Automotive Forums

I have been dealing with a random miss in my '98 Safari with 139,000 miles on the original 4.3L V-6 engine for three years now.  The miss only seems to occur on trips greater than 100 miles, and then only maybe 5 or 6 times per 100 miles.  I did the obvious of inspecting for vacuum leaks.  When the problem started, since the plugs and wires had never been changed, I started by replacing them.  Eventually, I also replaced the ignition module, coil, rotor and distributor cap.  So at this point, everything in the ignition system from the computer to the plugs are new.  I also checked fuel pressure which is within spec. and replaced the fuel filter.  I tried putting injector cleaner in the fuel.  The problem only occurs under constant speed and on level ground.  Accelerating makes it go away as long as I'm accelerating.  Over the three years, the problem has gradually gotten worse.

Since this has been happening for such a long time, I've gotten some more experience with it.  For example, on a trip last Christmas, I noticed that if I lightly press the brake pedal while maintaining a constant speed when the missing starts, the miss will go away.  I think this is an important clue, since when the brake pedal is depressed, the computer closes the EGR valve if it happens to be open.  So I removed the EGR valve and inspected it.  It appeared to be clean enough and not binding, but I replaced it anyway being happy that it had lasted over 100K miles, but it didn't help.  The vehicle has set a code only once, and that was also last Christmas on the same long trip, and the code was P300 - Random multiple cylinder misfire detected.  This engine has a crank position sensor and is able to detect specific cylinder misfire, so because of that and the P300, I've pretty much ruled out a mechanical problem, thinking that a mechanical problem would cause misfire in specific cylinders.  But back to the fact that pressing the brake pedal corrects the misfire: closing the EGR valve richens the mixture, pointing to a lean condition.  So I'm thinking that maybe one or more of the O2 sensors are bad enough to cause an overall lean condition but not bad enough to set an out-of-range code.  The miss never starts until the computer goes closed-loop.  The O2 sensors have never been replaced.  The engine still has good power with 139,000 miles on it, and there are no other driveability symptoms or excessive oil consumption.

Other than the O2 sensors, an intermittent connection at pin C3-9 of the computer, which supplies the ignition timing signal to the ICM, or pins C2-3 or C2-8 which supply the camshaft position signal to the computer I suppose could cause a miss not specific to one cylinder.  I unplugged both C2 and C3 connectors, examined for an obvious connector problem and reconnected them, but this didn't help.  This problem happens so infrequently, unfortunately I'm in the mode of just replacing the next-most-likely component(s).

Has anyone seen this type problem, and do you think I'm on the right track suspecting the O2 sensors?  There is no good way to test O2 sensors that I know of, so before I spend the money, I'd like some *** from you guys who do this all the time, that the investment would be worth trying.  There are three of them on the engine.

Thanks in advance for your advice.

Alan  

Your right about the lean mixture, the miss fire goes away with injector cleaner in it for awhile,(a day).Which makes me think its running lean. Haven't tried the brake thing, I'll give it a shot and let you know.  

Have you done a fuel volume test on it. have you taken the injectors out and cleaned them injector cleaner will not help there. also have an injectro balance test done.  

Johnswiftm,

No, I have not done a fuel volume test, injector balance test or removed the injectors for cleaning. Since the P0300 code indicates multiple, random misfires, I didn't suspect the injectors.  It seems to me that a bad injector or two would set one of the specific cylinder misfire codes.  Am I off-track with this *** for your help.  

The fuel volume test can cause any cylinder to miss fire also an injector can intermitaly miss fire.  

Since you have spent a ton of money as I have on my 1999 Safari, I would change the distributor drive gear. I fought this problem for over a year! The last time it started with a random misfire on cyl 4 then switched to cyl 5. I was going to change the cam sensor under the distributor cap but was informed either they work or they don't.  I keep changing cap and rotor and it would be okay for a 100 miles and start back up again. When you change the rotor it must change the location of the mating surfaces enough to take out the slop for a short time. It seemed to be an ignition issue but I couldn't put a finger on it. the last time I changed the rotor I noticed alot of slop so I pulled the distributor and noticed some slight wear so for $30 it was a cheap try and what do you know? I changed the gear and it hasn't missed a lick. 3.5 MPG increase and no hesitation. Good luck!  

i have a 1996 gm safari van. i think i have the same problem, but mine will do it when ever it wants to. i can go for up to 4 days with it running fine!! then it will decide to give me grief.  mine will rev up on its own and some time i cane drive from one end of town to the other without touching the gas, then it wants to stall.  if i give it gas it won't lately i have to put it into neutral at a stop light and give it the gas so it won't stall. my engine light has not come on, i have changed plugs, wires, co indicator and fuel filter, has an oil/filter change done but she still gives me greif.
help!!!  

If your engine has a magnetic sensor in the distributor, then tiny bits of metal will get stuck to the magnet.

This will weaken the magnetic field, causing a misfire. Or, it could be the reformulated gasoline, especially in California, which will lean out your fuel mixture. I think this is our states way of getting the older cars off of the road!  

well the monte's have a similar problem with misfires.  
http://mymonte.com/discussion/viewtopic.php?t=17233
Im trying to hunt it down right now with no luck yet.

I have:
-new plugs
-wires
-coils and coil pack
-fuel filter
-fuel injectors
-o2 sensor
-cat/full exhaust system

so far my best guess is its transmission related.  More specificly Tq Converter related.  that would explain why it goes away with pressure to the brake.  the trany unlocks the coverter when the brake is on.  my idea is the converter is intermitatly putting extra pressure on the engine causing additional load that is interpreted as a misfire.  i will continue to search for a cause and update the mymonte link as i learn more.  

We also have a 98 gmc safari that has a random misfire. It usually occurs on the highway after 60 mph but ours happens just about every time we drive it. we also have changed the spark plugs, wires, cap and rotor, fuel pump next we were told to change the injectors and regulator. We have had this for over ayear now and it is driving me insane. We changed the cap and rotor 2 times in 4 months because of the corrosion. So I was wondering if you have found out what is causing the problem?  

alanh:
I have been dealing with a random miss in my '98 Safari with 139,000 miles on the original 4.3L V-6 engine for three years now.  The miss only seems to occur on trips greater than 100 miles, and then only maybe 5 or 6 times per 100 miles.  I did the obvious of inspecting for vacuum leaks.  When the problem started, since the plugs and wires had never been changed, I started by replacing them.  Eventually, I also replaced the ignition module, coil, rotor and distributor cap.  So at this point, everything in the ignition system from the computer to the plugs are new.  I also checked fuel pressure which is within spec. and replaced the fuel filter.  I tried putting injector cleaner in the fuel.  The problem only occurs under constant speed and on level ground.  Accelerating makes it go away as long as I'm accelerating.  Over the three years, the problem has gradually gotten worse.

Since this has been happening for such a long time, I've gotten some more experience with it.  For example, on a trip last Christmas, I noticed that if I lightly press the brake pedal while maintaining a constant speed when the missing starts, the miss will go away.  I think this is an important clue, since when the brake pedal is depressed, the computer closes the EGR valve if it happens to be open.  So I removed the EGR valve and inspected it.  It appeared to be clean enough and not binding, but I replaced it anyway being happy that it had lasted over 100K miles, but it didn't help.  The vehicle has set a code only once, and that was also last Christmas on the same long trip, and the code was P300 - Random multiple cylinder misfire detected.  This engine has a crank position sensor and is able to detect specific cylinder misfire, so because of that and the P300, I've pretty much ruled out a mechanical problem, thinking that a mechanical problem would cause misfire in specific cylinders.  But back to the fact that pressing the brake pedal corrects the misfire: closing the EGR valve richens the mixture, pointing to a lean condition.  So I'm thinking that maybe one or more of the O2 sensors are bad enough to cause an overall lean condition but not bad enough to set an out-of-range code.  The miss never starts until the computer goes closed-loop.  The O2 sensors have never been replaced.  The engine still has good power with 139,000 miles on it, and there are no other driveability symptoms or excessive oil consumption.

Other than the O2 sensors, an intermittent connection at pin C3-9 of the computer, which supplies the ignition timing signal to the ICM, or pins C2-3 or C2-8 which supply the camshaft position signal to the computer I suppose could cause a miss not specific to one cylinder.  I unplugged both C2 and C3 connectors, examined for an obvious connector problem and reconnected them, but this didn't help.  This problem happens so infrequently, unfortunately I'm in the mode of just replacing the next-most-likely component(s).

Has anyone seen this type problem, and do you think I'm on the right track suspecting the O2 sensors?  There is no good way to test O2 sensors that I know of, so before I spend the money, I'd like some *** from you guys who do this all the time, that the investment would be worth trying.  There are three of them on the engine.

Thanks in advance for your advice.

Alan

Hi we purchased a 98 safari new from the factory and noticed it cutting out with about 50 thousand miles on it or miss fireing at random times injector cleaner temperarally fixed the problem we had it in the shop alot of times and the computer said it was a rp300 a random missfire over 30 thousand times since it was brand new. it now has 97 thousand miles on it and the light came on and it missfired and rocked the van going down the interstate so back it knocked the gears out of the transmission wich had to be replaced. there went 2500$. not even a week later it wouldnt start so here comes the tow truck again now they have to tear the engine down and replace the wiring harness all the injectors and gaskets with an estiment of 35 to 45 houndred dollars ouch. we called gmc direct and got the run around. yes it has 97 thousand miles on it but most engines are just getting broke in and we did everthing by the book on maintnence. by doing research on this problem it aint just the 98 safari but starts with the 96 gmc models and goes past the year 2000. i was told by a machanic that the miss fire keeps happening and eventually it will leak gas on to the wiring harness and then possibly catch the vehicles on fire and or knock the computer out. gmc was reaL NICE ON THE PHONE AND SAID TO GET IT TO A DEALER AND THEY WOULD HELP ON THE COST OF GETTING IT FIXED SINCE IT IS HAPPENING IN ALOT OF THE GMC ENGINES WITH UNDER 100 THOUSAND MILES ON THEM. SO WE DID AND CALLED THE BACK WITH THE TOTAL AFTER IT WAS FIXED. THEY SAID SORRY WE CANT HELP YOU BUT  WILL GIVE YOU A DISCOUNT ON A NEW VEHICLE. SORRY GMC AND THANKS FOR ALL YOUR HELP BUT WE WILL AVOID CHEVY GMC AND OUR NEXT NEW VEHICLE WITH BE A DIFFERENT TYPE AND MODEL. IF THEY KNOW ABOUT ALL OF THESE MISSFIRES AND PROBLEMS IT SHOULD BE A RECALL ON ALL BEFORE THE FIRE DOES HAPPEN AND KILLS A INICENT PERSON OR FAMILY  

Could you tell us what the dealer fixed on the van? There seems to be many of us having the same problem.
Thanks  

My 98 has a strange problem.   It will be just fine and one time when you go to get in you have no power to the fuel pump.  Nothing!  The first time for this was a week ago.  I change out the relay and fuel filter. No help.  Next day after work I was going to drop the tank and check the pump at the connection.  However when I got home and turned the key bang pump action and the thing ran just fine.  Had a second event on this past Wed.  Went out to go for an evening bow hunt and nothing no power what so ever to the fuel pump. Next day everything was fine.  So I have a new filter and relay in place any suggestions. Ancho  

Hey alan got mine fixed, it was distributor gear. it was worn down which caused rotor to bounce under light load, and hold steady when gave it gas. Take a close look at gears on distributor  

Other than the "spider"(injector control)the only thing that wll give you a random and multiple missfire without any other code is a worn distributor gear,seen this 6 or 7 times with the 5.0 and the 4.3