2001 GMC Sonoma - 4.3L Eng P0300 Code

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Tim Martin
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2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by Tim Martin »

I got this 2001 GMC Sonoma with a 4.3L engine, auto trans and 4wd. THe issue is it cam in for emissions inspection and it failed due to a whole host of codes in the PCM memory. I got to diagnosing the codes and addressed several of them. The issue I am having is that I cannot barely begin a OBD II Drive cycle until the CEL comes back on and it shows a P0300 (Random Misfire). On my scan tool it shows misfires on #1, #4, and #6. We got new plugs, new wires, new distributor, and new fuel injector spider assembly. The scan tool shows misfires as soon as the engine starts up. Everything I have done so far is failing to correct the misfire activity. What am I missing? The engine runs absolutely fine, fuel pressure is within specs. Cam Retard is at -7 at 1200 rpm. Oxygen sensor activity even when I check it with my lab scope is an excellent see-saw pattern that ranges from .12volt to .89 volt. The short term fuel trim varies from +4 to -6 and long term ranges from +10 to - 15 depending how long I run the engine and how hard I run it. I am concerned about the long term fuel trim variation as usually I do not see that much variation in the same drive cycle. I am greatly frustrated and the customer is puzzled why come this is not an easy job to resolved. What am I missing?
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Re: 2001 GMC Sonoma P0300 Code

Post by brianp87 »

If the misfires were on the same side id suspect a partially plugged cat. on that side but that #1 mis throws that off. How bout unplugging the front o2's reseting the fuel trims and trying. Id probally do a compression check.
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Re: 2001 GMC Sonoma P0300 Code

Post by steven kiser »

I've dealt with these issues a bunch of times. Usually I find a piece of hard carbon stuck in the EGR plate. This will cause the misfire code and send the trim readings all over the place. Remove the EGR and blow out the port ( be aware that if it's full of carbon it will blow back at you) I would also strongly suggest that you get the "Help" EGR gasket with a screen in it so it won't happen again. The first few times I cleaned the ports out and I really cleaned it, a piece of carbon worked it's way back into the plate. I'm pretty sure that this is where your problem is. The EGR can be removed by wiggling it around a bit and if it slips and hits your fingers it will hurt like heck. Good luck
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Re: 2001 GMC Sonoma P0300 Code

Post by Tim Martin »

brianp87 wrote:If the misfires were on the same side id suspect a partially plugged cat. on that side but that #1 mis throws that off. How bout unplugging the front o2's reseting the fuel trims and trying. Id probally do a compression check.
I am trying to figure out the rationale for a restricted cat being the cause of misfire codes, not saying it cannot be that way. My theory is I entertain any possibility until I prove or disprove the validity of it.
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by Tim Martin »

I am back to working on this again. How can I test for a lower intake manifold gasket leaking vacuum on the bottom side?
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by brianp87 »

Are we certain the misfires are on 1, 4 and 6? What are the fuel trims on each bank?
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by Tim Martin »

brianp87 wrote:Are we certain the misfires are on 1, 4 and 6? What are the fuel trims on each bank?
Great questions. To answer these questions, I defer credit from my scan tool that has each cylinder listed with their misfire counter numbers. 2, 3, 5, and 6 show zero misfires. Let me correct that: #6 shows extremely intermittant very low misfire numbers. 1 and 4 are running 80-90 on each cylinder. Cylinder #1 being the front cylinder on the drivers side and #4 being the middle cylinder on the passenger side. That is where I am getting my info from. Here's the thing: If it was any other cylinder, how would I go about identifying it, or them? What is interesting is that when I go into Mode 6, it does not bring up any misfires. Neither I nor the customer would be all that worried but as it is, the vehicle does not pass emissions inspection here in the state of PA. Last year he was granted a waiver and so this year the problem needs to be addressed. He brought it to my shop without scheduling it the last week of July and it run out on July 31. So, the owner is getting slightly irritated that I do not have it finished yet. His theory is that we have these tools that just tell you what to do. So he is having a hard time appreciating my inability to get this set of wheels back on the road. Ok, we got new plugs, new wires, new distributor including cap and rotor, new ignition module, new coil, new fuel injector spider assembly, new MAP sensor, new crank sensor, and what else? Late this afternoon I removed the PCV valve and felt good vacuum on the port where the PCV valve sets in the drivers side rocker cover. If I place my thumb over the hole there, there is noticeable vacuum there. Back to the question of fuel trims: Short term is running +5-10 on both sides and long term is running in the low twenties on both sides. These reading are indicating to me that the computer is trying to correct for a lean mixture, right? Or am I mixed up on that? Beings that I feel vacuum on the PCV port, I am thinking that there may be an intake manifold leak on the bottom side. No more difficult than these intake manifolds are to R&R, we have begun to remove it and examine the gaskets that GM put there when this ride was made. Will know more when we get it back together tomorrow (Friday). One question I have is that if I reflash the PCM, is there any possibility that it may help this situation?
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by steven kiser »

•Important Information - PIP 3081. This speaks of an occasional valve guide
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by Jeff @ Able Auto »

I think you are on the right track. The computer is trying to compensate for a lean condition. That along with the vacuum on the valve cover would lead me to an intake gasket leak. Maybe get a vacuum gauge on the valve cover to see just how hard its pulling and if it changes with throttle opening. How about using a smoke machine? Plug anywhere it could leak out including the exhaust pipe and look for smoke in the valve covers or the dipstick tube. Just a thought.
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by Tim Martin »

steven kiser wrote:•Important Information - PIP 3081. This speaks of an occasional valve guide
At the risk of seeming like I am uninformed, I am wondering where I get access to this bit of information steve.
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by brianp87 »

The fuel trims answered your question.
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Re: 2001 GMC Sonoma - 4.3L Eng P0300 Code

Post by steven kiser »

I'm thinking Brian is on the right track here. The more and more I look at it the more and more I think we are missing something basic. An engine needs four things to operate fuel, compression, intake and exhaust. Some people add ignition but being an old fart I look at diesel engines as not having an ignition system per say. Let's get back to basics here shall we. If this was at my shop somewhere along the line I would request a detailed compression report along with a detailed three step back pressure one as well. I've been reading about this a bit especially during my wind down period at the end of the day so I can go home more relaxed as opposed to going home still wound up. Anyway I'm starting to wonder if a partially restricted converter in the culprit. I would have my tech test it at idle and at 3k rpm before engine reaches operating temp and after it's been running for a half hour. I suspect the results after warming up will lead you to the answer. I don't know the configuration of the converters but if they are one each side and then another after the Y pipe the rear one being partially plugged may cause a feed back and partially restrict the exhaust causing a scattered misfire at the pcm's level. I would suspect that if this is an issue the restriction would be after the system joined. It could be in a pipe. a muffler and or a bent tail or other pipe causing a restriction. It wouldn't have to be much of one but just enough to throw this code. My basic understanding of the 300 code is the pcm knows there is an issue but at it's level can't be identified. I've seen a 300 being caused by a multitude if issues from ignition, exhaust, fuel, compression, emission related, cam shafts, heads, rockers, etc. check compression and exhaust back pressure. I would also ask the customer about any oil consumption along with if this issue came about after he used a flex fuel.
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