Van performance problem related to gas level...

[The Dartman]

I have a 1996 Dodge Ram Conversion van that I use to cart the family around and occasionally haul the old Mopar to the track. I have been experiencing what I assumed to be a problem with bad gas recently, and wondered whether anyone has had trouble like this.

Here's exactly what its doing:

On a full tank of gas the van's 318 V8 feels like its ready to go smoke some 5.0's.

Between 3/4 and a 1/4 tank the van seldomly has trouble under normal accelerating, but occasionally has some noticable performance loss under heavy accelerating. This loss is noticable but irregular. It acts like the van won't rap at all (like a rev limiter but no stutter).

At 1/4 tank the van starts to have problems accelerating. No real pops or backfires, and van will accelerate fine at 1/4 throttle and terrible at a 1/2 or more (like a rev limiter again). The van idles fine, and accelerates normally under normal driving conditions.

1/8 tank and under: The Van will not accerate under 1/2 or full throttle at all. When pedal is here, the van will slowly accerate and than top out until the accelerator is backed off. Still accelerates fine under "Grandma" driving. It is here that the motor also pops and backfires ocasionally under acceleration. Sometimes even driving down the road the motor will run so rough at 65 that the entire van will shake.

Again, this looked to me to be an almost clear cut fuel problem at first. I've added heet and I have run ethanol (my owner's manual tell's me this is great) in an effort to absorb any water in the tank. With these not really helping, last night when I ran all the gas out of the tank that I could (and tried to syphon the rest). I then filled it with about four gallons of amoco silver. This put me at less than an 1/8 tank. At this time the van still had a noticable power loss under heavy acceleration. I then filled the tank full, and went looking for 5.0's again.

I suppose it is possible I did not get all of the "bad" gas of the tank, but I really think I need to look elsewhere for the problem.

Could I have a pinhole in the pump sucking air when the tank gets low? Could I have a bad vent line or some other problem like that? Is the fuel pump trying to go bad? Do I still have bad gas in the tank? Could someone have put sugar or some other foreign material into my tank?

Let me stress the problem is DEFINITELY ralated to the level of gas in the tank. This much I do know. If anybody works at a parts store or dealer, could you get me a "ballpark" estimate on a new fuel pump? I've seen them run anywhere from $75 to $500 for just the part.

Thanks!!

[moparsofcamelot]

I know this sounds to easy and cheap, but it's worth a try. Fill it up and drive it to your 1/8 tank with the gas cap removed. If it doesn't give you a problem, then you have a vent problem where the pressure in the tank is competing with the pump. Hope it doesn't rain during this test.

[Christopher]

You need to check the fuel pressure also.This pump you have has the pressure regulator mounted on the pump,so that could be a problem also.On the other hand,you need to make sure that the canister purge valve is not sticking or not operating at all.If it sticks,it will cause vapors to build up in the tank(more noticeable when fuel is low)causing loss of fuel pressure.A pump runs about $200.00

[The Dartman]

OK - its got a full tank now, so I've got the rest of this week to get ready to try these two things.

I have one question, though. Where is the cannister? And will I be able to check this valve manually and free it up or do I need to do some other test?

When I tried running the gas out Sunday, I did have the gas cap off for a short while. I don't know how long or when though.

[Christopher]

If I recall correctly,it's under the van on the left side.The purge valve may be mounted on the firewall or on the side of the engine compartment.I'll look in the service manual when I go back to work.

[moparsofcamelot]

Dartman, I gave a possible solution but failed to mention that if it worked it was probaby a clogged charcoal cannister. I didn't notice that I left this out till Christopher posted his reply. Duh.