OK, my son has been driving the Blazer to college and he tells me that for the last few weeks, when he fills it up it will start fine, pull away from the pump fine but when he tries to accelerate in to traffic it just won't accelerate for about 10 seconds. Doesn't backfire or stall but just won't go. It only does it when you put gas in it and then it's OK after that. It's worse when you fill it up but also does it to some extent any time you add gas. I put a new AC Delco pump on it about a year ago so I doubt that's it (although I understand it can't be ruled out yet). He's bringing it home this weekend so I'll run a pressure test on it but I'm thinking it's probably EVAP related. He did scan it for codes and it's not throwing any. If anyone has some ideas about what I should be looking for, feel free to point me in the right direction.

 

What an interesting conundrum.

Let's think this through, adding gas causes the vehicle to mis-behave temporarily. She starts OK, she runs OK, but when you fill it up she gets balky...the act of introducing fresh fuel seems to be causing the problem. What is it about adding the fuel??? The fresh flood of fuel could be stirring up sediment in the tank, which could be temporarily plugging the intake screen on the pump (Guess #1) - did you flush the tank when you replaced the pump??? (read somewhere that somebody had major sludge and rust inside their tank), has the fuel filter been replaced (not a pleasant task but do-able) anytime recently??? Ruling out the fuel tank possibilities, the next thing(s) I would suspect are fuel pressure regulator (when was the last time that was serviced (replaced), clogged / leaking fuel injector(s), dirty intake plenum (all possibilities).

The act of pressing the accelerator simply opens the throttle body valve to allow more air into the engine. Sounds like your fuel delivery may not be keeping pace with the flood of air or the fuel delivery system is "dumping" too much fuel into the system temporarily flooding the engine causing it to lag. I would suspect the fuel pressure regulator ($64.00 +/- @ AutoZone). Could also have vacuum leak(s). Please let us know how you make out. Good Luck.

 

Sounds to me like guess #1. I've heard of this problem with other people to differing degrees before.

 

Sounds like it could be the vent valve on top of the fuel pump housing. The EVAP system pressurizes the tank to check for EVAP emissions. Does your son turn the truck off during refueling?? Some people leave the car/truck running and if the EVAP system is performing a system check it might cause a problem during/after fill up. When you scan the computer also check the pending codes that are stored in memory...these will not turn on the MIL/SES but will still let you know what might be happening...

 

Thanks for the replies. The vent valve was my first thought also and the techs at work seem to think thats it too. I did check all the lines and cleaned the tank when I changed the pump and I always change the filter on a regular basis. Of course I'm ruling nothing out because you never know what's in the gas you put in. He checked for codes and it's not throwing any (yet) and nothing pending. I read a post on one of the forums a while ago and I seem to recall the problem being the vent but I can't find it now. When I get the truck back this weekend I'll run it through the gambit of tests and pinpoint it as soon as I get my S15 out of the garage---it's getting a new 350. Just put a pump in my Silverado last week when it was 60 degrees and sunny. Now it's 30 degrees and snowing so I won't be doing this one outside. Michigan weather !
Anyways, thanks for the replies as every little piece of info helps. I'll let you all know what I find.

 

Finally got around to working on it. The vent valve was the problem. All is well again.