Random Misfire Diagnosis
Thread Starter
Intermediate
Joined: Nov 2003
Posts: 28
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Random Misfire Diagnosis
Hi all,
Looking for some help here -
My 96 LT1 started missing bad today. On startup, initially idles fine, revs fine, then less than a minute later, starts missing badly at idle, it continues to miss badly though all RPM ranges. Bought an ODBII scanner to pull the codes, just the one code came up for random misfire (P0300). I don't think it's spark or fuel since it is fine on initial startup. Something changes making it misfire when it switches from open to closed loop. I don't have a factory manual and the Haynes book I bought sorely lacks good diagnostic info on this car. What comes into play when the engine goes from open loop to closed loop that might cause such horrible missing?
Looking for some help here -
My 96 LT1 started missing bad today. On startup, initially idles fine, revs fine, then less than a minute later, starts missing badly at idle, it continues to miss badly though all RPM ranges. Bought an ODBII scanner to pull the codes, just the one code came up for random misfire (P0300). I don't think it's spark or fuel since it is fine on initial startup. Something changes making it misfire when it switches from open to closed loop. I don't have a factory manual and the Haynes book I bought sorely lacks good diagnostic info on this car. What comes into play when the engine goes from open loop to closed loop that might cause such horrible missing?
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Random Misfire Diagnosis (gramercyjam)
Though not sure how closely OBD-II is; on OBD-1:
-fuel leans out to 14.7:1
-ignition timing pulls back to normal
-TCC lockup can occur.
Put a timing light on several spark plug wires when it's misfiring, just to be sure it's not ignition. Here you aren't looking to read timing advance, just to see if misses in the engine missed coincide with flashes.
-fuel leans out to 14.7:1
-ignition timing pulls back to normal
-TCC lockup can occur.
Put a timing light on several spark plug wires when it's misfiring, just to be sure it's not ignition. Here you aren't looking to read timing advance, just to see if misses in the engine missed coincide with flashes.
Race Director
Joined: Aug 2000
Posts: 14,130
Likes: 7
Cruise-In II Veteran
Re: Random Misfire Diagnosis (gramercyjam)
I had a code 300 in my 95 3.8 3800 engine and it was a nightmare to find what was wrong.I changed everything and even re did the intake gaskets to no avail.It turned out to be a bad ECM not firing the injector pulse correctly.
Some days it did,others it did not.
If the scanner will allow you to see which cylinder will misfire or at least read as "weak",start from there.Man what a PITA to figure out a random misfire code of 300.I got every suggestion as to the egr,the injectors,the intake manifold gasket,wire,plugs,etc etc etc.However 02 sensors are another good place to check as well.Nothing seemed to fix it until I swapped the pcm.
Keep us posted and good luck.If your scanner cant show any info for real time driving,youll need a dealer to use a tech 2 and see if the misfire is more prominent in one area.
:crazy: :smash:
Some days it did,others it did not.
If the scanner will allow you to see which cylinder will misfire or at least read as "weak",start from there.Man what a PITA to figure out a random misfire code of 300.I got every suggestion as to the egr,the injectors,the intake manifold gasket,wire,plugs,etc etc etc.However 02 sensors are another good place to check as well.Nothing seemed to fix it until I swapped the pcm.
Keep us posted and good luck.If your scanner cant show any info for real time driving,youll need a dealer to use a tech 2 and see if the misfire is more prominent in one area.
:crazy: :smash:
Thread Starter
Intermediate
Joined: Nov 2003
Posts: 28
Likes: 0
Re: Random Misfire Diagnosis (gramercyjam)
Thanks for the input guys! Thankfully, I figured it out without too much unnecessary expense. The $150 scanner will probably come in handy in the near future, but I really didn't need it to tell me that I had a miss, that was very obvious. As suggested, I went to put a timing light on and bingo, when I stuck my hand down in there, I got a big old shock. Sure enough, a plug wire is arcing (due to wire chafe) to the EGR pipe coming off the drivers side exhaust manifold. So it doesn't seem to be the opti or the ECU or one of the other change-it-out-and-see-if-things-get-better items. I moved it off the pipe and it is running great again.
The scanner didn't tell me which cylinder was misfiring, and by the code, I assumed random meant various cylinders, not just 1. So that mislead me, supported by the fact that it seemed to run OK in open loop, but that was a red herring too.
So it is a simple old school fix - new plug wires. Changing the plugs and wires looks like it might be rather frustrating though. Not much room to work in there.
I guess my 96 CE is starting to self destruct in a rapid and orderly fashion, almost as if it designed to do so at precisely 60K miles, 7 years!
--John B
'73 F Prepared Datsun 240Z
'96 Corvette CE LT1
'02 Z71 Suburban
The scanner didn't tell me which cylinder was misfiring, and by the code, I assumed random meant various cylinders, not just 1. So that mislead me, supported by the fact that it seemed to run OK in open loop, but that was a red herring too.
So it is a simple old school fix - new plug wires. Changing the plugs and wires looks like it might be rather frustrating though. Not much room to work in there.
I guess my 96 CE is starting to self destruct in a rapid and orderly fashion, almost as if it designed to do so at precisely 60K miles, 7 years!
--John B
'73 F Prepared Datsun 240Z
'96 Corvette CE LT1
'02 Z71 Suburban
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Random Misfire Diagnosis (gramercyjam)
Ouch...that's finding the problem the hard way, but effective.
There have been some good posts on how to manage the plug wire change on LT1s; maybe the archives will be useful.
There have been some good posts on how to manage the plug wire change on LT1s; maybe the archives will be useful.
Race Director
Joined: Aug 2000
Posts: 14,130
Likes: 7
Cruise-In II Veteran
Re: Random Misfire Diagnosis (gramercyjam)
Gram,
Good to hear you found a easy fix!! I wanted to share a little more info...with my code 300 and it was random misfire,the scanner was not showing which cylinder was missing since it was "random"...but when I left the scanner hooked up one day when it acted up,and let the car miss at idle for a while,it began showing a "weak cylinder" on #5...so I switched the injector harness plug with number 3 and it moved the weak cylinder to #3!!Why the pcm did not store a specific misifire code for that cylinder in the first place,I dont know.But just so you know,dont always assume a random misfire wont show up in a specific place on a scanner in one form or another.Those OBD11 codes(at first) can be very misleading and drove me nuts.
Now im seeing OBD11 as a pretty good system once I got a little more familar with it.
Another form of code 300 misfires are also due to excess carbon on the valves.Most of the GM tech guys I spoke to wanted to run a top end cleaner to see if it fixed the problem!!
Anyways,that was just for reference for ya.Have fun with the Vette now.
:cheers: :crazy: :)
Good to hear you found a easy fix!! I wanted to share a little more info...with my code 300 and it was random misfire,the scanner was not showing which cylinder was missing since it was "random"...but when I left the scanner hooked up one day when it acted up,and let the car miss at idle for a while,it began showing a "weak cylinder" on #5...so I switched the injector harness plug with number 3 and it moved the weak cylinder to #3!!Why the pcm did not store a specific misifire code for that cylinder in the first place,I dont know.But just so you know,dont always assume a random misfire wont show up in a specific place on a scanner in one form or another.Those OBD11 codes(at first) can be very misleading and drove me nuts.
Now im seeing OBD11 as a pretty good system once I got a little more familar with it.
Another form of code 300 misfires are also due to excess carbon on the valves.Most of the GM tech guys I spoke to wanted to run a top end cleaner to see if it fixed the problem!!
Anyways,that was just for reference for ya.Have fun with the Vette now.
:cheers: :crazy: :)
Thread Starter
Intermediate
Joined: Nov 2003
Posts: 28
Likes: 0
Re: Random Misfire Diagnosis (Bill's86Coupe)
Appreciate the info. I am pretty old school when it comes to engines and don't have any real knowledge of what the ODBII diags on my 96 will and won't show. I more at home with primitive machinery, like good set of weber carbs that I can tune by ear, color of plugs, etc.
I picked up an Actron scanner and it wasn't real detailed about what the capablities of the thing are in the manual. Any idea how good the corvette factory manuals are? I would think they should be pretty good for $100+!
I picked up an Actron scanner and it wasn't real detailed about what the capablities of the thing are in the manual. Any idea how good the corvette factory manuals are? I would think they should be pretty good for $100+!
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Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Random Misfire Diagnosis (gramercyjam)
My shop manual is a 2-piece set; mechanical & electrical in a separate volume.
I consider it an indespensible tool. And it will explain well operation and diagnostics of your ECM & circuits.
I consider it an indespensible tool. And it will explain well operation and diagnostics of your ECM & circuits.
