P0300 Issues 1997 K1500

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Brocktoon

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97 K1500 5.7L MT
147,000 miles

Got a p0300 a few days ago going uphill, no other codes. Nothing really noticeable with drivability.
Recent history:
Replaced old lower manifold intake gasket with metal-backed one in 2020.
Changed plugs, wires, cap and rotor in 2021. (AC Delco)
New fuel pump, tank and filter this past May when I had the bed off to paint the frame.
New Oil pressure sensor, distributor gasket to correct oil leak in August.
Another new Delco cap and dizzy holding bracket last month. Rear of previous cap was loose and threw off cam retard offset. Offset is currently +0.6

Some DashCommand screenshots from this afternoon. I know I'm probably missing some information.

Misfire counter and Fuel trims a minute or so after startup:
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Two more after about 10-12 minutes. The STFT must not have registered digitally when I took the screenshot.

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Other notes:
-Upstream O2 sensor on bank 1 has some chaffing on one of the wires. I swapped that sensor with bank 2, and didn't really notice a difference.

-Tried doing the propane vacuum leak test, no change anywhere.

-Cleaned the MAF sensor.

-Twice over the summer, the truck would be hard to start after filling the tank, but would eventually fire up. Pump is Delco and is new, as noted above.

-AFAIK, the injectors are the original spider injectors. I took the upper and lower manifold off as one unit when I did the lower gasket.

- I tested the ignition coil Ohm measurement with the test found in the service manual. Everything was within spec.

Again, I know I'm probably leaving things out, but if anyone could point me in the right direction, I'd appreciate it.
 
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Schurkey

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Yeah, you seem to have a heap of cylinders with misfire issues. Is your scan tool capable of showing misfire codes cylinder-by-cylinder? Or does it call all misfires "P0300"?

An ohmmeter can tell you if an ignition coil is defective. It CANNOT confirm that the coil is "good". Get a spark-tester calibrated for HEI and test the output power directly.

What is the cranking compression pressure of all eight?

What is the fuel pressure? Does it hold pressure? Can your scan tool perform a cylinder-balance test?
 

Brocktoon

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Fuel pressure is 57 PSI at key on, jumped to 61 at key off, and dropped to 51 PSI after 10 minutes. Looks like it should be a little higher. I'll have to grind off some of the Chassis Saver on the frame where the pump ground is, maybe that might help, or maybe I'm just stupid.

I got an adjustable spark tester, coil output seems to be strong at 30 KV. As I backed it towards 40 KV, it would still arc but would occasionally jump down towards the electrical connector on the coil. I don't know if that's a fault with the tool or the coil or nothing at all.

I'll try to do a full, proper compression test tomorrow. I checked #1 before it got dark, got to 180, but that was with all other plugs still in and throttle not held open.

I don't think my scan tool can do a cylinder balance test, and it doesn't show codes cylinder by cylinder. I've only ever gotten P0300 for misses.
 

Schurkey

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Fuel pressure is 57 PSI at key on, jumped to 61 at key off, and dropped to 51 PSI after 10 minutes. Looks like it should be a little higher. I'll have to grind off some of the Chassis Saver on the frame where the pump ground is, maybe that might help, or maybe I'm just stupid.
I'm thinking 10 psi loss in 10 minutes is getting to be unacceptably high. But I don't know that for sure. Be worth checking the service manual for specs.


I got an adjustable spark tester, coil output seems to be strong at 30 KV. As I backed it towards 40 KV, it would still arc but would occasionally jump down towards the electrical connector on the coil. I don't know if that's a fault with the tool or the coil or nothing at all.
MAKE SURE there is no permanent scar on the plastic housing of the coil. Over time that voltage flashover will carve a groove in the plastic, and then it flashes-over at progressively lower voltage until the engine runs crappy all the time.

If there's no scar from the high-voltage flashover to the electrical connector, the coil is probably OK.

I'll try to do a full, proper compression test tomorrow. I checked #1 before it got dark, got to 180, but that was with all other plugs still in and throttle not held open.
180 under those conditions is...spectacular. Might want to make sure the gauge is accurate by connecting it to a compressed-air source; compare your pressure gauge to the gauge on the air-compressor regulator gauge.

I don't think my scan tool can do a cylinder balance test, and it doesn't show codes cylinder by cylinder. I've only ever gotten P0300 for misses.
That's unfortunate. Looking at your misfire counts, I'd kinda expect a P0307, and maybe others. But #7 seems to be the worst. Be sure to look at that plug while you have it out. Would be worthwhile to verify the plug-wire resistance, and insulation integrity, too.
 

Brocktoon

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So today I pulled all the plugs. Except for #1, they all had some burning near the electrode. I remember when I put them in, I forgot to put antiseize on #2 so the threads look clean, and I know that's an age old debate anyway. Number 4 seemed to break loose a little too easily, I probably didn't tighten it enough. I looked closely at #7, and it actually looked pretty good. None of the wires had any problems in the insulation.

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Here are the numbers I got for the compression test, all plugs out, dry test only.
1.) 150
2.) 180
3.) 155
4.) 125
5.) 140
6.) 100
7.) 145
8.) 115

So less compression overall on the passenger side. I'll try a wet test tomorrow, put the plugs back in torqued and see what happens. I might have to just take it to someone who's better at diagnosis than I am.
 

Schurkey

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Far as I'm concerned, a "wet" test is a waste of time and oil. The oil isn't going to travel around the piston to seal the ring when the piston is slanted-over at 45 degrees. But adding several cc of incompressible oil to the combustion chamber nearly guarantees increased pressure due to the "higher compression ratio" of a smaller chamber.

Re-test #2, #6 and #8. If you get similar readings, that engine has problems. #6 maybe has too much carbon in the chamber; #2 and #8 might have compression-leakage problems, or perhaps intake valves that don't open like they should.

Grab a propane torch and a vice-grip. Hold the dark, fouled plugs with the vice-grip on the tip that the plug wire snaps onto. Burn the deposits off with the torch. Yellow flame is carbon burning away. Orange flame is when the plug is getting clean, or fully-clean.
 

Brocktoon

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After retesting cylinders 2, 6, and 8, here's what I got:
2.) 130
6.) 115
8.) 120

I put everything back together and fired it up, and I'm still getting misfires, actually more than before. Mostly on cylinders 5, 6 and 7, and within the first 6-8 minutes after startup. The engine sounded like crap at first, but evened out.

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Brocktoon

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So, an update of sorts. I pulled the cap and rotor, which was pretty charred around the button. I ordered a no-name cap and rotor with brass terminals. (I know, I know, but the "old" ones were Delco and didn't even last a year.) I slapped them on and the misfires were reduced by a lot. They didn't go away entirely, and cylinder 5 is still the biggest culprit, but they only seem to be misfiring after cold starting. It runs great once it's fully warm. I drove the truck a couple hours last week and another hour and half this morning, and the money light has stayed off so far. I'm thinking the spider might be the source of the remaining misses, but that'll have to wait until spring. Even then, a new clutch will probably come first. It's dragging and the TO bearing is chirping like hell. Always something...
 

Brocktoon

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Well, I guess I spoke too soon. The P0300 is back, along with a P0161 Bank 2 Sensor 2. The misfires are worse on bank 1, and both downstream sensors are bouncing around too much, but only bank 2 threw a code. I'm about at my wit's end with this truck.

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