Ghost in the Machine...

Biggun · Jun 2, 2020

454cid said:
Some trucks have a pressure sensor on the fuel pump, that looks a lot like a MAP sensor.

So, would that sensor be on the pump inside the tank or on on top? If there is one, then the harness I do have must contain the wire that talks to the PCM.

454cid said:
When buying a fuel pump you have to get one with the right code.

Good Lord. Well, I have no idea...the original pump had been replaced already when I got the truck...could Chevy find out by VIN?

RichLo said:
Still haven't done anything with the EVAP system???

Yes. I replaced the purge solenoid and the gas cap. My truck doesn't have another solenoid on the charcoal canister, it just vents to atmosphere. I'd say all of the lines and connections are good, since it pulls (and holds) enough vacuum to suck your face off.

AK49BWL said:
Although the tank pressure sensor being disconnected is a pretty big red flag

someotherguy said:
I can vouch for the tank pressure sensor not being used on the 3500's as well. No harness there for it, just for the pump/sender connection.

someotherguy's picture looks exactly like what I see on my truck; there is no other harness that plugs into the fuel pump. That said, it would make sense that if it has a purge solenoid (which it does), then it must have some sensor to trigger the PCM to open the solenoid.

stutaeng said:
Doesn't EVAP require a tank pressure sensor? How is the ECU supposed to know when to vent to atmosphere or open purge solenoid for gas vapors from tank to be sent to intake for combustion?

I had an F-150 where the purge solenoid was sort of opened randomly by the computer and did not have a tank pressure sensor. I mean to say, it used some sort of computation to figure out when to open the solenoid, but it wasn't based on information from a tank pressure sensor.

I guess I could just leave the gas cap cracked open a little...I've driven 300 miles this week like that, and no CEL. It just bugs me because it's not right, and now I'm really curious as to what I've got going on here. I guess I'll try to find some diagrams of the different permutations of fuel/EVAP wiring/plumbing for these trucks.

As always, thanks for the input!

stutaeng · Jun 2, 2020

someotherguy said:
From the mid-90's and older, there were no tank pressure sensors on GMT400's of any GVWR spec. They all have EVAP systems though. 94 was first year for solenoid control of the purge valve. How those systems operate year vs. year, beats me; never needed to look into it.

Richard

Yes, indeed. I was helping my buddy wrench on his '90 Chevy truck and noticed it had the charcoal canister under the hood. By coincidence, we were chasing a bad fuel pump and removed the bed and didn't see any tank pressure sender. I should have taken a picture.

It's funny how people don't even know their vehicles have these systems, unless they fail, LOL.

OP, did check into the vent solenoid? Maybe just do a simple continuity check on it with DVM?

Erik the Awful · Jun 2, 2020

EVAP systems were an OBDII requirement.

The GMT-400s had the charcoal canister (and vent solenoid?) under the hood, and the components hardly ever failed. When they moved them to under the trucks in the newer generations they seem to fail more often.

If you're the type of person who has to click the gas handle a few times and get as much gas as you can in the tank, stop it. When the EVAP canister is mounted lower than the fill nozzle, you're not topping off your tank, you're filling the EVAP canister with gas. When I was a technician I changed a lot of EVAP canisters that were full of gas.

someotherguy · Jun 2, 2020

Erik the Awful said:
EVAP systems were an OBDII requirement.

If you're the type of person who has to click the gas handle a few times and get as much gas as you can in the tank, stop it. When the EVAP canister is mounted lower than the fill nozzle, you're not topping off your tank, you're filling the EVAP canister with gas. When I was a technician I changed a lot of EVAP canisters that were full of gas.

It is indeed why the pump instructions say, "do not top off"

Richard

454cid · Jun 2, 2020

Biggun said:
So, would that sensor be on the pump inside the tank or on on top? If there is one, then the harness I do have must contain the wire that talks to the PCM.

It's on top of the pump.

Good Lord. Well, I have no idea...the original pump had been replaced already when I got the truck...could Chevy find out by VIN?

Yeah, the dealership should be able to figure it out. They'll probably want to sell you the pump, Though. You may also get a pretty good idea yourself, buy looking at pump manufacturer websites, and looking at the specifics of each pump.

stutaeng · Jun 2, 2020

Erik the Awful said:
EVAP systems were an OBDII requirement.

If you're the type of person who has to click the gas handle a few times and get as much gas as you can in the tank, stop it. When the EVAP canister is mounted lower than the fill nozzle, you're not topping off your tank, you're filling the EVAP canister with gas. When I was a technician I changed a lot of EVAP canisters that were full of gas.

Thanks, I always wondered why that was. In all honesty, I've done this on more than one ocassion on my '99 Silverado, and I've never had an issue on that truck...

I thought there was a check valve that prevented liquid fuel from entering charcoal canister. Of course, anything can fail and go unnoticed, LOL.

Biggun · Jun 4, 2020

Erik the Awful said:
EVAP systems were an OBDII requirement.

If you're the type of person who has to click the gas handle a few times and get as much gas as you can in the tank, stop it. When the EVAP canister is mounted lower than the fill nozzle, you're not topping off your tank, you're filling the EVAP canister with gas. When I was a technician I changed a lot of EVAP canisters that were full of gas.

I'm not.

HotWheelsBurban · Jun 4, 2020

stutaeng said:
Thanks, I always wondered why that was. In all honesty, I've done this on more than one ocassion on my '99 Silverado, and I've never had an issue on that truck...

I thought there was a check valve that prevented liquid fuel from entering charcoal canister. Of course, anything can fail and go unnoticed, LOL.

Yes I had to replace the evap. valve on the 06 Denali. Since the spare tire was down, it wasn't hard to get to once I jacked the body up on the trailer hitch. What made it a 2 hour job instead of 15 minutes is the "quick disconnect " fitting on the line. I still don't know how I moved it to finally get it loose. Since then I haven't "topped off" any of our vehicles. When the pump nozzle shuts off, it's full. Texas used to have a vapor control system on gasoline filling hoses that was very obvious. I think they still have it, it looks different though.

2001ZR2 · Jun 5, 2020

alpinecrick said:
What the snarf is that? Did somebody stick a marshmallow in your filler?

No that is anti siphon ball swollen so I could only fill via the vent tube. Of course the Canadians haven't answered my emails.

alpinecrick · Jun 5, 2020

Erik the Awful said:
EVAP systems were an OBDII requirement.

If you're the type of person who has to click the gas handle a few times and get as much gas as you can in the tank, stop it. When the EVAP canister is mounted lower than the fill nozzle, you're not topping off your tank, you're filling the EVAP canister with gas. When I was a technician I changed a lot of EVAP canisters that were full of gas.

Aye yai yai, now he tells me......

My 96 and early 97 have the canister under the hood and well above the fuel tank, my 01 Sierra has the canister next to the fuel tank. I have always overfilled my fuel tanks to the brim. If the evap system stops working doesn't it throw a code?

Ghost in the Machine...

Biggun

Newbie

stutaeng

I'm Awesome

Erik the Awful

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someotherguy

Truly Awesome

454cid

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stutaeng

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Biggun

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HotWheelsBurban

Gotta have 4 doors..... Rawhide, TOTY 2023!

2001ZR2

OBS Enthusiast

alpinecrick

I'm Awesome

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