P0300 after extended idle

Re: P0300 after extended idle

Postby MechBob » September 12th 2020, 3:31am

Plugs look good.Gap looks wide?
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Re: P0300 after extended idle

Postby gman » September 12th 2020, 6:28pm

Oh yeah, that could be another cause, like mechbob said, badly gapped plugs
.Gaps temd to widen over time from either heat, or unideal cylinder conditions.
Ive had gaps that were so bad though, that they coild have beem caused by ham-fisting.
Most LAPS's sell gappers for $2.00.

Did you do the lower intake manifold gasket when you did all of the gaskets, or are you talking about upper intake/plenum?

Another possible culprit thats a quick check, is the crankshaft position sensor.
Its right below the timing chain gear on the front of the van, and ive seen the harnesses attach to them melt and cause all sorts of issues.
.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 13th 2020, 12:00am

Alright so I did a couple more tests. Checked gaps on 1 and 5 and they’re both .060 and I forgot to mention before that they’re AC Delco Iridium. No platinums for me. I did a compression test on 1 and 5 as well and they were both a hair above 170psi. That’s good news right? This was my first compression test but I pulled only the plugs from 1 and 5, pulled the fuel pump relay then cranked 4-5 times. I repeated cylinder 1 twice to make sure.

Checked the crankshaft position sensor and I didn’t see any melted insulation but it’s super dirty. I’ll brush it off and get a better look. Also I did replace the lower intake manifold gasket as well as plenum, throttle body, and valve covers.
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Re: P0300 after extended idle

Postby gman » September 13th 2020, 1:57am

Compression is excellent.
Though the test, from what ive read, gives more feedback wwhen you compare the ratings to all cylinders, but ifnthose are the ones misfiring, then you can at least be confident that the rings are good.

Compression can still be good with head issues.
You might want to find a way to do a leakdown test just to rule out your heads, but all of that is a lot of work.


There are some very experienced members here that migjt know of some electrical gremlins that you can take a look at.

If it happens after an extended period of time, it can be happening when the van is on a closed loop.

You could try unplugging the MAF once you have everything put together to see if your van is reading funny business after it goes into a closed loop.

You could just screw those plugs back in in a few minutes and unplug the MAF to double check.


Please keep in mind that I am relatively on the same level as you as far as car knowledge is conxerned. Im just tbrowing some stuff out there that you might not have thought of. Seems computer related to me , and the easoest first check as far as i know is to force the van into an open loop.

Being that it happens when the air is significantly more humid, i would start looking at sensors.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 13th 2020, 8:35am

Yeah I should probably check them all while I have the gauge. That's a good idea to unplug the MAF. Pretty quick and easy to do.
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Re: P0300 after extended idle

Postby MechBob » September 13th 2020, 11:45am

Don't rule out a knock sensor going pad.
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Re: P0300 after extended idle

Postby gman » September 14th 2020, 10:10am

Yeah, sorry bout the runaround, but it seems like you would have been doing these tests anyways.
If all cylinders show 170, that's an EXCELLENT sign that your motor is in great shape, and it's just great to have piece of mind.

Since the van runs fine in every other condition but the ferry, I, personally, don't think it has anything to do with the mechanical condition of the engine.
You have about as many miles on your van as I do on mine, so I have noticed that a lot of stuff has worn out, but it's ALL typical stuff that is expected of a vehicle that has probably never seen anything more than a single tune up, fluid flushes, and some oil changes.

So, since the conditions only happen when it is significantly more humid, and you are probably getting extremely moist, salty air in and around your vehicle, and also considering the fact that lots of stuff is worn, my personal guess (and again, just a guess), is that something is just not agreeing with all that moist air.

If the MAF was never worked on, that's a consideration.
Could take a quick peek at the air filter.

Also, could be grounds that need to be checked. I need to learn where all the grounds in my vehicle are myself so that I can give them a once over if I ever get anything really weird like you are experiencing.

Maybe you can start looking at sensors, specifically ones that consider airflow. In these particular vans, as far as I know, on the intake side it's pretty much just the intake air temperature sensor, which is on top of the intake tube right before it hits the throttle body, and the mass airflow sensor which is a square that tis attached to the same tube between that tube and the air filter tube.
If it's possible, you could probably hook up a scanner WHILE you are in similar conditions, and see if your short term fuel trim goes nuts or if you get any other clues.

You could probably recreate this condition without having to go on the ferry by just getting a spray bottle, filling it with salt water, and misting around your van while it's running.
Probably not a good idea to get any of it directly into the intake or any vital area, and probably not the best to get a ton of water in any really hot areas.

A boat is also not a road, but I find it hard to believe that any sensor would go nuts while floating around on a boat if they don't go nuts when you go offroad, but I could be wrong.

Really seems like either an electrical problem related to humid air, an air temperature sensor, or a MAF issue to me.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 14th 2020, 7:33pm

I haven’t been able to do anymore tests since the air quality has been so bad but it should be simple enough to monitor MAF and intake air temp. Luckily I still see misfires idling in the driveway so I think I got the code just because I was idling for such a long time. Getting it to repeat is sometimes the hardest part so now it’s just a matter of testing sensors. I did some research on the knock sensor and it seems I should be getting 5V to the connector, continuity on the sensor itself, and voltage output when a bang happens. I bought a new one on amazon as a backup. I hate throwing parts at it but I justify it to myself that it’s old and new parts are good. I can always return it if I find the actual cause first.

I also noticed my TPS reading was cutting out while I was getting misfires but I wasn’t sure if that could cause misfires. Bought one of these on amazon as well to test out. Here’s a plot of it happening.
7EF84CDB-8ACB-4AFD-B814-B715906DCE60.png


Luckily it hasn’t really caused other issues besides an SES light and some rough idling at stop lights. Still annoying and I’d like to track it down.

Hopefully get back to some more testing this week.
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Re: P0300 after extended idle

Postby gman » September 14th 2020, 9:23pm

That's great.
P0300 is a real doozy and can be almost anything.
But it seems like you've already knocked out most of the possiblities.

Yeah, if you have a garage or a place to store the vehicle, you can be pretty confident about throwing money at it (within reason), since a catastrophic failure (blown engine, transmission failure, or suspension gets completely destroyed) just means that it's going to be sitting in your driveway for a while.

I wish I had that luxury, because if I did, I would be getting polyurethane bushings/mounts, mevotech control arms, the rear of the vehicle would be really fancy, and I would probably get a paint job. It would be more of a long term investment, because my chassis is in great shape, and with just a few thousand I can have a vehicle that would last another 10 years. Heck, replacing the entire powertrain would only cost maybe $3-4k depending on how fancy i want it, and polyurethane everything, and an entirely new and super fancy suspension would be another $700 or so.
G80 locker, and upgraded brakes would be another $600. I can have something that would likely outlast many of the new vehicles on the road today, many of which will probably only get to 150k due to all of the extremely fancy new technology. It would take a few months of chipping away at it with some free time, but it would totally be worth not having to spend $30k on a new car, and totally worth not having car payments, extremely expensive insurance, exponentially more expensive maintanence costs, etc.


But in my case, if I have a catastrophic failure, It's going to the tow truck for a few hundred bucks, and i'm going to strip as much as I can off of the vehicle and put it into storage.
So most of the time it's just whatever keeps it on the road.

Anyways, it looks like we're really tracking down the problem here, and it's likely something electrical. TPS cutting out is weird.
It's mostly mechanical from what I can gather the last time I Looked at it. It's mechanically attached to your throttle body somehow, and the throttle plate moves DIRECTLY by cables from the pedals to the throttle body. Then, it likely uses the uses a round electrical contact of some sort to detect position.

Being that your comptuer reads the throttle position to adjust your idle, it's very likely that it can have something to do with misifres.
Misfires happen when there is incomplete, or off-time combustion. Basically the comptuer is just saying: "Hey! this cylinder is not doing it's part!".
If the TPS is going nuts and telling the computer weird stuff, then that could definitely contribute towards the problem.

Would be really nice if you had a misfire counter, so you knew which cylinders were the culprit.


Where did you find the information for testing the electrical stuff?
That's something that I understand, but don't know where to find all the pertinent information from.
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Re: P0300 after extended idle

Postby Rein » September 15th 2020, 10:59pm

Sounds like your ride may tend towards seasickness and knows when you’re getting ready to board the ferry. Next time have someone on the ferry reserve the very last slot for you, give him your cell number and have him call just before they close the gates. Then drive the last hundred yards as fast as possible and right onto the ramp and into the ferry. That way your vehicle won’t have that anticipatory yucky feeling and all will be well.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 16th 2020, 5:08am

I ended up downloaded dashcommand and the chevy PIDs so I was able to get misfires by cylinder. It's mainly cylinder 1 but 2-4 have some as well.

I replaced the TPS and put plug 1 back in but had some trouble getting the plug wire connector to snap on all the way. Fired it up and 1 was missing constantly. Took off the wire, pulled the boot back, tried connecting it to my old plugs and it was super difficult but I was able to get it on all the way. So then I connected plug 1, slid the boot back over the plug and hooked everything back up. Turned it on and cylinder 1 is still missing and now it doesn't go away with throttle. I also noticed a constant ticking from the distributor. Swapped the cap and rotor with my old one and still the ticking and still misfiring on 1.

Hopefully I just damaged the plug wire but I think I'll just go back to the basics and check for spark and fuel pressure. Starting to get a little frustrated with this one.
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Re: P0300 after extended idle

Postby gman » September 16th 2020, 9:38pm

At this point, i would do a leakdown on your cylinders before throwing any more parts at it.
Just so you can absolutely rule out the heads.
Or, at first you can just do a radiator pressure test to rule out engine gaskets like head gasket and LIM gasket.

Even though youve done those, woildnt hurt to test them because you already have everything apart

You could do that as you do the wires.

When is the last time you did your spider? SCPI is known to leak, and seems to do it on all of these vans at some point.

Anyways, these vans are very finicky to electrical parts, it wouldnt even take an LT1 ignition coil that i had.

Ive been here lol.

Theres way to figure it out without ripping everything apart and throwing parts at it, and IMO thats the real art to automotive maintenance, the deduction aspect.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 18th 2020, 5:53am

Alright well I'm an idiot.

I got my new plugs and went to replace them but decided to check compression on all 3 cylinders on the driver side (I had just that wheel off). All three were good so I finished the wires and started it up and it sounded better but not great. Started replacing the passenger side and I have plug 2 unplugged. No wonder it sounded to bad the other day.

Finished replacing the wires except plug 6, fired it up and it sounded way better. Checked the misfire count and I'm still getting a couple misfired every now and then at idle. No ticking like the other day but still a little roughness. I did notice short term trim was high, like 15-20%. I noticed I hadn't tightened the intake hose down so I did that but it was still a little high. I sprayed some electronics cleaner on some hoses since I already had it in the van but didn't notice any change. I'll have to search around more for vacuum leaks.
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Re: P0300 after extended idle

Postby erikw9 [OP] » September 18th 2020, 5:04pm

Oh I also noticed the resistance of the new AC delco wires was about 1/3 of the autozone ones I've had in there for less than a year. Didn't expect it to be that big of a difference. Not sure what that'll do for performance but it's great to see some empirical differences between products. No wonder everyone here recommends them.

These are the ones I went with: https://www.amazon.com/dp/B000S2SSYK/re ... zFbK46B1FN
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Re: P0300 after extended idle

Postby CopperFiremistCopperFiremist is online! » September 18th 2020, 7:21pm

PM Wimptech about misfires at Idle, I would run some seafoam in the crankcase, fuel tank and down the intake, the common issue is the valves gum up and and don't seal properly especially on the bank 1 aka drivers side. But with a little bit of throttle it smooths out..


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Re: P0300 after extended idle

Postby erikw9 [OP] » September 18th 2020, 10:46pm

Thanks Copper, I'll send him a message. Makes sense to me, seems like seafoam is a pretty harmless and cheap thing to try.
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Re: P0300 after extended idle

Postby gman » September 19th 2020, 5:28am

Please update the thread with the results.
Im really curious as to where this goes.
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Re: P0300 after extended idle

Postby CopperFiremistCopperFiremist is online! » September 19th 2020, 5:42am

Honestly from my experience with the 4.3L if you've thrown every ignition part at it and chased your tail for weeks on end for a P0300 Code and it still misses at idle. It needs a head job.
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