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!

  • Welcome to Motor Vehicle Maintenance & Repair! Jun 2, 2020 at 14:31

2 Answers 2


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.

  • Thanks Paulster! I noticed when I was doing the MAF that the original oil filter had been replaced at some point by a paper one. Is there a way to check that its good by just looking at it? And I assume id need to buy a diagnostic obdII thing to check the van in motion, correct?
    – Beefi1123
    Jun 3, 2020 at 15:03
  • @Beefi1123 - Not sure what the oil filter has to do with anything? Did you mean the fuel filter? Also, no, not an OBDII device. You'd need a mechanical gauge which attaches too the Schraeder valve on the fuel rail. It's tricky and a PITB, but about the only way I know if to test it under load. A fuel pressure tester shouldn't be overly expensive. Jun 3, 2020 at 16:13
  • Hmm I edited that previous comment, but it didn't go through. Yes, the K&N fuel filter had been replaced, probably by someone paranoid. For the gauge, I hadn't read your comment correctly the first time. I'm going to use a bike pump if that works and check with the engine just turned on, a steady throttle, and a quick throttle.
    – Beefi1123
    Jun 3, 2020 at 18:06
  • Oh, I was thinking of the wrong filter. The (former) K&N is the air filter. This is different, underneath the driver side.
    – Beefi1123
    Jun 3, 2020 at 18:08
  • You CANNOT use a bike pump to measure the pressure. The Schreader valve is too big, and you're talking about GASOLINE. Please, if you are unaware of what I'm talking about, get some help (mechanic of friend who does know). And no, I'm talking about the fuel filter, not the air filter. Jun 3, 2020 at 18:13

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.

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