p0300 mult cylinder misfire....

I have an 03 1500 silverado, 125000 miles, 5.3V8..... this morning once started it ran real rough, erratic idle, my check engine light flickered many times, but didnt stay on. I checked the codes after driving 25 miles and it had a p0300...which looks like multiple cylinder random misfire......so what to do? Plus are ac delco iridiums, changed about 50,000miles ago, new wires then as well.....

I have an 03 1500 silverado, 125000 miles, 5.3V8..... this morning once started it ran real rough, erratic idle, my check engine light flickered many times, but didnt stay on. I checked the codes after driving 25 miles and it had a p0300...which looks like multiple cylinder random misfire......so what to do? Plus are ac delco iridiums, changed about 50,000miles ago, new wires then as well.....

check the fuel pressure should be 53-62psi. could be iced up fuel in tank / filter especially if you keep fuel level low in the tank .

need scan tool to reset this code . engine running poorly will create this code as the cam gets jolted ..

Random misfires on random cylinders -

Air - MAF sensor, EGR system, PCV system, and Air Intake system.
Fuel - Fuel filter, fuel pump, flaky fuel pump relay.
Exhaust - A plugged catalytic converter can cause random misfires.
Ignition system - Crank sensor, and cam sensors.

check the fuel pressure should be 53-62psi. could be iced up fuel in tank / filter especially if you keep fuel level low in the tank .

need scan tool to reset this code . engine running poorly will create this code as the cam gets jolted ..


I did have a full tank of gas, will have to get a fuel pressure kit and see what I get. I almost wish the engine stays running rough so I know its fixed.
I had to jump my truck this morning due to the extreme cold, could that make it run rough?

I did have a full tank of gas, will have to get a fuel pressure kit and see what I get. I almost wish the engine stays running rough so I know its fixed.
I had to jump my truck this morning due to the extreme cold, could that make it run rough?

with low voltage and cold temps the fuel pump will not create the correct pressures quickly...

correct the battery/cable power issues first. check the alternator for proper output. have battery load tested.

then check the fuel pressures..

with the vehicle sitting it may show good pressure . driving may cause the pressure to drop out ... this is common.

voltage to the fuel pump [pump relay] also power and ground connections to the / of the fuel pump.

even fuses improperly removed causing loose sockets can create problems ..

Ok update....cleared the p0300 code last week and stayed away...yesterday light back on, now getting p0174 and p0171.....truck appears to be running ok.... I read somewhere that these codes are related to the initial p0300 code.....I checked fuel pressure and get 50/51 at idle and around 53 when I rev the motor...

intake............

intake............


I was afraid of that, had read it was common to have that problem....any way to test/check for sure? I heard spraying around intake and looking for bubbles? Would i need to replace to the entire intake with the newer redesigned, or just the gasket?

I did have a full tank of gas, will have to get a fuel pressure kit and see what I get. I almost wish the engine stays running rough so I know its fixed.
I had to jump my truck this morning due to the extreme cold, could that make it run rough?


I get about 51psi max 54psi fuel pressure when revving motor...that seems low relative to what you mentioned jcat?

I get about 51psi max 54psi fuel pressure when revving motor...that seems low relative to what you mentioned jcat?

the fuel pressure is low . if below 53psi then with these codes I would also check the pressure on how long it stays around this 50 psi with the engine off..if it drops quickly 90% its the pump.

your problem of intemittent running poorly is not a gasket leak.. gaskets that have failed are a constant..a motor / electric motor with bearings/brushes /pump impeller can cause pressures to vary as well as the feed voltage and the ground supply ...


with a new fuel filter and you get 50 psi I would replace the pump..this is not horse shoes game .close does not help you .

intake manifold leaking can ALSO be an issue here. the fuel pressure must first be corrected. this is not at the spec.

the fuel pressure is low . if below 53psi then with these codes I would also check the pressure on how long it stays around this 50 psi with the engine off..if it drops quickly 90% its the pump.

your problem of intemittent running poorly is not a gasket leak.. gaskets that have failed are a constant..a motor / electric motor with bearings/brushes /pump impeller can cause pressures to vary as well as the feed voltage and the ground supply ...


with a new fuel filter and you get 50 psi I would replace the pump..this is not horse shoes game .close does not help you .

intake manifold leaking can ALSO be an issue here. the fuel pressure must first be corrected. this is not at the spec.

Ok rechecked fuel pressure....I definitely am getting 51psi....when I shut the motor off I instantly get 48.5, then in 7 minutes it was down to 38psi..... I bought a new fuel filter which I will do if the snow ever stops here, I have to do it outside unfortunately. But, could it just be the fuel pressure regulator? The fuel filter is an ac delco with only about 30,000miles on it, truck isnt rusty and I run lucas fuel cleaner through it regularly, along marvel once in awhile as well. The tank would have to be dropped for the fuel pump on this truck right? Could low voltage/amps cause low psi?

Ok rechecked fuel pressure....I definitely am getting 51psi....when I shut the motor off I instantly get 48.5, then in 7 minutes it was down to 38psi..... I bought a new fuel filter which I will do if the snow ever stops here, I have to do it outside unfortunately. But, could it just be the fuel pressure regulator? The fuel filter is an ac delco with only about 30,000miles on it, truck isnt rusty and I run lucas fuel cleaner through it regularly, along marvel once in awhile as well. The tank would have to be dropped for the fuel pump on this truck right? Could low voltage/amps cause low psi?

the fuel regulator check is by seeing if the vacuum line to it has fuel in it. this would mean it has a torn diaphram. this works with vacuum to slightly change the fuel pressure. no vacuum max fuel pressure like when cranking engine..high vacuum like at idle lower fuel pressure..

so key on engine off fuel pressure should be 60 psi. with the engine idling should be above 53psi. pull the vac line off the regulator fuel should rise to 60 psi area.

I use the purolator fuel filters now. same as ac delco in quality , much lower price. you will need a small cup to capture the leaking fuel when you do this . park so front end is higher than the rear.

replacing the fuel relay with over 100,ooo miles is a good idea. even if the pump is the cause as a bad relay can damage a new pump motor. obviously replacing the fuel filter is needed anyway with the miles on it and with a new pump.

dropping the tank is one way to do this pump or as others have found removal/slide back about 2ft, of the truck bed with 3 guys .


rust in the filter is from not your tank , but that of the fuel supplier ! alcohol also GROWS crap in it ...

about 3 months ago I looked at my records for the vehicles and realized it was almost 4o,ooomiles on the fuel filter.

I bought filters for the fleet of vehicles and replaced them all..now I find they all start with only one revolution of the start crank cycle. about 1.5 sec. before it was taking about 2 revolutions to get the engine to start...

My money on intake, i have never had a driveability prob with over 50psi. I cant count how many times I have done these and no comebacks so far.

You need to do some more diag though before you go out and drop $250 on an intake.

about 3 months ago I looked at my records for the vehicles and realized it was almost 4o,ooomiles on the fuel filter.

I bought filters for the fleet of vehicles and replaced them all..now I find they all start with only one revolution of the start crank cycle. about 1.5 sec. before it was taking about 2 revolutions to get the engine to start...

Ok just went out....I get 55psi vacuum line off truck in run (not running), I get 59.5 with truck running and vacuum line off, when running I can hear the sucking of the vacuum line, sounds strong. Hooked vacuum back up while running and pressure dropped down to 50/51 again. I could hear a difference in the motor with the vacuum line hooked up and without it hooked up. Btw I did buy a Purolator fuel filter, I like them as well.

My money on intake, i have never had a driveability prob with over 50psi. I cant count how many times I have done these and no comebacks so far.

You need to do some more diag though before you go out and drop $250 on an intake.

What could I do to isolate that its the intake though? If it is, would you recommend the Dorman aluminum replacement?

I found a good video, see http://www.youtube.com/watch?v=FfiQ7qWiG-M

Unmetered air could lead up to random mifires, and DTC's.

I found a good video, see http://www.youtube.com/watch?v=FfiQ7qWiG-M

Unmetered air could lead up to random mifires, and DTC's.


Thanks...Yeah if only I had a smoker! I am going try the carb spray near the intake gasket and see if that shows anything as a test!

what we do is smoke the intake but you prob wont have access to one of those. try carb clean or propane from a small torch and see if there is an idle difference. also need to do this when the engine is cold.

I just use the plastic kit from dorman, comes with everything and is thicker than oem so they dont warp.

what we do is smoke the intake but you prob wont have access to one of those. try carb clean or propane from a small torch and see if there is an idle difference. also need to do this when the engine is cold.

I just use the plastic kit from dorman, comes with everything and is thicker than oem so they dont warp.

Now just to be clear, we are talking about the black plastic, upper intake manifold thats long and narrow like this one....

http://cgi.ebay.com/ebaymotors/99-07-GM-4-8L-5-3L-6-0L-Intake-Manifold-incl-hardware-_W0QQcmdZViewItemQQhashZitem1e612e2c6cQQitemZ13047 9434860QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccesso ries

Ok just went out....I get 55psi vacuum line off truck in run (not running), I get 59.5 with truck running and vacuum line off, when running I can hear the sucking of the vacuum line, sounds strong. Hooked vacuum back up while running and pressure dropped down to 50/51 again. I could hear a difference in the motor with the vacuum line hooked up and without it hooked up. Btw I did buy a Purolator fuel filter, I like them as well.
What do you think jcat?

Back yard smoke Test-
1) Take the air filter out, wrap with glad wrap, and put back in.
The Idea is to block all air going into the Inake system.
2) Disconnect the brake booster hose going to the Inake.
3) Light a Black & mild cigar, blow smoke into the brake bosster vacuum hose going to the Intake, and cover with thumb inbetween puffs.
Any smoke leaks is unmeter air leaks, that needs tobe repaired.

danielsatur

He doesnt smoke.

When ever you create a vacuum leak like when you take the line off of the regulator it will affect idle b/c you are letting unmetered air into the system.

Smoke Test's are done with the engine off!

Caution - Starting fluids, and other sprays canbe very flammable.

Ok just went out....I get 55psi vacuum line off truck in run (not running), I get 59.5 with truck running and vacuum line off, when running I can hear the sucking of the vacuum line, sounds strong. Hooked vacuum back up while running and pressure dropped down to 50/51 again. I could hear a difference in the motor with the vacuum line hooked up and without it hooked up. Btw I did buy a Purolator fuel filter, I like them as well.

the regulator is working correctly. problem is the fuel pressure is too low.

the spec is 53- 62psi. this is required to get the engine running correctly..the guage your using must be correct/accurate.

many have had this problem as the pump slowly dies.like you it is intermittent as the pressures are low but not so low it will not run.

normally 180,ooomile is what this componet fails at..running on low fuel it will fail sooner.

replace the fuel filter and the relay first . check all connectors grounds to the fuel tank/pump assembly..then replace the pump ..now you will be good for another 180,ooomi...

the regulator is working correctly. problem is the fuel pressure is too low.

the spec is 53- 62psi. this is required to get the engine running correctly..the guage your using must be correct/accurate.

many have had this problem as the pump slowly dies.like you it is intermittent as the pressures are low but not so low it will not run.

normally 180,ooomile is what this componet fails at..running on low fuel it will fail sooner.

replace the fuel filter and the relay first . check all connectors grounds to the fuel tank/pump assembly..then replace the pump ..now you will be good for another 180,ooomi...

Ok I got the check engine light to go off! I took the air cleaner assembly apart and cleaned the mass air flow sensor, I could see that it was cleaner afterwards! The truck sounds much better and runs stronger and smoother, within 2 miles of driving light went out and all codes were gone! It must be the oil from my airaid intake filter?
But my issue is that I reset my mpg calc and I get only 13-13.5mpg around town/country roads. Could my possible fuel pressure issue relate to this?

Back yard smoke Test-
1) Take the air filter out, wrap with glad wrap, and put back in.
The Idea is to block all air going into the Inake system.
2) Disconnect the brake booster hose going to the Inake.
3) Light a Black & mild cigar, blow smoke into the brake bosster vacuum hose going to the Intake, and cover with thumb inbetween puffs.
Any smoke leaks is unmeter air leaks, that needs tobe repaired.

danielsatur

What would happen if you lit a stick of incense and push it down the brake booster tube, then held thumb over the tube.

If that doesn't work , plug the air filter, and drop your favorite color smoke bomb down the air duct work!

you should have said you had an a/m intake on, these are very common b/c gm maf's dont like oil.

Ok I got the check engine light to go off! I took the air cleaner assembly apart and cleaned the mass air flow sensor, I could see that it was cleaner afterwards! The truck sounds much better and runs stronger and smoother, within 2 miles of driving light went out and all codes were gone! It must be the oil from my airaid intake filter?
But my issue is that I reset my mpg calc and I get only 13-13.5mpg around town/country roads. Could my possible fuel pressure issue relate to this?

after the fuel pressure is corrected , you may have other failures. a restricted converter will give a miss fire code. also lower power and MPG. O2 sensors too old also cause poor running ,but with this engine the PCM will quickly throw O2 codes.

dirty maf sensor pick up wires will throw the fuel mixture off some what but ,,, the O2 sensors will have the last say on fuel trim..unless the MAF is defective or not connected.

ON non OEM air filter housings etc....

when in a dry , warm climate you can get some benefit with these devices. when it is very cold , dry, and lots of snow blowing around it is best not to use these air intake devices..

you suck up too much snow/water then the media cannot pass air...an air filter that is wet will not pass much air.

you should have said you had an a/m intake on, these are very common b/c gm maf's dont like oil.

Sorry bout that! Do you think that could lead to my bad gas mileage? Wishing I kept my original filter kit to just see if that improves my gas mileage about 13....I use to get 15.5-16 when I first got the truck.

could be, the maf controls fuel consumption along with some other things.

could be, the maf controls fuel consumption along with some other things.

Was thinking of switching to synthethic oil or maybe a blend to improve gas mileage, think its worth the money??

Was thinking of switching to synthethic oil or maybe a blend to improve gas mileage, think its worth the money??

these engines run better with synthetic..they also run cooler..

since 2001 I have been using mobil one 5-30wt oil..replace oil at 8,ooomiles..filter at 3500-4,ooomi...filteration is also very important ..Use a good filter wix or pure one purolator...

I use 1/3 qt over the 8,ooomi oil change interval 130,ooomi on it !

these engines run better with synthetic..they also run cooler..

since 2001 I have been using mobil one 5-30wt oil..replace oil at 8,ooomiles..filter at 3500-4,ooomi...filteration is also very important ..Use a good filter wix or pure one purolator...

I use 1/3 qt over the 8,ooomi oil change interval 130,ooomi on it !

I actually just bought mobil 1 synthethic high mileage 5w30 and a mobil 1 filter, $30 for 5 quarts and the filter, sale at autozone! I should be ok doing every 5,000 miles with the mobil 1 filter and oil? I also bought ac delco new plugs, iridiums and bosch wires, I will do all of this on Sunday and see if that helps with my mpg...12.5 currently is just killing me!

I actually just bought mobil 1 synthethic high mileage 5w30 and a mobil 1 filter, $30 for 5 quarts and the filter, sale at autozone! I should be ok doing every 5,000 miles with the mobil 1 filter and oil? I also bought ac delco new plugs, iridiums and bosch wires, I will do all of this on Sunday and see if that helps with my mpg...12.5 currently is just killing me!


I gained 1.5MPG when I used engine oil and the transmission fluid with synthetic.. transmission shifts faster ..also runs cooler..

In cold weather you will see an improvement in performance and slight MPG gains ///less friction..

if you do the oil changes on the first time you remove the oil filter with synthetic it is very hard to hold the filter as it is very slippery...

I do the 8,ooomi change others do the 15,ooomi change ..

oil filter at 4,ooomiles...............

I would not go 5,ooomi on the filter change..since you have been using dino oil this synthetic may loosen up dino sludges , fouling the filter..

I gained 1.5MPG when I used engine oil and the transmission fluid with synthetic.. transmission shifts faster ..also runs cooler..

In cold weather you will see an improvement in performance and slight MPG gains ///less friction..

if you do the oil changes on the first time you remove the oil filter with synthetic it is very hard to hold the filter as it is very slippery...

I do the 8,ooomi change others do the 15,ooomi change ..

oil filter at 4,ooomiles...............

I would not go 5,ooomi on the filter change..since you have been using dino oil this synthetic may loosen up dino sludges , fouling the filter..

I already have dexron 6 in the tranny and lucas synthethic gear oil. I actually have never changed just a filter without oil, how much oil dumps out when spinning the filter out without draining the oil first? Have you tried the 0w30 weight oil, is that ok to use for even better economy?

I already have dexron 6 in the tranny and lucas synthethic gear oil. I actually have never changed just a filter without oil, how much oil dumps out when spinning the filter out without draining the oil first? Have you tried the 0w30 weight oil, is that ok to use for even better economy?

the filter will drain out about 1/2 qt ..I fill the new filter then install it..
never tried 0-30wt //5-30 or 10-30wt only..If it is below zero f temps 0-30wt might help dino oil...synthetic probably not much diff...........

the filter will drain out about 1/2 qt ..I fill the new filter then install it..
never tried 0-30wt //5-30 or 10-30wt only..If it is below zero f temps 0-30wt might help dino oil...synthetic probably not much diff...........


Ok so today I changed my plugs and wires, used AC Delco Iridiums and bosch wires and also changed the fuel filter. I also did an oil change, switched to mobil 1 synthethic high mileage and mobil 1 filter, 5w30. I then drove the truck 30 miles, mostly country roads I got an average of 17.4mpg! Mind you, I was lucky to break 13.5mpg before this. Granted i didnt hit much traffic so it will probably go down to 15-16, either way I literally will gain 60-95 miles a tank! I dont get why my mileage was so bad, from just the plugs? My plugs were changed 50,000 miles ago with the wires. Either way I am happy and hope the mpg stays in this good range!

Thats why I change plugs and wires at 25k instead of 50 it pays for itself in fuel in the long run.

Ok so today I changed my plugs and wires, used AC Delco Iridiums and bosch wires and also changed the fuel filter. I also did an oil change, switched to mobil 1 synthethic high mileage and mobil 1 filter, 5w30. I then drove the truck 30 miles, mostly country roads I got an average of 17.4mpg! Mind you, I was lucky to break 13.5mpg before this. Granted i didnt hit much traffic so it will probably go down to 15-16, either way I literally will gain 60-95 miles a tank! I dont get why my mileage was so bad, from just the plugs? My plugs were changed 50,000 miles ago with the wires. Either way I am happy and hope the mpg stays in this good range!

the reason you MAY have gained some MPG is because the pcm/computer is programed with these components working as designed. If the plugs or wires had issues you can get the engine out of balance causing this poor performance.

the combustion chambers in these aluminum heads is quite high in temp. 50,ooomi is max life for me. I can see how the plug tips get damaged from this.

plug wires need dielectric grease in the boot ends. this prevents poor connections and damage to the conductor.when ever you mess with the plug wires use a ohmmeter and make sure the resistance is correct. normally as wires age the resistance increases, now you get lower spark power on the plug tips..more unburned fuel to damage the converters.

synthetic oil reduces friction ..this is why I am believer in using this type of oil...when it drops to below zero F and that starter spins the engine like summer you never will go back !

Random misfires on random cylinders -

Air - MAF sensor, EGR system, PCV system, and Air Intake system.
Fuel - Fuel filter, fuel pump, flaky fuel pump relay.
Exhaust - A plugged catalytic converter can cause random misfires.
Ignition system - Crank sensor, and cam sensors.

1) The MAF/IACT sensors get contaminated by a bad PCV system.
Try cleaning with MAF sensor cleaner, and go with a external PCV system.
2) Check fuel pressure again.
3) Get your catalytic converters back pressure tested.

Do synthetic oil yourself today it's cheaper, better job, less oil changes, and better fuel economy.

1) The MAF/IACT sensors get contaminated by a bad PCV system.
Try cleaning with MAF sensor cleaner, and go with a external PCV system.
2) Check fuel pressure again.
3) Get your catalytic converters back pressure tested.

Do synthetic oil yourself today it's cheaper, better job, less oil changes, and better fuel economy.




its all fixed already ! read his replies .........

its all fixed already ! read his replies .........


What do you know about fuel injector cleaning systems? Was thinking of getting the otc kit and using 3m fuel injector cleaner.....think its worth the money, would that help with my fuel economy/performance?

What do you know about fuel injector cleaning systems? Was thinking of getting the otc kit and using 3m fuel injector cleaner.....think its worth the money, would that help with my fuel economy/performance?

I use techron fuel system cleaner every few thousand miles.

I also add 1 ounce per 10 gallons of fuel of marvel mystery oil every 1,ooo miles. never had any fuel issues. all vehicles run very smooth..

my opinion ,,,raming too much cleaner into your engine is damaging...best to slowly remove these deposits...