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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 5787
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have a 2003 chevy LS s-10 pickup and replaced air mass

Customer Question

i have a 2003 chevy LS s-10 pickup and replaced air mass flow control, all vacuum hoses,spark plugs,cap,rotor,ignition wires, fuel pump,ecu and when driving my truck it seem to run good no missing lots of power but yet check engine light goes on when i go over 50 mph and truck runs good. reset the po300 code which is multiple misfire and it will activate again at high speed but truck runs good. don't know what else to do.sorry engine 4.3 vortec v-6
Submitted: 1 year ago.
Category: GM
Expert:  Juan Crespo replied 1 year ago.

Hi there. Welcome to our site!

Well, sometimes the onboard computer (PCM) can detect faults while we think everything is OK and don't feel anything wrong going on. In this case, the PCM is seeing the crankshaft randomly "slowing down", which could be just a couple of miliseconds out of parameter without a specific cylinder being the cause.

Most of the times we used to see this type of issue associated with MAF sensors and/or fuel pressure/volume. Being in a professional shop, we had access to a scan tool similar to GM's TechII and a good 4-channel scope we used to pinpoint these small faults. Do you have anything similar that could look at the different fuel trim values, fuel pump voltages and mass air data?

Customer: replied 1 year ago.
M.A.F. 5.20 , ST-FTRM1(%) 0.0 , LT-FTRM1(%) -1.6 ,ST-FTRM2(%)-1.6 ,LT-FTRM2(%)-2.3 , CAN'T FIND FUEL PRESSURE /VOLUME
Expert:  Juan Crespo replied 1 year ago.

I appreciate your effort, but what you just posted alone can't reveal the root cause of the problem - if that was it, then I wouldn't have mentioned the TechII and the scope as must-have tools to diagnose this type of driveabilty issue.

I think you misunderstood what I meant; in order to determine what's causing these events that are causing the random misfires, I would need to see fuel trim values at idle and at cruising speeds - another thing that could be crucial to the diagnosis would be the freeze frame data for a couple of times when the P0300 is triggered. Then we would use the scope to confirm what the scan tool shows.

At this point, I don't think I can provide the kind of help you need. I'll opt out so other experts may try. Please don't reply until another expert responds or the site wil revert the request back to me.

Good luck with your truck.

Expert:  Roy Belding replied 1 year ago.

do you still need help??