After engine swap there is a code PO300 and engine is misfiring badly?

MUSDAKOTA8500

MEMBER
2003 CHEVROLET SILVERADO
6.0L
V8
4WD
AUTOMATIC
192,000 MILES

I have the truck listed above 2500 hd. It runs rough at an idle when it idles down also, I can give 1/4 throttle and it will run decent but anything after that misfires badly. The code is po300 multiple misfires. I took it in to a shop and they said the fuel pump is weak. So, I checked the pump myself and it is at 60 PSI under load and at idle it is at 48 PSI. I have new plugs and new o2 sensors and spark plug wires on it. I did just change the engine, I put a 6.0-liter LQ9 in it. It was originally an LQ4 6.0. But I am using the intake off my original engine. I am at a loss for what is wrong with my truck. Any help would be greatly appreciated.

1 Reply

BRENDON S

MECHANIC
653 POSTS

Good morning, MUSDAKOTA8500,

Nice upgrade! I did the same thing to my 2001 Suburban. Ripped out the 5.3L LM7 and put the 6.0 LQ9 in it. Ran fine off the computer for the 5.3 but wasn't great. I ended up getting HPTuners and copied the tune from my buddy's 04 Escalade EXT and put it into the truck. I know you had the LQ4 before so the tune shouldn't be that far off. This tune package also comes with a pretty good diagnostic tool.

Here is a link to their site:
https://www.hptuners.com/

They have a tune repository where you may be able to find a tune for the LQ9, but you have to be careful because some people out there put bad tunes in there to mess with people. I would find a friend to get the LQ9 base tune and perfect it from there.

The spec for the fuel pressure shouldn't go below 55 PSI is what my information is telling me. So, a weak fuel pump could be an issue, but I think it is best to check the wiring and make sure there are no issues with the connector or any resistance in the power circuit that could cause the pump to slow down before condemning the pump.

I remember about 8 or so years ago there was an updated harness that came with a new GM pump for these vehicles. From what I remember there was an issue with something called fretting on the terminals from it not connecting correctly, my assumption is excessive movement.

Here is a link to a resource about the cause and implications of this issue:

https://titanwnc.com/2022/05/fretting-a-common-cause-of-connector-failure/#: :text=What%20Is%20Fretting%3F%20Fretting%20is%20a%20phenomenon%20in,by%20motion%20and%2For%20repeated%20heating%20and%20cooling%20cycles.

I would check the wiring to the pump. I will include a wiring diagram for you. There is a ground on the outside of the frame rail under the driver's side door right behind a body mount. I would clean that up to make sure you have a good ground. This runs the Fuel pump and the ABS Module.

I would also replace the fuel filter. I use a K&N High Flow filter.

If you replace the pump, get it from GM. It's actually cheaper or at least was than going to an auto parts store and it comes with the updated harness. I think mine was under $400.00.

I will include some wiring diagrams, images and diagnostics procedures for you. In one of the pictures there are some pins called T-pins. I use these myself to back probe connectors. I suggest using them as it makes it easy to clip and alligator clip to for a multimeter.

I mean it isn't that far off so changing the fuel filter might help.

Here are a couple articles that may help as well:

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

https://www.2carpros.com/articles/how-to-check-wiring

I know it's a lot of information. Let me know if I can help you and also let me know what you come up with.

Thank you,
Brendon

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