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Discussion Starter · #1 ·
Hi, I'm new here but not new to working on vehicles. I have a 1999 chevy Silverado with 5.3 vortec ...the problem: engine has poor performance (sometimes) almost like its running on four cylinders mash hard on throttle and it thinks about it then starts pulling suddenly? The cure: when it starts running like this I can pull over, shut the engine off, get out and unscrew gas cap then reinstall it and start truck and usually it will run fine? Leaving truck running and removing gas cap doesn't work or just shutting off truck doesn't work. Truck always starts, idles fine, fuel millage isn't bad. I have replaced all normal stuff wires, cap, plugs, coil, thermostat sensor, Gas cap, new fuel pump and filter? Any input? do not have a scan tool nor have I tried one.
 

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I would hook up a scan tool and see if there is any codes stored in the computer to start with. My truck is doing similar things at the moment, the check engine light never came on but when I hooked up the scan tool I had codes for running lean. Still trying to figure out what my issue is. Did you replace your fuel pump and filter or was that just a question? If I clear my codes it starts to run better for a second but then they come back on. At least if you have some codes popping up you know where to start looking, steers you in the right direction.
 

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Discussion Starter · #3 ·
Replaced fuel pump as previous one failed, replaced fuel filter as I thought it could be the problem. Often I see peoples posts that are similar issues and they can tell you what the scanner is giving them for codes but often no resolution, kinda funny. Soooo ....if I came up with a code of P300 random miss? It would help? I already know it has a random miss :)
 

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haha yes I hear what you are saying. It doesnt always give you a solution right off the bat but it can at least point you in a direction rather then checking everything. My codes have a ton of things that can cause it and I am knocking them off the list as I go without actually replacing parts. On my list for this week is check my fuel pressure and make sure my MAF is working properly. My symptoms are similar to yours, slight misfire at idle, seems to lack power during acceleration and then it all of a sudden kicks in. Yours kind of sounds like some sort of EVAP issues if that gas cap trick works for a bit.
 

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Discussion Starter · #5 ·
I hate parts changing in hopes I change the right part. I was thinking the evap system also, when I changed fuel pump the evap hose was off ... maybe got clogged? There are so many different sensors and the main computer? Guess I can see if someone will scan it.
 

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That is a possibility for sure. I hate all these sensors as well. It may make the engine run more efficiently when things are working, but for troubleshooting it is a nightmare haha. But yes I would start with a scan and see if anything comes up at all. Then we know if your looking for EVAP issues or something else.
 
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