P0300 Random/multiple misfire

[troverman]

My 2005 GMC 3500 c/c 6.0L was running good. I was driving it up a hill the other day and all of a sudden it started running like crap and the engine light is blinking. Now it runs like crap misfiring away every time I start it. I changed the spark plugs (Delco platinum) and the MAF today...no change. I do have two codes for oxygen sensor heaters but those codes existed before when it was running good. What could it be? 

[casey.mikl]

Could be coil packs. If you have a flashing check engine light. It should store a code and tell you what cyl is misfiring. Start the truck and run it. Shut if off, pull 1 coil pack so it kills the cyl. Start the truck again. See how it runs. Plug coil pack back in, move to the next. Keep doing it till you find the one that makes no change. Then swap it to another cyl to confirm it’s not functioning properly.


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[Supreme Pizza]

I am an able mechanic, but I am not an expert diagnostic specialist. If I had to spend my life diagnosing engine problems that normal mechanics could not solve - then I would kill myself out of frustration. I paid a highly recommended "super duper" diagnostic shop $300 today to fix a problem on a customers vehicle. The problem is still there... I paid money for nothing. Plus I lost 5 hours of my life waiting for the shop to do what they do.  Well, its time to get back to the real world. Here is what I can tell you about your rig: 

 

 Random misfire:   Not related to one specific cylinder (such as a bad plug, wire, compression, injector, etc)

 

Could be an issues that is common to all cylinders. (fuel pressure, air intake, fuel quality, MAF, O2 sensors, etc)

 

You have to be a detective to find the problems. First of all, solve all of the OBD2 codes. 

[Supreme Pizza]

What are the O2 sensor codes indicating? Is it a pre, or a post cat sensor code? If its a pre-cat, you have an issue. If its post cat, you MIGHT not have to worry about that code, because it could be just a bad cat or post cat O2 problem. If its a "pre", then your motor is producing the wrong gases and needs to be adjusted / repaired. 

[Formulabruce]

I would take OFF the throttle body and clean it VERY VERY well, WITH TB cleaner, NOT carb cleaner, go buy the right stuff. Clean the plate, BOTH sides and the whole area in there. This would be my First thing to do, has fixed many of this symptom, including stalling at a stop sign.

[troverman]

O2 codes are P0141 and P0155...both are heater circuit malfunctions, both are on different cylinder banks, and one is upstream and one is downstream. If its just the heater, my experience is that any symptoms would go away by the time the truck was fully warmed up. That is not the case.

 

Some people have mentioned a vacuum line off the fuel pressure regulator. I do not see any regulator on this truck.

 

The MAP sensor seems very loose, but connecting it and disconnecting it doesn't seem to make any difference.

 

I'm thinking about doing a compression test. I'd like to test fuel pressure, but don't have a tester. Even if pressure was a little low, wouldn't it run well at idle and poorly when more throttle was needed? This truck is misfiring at idle (light shake) and right on up through the RPM range.

 

So to summarize:

 

-Truck runs rough and misfires at all RPM ranges

-Exhaust smells hot

-Truck does not have great power

-P0300 code stored

-P0141 and P0155 codes stored

[Supreme Pizza]

I would replace the O2's. They are not very expensive, and need periodic replacement anyway. 

 

These are not price shopped. Make sure to confirm fitment if you order them. 

 

https://www.amazon.com/ECCPP-Downstream-2011-2013-Chevrolet-2003-2006/dp/B01N593XPP/ref=sr_1_19?s=automotive&ie=UTF8&qid=1525788292&sr=1-19&keywords=2005+GMC+3500+o2+sensor

 

https://www.amazon.com/ECCPP-Upstream-Downstream-1996-1998-Chevrolet/dp/B01NCTMFRA/ref=sr_1_18?s=automotive&ie=UTF8&qid=1525788292&sr=1-18&keywords=2005+GMC+3500+o2+sensor

[troverman]

Those don't fit. Either for the 8.1L engine or older style GM engines. You've got to remember, these codes existed before, when the engine was running good.

 

I'm going to rent the auto-zone fuel checker and see what the fuel pressure is. I'm also going to remove the upstream O2 sensors and see if it runs any better with a "vent" ahead of the cats.

[Supreme Pizza]

I have used tire pressure gauges on the fuel rail shrader valve to check fuel pressure. Normally it would not work, because the tire gauge doesn't reach far enough down into the shrader valve to push the valve open, but all you have to do is drop a small pebble into the tip of the valve before using the gauge. The pebble works the same way as a firing pin does between the hammer and the primer. 

 

You might be right about the o2's being unrelated, but according to the OBD2's - they are at least partially failing. Perhaps they have failed further. 

 

To quote Google: 

 

"Vehicle engines that have a bad oxygen sensor often run irregularly or roughly.Oxygen sensors can control or contribute to many different engine functions, including fuel/air mixture, engine timing and engine combustion intervals. A faultyoxygen sensor can disrupt any of these things and cause a rough engine idle."

 

 

 

[troverman]

Thanks for the tip, starman8. I'm just convinced with the codes already existing and the engine running smooth as silk they aren't causing this. Perhaps I'm sick of just throwing money at this truck. Plus the codes are for the heaters, not a failed sensor.

[Supreme Pizza]

 

Would they take the MAF senor back? 

 

If you say that you plugged it in as a test, and it didn't help - they wont allow the return. 

 

They do allow returns for: 

 

Didn't fit vehicle. 

You didn't install it on engine. (they look for marks on the terminals)

Installed, but tested bad - and you already got another one from some other place. 

 

[troverman]

I doubt they'll take it back. I try to be honest, so even though I appreciate the sentiment I'll just suck it up.

[Dixon700]

Only code being random misfire, possibly a cam or crank sensor, perform a crank position relearn so if it makes any difference.

[troverman]

I figured the cam or crank sensor would throw their own codes. I have no way to do a crank position relearn without taking it to a dealer (ways away from me).

[1st diesel truck]

Just went through code P0300 on an 2010 chrysler town and country van 4.0L with occasionally very intermittent P0306 popping up to start then eventually P0304 showed up roughly half a year later.....This took awhile to figure out as it did it mostly at idle but occasionally at higher RPM's too.

 

I started off with new spark plugs which the old ones looked fine and one new coil for cylinder #6......Ran okay after that but you could still feel some roughness to the engine at idle mostly....Codes stopped for awhile then P0300 came back strong with intermittent P0304.... I now wish I did a cylinder compression test while the plugs where out but didn't think it was really needed suspecting injectors, sensors or coils, pcv valve, EGR valve which was replaced as it was questionable while rattling it and cheap enough while in there.......Cleaned throttle body, and all intake plenum related components....MAF sensor seemed okay so didn't mess with that too much plus with no codes for it was hoping for the best though it's on a big list of possibilities when thoroughly reading through this code online from different websites as the list kind of varies.

 

Ran a bunch of fuel cleaners through it for fuel treatment and injectors, etc. as the van did at one point get a bad load of fuel from a high capacity chevron station. 

The exhaust smelt funny/odd too throughout the ordeal.  

 

Started to suspect injectors being dirty, low fuel flow, etc. and almost pulled them for inspection, cleaning or replacement if needed but held off because the van is still technically under chrysler lifetime warranty so cost wise wasn't worth it... yet.

 

Also sort of suspected a possible voltage diode type problem say within the alternator as the headlights seem to slightly flicker during the issue of the code popping up after clearing. For the heck of it I temporarily tried a new alternator as it was easy enough to get too to no avail and all battery posts where clean and tight etc..   

 

Though it took the dealer close to a month to find the issue; 'Cylinder # 4 and 6 where low on compression down to 105 psi while the rest where just over 175 if I recall them saying correctly.....Under warranty the van ended up with a new engine and it runs great now with no more rough idle, etc..

 

Moral of my story is you may as well perform the compression test and hope it's not it before going after the sensors, injectors etc. for code P0300.

 

Let us know, good luck.