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P0300 at high speed/incline

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  #1  
Old 04-14-2018, 08:12 PM
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Default P0300 at high speed/incline

I've been having this problem for about 6-8 months now, and I cannot remedy it for the life of me. Literally will only happen at RPMs in excess of 2400-2500, speeds of 60+mph, while on an incline (literally only gives a flashing SES light and pending P0300 when all three of those conditions are met.) Other than that, a Snap On and a dinky Bluetooth + Android app scanner show low numbers of "misfires" (gotta put it in quotes, because you can NOT feel it cut out what's ever, could literally balance a glass of water on the damn thing and it wouldn't spill) every few cycles, random cylinders, really no rhyme or reason, more at idle than cruising speed (and 9/10 times it's no more than 10 "misfires" in a cycle.)

I'll start rattling off everything I've done so far to try to remedy this. Replaced plugs and wires, cap and rotor, fuel filter, distributor (which also came with another new cap and rotor), ignition coil, MAF sensor (with good reason, I did also get a P0101 the last two times I got the P0300), spider injector (MFI style), cleaned throttle body, I'm pretty sure that's it.

Fuel pressure comes back good at the Schrader valve (if I'm remembering correctly it's 60 with key on engine off, stays that way for at least 5 minutes, and I think when I checked it was at 52-54 when running). Distributor was definitely installed properly, cam retard is showing at -2. One thing I know for sure is my cat is partially plugged, I have a nasty rattle coming from it. Could that really, after all this time, be the cause of this issue? I'm losing my mind and as much as I love the Blazer life, I'm half tempted to ditch this thing even though it's the last thing I wanna do.
 
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Old 04-15-2018, 07:28 AM
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The p0101 is most likely from the clogged cat, Its a good place to start. You will notice a throttle response increase
 
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Old 04-16-2018, 08:26 AM
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Especially since the misfire seems to be "false misfire" lets start by verifying that your misfire detection is calibrated and working properly. Disconnect various plug wires at the distributor and see if the misfire counts for those cylinders increase rapidly. This is a functional check. The assumption is that if it is shown to count misfires correctly on specific cylinders, then any misfires counted during normal operation are likely real. I believe the assumption is typically valid.

There are also some BAD things that can show up as a false P030x. Excessive bearing clearance in the engine, and transmissions on their way out can masquerade as pesky false misfires (in the beginning). I have had both situations on mine.

2400 rpm at 60 mph sounds like 4th gear with TCC off. How does your TCC work on level road at steady speed? Does it apply? If you lightly tap the brake do RPMs jump up? Strange P0300 only at speed.
 

Last edited by LesMyer; 04-16-2018 at 08:45 AM.
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Old 04-16-2018, 11:31 AM
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Originally Posted by Lesmyer View Post
Especially since the misfire seems to be "false misfire" lets start by verifying that your misfire detection is calibrated and working properly. Disconnect various plug wires at the distributor and see if the misfire counts for those cylinders increase rapidly. This is a functional check. The assumption is that if it is shown to count misfires correctly on specific cylinders, then any misfires counted during normal operation are likely real. I believe the assumption is typically valid.

There are also some BAD things that can show up as a false P030x. Excessive bearing clearance in the engine, and transmissions on their way out can masquerade as pesky false misfires (in the beginning). I have had both situations on mine.

2400 rpm at 60 mph sounds like 4th gear with TCC off. How does your TCC work on level road at steady speed? Does it apply? If you lightly tap the brake do RPMs jump up? Strange P0300 only at speed.
I'll have to pull a plug wire and check and get back to you on that. I bought it used, the tranny was rebuilt but I do know the shop did some questionable work to it when the previous owner took it there, fuel lines are from God knows where because it's not using the correct filter (it's using a filter from a 3500 van). TCC works as it should, so I don't suspect that being an issue, just has the higher RPMs after it shifts down to 3rd to climb a hill. I'm definitely hoping it's nothing as serious as bearing clearance but then again you never know. I haven't checked compression on any cylinders either but I'm doubting it would be that. Engine runs pretty smooth.

A new update though, after thoroughly searching for vacuum leaks yesterday and running down the highway with a gauge connected to a vacuum line to see if I was by chance losing any, it would hold vacuum even when I could get the P0300 to come through, but I noticed I would lose vacuum when the AC was on and I would press the gas pedal.

Also, out of the blue, yesterday it started idling horribly and and misfiring constantly, a true misfire that could be felt and heard, truck shaking and all. Had to nurse it home misfiring the whole way with barely any power, noticed decently high fuel trims.
 
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Old 04-18-2018, 01:52 PM
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I have the exact same prob no idea where to begin as it seems like a pain in the *** to get to the spark plugs and I havent seem to found a cohesive agreement on the issue thru many forum posts n videos. My ses is the po300 where would begin to stsrt narrowing down? Retracing ur steps if u will. Keeping my eye on this thread tho good luck!!
 
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Old 04-18-2018, 07:39 PM
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Originally Posted by humblaze View Post
I have the exact same prob no idea where to begin as it seems like a pain in the *** to get to the spark plugs and I havent seem to found a cohesive agreement on the issue thru many forum posts n videos. My ses is the po300 where would begin to stsrt narrowing down? Retracing ur steps if u will. Keeping my eye on this thread tho good luck!!
Plugs aren't as bad as they seem outside of the #3 plug, I got that one out with an extension and two universals. It's not fun to just throw money at a vehicle to try and fix it, but I'd recommend starting with plugs and wires, see where that gets you, especially if it hasn't been done for a while (30-50,000) and then move on to distributor cap and rotor, those would be the most common causes of misfires.
 
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Old 04-25-2018, 02:31 PM
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Originally Posted by strosniderd View Post
Plugs aren't as bad as they seem outside of the #3 plug, I got that one out with an extension and two universals. It's not fun to just throw money at a vehicle to try and fix it, but I'd recommend starting with plugs and wires, see where that gets you, especially if it hasn't been done for a while (30-50,000) and then move on to distributor cap and rotor, those would be the most common causes of misfires.

any updates on ur issue?
 
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Old 04-26-2018, 04:50 AM
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Originally Posted by humblaze View Post
any updates on ur issue?
Yep, actually fixed the misfire yesterday. Ended up putting a 3rd cap and rotor on it, only this time I used AC Delco parts, and it magically went away. My scanner is still showing a ghost misfire that cannot be felt or heard, and it shows more when idling than driving, but I'm pretty sure I just need to adjust my distributor as the cam retard is reading at -7
 
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