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GMSEE
04-04-2009, 11:09 PM
1996 GMC K1500 XC 5.7L SFI

My truck is misfiring. It is giving the code of P0300, which in my Haynes book says "Engine Misfire". It is running really rough consistent with a misfire. I've driven it around town and it is really down on power. I can't figure out what could be the problem. The motor is shaking pretty bad a certain RPM's. I have dual exhaust that is completely separate on both sides. One side of the exhaust is throbbing a lot more than the other side. Can't hear or see anything else weird in the motor.

OK, so I've replaced:

1. Both O2 sensors (pre-cat)
2. Camshaft Position sensor
3. Crankshaft Position sensor
4. Spark Plugs
5. Plug Wires
6. Cap and Rotor
7. Fuel Filter
8. Cleaned MAF sensor
9. Ignition Coil

I don't know what there is left to do... I've done everything I can think of.

Is there anything else that could be causing a misfire??? Or I could try changing????

Aloicious
04-04-2009, 11:14 PM
1996 GMC K1500 XC 5.7L SFI

My truck is misfiring. It is giving the code of P0300, which in my Haynes book says "Engine Misfire". It is running really rough consistent with a misfire. I've driven it around town and it is really down on power. I can't figure out what could be the problem. The motor is shaking pretty bad a certain RPM's. I have dual exhaust that is completely separate on both sides. One side of the exhaust is throbbing a lot more than the other side. Can't hear or see anything else weird in the motor.

OK, so I've replaced:

1. Both O2 sensors (pre-cat)
2. Camshaft Position sensor
3. Crankshaft Position sensor
4. Spark Plugs
5. Plug Wires
6. Cap and Rotor
7. Fuel Filter
8. Cleaned MAF sensor
9. Ignition Coil

I don't know what there is left to do... I've done everything I can think of.

Is there anything else that could be causing a misfire??? Or I could try changing????

common causes of P0300 in the L31 motors could be the injector spider problems, fuel pressure regulator leaking, and intake manifold gasket leak. good luck, these radom misfires can be a pain to track down sometimes.

EDIt- forgot, the dizzy gear can get worn in these trucks and cause the p0300 also.

86GMC4.3
04-04-2009, 11:15 PM
does it still have cats? im assuming it doesent but you know where that can get you.

GMSEE
04-05-2009, 12:21 AM
Yes still has cats. It has two separate cats. What is the spider gear?

Aloicious
04-05-2009, 12:46 AM
Yes still has cats. It has two separate cats. What is the spider gear?

spider gear? no, the dizzy gear is the gear on the bottom of the distributor that mates with the cam, the injector spider is the fuel injecting aparatus on the L31 motors, it consists of a fuel metering body and 8 poppet valves to act as fuel injectors, it looks like a spider is why its named that.

here's some posts to help:
http://www.fullsizechevy.com/forums/general-discussion/technical-maintenance/329824-finally-fixed-my-random-misfire.html

http://www.fullsizechevy.com/forums/general-discussion/technical-maintenance/260251-stumbling-misfires.html

GMSEE
04-05-2009, 03:04 PM
OK, thanks for the info. Does anyone know about the MAP sensor?Would this be causing a misfire? The Haynes book says the MAP sensor uses intake pressure to tell the PCM how to control fuel delivery and ignition timing.

I've already replaced everything else that has to do with ignition timing.

One thing to mention is when I put a spark sensor (non-invasive) on my wires and cap it only shows a spark every so often and very very random. Sometimes I won't see a spark for up to 10 seconds . It's completely random on every wire. When I put it to the coil wire it shows a constant light. Why would the spark to the plugs be so random?

Aloicious
04-05-2009, 04:27 PM
OK, thanks for the info. Does anyone know about the MAP sensor?Would this be causing a misfire? The Haynes book says the MAP sensor uses intake pressure to tell the PCM how to control fuel delivery and ignition timing.

I've already replaced everything else that has to do with ignition timing.

One thing to mention is when I put a spark sensor (non-invasive) on my wires and cap it only shows a spark every so often and very very random. Sometimes I won't see a spark for up to 10 seconds . It's completely random on every wire. When I put it to the coil wire it shows a constant light. Why would the spark to the plugs be so random?

the MAP sensor is the "Manifold Absolute Pressure" sensor, it tells the PCM what the vacuum is in the manifold so the PCM can calculate the fueling. its possible that it could cause a misfire, but pretty unlikely. I'd really bet it is the distributor gear, that is a very common part on our trucks that will cause a P0300.

I don't know abou the spark light, I would manually pull a few (one at a time) plugs, ground them and check for spark while someone cranks the engine over, if they were firing in 10 second intervals, the truck wouldn't even run. however, the goofy readings do suggest a spark problem over a fuel issue. which again, would point to the distributor gear being worn or a cap/rotor issue since the readings at the coil are consistant.

does the cap bolt down tightly? there is a high likelyhood that the screws that hold the cap down have stripped or broken the plastic tabs on the distributor that they screw into, I had that happen to me and it wouldn't let the truck run at all. I have since put in a aluminum distributor to stop that from happening again.

86GMC4.3
04-05-2009, 11:21 PM
the way you described the exhaust would make me think about possibly a clogged cat but idk.

GMSEE
04-06-2009, 03:37 AM
on my motor the timing is set by the computer, so if I take the distributer out to replace the gear, how do I know how to put it back in correctly? Is it just by making a mark on the housing of the distributer where it meets the motor?

Da Burb
04-06-2009, 02:22 PM
Mark iot all before you take it out. If the gear needs changing, do so and then re-install exactly as you removed it. I always take a heap of pic's of everything I do....handy reference!
You may need to get the CMP retard set by someone with a scanner once you replace the dizzy gear.

Aloicious
04-06-2009, 02:47 PM
Mark iot all before you take it out. If the gear needs changing, do so and then re-install exactly as you removed it. I always take a heap of pic's of everything I do....handy reference!
You may need to get the CMP retard set by someone with a scanner once you replace the dizzy gear.

yup, mark the dizzy before you pull it, that way it will run when you put it back in, I would take it somewhere that has a scanner able to read the CMP retard and have the timing set, shouldn't cost much, even at the dealer I would think it would be a pretty routine thing.

GMSEE
04-07-2009, 12:39 PM
If I take it out and replace it exactly the same way why then would I have to have the timing reset?

Aloicious
04-07-2009, 04:59 PM
If I take it out and replace it exactly the same way why then would I have to have the timing reset?

it would run but the timing on these is VERY sensitive (VERY we're talking a couple degrees could be like 1/16 of an inch turn), when I pulled my dizzy last time, I put it back in the same as when I took it out, and in reading the CMP retard with my efilive, it was still 14* off. you may have it close when re-stabbing it but chances are its not super precise, best just to get it set. the CMP retard needs to be at 0* +- 2*. you can definantly get it close so that it will run just fine w/o codes, but I'm too OCD to not have it set correctly on my truck, Howard will agree with that I believe. :aniteef: .

Da Burb
04-07-2009, 05:59 PM
Agree 100%. The damned things are so sensitive that just tightening the hold down clamp can throw them out a few degrees.

GMSEE
04-08-2009, 01:37 AM
cool, I'll let you know how it goes. Thanks for the help.

CscottsSS
04-08-2009, 06:28 AM
fuel pressure regulator........

GMSEE
04-10-2009, 02:32 AM
So, got a new dizzy gear and noticed that the teeth were not the same as the original ones. They were a little more curved and thinner. I chose to install it anyway and holy crap that was a mistake. I heard a loud clanking while trying to start it. I pulled it back out and reinstalled the original which actually didn't look too bad. When I reinstalled the original I put it back in at TDC on the exhaust stroke. So had to reinstall it again in the real Top Dead Center, but couldn't get it to start. The timing must have been way off. I put it all back together correctly, and it would start but would just sit there "dieseling" (if that makes sense) no throttle response at all. Basically was not running. Had to get it towed to a local mechanic and will update you as to what the problem ends up being.

vcrazy8
04-10-2009, 09:27 AM
I have seen this more than once. Here is a GM TSB
Subject: SES Light and P0300 when Towing, Cruisi #PIP3081: SES Light and P0300 When Towing , Cruising Uphill or on Hard Acceleration - kw 4.3 5.0 5.7 accelerate cruise cylinderhead DTC L30 L31 L35 LF6 LU3 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 - (Jul 23, 2004)

Subject: SES Light and P0300 when Towing, Cruising Uphill or on Hard Acceleration



Models: .

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
Condition/Concern:

The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3,4,5 or 6 will be the ones to experience this.
Recommendation/Instructions:

If the P0300 SI diagnostics did not isolate a concern, the following may help:

Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:


Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.


Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.

Reassemble the cylinder heads using new valve seals.

.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.
Models:

(96 - 02 Chevrolet Astro) and (96 - 02 GMC Safari) and (96 - 00 Old Body Style Chevrolet Suburban) and (96 - 00 Old Body Style Chevrolet Tahoe) and (96 - 02 Chevrolet Express) and (96 - 00 GMC Yukon) and (96 - 00 Chevrolet Old Body Style C/K Truck) and (96 - 00 GMC Old Body Style C/K Truck) and (96 -02 GMC Savana) and (96 - 99 P32 Cab and Chassis) and (96 - 02 Chevrolet S-10) and (99 - 02 Chevrolet Silverado with 4.3L V-6 Engines (96 - 02 GMC Sonoma) and (96 - 02 GMC Sierra with 4.3L V-6 Engine

GMSEE
04-10-2009, 10:18 PM
So, I had to take my truck in to a mechanic, not to mention have it pulled from an underground garage and loaded on to a flatbed. Let me tell you, seeing that almost made me cry... Took it to a good mechanic and he told me that the #5 injector is bad. So he is replacing the whole injector spider with a new acdelco setup its going to cost me $1000 in all including diagnostic and reseting the computer to the distributer shaft, but in the end I think it'll be worth it. I've learned a whole lot about this beauty of a truck, pored my blood sweat and tears into it and now love it more than ever. Hopefully it'll run better too! I called the dealership and they told me it would be $1100 all together to replace the injector spider. I opted to do it with the mechanic because he was only charging me 809 for the whole injector replacement with the new updated MPFI injectors. I found this site (link below) that explains a little about Vortec CSFI to MPFI conversion. Look at it theres tons of clear pics...

http://www.theautoshop.net/VortecMPFI.htm

GMSEE
04-11-2009, 02:21 PM
The truck is done! Runs really good compare to how it ran before. The price total from all work and diagnostics done was $993. Anyways they gave me the old injectors and they looked like crap. I'll take a pic before I toss them and post. The truck, however, still has a slight misfire... Pretty much when driving on the freeway under cruising load I can feel it missing. Sometimes it's worse than others. The check engine light only comes on if I drive it long enough and its the same code, P0300. I took it back to the shop and they say that there is nothing they can do. I think they are just screwing me over. Any Ideas?? Also, on start up about 50% of the time it misses then fixes itself. The other 50% it lights up just fine. Thinking dizzy gear now??? But if it were something like that (mechanical) wouldn't it be happening the same way constantly? It seems to me with the intermittent misfiring it's electrical of some sort. But what?

Aloicious
04-11-2009, 05:03 PM
The truck is done! Runs better than ever. The price actually total from all work and diagnostics done was only $993. That was such a good deal. If anyone lives in Burbank and needs auto work, go to "Cornejo's Automotive" aka "Fuel Depot". Anyways they gave me the old injectors and they looked like crap. I'll take a pic before I toss them and post.

weet, glad you got it going good. those P0300's are so vague its sometimes hard to track them down. I'm glad you went with the upgraded spider, they should work much better and offer better longevity than the poppet valves.

GMSEE
04-14-2009, 01:57 PM
Here are some pics of my 1996 GMC 5.7l Vortec Injector Spider. Looks pretty crappy. Lots of grime build up...
http://i188.photobucket.com/albums/z211/Sootkase/injectorspider3.jpg
http://i188.photobucket.com/albums/z211/Sootkase/injectorspider1.jpg
http://i188.photobucket.com/albums/z211/Sootkase/injectorspider2.jpg
http://i188.photobucket.com/albums/z211/Sootkase/injectorspider4.jpg

Aloicious
04-14-2009, 06:11 PM
yeah thats pretty gross, did you have a coolant leak of some sort? I'm wondering what the yellow/orange crap is around the tips.

GMSEE
04-15-2009, 03:39 PM
Yes, I did have a coolant leak but was never able to sort out where it was coming from. I have noticed that since the upper air intake now had a new gasket, I haven't seen any more leaking. Could that be where the coolant leak was coming from? I didn't know that coolant could leak from there. Where does it flow from? Those yellow and orange specs on the tips are just from rolling around in my truck bed.

Aloicious
04-15-2009, 04:30 PM
Yes, I did have a coolant leak but was never able to sort out where it was coming from. I have noticed that since the upper air intake now had a new gasket, I haven't seen any more leaking. Could that be where the coolant leak was coming from? I didn't know that coolant could leak from there. Where does it flow from? Those yellow and orange specs on the tips are just from rolling around in my truck bed.

no, coolant can't leak from the upper manifold, only the lower. if its coming from there, which is pretty likey since those lower intake gaskets are known to leak, you'd need to pull the whole intake to replace them.

GMSEE
04-17-2009, 04:59 AM
that's what I thought, but it seems the leak is gone now...

Aloicious
04-17-2009, 10:14 AM
that's what I thought, but it seems the leak is gone now...

thats good, keep an eye on how your oil looks, and how your coolant levels are, it could be leaking into the oil which is not good.

GMSEE
12-10-2009, 08:40 PM
still misfiring. Intermittent and throws P0300 when I drive it long enough. Totally pissed. Sometimes it misses at start up... but not all the time. Does this sound electrical or mechanical?

Aloicious
12-10-2009, 08:56 PM
still misfiring. Intermittent and throws P0300 when I drive it long enough. Totally pissed. Sometimes it misses at start up... but not all the time. Does this sound electrical or mechanical?

did it stop for a while and now come back? check your distributor cap again, my truck used to eat caps and rotors, I needed one every 8 months or so, back when I used a distributor.

can you FEEL the misfire now, or is the SES light the only way you know its misfiring?

red_vortec
12-28-2009, 12:53 PM
Wow! what a nightmare...I am about to do the MFI conversion as well

GMSEE
01-09-2010, 03:18 AM
Ok, sonofabitch! It was the plugs!!!!!!!!! I f-ing changed the plugs back to the stock ac delco platinum's and viola...A non-misfiring truck!

Well, again that's not entirely true. I drove from LA to Vegas (about 270 mi) with the same E3 spark plugs and the truck was misfiring pretty bad at freeway speeds. While in Vegas I talked to a few mechanics and they immediately said to change the plugs back to stock. So, thinking the plugs I had were superior to the stockers, I did it anyways. Drove back to LA and it only slightly misfired 3 times right in the beginning of the drive. After that it's been misfire free.

Oh ya, I also had it tuned with the Hypertech power programmer this whole time even on the way back from Vegas when it slightly misfired 3 times but when I got home I took the power tuning out of it and NO MORE MISFIRES. Been driving it all around and its as smooth as butter. I'm planning on driving to Santa Barbara soon so we'll see if it does it anymore. Keep you posted.

GMSEE
01-09-2010, 03:22 AM
Do not buy E3 spark plugs. For my truck they did not work what so ever. I carefully monitored fuel mileage and they did not help at all. Not to mention they look like the cheapest plugs ever made. Stick with the stock AC Delco Platinums. You'll thank me.

xch3no2
01-09-2010, 04:10 AM
Not enough resistance in the plug will do it..leakers! (I have seen more broken broken plugs cause this than anything else).

GMSEE
01-14-2010, 06:18 PM
Now I'm starting to wonder if this has been my problem the whole time and I just got taken for a grand???

86rcscc
02-08-2010, 06:14 PM
I had the same misfire and GM had to reflash my PCM with the update software and it fixed it. Got it done last week and i keep checking eith a OBD2 readed and has not come back . But i will keep my fingered crossed ..The was a tsb issued for this .

GMSEE
02-12-2010, 08:42 PM
Was it free to get it flashed? Where can I find info on the TSB?

86rcscc
02-13-2010, 05:12 PM
No, I had to pay for it . $70.00 I can email you a scan of the tsb

Crewdawg141
04-23-2010, 05:21 PM
Can either of you tell me about this TSB that calls for a new Flash on the PCM? I am having a P0308 constantly after doing a whole lot of work . The mechanic that did a relearn for me also told me that my whole right bank was misfiring. I am at a loss as I just dropped this engine into the truck, it was built with all new everything so I am at a serious loss.

jzpp904
04-23-2010, 06:23 PM
had this code plus po155 po154 get some carb or break cleaner with a tip start engine spray between intake to head if idles up or down replace intake gaskets original gaskets are orange seals get the new teal green colored ones something about ethonal in the fuel eats them away lucky i found this before i replaced everything but make sure you spray the intake area first to see mine started to idle down almost stall do it when engine is cold stuff is explosive flammable check fuel filter if nothing happens after spraying most likely the problem