SOLVED: 98 K1500 5.7L Vortec random misfire P0300 P030x

[THerm]

Hello everyone, I'm hoping to get some help with my truck. It's been a great beater and I'm hoping to keep it running for at least a few more years. Currently, it has ~240,000 miles on it.

For starters, here's the list of things I've replaced since I've had it:

Part Replaced

plugs 2018
wires (ac delco) 2018
coolant temp sensor 2020
distributor cap (Oreilly, brass pins NOT AC Delco) 2020
Fuel filter 2020
rotor 2020
spider injector (mpfi conversion, Oreilly's) 2020
thermostat (195°F) 2020
Water pump 2020

I've also recently flushed/replaced coolant, the #4 spark plug with an AC Delco. It doesn't lose coolant (that I can tell), and the exhaust seems ok. I'm far from an expert but I'm pretty handy and good with data/numbers. I have this: https://bafxpro.com/products/wifi-obdii-reader-scanner-for-ios-devices, and an app called OBD Fusion. I don't think this app has the ability to tell me misfire counts or detailed data, but if someone knows how to do that with this tool I would love to hear it.

For the last few weeks I've been tuning this truck up in preparation for deer season, and overall it runs great. Especially since replacing the thermostat, it now heats up fast and holds at around 190 and gets about 17 mpg on short trips. Not too bad, and an improvement from the 14-15 it was getting with the old thermostat ( I don't know what specs were).

The truck sat for awhile this summer, was hard starting when I needed it but would ultimately always start. Never really noticed hesitation or anything until I got the code reader and read the codes and noticed the P0304 and P0300. It also throws a P0440 sometimes, but I think that's the gas cap. When I replaced the injector assembly, i found fuel in the electrical connector ( leaking by the seals on the solenoids) which now makes me suspect the harness (maybe corrosion). Problem is, if I start it and immediately put it in gear and go it won't throw the code and it seems to run fine. From my view, fuel trims and all other data look good. Please let me know if you see something different here. I'm including a couple of charts from a recent 15 minute drive that may be helpful.

The things I know I need to check:

1. Fuel rail pressure (though this wouldn't explain one cylinder misfire with new injectors)
2. Compression: but if there was a stuck valve or broken spring wouldn't this show up in performance too? the truck runs strong, no performance issues that I can tell.
3. Distributor cap: I bought one of the bargain ones with brass pins, maybe a mistake. I hear crossfire could be a problem, any way to check for this without buying more parts?

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Long term fuel trim averages (approximate):

Bank 1: -3.0%
Bank 2: 5%

Short terms (approximate):

Both ~0.3%


I've also been running Marvel Mystery Oil in the crankcase, and Seafoam in the fuel tank. Fuel is premium (not ethanol free though).

I'm at a loss, but would like to keep the check engine light off if possible. Seems to be quite difficult though, with this truck. Previous owner had a piece of tape over it. Got a heck of a deal on the truck, PO thought it had major problems but replacing cap, rotor, wires, fixed it right up a few years back.

I've been browsing these forums for awhile, trying to find similar situations and I know there's a lot of information out there. Now hoping for some personalized advice. What would your next step be?

Thanks!

-Trent

Edit 10/11/20: Removed irrelevant maintenance items from the list, I originally copy pasted everything from my personal list of things I've replace

 

[stutaeng]

Ok, so you replaced the spider injectors? Please don't list things like handle and taillights, that has nothing to do with misfire! LOL

I agree that fuel trims look REALLY good, so likely not injector issues.

Compression check would be good. What about intake gaskets? I'm assuming those have been replaced at some point? Basic things like spark check at cylinder with misfire check ok?

 

[THerm]

Yeah sorry for that, I copy pasted the list from the list I use to keep track of everything (edit: removed the irrelevant ones).

I don’t know about head or intake gaskets. I haven’t done them but at 250k miles they *must* have been replaced by now. Wasn’t the problem with the originals from the factory? I saw a suggestion the check the spark plug for rust after sitting overnight, seems like a good suggestion that I might try this morning. I’ll keep track of coolant level closer once I get the system completely topped off from the refill. I knew I was losing it before because the water pump was leaking from the weep hole. I haven’t had enough time to monitor the level since replacing it all, so I’m sure I’ll be able to tell soon.

Yes there is spark at 4.

Thanks for your suggestions!

 

[stutaeng]

Yeah sorry for that, I copy pasted the list from the list I use to keep track of everything (edit: removed the irrelevant ones).

I don’t know about head or intake gaskets. I haven’t done them but at 250k miles they *must* have been replaced by now. Wasn’t the problem with the originals from the factory? I saw a suggestion the check the spark plug for rust after sitting overnight, seems like a good suggestion that I might try this morning. I’ll keep track of coolant level closer once I get the system completely topped off from the refill. I knew I was losing it before because the water pump was leaking from the weep hole. I haven’t had enough time to monitor the level since replacing it all, so I’m sure I’ll be able to tell soon.

Yes there is spark at 4.

Thanks for your suggestions!

Well maybe you on to something with intake manifold gasket leak.

They sometimes leak ONLY when cold, as the engine heats up the plastic gasket seals up again. That's consistent with your symptoms.

You can check with cold engine running by spraying with carb cleaner and seeing if RPM changes. Or better yet, monitor fuel trims and O2 sensors when spraying the carb cleaner, you should see some sort of spike in the graphs if the carb cleaner makes it inside the engine.

My 4.3 Vortec Made it to like 225k miles on the original gaskets. I just had a bit of coolant on the passenger side against the valve cover. Seems like passenger side is more prone to leak for some reason.

 

[THerm]

So did a little more digging this morning. Pulled the #4 plug to check for rust, seems reasonable that if the manifold gasket was leaking that there would be some slight rusting or moisture present on the plug after sitting overnight. Dry as a bone. No fuel odor present at all.

I can't stop thinking about the fuel that I found in the spider injector harness. Today I opened the cover on the harness and it was...not great.

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No idea what the white residue it, and it doesn't matter. I think what I need is a "noid light" but multimeter test should work too I would think? Just have someone crank the engine while I watch for the pulse? Any time I've ever seen corrosion like this, and visible copper I get a bad feeling. How could this explain a misfire only at startup?

 

[THerm]

Tested each injector circuit at the harness with a multimeter, 12V at each one during crank. It's an ugly mess in there and it's hard to believe it's not the problem but I don't think it is.

The thing that's most frustrating is that it seems to run fine, or as fine as it ever has for me. Especially under load. Distributor, timing chain, all that stuff would cause noticeable performance problems, right? It's literally only at idle that I notice the misfire (barely). I think the reason I don't get the code if I put it in drive and take off immediately is that it doesn't run the misfire diagnostic if it's not allowed to sit at idle for a certain amount of time (something about the GM driving sequence).

I'm open to any input. I know I need to open this thing up and do the intake manifold gaskets but I don't have good reason to believe they're leaking. I'm not seeing any leaking, no noticeable loss of coolant, no wet/rusty plugs, no stumbles when I spray carb cleaner, nothing.

The codes change from time to time, from confirmed p0304 pending p0300 to just confirmed p0300.

The only other code I see from time to time is the p0440, and I'm pretty sure it's my cheap locking gas cap not sealing properly.

 

[THerm]

Update:

Opened it up for intake gaskets today. Not as bad as I expected, and not in the places I expected but you can see where #4 was sagging. These were aftermarket FelPro gaskets but they were not the metal ones, just plastic. Broken in several places. The set I am installing is much more robust.

I did find in my app that I could see misfire history, so when I started watching I noticed that #3 and #4 were misfiring but #4 the most by a long shot. Really hoping this clears it up. I found where someone suggested opening the crankcase breather hose and the pcv grommet at the same time, if there's vacuum at both valve covers there's a good chance this gasket's shot and that's what I found. Lots of vacuum at the breather hose and at the pcv grommet. Time will tell.

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[PlayingWithTBI]

Was this log done while idling or did you blip the pedal sometimes? The reason I ask is, at 19.5 it shows a spike in RPMs that may be due to your coil and/or ICM. If your issue doesn't go away after you get it back together, try spraying some water on the coil, not the wires, and see if it stumbles. A bad coil can take out the ICM. When it goes bad it'll throw a false pulse to the PCM which will think it's revving higher so, it'll change fuel and timing causing a surge that may be felt like a misfire.

 

[THerm]

The log was taken on my drive to work. No surge really noticeable, worst I could say I noticed was a rough idle. But by rough, it was really pretty consistent just a good amount of vibration consistently.

Part of my problem might be that it’s been this way ever since I got it with over 200k miles. I don’t have any other experience with these trucks and it had enough power for me so didn’t think much of it. Really hoping I’m pleasantly surprised when I get it back together.

One other thing: on the 98’s, if I stab the distributor in exactly the same as it came out (marked, carefully removed) does the truck’s computer take care of any timing or will I need to take it for a cam/crank relearn? Really hoping it just runs right, but if there’s some way to get it right without forking out cash to a shop I want to do it that way.

 

[east302]

It’ll need the CMP Retard reset.

There are posts here where you can use an OBD Bluetooth connector and an app to get it set. DashCommand and CarGaugePro are two that worked on my 98. Maybe $50 for everything.