5.3 Gen 3 - Misfire/rough idle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

AnimalMother · 08-24-2021, 09:53 AM

I just purchased a 71 C10 that has a 5.3/4l60 swap, BTR stage 3 cam w/ LS6 springs. The truck has a misfire with an intermittent CEL. I should also state that the wiring of this engine was done haphazardly, to the point in trying to connect a code reader to the OBDII port a couple of wires sparked together.

The engine starts and runs, but under any load it stutters at wants to die. I've checked for vacuum leaks with MAF cleaner sprayed around the intake and TB while running, checked all plugs/wire and all cylinders are firing, just looking for any other suggestions before I start throwing parts at it. I'm also planning to buy a new harness for it to replace the spliced and gutted harness that it currently being utilized, which could be the problem all together. Appreciate any help.

**MySons68C20** · 08-24-2021, 01:24 PM

I would start with a basic compression test since this engine is new to you.
Fuel pressure/filter and spark plug condition next.
As you stated, the harness is butchered get that straightened out after checking the easy stuff 1st.
Can you post a pic of the engine? There are a bunch of sharp dudes on here that may pick up on something they see.
Hope this helps!

Blake

**biketopia** · 08-24-2021, 02:28 PM

Was it tuned for the cam, need to know which cam too. Just "stage 3" doesn't help as BTR sells 3 or 4 cams with stage designations. It most likely will have a lot of chop and a rough idle regardless. The first thing is to tidy up the harness, a new one, rework what you have, something. Crossed wires, poor splices, and bad grounds will wreak havoc on the system. Coil harness' not plugged in right or fully are a common problem as well.

AnimalMother · 09-13-2021, 03:05 PM

Found the problem, turned out to be a bad MAF sensor. I read that if you unplug the MAF that the ECU will run in a safety mode, and in doing so the truck instantly ran better. Replaced the sensor and it's solved the majority of the issues. It still starts rough and needs properly tuned for the cam, but it's now drivable.

08-24-2021, 09:53 AM	#1
AnimalMother Registered User Join Date: Sep 2011 Location: OKC, OK Posts: 5	5.3 Gen 3 - Misfire/rough idle I just purchased a 71 C10 that has a 5.3/4l60 swap, BTR stage 3 cam w/ LS6 springs. The truck has a misfire with an intermittent CEL. I should also state that the wiring of this engine was done haphazardly, to the point in trying to connect a code reader to the OBDII port a couple of wires sparked together. The engine starts and runs, but under any load it stutters at wants to die. I've checked for vacuum leaks with MAF cleaner sprayed around the intake and TB while running, checked all plugs/wire and all cylinders are firing, just looking for any other suggestions before I start throwing parts at it. I'm also planning to buy a new harness for it to replace the spliced and gutted harness that it currently being utilized, which could be the problem all together. Appreciate any help.

08-24-2021, 01:24 PM	#2
MySons68C20 Senior Member Join Date: Sep 2014 Location: Wenatchee, Wa Posts: 929	Re: 5.3 Gen 3 - Misfire/rough idle I would start with a basic compression test since this engine is new to you. Fuel pressure/filter and spark plug condition next. As you stated, the harness is butchered get that straightened out after checking the easy stuff 1st. Can you post a pic of the engine? There are a bunch of sharp dudes on here that may pick up on something they see. Hope this helps! Blake

08-24-2021, 02:28 PM	#3
biketopia Senior Member Join Date: Jul 2015 Location: Warrenton VA Posts: 1,106	Re: 5.3 Gen 3 - Misfire/rough idle Was it tuned for the cam, need to know which cam too. Just "stage 3" doesn't help as BTR sells 3 or 4 cams with stage designations. It most likely will have a lot of chop and a rough idle regardless. The first thing is to tidy up the harness, a new one, rework what you have, something. Crossed wires, poor splices, and bad grounds will wreak havoc on the system. Coil harness' not plugged in right or fully are a common problem as well. __________________ 71 c-10 Long Bed, under the knife for a No Limit Engineering Wide Ride Chassis Pack and coil over static drop. 07 Silverado Classic 2wd 95 Camaro 427SBC/TH350

09-13-2021, 03:05 PM	#4
AnimalMother Registered User Join Date: Sep 2011 Location: OKC, OK Posts: 5	Re: 5.3 Gen 3 - Misfire/rough idle Found the problem, turned out to be a bad MAF sensor. I read that if you unplug the MAF that the ECU will run in a safety mode, and in doing so the truck instantly ran better. Replaced the sensor and it's solved the majority of the issues. It still starts rough and needs properly tuned for the cam, but it's now drivable.



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