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possessed fuel pump on '96 hardbody vg30e engine and bad mileage

Hi, I've been having a weird problem with my truck. The fuel pump will turn on and off at random times with the ignition off. Most often, it is after I have just turned off the truck. Once in a while I am in the vicinity of the truck and hear the fuel pump just turn on. The span that it is on is usually about 10 to 30 seconds. The truck runs great and I've never had a problem with it not starting, or stalling, or running rough. However, my fuel mileage is crap. I get

18.5L/100km or less than 13MPG right now and it seems to be a little short on power.

To give some background... I had the check engine light come on, checked the codes, got the code for the oxygen sensor and replaced the oxygen sensor, which fixed the check engine light. I recognized that the sensor was just a symptom. I had a manifold leak, which I got fixed and thought that was the cause of the oxygen sensor misreading. There was a lot of carbon on the sensor and in my exhaust, probably cause I ran it for about a year before deciding to shell out and fix the manifold and oxygen sensor. I thought that fixing the manifold and oxygen sensor would fix my rich running problem. It didn't. Now I suspect that maybe my fuel pressure regulator is broken causing too high of fuel pressure and therefore causing a rich mixture, which will eventually lead to the oxygen sensor going again. So, I'm going to check the fuel pressure, and also the catalytic converter backpressure so make sure it's not plugged with carbon a little (exhaust does kind of smell worse than it should).

However, I want to know if the fuel pump going off at random times could be a factor in this. As I understand, the fuel system stays pressurized even with the vehicle off. If the pressure regulator is broken and the pressure drops with the truck off, is there a sensor that will turn the fuel pump on to bring the pressure back up to par? Will the fuel relay be a problem in this case, or is it maybe a short somewhere? Maybe the fuel pump going on with the truck off is normal.

Anyways, any insight into why my fuel pump has a mind of its own and if this could in any way be a cause of my bad fuel mileage would be greatly appreciated.

Thanks in advance, Victor

Reply to
Victor
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My hardbody used to make an ocaasionaly whining noise from the gas tank area ocassionaly when the truck was parked. I thought maybe it was the fuel bleeding back into the tank. I never gave it any thought that the fuel pump maybe on. It did it for the life of the truck and I always got 19mpg city or highway.

Reply to
Rob Munach

Ok, as a follow-up, I checked the fuel pressure which I thought was the problem. It's supposed to be 43psi without vacuum attached to the pressure regulator and 34psi with vacuum attached. Instead it was 46 and 36psi respectively. I don't think this is enough of a problem to be resulting in my 150km less per tank than I should be getting. I checked the ECU in mode 1 and the exhaust gas sensor was alternating between lean and rich like it's supposed to. In mode II red and green lights flashed simultaneously like they should indicating the mixture is being regulated fine. No error codes... no nothing. When the fuel pump turns on while the ignition is off, the fuel pressure goes up to 10psi, but this is no problem I don't think.

It runs fine, but seems to be lacking a bit of power.

I guess next thing is checking to make sure all my cylinders are actually firing, although I would think that a cylinder not firing would result in a rich reading somewhere.

Thanks for the comments so far, Victor

Reply to
Victor

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