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Engine Stumbles at take-off

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August 19th, 2009, 1:58 PM   #1  
Engine Stumbles at take-off

I have a 97 K1500 Z71 (5.7L Vortec) that has been acting up recently. In the afternoon when I leave work to head home my truck will sometimes stumble as a begin to accelerate. If I keep the accelerator pressed the engine will eventually catch up and the truck will takeoff. If I really get into it the engine almost seems like it is going to die (never has) but will eventually rev up as it should when you floor it.

I am going to change the fuel filter (which I did about this time last year along with the pump) and wanted to check the fuel pressure to make sure it wasn't the pump or regulator. Can anyone tell me what the idle pressure is supposed to be? Has anyone else had/heard of this problem before?


Last edited by bioDAWG; August 19th, 2009 at 4:12 PM. Reason: Edited to add engine information
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August 19th, 2009, 5:20 PM   #2  
Two things come to mind. Check all your vacuum hoses carefully. Make sure your distributor is OK. Checked for a burned up rotor, carbon tracked or cracked cap, or loose timing.

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August 24th, 2009, 3:09 PM   #3  
Thanks for the info. I will take a look and see if any of these are an issue. I also wanted to check and see if anyone could tell me what the proper output of the O2 sensor should be at idle. I have been noticing that my engine has some what of the rough idle lately so I hooked a scanner to my OBDII port. I have an existing issue (P1870) that periodically sets my light so I did not really think I would learn anything from the scan. To my surprise I had codes thrown stating I had a Misfire detected and fault of the O2 sensors 1 and 2. So a started the truck, cleared the codes, and watched the O2 sensors and noticed that they were oscillating between 0.3V and 1V. Is this normal? Could this be a contributing to my stumping engine?


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August 24th, 2009, 8:43 PM   #4  
i had a jeep cherokee 96 that problems that sound similar to this one. When i accelerated and when I wasn't moving, the vehicle like "stumbled" every so often. I changed the spark plugs and wires and the problem was completely gone. Someone else had the same issues and changed out the plugs and it ran fine. IDK thats my thoughts on the problems. good luck and let us know what happens : )

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August 24th, 2009, 11:18 PM   #5  
fuel pressure should be 60psi

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August 25th, 2009, 9:28 AM   #6  
Thanks for all the infomation. Two things that I have found in the last couple of days.

1. The fuel pressure at idle read around 55 psi. When I step on the accelerator the pressure jumps up to about 61 psi and holds steady. I zip-tied the gauge to my windshield wiper and drove around and did not see anything that look like it could be the problems

2. Last night I took a peak at my plugs, wires, and distributor cap. The plugs and wires looked good (I changed them about a year or so ago). When I removed the distributor cap I noted that there was a little bit of gray build-up on the metal tabs directing the spark. I also noticed a bit of black (i guess carbon) build up on the rotor. My plan is to replace the cap and rotor this afternoon and see if the helps.

Thanks for all the help. I will let everyone know if this works!

Thanks again.

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August 25th, 2009, 6:30 PM   #7  
you could also try just cleaning the cap and rotor of the build up and see how it runs.

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September 9th, 2009, 10:37 PM   #8  
your CATS my be clogged up...

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September 16th, 2009, 1:48 PM   #9  
Engine Stumbles at take-off - WHAT I HAVE LEARNED

So after a couple of weeks of testing, experimenting, and tinkering this is what I have learned!

1.) If I force my truck fuel delivery system to run open loop by disconnecting the MAF sensor the truck does not stumble. I did this over a week period and for the first time in a couple years had to be light footed on the accelerator to keep for rolling the tires.

2.) When I used a scan tool (CarCode) to look at all of the enhanced PIDs for the ECU I found out that my O2 sensors were all over the place. In particular the BANK 1 sensor. While at idle the BANK1 sensor would be reporting a very rich fuel reading while the BANK2 sensor was reporting a lean reading. Driving around I noticed that the BANK1 sensor would read densently compariable to the BANK2 sensor but when I came to a stop the BANK1 sensor would again read RICH and would remain that reading through an acceleration and slowly begin to lean out as I reached a steady speed.

I am planning on changing out this sensor in hopes that this will slove the problems that I have been having.

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September 16th, 2009, 2:27 PM   #10  
Id say you have a bad MAF, when mine went bad it did the same thing, bank 1 lean codes & such. Man, when I unplugged my MAF it ran like a bat out of hell, never felt it so fast, it felt like a camaro. The computer would adjust the best it could and it would go back to normal accel after a few hours. I installed the new MAF and all codes went away and it ran normal. I do miss the way it ran confusing the computer by unplugging it, that was fun, but I like it to run the way its designed to.

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