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Discussion Starter · #1 · (Edited)
5.3 vortec buggy no power--- UPDATE!!

I tried to search this but found nothing----------I've got an '02 5.3 vortec in the buggy. I went with Howell harness, computer, fuel pump, and throttle body. No low end or mid range. Little top end power. Idles OK. I've checked fuel pressure, coil packs, plugs, vacuum leaks, changed injectors (out of new truck that was converted to propane), mass air flow looks to be ok. Could this be from the knock sensor? mass air flow itself(not sure how to test)? injectors? The motor runs just not well. Poor throttle response off of idle. I've just simply ran out of ideas and don't have alot of knowledge about these motors in general. Any advice would be appreciated.

Problem solved! Read on...
 

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I would double check the injector's, after its running for a while if you have one shoot a pyrogauge it at each of the exhaust ports coming off the heads, ive watched people change injectors over from practically brand new motors that sat for a month or so and have the same problem with clogged injectors. If all primary's are the same temp then you know the injectors are working, if one or more are cool as in you can touch them, then your not getting fuel on that cylinder(s).
 

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My suggestion would be to take it to a tuner that knows vortec/LS motors.

First thing as mentioned, would be to scan the PCM for diagnostic trouble codes. If no codes, it could still possibly be an intermittant sensor problem. I would check the MAF sensor first. Unplug the MAF start it and see if it still stumbles on throttle tip-in.

If not the MAF, I would check the IAC next.... what's the IAC counts at idle? Typical for an auto trans in park would be 10-30 steps.

What's the MAP read?... should be about 45 kpa at idle... Check the TPS, IAT, TPS, ECT.... check all the sensors. Once the motor is at temp, See if the PCM is going into closed loop and adjusting the fuel trims....

If still nothing obvious, it most likely just needs to be tuned. My advice is unless you're really an expert at tuning, take it to a professional tuner. He can put it on the dyno and log everything going on in the PCM and see exactly what needs to be adjusted. Getting the right Air/fuel ratio is so critical. Too rich and it runs like shit.... too lean and you can get detonation.... not a good thing ever.

There are over a hundred tables used to arrive at the right calibration or tune in a stock GM PCM. An engine that stumbles at throttle tip-in could be caused by a number of things.... but absent an obvious bad sensor, it's usually the result of a problem with the VE table.... Getting the VE right is key to a good running engine with crisp throttle response.

It may cost a couple bucks to take to a good tuner, but it's worth it.... every penny.
 

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Discussion Starter · #10 ·
i have no idea , but have you had it scaned for codes?
Only code it had was -- High Idle RPM

After sending comp. back to Howell, they replaced with new unit but said the original 1 had no codes. They said they wanted to replace so there were no doubts about unit condition. That was just last week.
 

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Vacuum leaks behind throttlebody? Or maybe around the butterfly shaft, ? use a propane torch and see if your idle increases or changes while you move the torch around near the throttle body area/intake etc. (torch off but flowing gas..duh)
 

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Vacuum leaks behind throttlebody? Or maybe around the butterfly shaft, ? use a propane torch and see if your idle increases or changes while you move the torch around near the throttle body area/intake etc. (torch off but flowing gas..duh)
Brake cleaner works just as well.
 

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Discussion Starter · #15 ·
Vacuum leaks behind throttlebody? Or maybe around the butterfly shaft, ? use a propane torch and see if your idle increases or changes while you move the torch around near the throttle body area/intake etc. (torch off but flowing gas..duh)
Forgot to mention... I tried that as well. I've tried so many things that I forget until someone suggests it. Thank you for the advice though...:)
 

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Did you change the intake manifold gaskets? I was warned that they have a tendency to leak.. perhaps that's messing with one of the sensors. When the intake is off, you could also take a look at the knock sensors as well.

If it lopes or stumbles at idle, you could try wiggling the wiring harness in different spots, see if there might be a broken wire somewhere.

Jim's performance in MD did my harness and computer (went with him after a few others I know were all 100% happy with theirs), and so far, so good.

If it runs OK at WOT and/or when it's first started, that could help eliminate certain sensors that are ignored at times..
 

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I second on checking the intake manifold gaskets for a vacuum leak. Usually each runner will leak just a little causing the engine to run lean. Also if you have a scan tool hooked up, look at the long term and short term fuel trims and both upstream o2 sensors. These will indicate how well the engine is running in relationship to the base program. Fuel trims should be around -10 to +10 at all times, with a vacuum leak you will see fuel trims in the + indicating having to add fuel for a lean condition and opposite for a rich condition.
 
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