P0300 won’t go away!!!

[1998OBS]

This is a scan through of the engine data, I don’t know what I’m looking at I’m hoping someone see’s something I don’t. It’s yard to see I apologize but it’s 7 & 8 misfiring and then the rest of the data…

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[Schurkey]

Why are there two "BARO" readings, one about half of the other (29.x vs. 13.x)?

This is "EVAP / EGR Data", there's other screens to look at.

I don't have a '98 to compare to--I have a '97, but the computers are different in '98.

Any chance the 7 and 8 plug wires are crossed? They're not next to each other, but it'd be easy to check.

 

[1998OBS]

Just did a compression test on cylinders 7 & 8 (both are misfiring)

110psi max on both

 

[Schurkey]

"BARO" would be barometric pressure. Weather and altitude, in other words. SOME vehicles use what amounts to a MAP sensor, that's NOT connected to manifold vacuum. It just reads atmospheric pressure all the time. Other vehicles use the MAP sensor, doing double-duty--when you turn the key from "Off" to "Run", the MAP sensor is powered-up but there's no manifold vacuum because the engine isn't cranking yet. So the MAP sensor has an instant to read atmospheric pressure before the key goes to "Crank" and the engine starts. Those cars get into trouble when the engine is started at one altitude, but drives to a much-different altitude. So you start your drive at 1000 feet altitude, then drive up the mountain to 6000 feet--the engine runs shiity as you climb. At some point, you shut off the engine, and then re-start. Now the computer has a fresh update to the barometric pressure, so it can fuel appropriately again. The engine runs good--until you go back down the mountain.

Anyway, I don't understand why your scan tool is showing two readings for "BARO", with one being half the value of the other.



Depending on your altitude and the accuracy of the gauge, 110 psi cranking compression meets GM specs but is definitely on the low side.

Measure a "good" cylinder or six, and see how it (they) compare.

 

[1998OBS]

"BARO" would be barometric pressure. Weather and altitude, in other words. SOME vehicles use what amounts to a MAP sensor, that's NOT connected to manifold vacuum. It just reads atmospheric pressure all the time. Other vehicles use the MAP sensor, doing double-duty--when you turn the key from "Off" to "Run", the MAP sensor is powered-up but there's no manifold vacuum because the engine isn't cranking yet. So the MAP sensor has an instant to read atmospheric pressure before the key goes to "Crank" and the engine starts. Those cars get into trouble when the engine is started at one altitude, but drives to a much-different altitude. So you start your drive at 1000 feet altitude, then drive up the mountain to 6000 feet--the engine runs shiity as you climb. At some point, you shut off the engine, and then re-start. Now the computer has a fresh update to the barometric pressure, so it can fuel appropriately again. The engine runs good--until you go back down the mountain.

Anyway, I don't understand why your scan tool is showing two readings for "BARO", with one being half the value of the other.



Depending on your altitude and the accuracy of the gauge, 110 psi cranking compression meets GM specs but is definitely on the low side.

Measure a "good" cylinder or six, and see how it (they) compare.

 

[1998OBS]

Here’s the new after today:

After hooking it up to a legit tester I’m getting cylinders 7 & 8 misfiring simultaneously which is weird because they’re on opposite sides of the block. Their numbers in misfire go up simultaneously…

I ran compression tests on cylinders 7 & 8 and they’re both 110psi (I didn’t check any of the other ones)

I checked the temperature of the exhaust all the way down on both banks and it seems to be fine from what I’m gathering online… after test drives to heat up the exhaust and then letting it idle with 2500 to 3000 rpm bumps every now and then to get Seafoam through the system sufficiently I noticed that once I shut off the truck then immediately try to start it, it would just crank and crank with no start, after about 10 to 15 minutes it would start after 2 chuggas… then I rev it, it seems fine, idles but then shut it off and then try to restart it and it just cranks and cranks… like it’s out of gas.

Now I’m definitely leaning towards the injectors but someone mentioned that it seemed like the ignition control module or the original ignition coil that are both still in it but I don’t see how a single ignition coil could misfire specifically cylinders 7 & 8 simultaneously and if it were the ignition control module wouldn’t it have just not started or stalled before it just started cranking and not starting recently? I’m so done with this truck…

 

[1998OBS]

Update:

So I have a P0300 code still and I’ve been pulling my hair out, as I sit, my hands are raw and sliced up here and there… I JUST got back inside from a 40 minute test drive and have the same symptoms as before, I just spent 3.5 hours to take the manifold off, replaced the spider injectors and intake gasket and I’m still getting a choppy idle and P0300 with a P0155 that disappeared after the initial scan…

I torqued the manifold down to 44 inch pounds in sequence on the first go then 88 inch pounds on the final torque sequence…

I’ve changed the EGR, plugs, wires, distributor, MAF, air filter K&N, crank shaft sensor, fuel filter and now spider injectors and intake gasket…

I really seriously could use a fresh look, advice, anything…

I live in Southern California I’m not sure if you know of anyone here that you trust to help me diagnose this but I’m desperate now I’m over $1,600 and I’ve had it…

Everyone is saying it’s the tps, it’s the ignition control module, it’s the cats delete the cats, delete the egr, go straight piped but I can’t mess with any of that in Comifornia…

It ran WAY better on the drive but at idle it’s super choppy and it does sound muffled but idk if it’s just the sound of the exhaust or what or how to check for a bad cat, no sulfur smell, no rattling… I’m out of ideas…

Note: After the 40 minute drive, WOT a couple times on the freeway where it ran better than before after coming to a stop it was still choppy and rough idling then while waiting in the drive thru it just shut off, tried to start it and it acted like it was it out of gas, pushed it, waited 10 minutes, cranked it and it started right up, drove a mile, shut it off then tried to restart it and it acted out of gas again, shut everything down, started googling again… HEELP!

 

[Schurkey]

1. Is it still 7 and 8 that are the problem children? Have you checked for broken valve springs especially those two cylinders?

2. Is the MAP sensor showing steady reading at idle? The Solus Pro will graph the output--look for a steady line.

3. What is your fuel pressure just before/as the engine stalls?

4. K & N air filters and MAF sensors sometimes don't play well together.

5. You had the top of the intake off. Did you ever have the bottom part off the heads? My TBI idled poorly. The intake gasket distorted, created an air leak into #1 and #3 intake ports. The original Vortec intake gaskets were FAMOUS for doing that, too. Misfire worst at slow idle, less noticeable as speed increases.

6. It's running better. You're making progress.

 

[1998OBS]

1. Is it still 7 and 8 that are the problem children? Have you checked for broken valve springs especially those two cylinders? I haven’t taken the valve covers off of it, would low oil cause issues like this?

2. Is the MAP sensor showing steady reading at idle? The Solus Pro will graph the output--look for a steady line.

3. What is your fuel pressure just before/as the engine stalls?

4. K & N air filters and MAF sensors sometimes don't play well together.

5. You had the top of the intake off. Did you ever have the bottom part off the heads? My TBI idled poorly. The intake gasket distorted, created an air leak into #1 and #3 intake ports. The original Vortec intake gaskets were FAMOUS for doing that, too. Misfire worst at slow idle, less noticeable as speed increases.

6. It's running better. You're making progress.

1. I need to get a legit scanner back on it first.
2. It’s the original MAP sensor, I can’t take live readings, I’ll have to wait until tomorrow.
3. No idea because it was unexpected and I wasn’t scanning it when it happened.
4. I’ll have to throw my OE filter back on and see if that makes a difference.
5. I’ve never had the lower intake off of it, just replaced the upper intake gasket with the blue Felpro one. Torqued the intake to 88 inch lbs.
6. It’s more powerful yeah but you can still feel and hear the misfire. It’s frustrating AF.

I’m thinking sensors now, it suck’s because technically I’m going backwards but I figured why not do the things that end up being the last resort type issues, the big stuff… turns out I’m still spinning my wheels (pun intended) I’m seriously pissed after the injectors, I really thought that would be it and I would have been stoked after a test run I dreaded the spider injectors job so doing it with the same results are the end of the day is infuriating.

What sensor’s would cause a P0300? What’s the cheapest route from here out? Why does this have to be such a huge pain in the ass? It’s starting to effect other parts of my life now because now I have to rely on this truck for work.

I heard you’re supposed to do a crank shaft sensor relearn if you ever change it out, does anyone know how to do that?

 

[Supercharged111]

Choppy idle sounds like it may be a lean idle. Is your EGR valve maybe stuck open? Check your O2 mV when you get a scanner on it.