pettsir

here is my situation. i bought this car about a year and a half ago. after a couple of months of owning it the check engine light comes on. code P0300 random misfire. i do a quick inspection and find two bad plug wires they were broken and burned thru. so i replace the wires with GM LT4 wires and the plugs with AC Delco stock replacements. car is fine for a couple of weeks then the light comes on again P0300 again. so i figure at 50K miles the opticrap is going, so i replace the optispark distributer and the coil and the ignition module. guess what the light is back on again. i figured maybe a bad opti right out of the box so i install another new one. (getting good at them now!). the light came on again with a P0303 (misfire cyl #3) so i check wire routing and spray them down with water looking for arcing and found none. i checked all 8 plugs, they seem to be burning fine, but if i listen closely i hear it spitting out the tail pipe. i switch the #1 and #3 plug and reset the light(i have a cheap code reader and clearer) the light is back on with a code P0300 again. i also replaced the crank sensor and the four O2 sensors. i ohmed out the fuel injectors they are all about 12.5 ohm with in .1 of each other. the car seems to run strong, it dynoed 307 rwhp and runs the 1/4 mile at 13.3(due to driver, and tires) and 107 mph. strong mile an hour and dyno curve for a car with a random misfire. my list of mods are as followes, open air lid-K&N air filter-relocated intake air temp sensor-graniteli MAF-throttle body air foil-Hypertec power programmer III-SLP shortie headers. i checked vacuum hoses and found some surface cracks but not all the way thru. the car doesn't surge at idle, but tends to go from rough to smooth idle and back and forth. starts right up, doesn't hesitate or die or stumble. only indication of a misfire is a slight rough idle and a spitting noise out the tail pipe. can more than one injector be bad but still ohm out o.k? my fuel pressure looks good at about 35 p.s.i at idle and about 47p.s.i with the regulator disconnected. any one know of bad intake manifold gaskets causing this problem or any of the list of mods i gave earlier? most of my problems happened before i even modified the car. :mad :confused: :U :reddevil :cry :banghead:

VR'92

:confused: :cry

tjwong

It could be a bad injector. Even though you checked the resistance you are only checking the coil and not any of the mechanical parts of the injector. What you could do is to move the suspect injector to another spot and see if the miss firing cylinder moves with it. It could be that the injectors are fouled and you are getting a lean miss.

rcolling

I'm just taking a wild stab but have you tried the obvious and changed the fuel filter?

MJ

Okay, this is from way out in left field but, I had an idle stability problem that set misfire codes on my 96 also. I traced mine back to the descreened MAF with the high flow ends. After i replaced it with my old MAF the problem went away. The car made plenty of power tho. I've never used the GMS MAF so I can't say anything about it. But Ed (the programmer) Really wanted me to use a stock MAF on my 396.

Aggravated4life

Code 300 is a pain to track down...you need a scanner hooked up when it happens.I had this problem on my 3.8 3800 V6 minivan and it threw a code 300 and would misfire every once in a while.I threw every new part at it I could think of.I even replaced the intake gaskets,which can sometimes cause a code 300.Nothing worked.I finally tracked it down to a bad ECM.
The fuel injectors were not firing properly due to a bad ecm signal.It drove me nuts and I tried everything.Even called the GM dealer to speak to their class A tech over the phone and he told me to try everything I already replaced.

Heres how we found it...when the car was misfiring,and I had the snap on scanner hooked up,it showed cylinder #3 as "weak".When we switched the injector harness from cylinder 3 to 5,the weak cylinder MOVED on the scanner to #5.
NOTE:It did not record a specific cyliner misfire like a code 301 or similar when it happened! Thats what made it hard to track down.

I had already swapped injectors out there and it made no difference.
The harness was OK so it had to be the ecm signal to the injectors.We replaced the ecm with a new one and it was fixed.SIGH!!!!!!

You may need a TECH 2 scanner hooked up to see if the random misfire code is in fact occuring at one or two cylinders.

I read your info and Im unsure if in the beginning,if the codes got cleared the first few times and they came back later.They have current codes and "Historical codes" which needs to be removed by the scanner.
If your car ran fine and then just threw the code,Im betting it didnt actually misfire but was throwing it back out.Is this what happened?Did your car misfire and you could feel it or was it the light that kept coming on?

Here are other causes of the code 300 misfires for GM cars.

Bad O2 sensors
carbon on the valves
bad egr/leaking egr(LT4 doesnt have it so rule that out)
Intake gaskets leaking or going bad
bad injector(s)
bad injector harness someplace
bad ecm
bad plugs,cracked plugs/bad wire(s),opti
Plug Wire arcing or shorting out someplace

Im sure theres a few more but I cant remember them right now.

Youve got OBD2 and while its supposed to help,it can drive ya nuts sometimes over them misfire codes.Which is why a Good scanner that can show you everything can help narrow it down to which cylinder(s) it may be happening at.
Hope this helps some...I know the pain and been through it myself and it was a costly expense before I could figure it out with my other car.Since the fix,it runs like it never had the codes!!

:crazy:

HammerDown

How does it run in the upper rpm range??

Corvette Kid

A lot of good advice here and all valid. If there's anything here you haven't tried yet that's cheaper than an ECM, try it including checking all injector wiring from end to end, ohms, corrision, loose pins, connectors, etc. That goes for other engine wiring too.This kind of thing can be very time consuming and make you pull your hair out. My experience with modern computerized engines is more with diesels but the principles are the same. We replace a good number of ECMs for various reasons, many being erratic running and driveability problems. Obviously if you can beg, borrow or steal one to bolt on and try it can save you a lot of time.

DieL

i hope you dont have a similar issue i had. # years ago my car ran 12.8-13.0 @ 110-111 mph consistent ... then all of a sudden.... i started replacing everything.... the car ran good but just felt a little down on power.. ran 13.3s @ 105 mph for 2 YEARS :mad my car used to spit out the xhaust... i always heard it whenever i listened to the tailpipes. kind of like a stumble or roughness..

i ended up rebuilding the entire motor from scratch... thats how :mad i got.

DieL

check your rockers?? weren't early lt4 rockers having some reliability issues?? just a quick guess

pettsir

the car runs great in the upper RPM ranges as both my buttometer and dyno graph will attest. i,m gonna take all your advice and try a new fuel filter, re-install the factory MAF, check the rockers. though i would think if it was a mechanical problem then the PCM would be able to identify one down cylinder. and if the new gasket gets here in time i,m gonna swap the intake gasket out, wanted to put the better 7777 intake gasket on anyway. also gonna look for carbon on the back of the intake valves while i,m there. any ideas on how to clean them while still in the head and on the car? also gonna re-check my fireing order AGAIN, and the injector harness. thanx all for your help, i will keep u posted. :banghead:

FD2BLK

Man, can I feel your pain. I had the same problems with the P0300 on my wifes sebring. I bought a bad dist. and trouble shot it for a month before I replaced it for the second time and fixed the problem. Random misfire is usaly in the ingnition and you are on the right track. It looks like you had a bad Opti the first time and I wouldn't rule out another one since your symptoms changed when you installed the new one to #3 misfire. Run a compresion check on the 1,3,5,7 bank just to see if #3 is Ok, ( I suspect it is fine. I don't remember seeing where you replaced the coil, If not I would drop the $30 and replace it since you have replaced everything else in the ingntion system. I would think if a bad injector was the problem you may get a O2 code (lean LB for instance)