound with a lot of high frequency content to the bang. Normally when an engine runs rich, the extra fuel dampens out the higher frequencies, so what we hear is that fat sound of midrange and low end. This has a very brittle thin "distorted" sound. I assume the "distortion" is misfires. I'll report back tomorrow.

You may have a situation where the ECU ccannot remove fuel as it is at a BLM that can only remove 30% of fuel NB02 sensor sees. A 90 BLM is maxed out. You may need to reduce FP or change your VE table settings.

Ok, so I got underneath it and dropped the tanks over the weekend. I did in fact find that I was using the wrong return port on the tank. It has a check valve in it. Both the pickup tube and the return tube look identical, even when removed from the tank, however one of the tubes has a check valve. Whodathunk. When I installed the tank, I just chose the lines by what was most convenient by the hose routing, as both tubes looked identical. So, the return line was closed off, resulting in very high fuel line pressure. I never did check how high the pressure was, I hope I didn't stress the pump or the injectors. I've not had a chance to test drive it yet, but I would assume that its fixed. The tanks are marine tanks, so I don't know if that has anything to do with why it has a check valve. Maybe they figure boats sit for long periods of time, so they want to prevent drain back.

I'm not driving a 3rd gen vehicle, but I could see someone replacing their fuel tank with a marine unit, as they are cheap and come in all sorts of sizes and shapes. Be great for a custom street rod.