new member with a code P0300

[pettsir]

i will make a long story short. i have a re occuring P0300 misfire code on my second consecutive highway drive cycle. i have since replaced the water pump and distributer. all the plugs and wires, the coil, the ignition module, all four o2 sensors, the crank position sensor. i run 93-94 octane fuel and the car seems to run fine. it dynoed at 307 rwhp basically stock. the light only seems to come on duiring highway cruise. do i have a faulty new disributer? or a PCM? or something else. i doubt the chain is stretched with the dyno numbers i got and besides it is a steel rollor chain and gears. please help. also are the zf's noisy or what?

 

[KOPBET]

I'm assuming that since you are posting a ODBII code in the C4 section you must have a '96 vette.

edit: Corvette ownership in profile above was blank when this post was made.

Possible vac leak, open EGR, maybe fuel pressure or multiple injectors. Who changed the crank sensor?

 

[pettsir]

the crank sensor was put in by yours truly. i would think that a vacuum leak would either throw a fuel trim too rich code as the pcm tries to compensate for the extra o2 it sees in the exhaust. or a code for o2 sensor voltage too low. i don't feel any misfire or problem under cruise when the light comes on. and it only seems to come on after a couple of consecutive highway cruises. i'm going to check fuel pressure and maybe replace the fuel filter this week end. i also noticed that the plug wires appear to have twisted themselves up and around each other behind the power steering pump bracket. maybe they are arcing. they are new but you never know. i will try and rout them a little better. see if that helps. i'm a little limited as i do not have a scan tool redily availible to me at this time. my only other guess is that the high resolution signal from the distributer that the pcm compares to the crank sensor to make sure the cam and crank are in sync is somehow being interupted or changed or the pick-up in the distributer is faulty. i hope not as i really don't want to do the distributer again.

 

[pettsir]

one mor thing, the LT4 doesn't have an EGR valve. so that possibility can be ruled out. and yes it is a 96 vette.

 

[KOPBET]

one mor thing, the LT4 doesn't have an EGR valve. so that possibility can be ruled out. and yes it is a 96 vette.

Yes, but when you posted, that information was not made or in your profile. Thanks for the update.

 

[KOPBET]

the crank sensor was put in by yours truly. i would think that a vacuum leak would either throw a fuel trim too rich code as the pcm tries to compensate for the extra o2 it sees in the exhaust. or a code for o2 sensor voltage too low. i don't feel any misfire or problem under cruise when the light comes on. and it only seems to come on after a couple of consecutive highway cruises. i'm going to check fuel pressure and maybe replace the fuel filter this week end. i also noticed that the plug wires appear to have twisted themselves up and around each other behind the power steering pump bracket. maybe they are arcing. they are new but you never know. i will try and rout them a little better. see if that helps. i'm a little limited as i do not have a scan tool redily availible to me at this time. my only other guess is that the high resolution signal from the distributer that the pcm compares to the crank sensor to make sure the cam and crank are in sync is somehow being interupted or changed or the pick-up in the distributer is faulty. i hope not as i really don't want to do the distributer again.

Well, I am one of those inclined to fault random/multiple misfires caused by a lean mixture due to a variety of conditions such as unmetered air entering the intake, varnished or dirty injectors or other fuel delivery problems. Even bad gas, diluted premium gas from dishonest vendors or distributors, gas with too much water or alcohol could be problematic, but you indicate the car seems to run fine otherwise, so ?. Draining gas tank and refilling from other sources would fix that of course.

From what you have indicated, I could only continue on the same course you are on and eliminate potential causes by process of elimination.

Does the engine burn oil at all?

 

[tazdevl35]

I'm guessing one or more injectors have gone bad. This isnt unusual at all, in fact its very common . Egr would be a good second guess. This is assuming all the other ignition components are good.

Later

Taz.

 

[KOPBET]

BTW - I would try to track down a scan tool so that you can check CKP output, misfire history, etc. Without it you are probably shooting in the dark. Don't know how many miles are on the car but I also wouldn't completely discount excessive timing gear and chain wear which could cause erratic CKP signals.

 

[pettsir]

i did find a couple of slightly damaged vacuum lines at the manifold. i repaired them but have not had a chance to road test the car extensivly to see if the problem is solved. how often is it common for LT4 injectors to go bad or get clogged? if i'm gonna replace them does anyone have any recomendations as to what are good ones? may as well do the filter too, and maybe put an adjustable fuel pressure regulator on it. what is a good brand? and what is a good fuel pressure? the car has the cut-out air lid with K&N filter, a relocated air temp sensor, throttlebody coolant bypass, a granitelli MAF, an airfoil,a hypertech PPIII,and a set of SLP headders. i plan on doing the rest of the exhaust later depending on what it takes to solve my problem. thanx in advance. the car has 51K on it. and i have used fuel from several vendors since this problem has started, they can't all be bad can they? also at only 51K i'm not inclined to go after the chain considering it is a roller chain with steel gears, and is supposed to last longer than the conventional chain.