Riddle me this

R Q

R Q

Well-Known Member
Joined
Apr 4, 2005
Location
Charlotte
'97 Chevy 3500 cab/chassis with dump body, 350 Vortec, 5 spd, dual tanks.
When I fill the tanks all the way, it loses power as the pedal is pushed about halfway, at about 3000rpm. As the fuel load goes down, it remedys itself.
I've changed gas caps and let them loose and tight. No other repairs for this problem but have replaced O2 sensors for other probs. No "Check engine light" on right now
Was thinking MAF sensor maybe. I do have a K&N filter and it has been cleaned.
 
Is there a filter in the tank? Sending unit sticking? Sounds like it's fuel tank related and those are about all I can think of to check.
 
CHeck the Evap lines comming off the tanks, you may be sucking in liquid fuel into the charcol canister when the tanks are full.

This can cause a flooded engine effect as the vapor are being sucked out of the canister on acceleration. ( this is what is supposed to happen to the vapors, but if it's raw liquid fuel, it's actually flooding the engine)

With there being 2 tanks, there could be an issue with the rear tank OVER pumping into the main casing the same situation.

Rear sending unit fault, or fuel tank switching module fault, ( you know the rear pump is good, as you have to fill the tank)

Try filling only the front tank one day, see if the problem persistes or changes at all.

THe fuel tank switching module is on the frame, just infront ot the fuel filter, it read fuel level in both tanks, when there is a 2-4 gal differance between the 2 tanks, it pumps fuel from the rear to the front tank.

Sometimes the sending units puke and the module reads the variance wrong and can over pump the front tank, this could send fuel into the evap system and flood the engine

I am surprised there isn't a check engine light
 
Check engine light came on today and the truck would barely get me home. I stopped at AutoZone and put the OBDII on it and it said low voltage to the #1 O2 sensor in bank #1. Gonna go check the manual and try to see which one that is. I replaced all O2 sensors two weeks ago.
Good info Blkvoodoo, that gives me some more things to check.
Out to the garage. Spoiled a trip I had planned tonight though.
RQ
 
R Q said:
. I stopped at AutoZone and put the OBDII on it and it said low voltage to the #1 O2 sensor in bank #1. Gonna go check the manual and try to see which one that is.
RQ
Click to expand...
that may be a symptom and not the problem, intake vaccum leaks along with several other things will throw O2 sensor codes as well, I'd be checking that charcol canister, Voodoo did have some good info.
 
I checked wiring to all O2 sensors, all good.
Pulled the charcoal canister out and it has no fuel in it and neither do the lines running to it and to the engine.
I'll check for vacuum leaks. What else could be throwing the codes?
 
Just checked vacuum leaks with propane and didn't notice any.
Wonder if my fuel pump is going south? I haven't checked fuel pressure. Not sure if my guage will work. I have one for a TBI motor. Will go and see.
Please keep comments coming.
Thanks,
RQ
 
Also, I didn't mention that it runs good when I first start it. Got about 15 miles this morning parked, then an hour later, go to next job and start having problems. It gets progressively worse from there. Sounds like a vapor lock doesn't it? I blew through the line at the canister going back to the tank and it wasn't clogged.
My pressure guage didn't work, I'll have to get one for the Vtec motor.
 
replace the fuel filter? Pull it out and pour it out from the tanks side, bet what comes out is nastier than your last shower.

then start saving yer box tops for a fuel pump (main tank), cuz it'll be going south soon after (get the whole fuel module, not just the pump, comes with new fuel sending unit, about $250ish at Carquest)

As for low voltage on the O2, check the wiring to the drivers front sensor (inside the plug as well) and for giggles, there is are fuses for the sensors inthe under hood power center as well, check those.

as for vacuum leaks, look at the front of the head on each side where the intake and head meet, if it is wet there that'll be coolant leaking, and there is a pretty good chance you have an air leak too (Vortec intake gaskets are shit ) Propane may not do much, but have had decent results with carb/brake cleaner along the same areas. ( propane disipate too quick for the slight leak you may have, where the fluid puddles and would be sucked in more readily)
 
Thanks Kevin,
I really appreciate your help. Had to stop for now but will get back into it tomorrow and the weekend. Will post results.
RQ
 
any chance right that right after you fill up...and then go for a quick ride...that when you take off the gas cap there's a big "whooshing" sound, and a decent amount of air built up in the tank?

Greg
 
What should the fuel pressure be? It's not in my manual.
Thanks,
RQ
BTW, I replaced fuel filter, it was dirty, checked for vacuum leaks with brake cleaner (none), double checked wiring to #1 and it looked good, and couldn't find fuses at either panel for the O2 sensors.
Looking for a fuel pressure guage tonight and will go from there.
 
I got a fuel pressure guage and have 50psi on the truck. Just for comparison, I checked my pickup with the same motor. It had 54psi.
I've read on searches that anywhere from 54-66 psi will work.
So does this pretty much sound like the fuel pump?
Thanks
 
Run it (locally if at all possible) see how it runs. Odds are yeah, it's the fuel pump, are real anxious to spend the coin for that ?
Best part is you have a dump bed, that makes swapping the pump assy cake !

I replaced same assy on a 1/2 ton pick up 2 weeks ago, whole deal was $255 in parts from CarQuest, including fuel filter, and fuel module (pump assy and level sending unit)

pump is available seperate, if your fuel gauge is working ok, you may want to go this route (cheaper)

Toughest part of the deal will be cleaning the sealer off the fuel sending unit lock ring (brake cleaner and a small screw driver)
 
RQ, when the Pump on my Zj went south, Anything other than barely touching the gas pedal it would act up and not get out of it's own way. I had teh fuel pressure checked and it was within specs at 50psi. Caver and I pulled the return line off thefuel rail, hooked a hose on the return port off the rail and ran it into a bucket. Cranked the engine in order to drain the tank and nothing came out. not enough flow to return anything.

AFter the pump swap we tried it again as a test and it shot fuel out teh return pretty good. That might be a test to try. not that I think it's a be all end all test, but might be an indication.

brent
 
Well the bitch acted up on my way in this afternoon. I checked the pressure and it was still at 50#. It did leak down pretty quick after I shut it off so the check valve in there may be bad too. I work close to home Monday and Tuesday so I'm going to replace the whole module tomorrow afternoon.
 
Update:
Today replaced the whole sending unit, pump, etc. Little over $300.00 due to no one having it in stock except the dealer and I had to chew him down from $450.00!
After the install I measured pressure and was still at 50-51 psi but went up to 60 real quick when I revved it. Also it did not leak down near as quick after I cut it off.
Guess we'll see tomorrow when I hitch up the trailer and go to work.
Thanks for everyone's comments and help.
RQ
 
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