LM7 P03000 misfire
Thread Starter
Registered User
Joined: Jan 2026
Posts: 1
Likes: 0
LM7 P03000 misfire crazy backfires on start up, backfire goes away if RPMs are above 1000-1500
Replaced my motor with a crate motor that had a bigger camshaft in it with the supporting head components, as well as flat top pistons. 10.5/1 compression.
for fuel I’m using a 400lph pump and 42lbs injectors.
intake wise a S&B with a enclosed case, and a stock LM7 truck intake.
tune wise all that’s done is the injectors are scaled
now I still have a misfire code even with RPM’s higher. I pulled all plugs, all plugs have build up on them besides cylinder 1&5. I checked wires and plugs, and I have spark in both cylinders.
i checked injectors outside of truck with a 12v source they are clicking and working. Also used node light to check if the injectors were getting signal from computer, that passed
no other codes are generating. And I know truck is not tuned for the bigger camshaft. But any reason these two cylinders would have squeaky clean plugs? And any correlation in the backfires? (Like unburnt fuel in exhaust getting burn in collector or y pipe)
Replaced my motor with a crate motor that had a bigger camshaft in it with the supporting head components, as well as flat top pistons. 10.5/1 compression.
for fuel I’m using a 400lph pump and 42lbs injectors.
intake wise a S&B with a enclosed case, and a stock LM7 truck intake.
tune wise all that’s done is the injectors are scaled
now I still have a misfire code even with RPM’s higher. I pulled all plugs, all plugs have build up on them besides cylinder 1&5. I checked wires and plugs, and I have spark in both cylinders.
i checked injectors outside of truck with a 12v source they are clicking and working. Also used node light to check if the injectors were getting signal from computer, that passed
no other codes are generating. And I know truck is not tuned for the bigger camshaft. But any reason these two cylinders would have squeaky clean plugs? And any correlation in the backfires? (Like unburnt fuel in exhaust getting burn in collector or y pipe)
Moderator
Joined: Aug 2010
Posts: 7,320
Likes: 526
From: Ann Arbor, MI
I'm certainly not a expert like some people here, but I would start by verifying that the wiring to the #1 and #5 coils is correct. Heck maybe even reversed. You tested that they have spark, but it also needs to be at the right time. There could be a mixup in the sub-harness for the odd cylinder or in the connector going to the sub-harness. While bit of a guess on my part, if true, it would be at least the 3rd time this has happened to someone posting here.