p0300 mult cylinder misfire....

[someday31]

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.....

[j cAT]

Originally Posted by someday31

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 ..

[danielsatur]

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.

[someday31]

Originally Posted by j cAT

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?

[j cAT]

Originally Posted by someday31

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 ..

[someday31]

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...

[kahjdh]

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

[someday31]

Originally Posted by kahjdh

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?

[someday31]

Originally Posted by someday31

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?

[j cAT]

Originally Posted by someday31

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.

[someday31]

Originally Posted by j cAT

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?

[j cAT]

Originally Posted by someday31

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 ...

[j cAT]

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...

[kahjdh]

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.

[someday31]

Originally Posted by j cAT

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.