Misfire problem

[Jeremy the 3]

I've got an 03 Denali xl that is giving me misfire problems. Changed the plugs and wires before I had my buddy hook it up to his readers. Cheap reader gives P0300(random misfire) and P1380(misfires repeated) codes. His other reader is a little bit more in depth. Shows all the cylinders misfires. This cant be all the coils CAN IT? It's got to be further in then the coils right? Any input helps thanks

 

[Tonyrodz]

Curious myself.

 

[Scottydoggs]

clogged cats would do this, how high your miles?

 

[Doubeleive]

possible intake leak, try to do a leak test, spray some starter fluid or something volatile along the edges of the intake and see if the idle changes

 

[Bear77]

Also got the p0300 code on mine and was going to change plugs on it tomorrow


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[SnowDrifter]

Things to check in no particular order:


Vacuum leak

Bad MAF

Low fuel pressure - pressure regulator, filter, or pump

Worn cam

Skipped timing

Worn timing chain

EGR issues

Poor fuel quality

A horrifically clogged air filter

Bad plug gap. You did gap them right?

Bad ground(s)

Exhaust issues

 

[SnowDrifter]

Also: I see it's -39 degrees where you are. If you have a flex fuel vehicle, e85 doesn't do well in the cold

 

[Jeremy the 3]

Also: I see it's -39 degrees where you are. If you have a flex fuel vehicle, e85 doesn't do well in the cold

The -39 was because I forgot to plug the radiator temp sensor back in. plugs are new and pre gaped as well as the MAF 6 months ago. It has 175K on it. Was also told the fuel filter or pump. How do I check the fuel pressure?

 

[M1Gunner]

The engine coolant temp sensor can royally skew A LOT! The sensor can even keep a truck from starting. If the computer thinks it’s that cold outside it’s going to dump fuel to that engine to compensate. You need ALL sensors plugged in for proper data.

 

[Doubeleive]

ya thinking it is -40 below certainly cant be helping the matter

 

[Jeremy the 3]

I get that, the sensor was just from changing rhe plugs, the misfire existed before the sensor was unplugged

 

[SnowDrifter]

There's a valve on the fuel rail that a gauge attaches to.

Plug that sensor back in. It'll invalidate any testing you do until then. What else did you do? A plug job doesn't necessitate unplugging it

 

[xDan]

Wow curious about this one

 

[89Suburban]

When I went through this my mechanic had some kind of magic wand tool that read the coils for spark. Can't find that tool online to save my life. Wound up being a bad plug in my situation.

 

[dtpgoose]

I've got an 03 Denali xl that is giving me misfire problems. Changed the plugs and wires before I had my buddy hook it up to his readers. Cheap reader gives P0300(random misfire) and P1380(misfires repeated) codes. His other reader is a little bit more in depth. Shows all the cylinders misfires. This cant be all the coils CAN IT? It's got to be further in then the coils right? Any input helps thanks

View attachment 209674

I have had a 2004 Sierra 5.3 and Currently own a 2003 GMC Yukon XL both motors presented the say as yours and on both truck the same answer after tracking it all down. On the Sierra I put plugs in, Tried replacing the injector on the 2 cylinders that were misfiring and also a coil on both of them and no change, did a leak down test for compression no leaks no fuel filer problems no fuel Pressure problems and Finally we tore into the motor and found one lobe on the CAM worn bad so it was missing bad at low RPM but running fine going down the road. I had already found a motor in a wreck to replace it but I had to tear into it to prove it to myself this was also the case on a friends truck a year after 2005 and in mine now 2003 anyone elese have the problems with the CAM Lobes wearing down to a point of causing the misfire at low idle...... Also I replaced the Cat. (however I would plug that sensor in for coolant then have your buddy run the same test again)