P0300 Requesting Advice

[Hair]

1998, GMC Sierra, 2WD, 140k miles, 305 Vortec
Drive it every day, no issues ever, all stock.
It started missing on me, and the ONLY code that it shows is P0300.
All cylinders are getting fire.
The plugs look fine (#3 slightly wet/smelled like fuel, but looked good other than that.)
As I pulled plug wires while running, it seemed like the #3 didn't make much of a difference, but when I pulled it out and laid it against the manifold, as the truck ran, it was firing good. So I'm thinking that fuel injector may be bad.
Every cylinder has 190/195 PSI.
I do not see or hear any vacuum leaks. (Tried my best to fine something like that.)
At this point, I am thinking that it is probably the spider fuel injector assembly.
Before I start taking the upper intake off, do you guys agree, or is there something else I can check to make sure before I spend a day working on something that aint the problem? Any advice much appreciated. Thanks in advance.

[hutcholdschool]

oh the dreaded P0300. I have also been chasing this same issue for quite some time and i have thrown every part at it that I can think of. I also have great compression on all cylinders. So far I have changed plugs, wires, cap, rotor, distributor, coil, spider fuel injection assembly, crank sensor and no change for me. The one thing I found on my truck and I thought it was going to be the issue was my crank sensor had been rubbing on the reluctor wheel so it was gouged out on the crank sensor. I got a new one and shimmed it down so it would not hit the reluctor wheel any longer and that did take away my flashing check engine light missfire but the P0300 still pulls up. I would check that crank sensor if I was you as it is easy to check with one 10mm bolt holding it in and see if it has been rubbing. On my 1997 there is no re learn procedure I have found for a crank sensor replacement but on your 1998 when I used a scanner capable of a re learn on the crank sensor it did pull up for your year but when I select my 1997 there is no re learn procedure available so keep that in mind if you need to change yours out. While researching this issue I found most of the time its the spider fuel injection pressure regulator goes out and floods out the cylinders in the back of the plenum. This issue is easy to diagnose with a fuel pressure gauge hooked up to the fuel rail service port and I think it is supposed to hold at 60psi.

[Hair]

The fuel pressure is 60psi with the switch on and at idle. When I turn it off the pressure remains steady at 60psi and does not drop. However, when I rev it up the pressure only goes up to 65psi regardless of how high I rev it. Is that correct, or should the pressure continue to increase with the rpms?

[speedygonzales]

You mentioned NOTHING about the cap and rotor. I'll put money on 2 things. That's the problem and the other is your vents are plugged.

When I had the original factory distributor, I was running fine one day and it wouldn't start the next. Changed both and good to go.

Check it out and see how bad they both look. You'd be surprised.

[Hair]

I did not check because I'm getting good fire at spark plugs. But I will check tomorrow anyway. On the scanner it shows that it is cylinders 3 & 5 that are misfiring, if that helps anyone help me. Also, the long range fuel trim is 17% on bank 1 and -5% on bank 2, but I don't know if that is good or bad or if it means anything.

[crsgmc]

This may not be helpful. I replaced the engine on a 1996 K1500 with a factory crate engine. When replacing the engine, I replaced a lot of parts including cap, rotor, and wires. A couple years later I was on the on ramp accelerating on to I80. The truck totally died and would not restart. I had it towed home and began working on the problem. I had 16,000 miles on the cap and rotor........so I was sure it wasn't that. I did a post with pictures on this site years ago. The cap and rotor looked fine. I tested the continuity on the cap and it was also fine. I checked for spark on all cylinders and had zero spark. Bottom line I replaced the cap and rotor and the truck fired up and ran perfect. It was not a cheap cap, and had bronze contacts. You have spark, but is a full spark or weak. I have a hard telling the difference.

[Hair]

SOLVED!!
I replaced the Cap and Rotor Button and it fixed it!!
Thank you guys for recommending that I try replacing those!!
Y'all were right!! I would have bet $10,000 that was not the issue.
When I pulled the plugs out and started it, fire would jump two inches to the manifolds. There was no way that I thought that fire was the issue, but it was, the cap looked corroded and the rotor was a little burned on the end. I put in new Cap and Rotor and blew out the vents (they were not stopped up) and only #5 was showing a misfire. I pulled that plug out and hit it with a wire brush. fired it up and BOOM! No code and no misfires on the scanner!! That #5 plug would have probably cleaned up going down the road, but anyway, I'm good now and thank you guys so much, because I was within a hair of buying intake gaskets and a new fuel spider and spending my Christmas break on that.
One last note for future people: Back when I was trying to figure out what was wrong, trying to diagnose, another thing I noticed, #3 & #5 at the manifold would never get over 200 degrees and all other cylinders were about 400 degrees. That could have been a weak spark that made that difference. IDK, but I'm glad I got my truck going again. Thanks Again guys!!