4.8 to 5.3 Swap. Engine in but I need troubleshooting help.

Hey everyone. I haven't posted on this site in a while. Hope everyone's well.

So let me give you some history on this truck. 2002 1500 Silverado, supercharged 4.8ltr V8 w/ injectors out of a 6.0ltr truck (recommended for supercharger). Supercharger blew piston rings on two pistons after about 35k miles of usage.

Now for the swap. I swapped in a 5.3ltr engine w/ trans out of a 2003 Avalanche. The Avalanche 5.3 went in complete with it's original intake manifold. The parts transferred over from the 4.8 were throttle body (cable type, to eliminated drive by wire), alternator, ac compressor (kept to avoid disconnecting freon lines), fuel rail w/ 6.0ltr injectors (needed the original fuel rail w/ fuel return line), and fuel pressure sending unit (located on top of the block behind intake manifold. Did this to keep compatibility with 4.8's original wiring harness), headers w/ new gaskets, and 4.8 computer (temporarily running with 4.8 tune, but plan to reflash with 5.3 tune). The only modification I had to make was running a new vacuum line to the fuel pressure regulator on the fuel rail. The 5.3's intake manifold didn't have vacuum adapter needed so I had to run a vacuum line to the other side of the intake manifold.

The truck starts easily, throttle responds very well, truck drives great, and engine sounds overall very healthy. The problem is that when the engine starts, from being cold, it idles @ 1k RPM for a few seconds and then shoots up to approx. 2600 RPM and stays there. The butterfly, in the throttle body, is completely shut, MAS is intact and connected, coolant lines are purged, no vacuum leaks, engine does not show 'Check Engine' light, and the engine runs for 10 min plus and doesn't overheat. However, the problem persists. The only thing I can think of is that I'm using the 4.8's computer, which BTW has a tune for the supercharger. It was suggested to me that I need to install the original 5.3 injectors instead of the 6.0 injectors. I was told this might be shooting too much fuel into the system, but I don't think I agree.

I would appreciate anyone's input. Thanks in advance for any help.

Sounds like you have a vacuum leak somewhere or the IAC motor needs to relearn. First off I would start with getting the tune corrected before you kill this engine, too. You made too drastic of a change to keep running the same tune even limping it around. You say it's overheating too? Definitely get the tune done since it could be doing all kinds of strange things causing overheat issues. Have you removed the air bleed hose that goes from the heads up to the TB and make sure you have a good stream of coolant coming out? If air is trapped in there you will overheat. Make sure those lines flow. If you added headers and didn't have them before you undoubtedly moved the 02 sensors. If they don't warm up properly you won't get a good reading from them. The most common fix for 02 issues on trucks with headers is getting the after cat 02 sensor from about a 2002 Corvette and wire them in instead of the truck 02s. They are made to read at cooler exhuast temps. An alternative is to wrap your headers with exhaust wrap to help keep the heat in the airstream.

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Originally Posted by RedHeartbeat

Sounds like you have a vacuum leak somewhere or the IAC motor needs to relearn. First off I would start with getting the tune corrected before you kill this engine, too. You made too drastic of a change to keep running the same tune even limping it around. You say it's overheating too? Definitely get the tune done since it could be doing all kinds of strange things causing overheat issues. Have you removed the air bleed hose that goes from the heads up to the TB and make sure you have a good stream of coolant coming out? If air is trapped in there you will overheat. Make sure those lines flow. If you added headers and didn't have them before you undoubtedly moved the 02 sensors. If they don't warm up properly you won't get a good reading from them. The most common fix for 02 issues on trucks with headers is getting the after cat 02 sensor from about a 2002 Corvette and wire them in instead of the truck 02s. They are made to read at cooler exhuast temps. An alternative is to wrap your headers with exhaust wrap to help keep the heat in the airstream.

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Okay, sorry. I meant to say the engine 'doesn't' overheat. I don't have heating issues. I did remove the air bleed hose and it is flowing well. As far as the headers go, the original 4.8 had the headers installed and I retained all four 02 sensors. I did check for vacuum leaks, but found nothing wrong. How do I get the Idle Air Control Valve to relearn?

I'm sending out an extra 4.8 computer that I have and having it reflashed for the 5.3 tune. I guess I'll wait for that and see where it gets me. Thanks for your response.

well, if the tune you're running is programmed for the 42# injectors (or whatever they are), don't put the smaller ones in there.
you sure there ain't any vacuum leaks? i just installed my headers last night & the first start surged to about there & promptly died. thats when i remembered i forgot to plug the hole from the EGR delete

Okay. So I made a few changes. I now have a 5.3 block, with the 4.8 intake manifold, cable operated throttle body, original 5.3's injectors, and a properly tuned ECM. The idle is now at about 2,000 RPM! I've checked for vacuum leaks, with a gauge this time, and it's showing normal at about 20. I cannot think of any other issues. Any ideas? Can anyone tell me how to make the IAC motor relearn?

you ever figure out the problem?

mine was doing something similar. I replaced the iac, went thru the idle learn procedure. Replaced the maf sensor. still had the prob. I had a ver slight vacuum leak at the intake gaskets. Replaced the intake gaskets and it idles smooth as new.