5.3 acting like a 4cylinder
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Joined: May 2015
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5.3 acting like a 4cylinder ok I have been working on a 2000 gmc sierra that was orig, a 4.3 v6 that I swapped to a 5.3 out of a 04 gmc sierra everything from the 04 was swapped to the 2000 4.3 and the truck ran great until the tranny went out. Ive since then replaced the trans and torque conv. with new ones engine harness was changed because of fire damage from a faulty fuel line and open headers....stupid I know. but anyway ive replaced the harness with a 06 4.8 gmc sierra engine harness as ive been informed they are the same. pcm was bad so I got a reflashed pcm coded to my vin. Ive already done the relearn and got it to were it will start and run, but.....It runs like **** driver side of the motor headers are nice and hot, passenger side headers are cold ive got spark and ive got fuel, but what I don't have is a nice running engine. I get reduced power warning on the dash, when I try and make the throttle w/o (wide open) and throttle shuts and wont open unless I kill power to the truck. then I can push it open with my hand....Ive hit a wall 
. Please help me Ive spent atleast 7,000.00 building this truck and doing it the right way and then this happens. I have two kids and a wife to support and im running low on funds any help would be greatly appreciated and prolly keep me from getting a divorce lol.
. Please help me Ive spent atleast 7,000.00 building this truck and doing it the right way and then this happens. I have two kids and a wife to support and im running low on funds any help would be greatly appreciated and prolly keep me from getting a divorce lol. 
Teching In
Joined: Mar 2007
Posts: 27
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From: Schererville, IN
Last time I saw something almost exactly what you are describing was on my friends car we swapped an LS1 into.
Cam sensor wiring does not like being by the other wiring on the harness, so try moving it out by itself a bit and see if that changes anything for you.
Dumbest fix I ever saw and we verified over and over again the wiring was good,etc,etc.
Cam sensor wiring does not like being by the other wiring on the harness, so try moving it out by itself a bit and see if that changes anything for you.
Dumbest fix I ever saw and we verified over and over again the wiring was good,etc,etc.
Teching In
Joined: Mar 2007
Posts: 27
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From: Schererville, IN
Crank sensor, not cam sensor, sorry
Thought about it for a bit after reading your reply.
Long story short, the way he ended up routing the harness in the area caused issues later.
Car was running on one bank only.
Went thru everything and it vanished after we separated parts of the harness.
It has never happened again once we did a little rerouting. Never found a reason, pierced wire, suspect area, nothing...
I'll give him a call t'row and ask which bank was running and which was not.
Thought about it for a bit after reading your reply.
Long story short, the way he ended up routing the harness in the area caused issues later.
Car was running on one bank only.
Went thru everything and it vanished after we separated parts of the harness.
It has never happened again once we did a little rerouting. Never found a reason, pierced wire, suspect area, nothing...
I'll give him a call t'row and ask which bank was running and which was not.
I had the same exact thing happen after heads/cam/intake. My problem was the plug where the harness plugs into the coil pack, one of the little prongs bent over when I plugged it in. I bent it back plugged it in and it ran fine. Good luck!
TECH Addict
Joined: Aug 2005
Posts: 2,305
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From: Monroe,NY
If your headers aren't getting hot on the passenger side then those aren't firing. I would check your connections to those coil packs. Could be the same thing that happened to silplu83. There is either no fuel or spark on that side and I would have to say spark.
Registered User
Joined: May 2015
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I have weak spark on that side, #2 and #8 are sparking pinkish purple, and #4 & #6 are kinda orange, #8 spark plug looks like I just pulled it out of the box brand new i don't know if that helps yall but i will check my prongs on the coil packs tomorrow and see what they look like. What about the throttle body issue when i press the gas with key on engine off it opens for a split second then closes and stays closed the motor inside keep the flange from being opened by hand until truck is off, I've been told it can be a bad throttle body all together is that true???
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Verdad Gallardo TECH Addict
Joined: Aug 2005
Posts: 2,305
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From: Monroe,NY
I cleaned a friends 2007 throttle body a month ago. With the key on (car not running)see if the throttle body moves when push the gas pedal. It should work that way if not you have something wrong.
Here is some good info on the po 200 code injector circuit
https://ls1tech.com/forums/fueling-i...r-circuit.html
Here is some good info on the po 200 code injector circuit
https://ls1tech.com/forums/fueling-i...r-circuit.html
Registered User
Joined: May 2015
Posts: 23
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Well I got a new harness after finding a page that showed me how to test my harness I believe the harness is my problem I just got a new harness and I grabbed some new injectors to be on the safe side seeing how mine have quite a few miles on them as soon as the rain quits here I'll install and update on how it turns out, thank you all for your help and I hope I can get this figured out
2006 and 2007 classic V8's use a different Throttle body, Pedal, and Tac module.
2003-2005 are interchangeable and the harness should be as well.
You can not flash an 03-05 operating system and use an 06-07 classic TB, Pedal, and Tac module or Vice Versa.
The main Coil pack connection on the valve covers is also a good place to look as it is easy to bend a pin there.
Regarding the old PCM, How do you know it was bad? was it burned?
You stated you had the replacement PCM was reflashed with your vin. Was it done with tuning software? Are you sure it has a V8 calibration in it.
Where are you located?
2003-2005 are interchangeable and the harness should be as well.
You can not flash an 03-05 operating system and use an 06-07 classic TB, Pedal, and Tac module or Vice Versa.
The main Coil pack connection on the valve covers is also a good place to look as it is easy to bend a pin there.
Regarding the old PCM, How do you know it was bad? was it burned?
You stated you had the replacement PCM was reflashed with your vin. Was it done with tuning software? Are you sure it has a V8 calibration in it.
Where are you located?
Registered User
Joined: May 2015
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The pcm was bad it was shorted out when the fire happened, the harness I just pulled off was off of a 06 sierra, I just got one off of a 2004 Yukon which is the same year as the motor so there's no question of difference in year of harness. I got the new pcm from flashmasters on eBay for only 100.00 they probably used a tech2 on it
Registered User
Joined: May 2015
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OK I've changed wire harness and injectors and the truck is running on all 8 but it has a kinda rough idle stays around 450rpms, I can rev the engine a few times low and once high and it still goes into reduced power mode...what gives...I have a p0101 and p2135 code the 2135 is throttle circuit i believe and 0101 is Maf... but i believe my Maf is in good condition seeing how its not even 2years old...if I leave the gas alone the engine runs decent but ses light flashes for a few min then goes off then comes right back on and repeats any clues?
Read this . It explains what to look for.
http://www.obd-codes.com/p2135
The P0101 could be a secondary condition to the Rep mode.
I would focus on the P2135 first.
http://www.obd-codes.com/p2135
The P0101 could be a secondary condition to the Rep mode.
I would focus on the P2135 first.
Registered User
Joined: May 2015
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Do any of you know where I can find the relearn procedure for the throttle I think that may be the problem because I didn't have this problem until after the new pcm, and I believe the pcm has the proper injector data, it wouldn't rub with out it and it runs smooth, but earlier when I went to give the truck gas usually I can open the throttle half way before I get the REP message. well when I went to move the truck earlier I pushed the gas before I moved it with it still in park and nothing no slight rpm climb just push the petal and nothing no change what so ever just the REP code and Service Brake Booster message...Why would it say service brake booster never had a problem out of it, it holds vaccum and stops the truck on a dime.....
.... If the PCM was programed for a 2004 PCM and it had a Vacuum assist canister, and the harness you have does not have that, that could be the reason.
Disconnect the battery and let it set for 10 minutes then reconnect.
Fire up engine and let it idle on its own for at least 10 minutes do not rev durring this time. The computer will adjust itself automatically. You can give some gas if need to get it started but slowly bring it down till it idles on it own. again it must idle on its own for 10 minutes from a cold start.
Other wise you need a scan tool with bi directional controls to do it properly.
Disconnect the battery and let it set for 10 minutes then reconnect.
Fire up engine and let it idle on its own for at least 10 minutes do not rev durring this time. The computer will adjust itself automatically. You can give some gas if need to get it started but slowly bring it down till it idles on it own. again it must idle on its own for 10 minutes from a cold start.
Other wise you need a scan tool with bi directional controls to do it properly.
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getting mad well ive changed intake gaskets due to a minor intake leak and got a new throttle body changed crank pos sensor yet no luck... it does idle smoother but still if I give more then 15% throttle it goes into reduced power mode....I can max out the revs all the way to 6500 by intermittently tapping the gas pedal and make the rpms climb....so what gives....new pcm, new trans, new throttle body, new gaskets no vacuum leaks found anywhere ive done idle relearn still no luck.. Idk why im having this damn problem. can anyone help before I end up having to take it to a dealership to get crucified by the so called techs for doing this swap.
TECH Addict
Joined: Aug 2005
Posts: 2,305
Likes: 126
From: Monroe,NY
Sounds like you have something wrong between the dbw setup and the pcm. I would try in the conversion section or the tuning section . Someone else that has done a similar swap should be able to help. I have dbc so mine is different.