Chasing 05 Chevy Express Flashing Engine Light P0300

Question

Alright, I'll try to keep it brief (been through tons of threads and I thought I had it). 4 mechanics have looked at it to no avail.

2005 Chevy Express 3500 4.8L V8 130k miles
Code P0300 (not sure which cylinders)

Engine light flashes, but only on the highway after going above 52 or so for a couple minutes. If I slow down, it turns off. Sometimes, it stays on solid for about a day afterwards. Nothing at all seems off about the van other than the seatbelt light stays on always.

Fixes tried:
New spark plugs/wires, 
Cleaned MAF/TB and ran some SeaFoam through gas tank, 
Crank Relearn

After tons and tons of research, I really thought the crank relearn was gonna do it, but I tested yesterday and...no good. Next up, I was planning on trying:
Distributor cap? Whole distributor, haven't looked into it yet, 
O2 sensors (trying to start with cheap fixes first)

Thought of getting a diagnostic tool that I can use in motion, but I want to hear what you guys think. Been driving me crazy and I really want to get the stupid thing fixed. As a side question, I usually just drive under 50 so that it doesn't come on, but I have a feeling that it's probably still doing damage.

Thanks a bunch for any replies!

Beefi1123 · Accepted Answer

From the comments:
"After trying (and failing) on multiple occasions to reattach the fuel gauge, I gave in to my gf and took it to my friend's mechanic. After giving him the low-down, he test drove on the highway and saw the multiple misfires (mostly on 5 and 6, but some on 8 as well). Interestingly, he mentioned that he didn't feel them though. He stopped and did another crank relearn and then it was all good. Having been less than a week ago, I'm a little skeptical, but so far so good. Who knew the fix was something I'd tried already!"
I originally learned about the Crank Relearn fix from online forums, particularly this one: https://www.plowsite.com/threads/help-check-engine-light-flashing.83879/, so I guess the internet wins again.

Pᴀᴜʟsᴛᴇʀ2 · Answer

You need to check your fuel pressure while the engine is under load to ensure it's correct. Should be ~58psi on an LSx engine when functioning correctly. The P0300 when not pointed at a specific cylinder is indicative of a misfire due to lean condition while under load. What it sounds like is, your pump isn't keeping up at speed or possibly you need to change out your fuel filter.

Your 4.8L V8 is an LSx engine. It has coil-over-plug ignition system, which means it doesn't have a distributor cap, but rather, an individual coil per cylinder.

Chasing 05 Chevy Express Flashing Engine Light P0300

2 Answers

Add your own answers!

Ask a Question