Misfiring after 45 minutes of driving..........

kcotruck · 10-25-2009, 01:38 PM

1995 GMC 1500 4.3L V6
2-3 Months ago I changed all the plugs, wires, dist. rotor, dist. cap, ignition coil and ignition module. Truck has been running fine, no misfiring on regular 20-30 minute drive to work. I have now recently been driving to school an hour away and the vehicle has lately been starting to misfire after about 45 minutes of driving. Seems to only be misfiring when cruising at 70 mph and giving it gas and sometimes even holding steady. I is like a surge when the accelerator is applied when cruising. Could this be a BAD ignition module or coil that is not working properly when it gets warm. Could this be a BAD distributor shaft (main part) I don't think the distributor is the problem but I'm not sure. The truck runs fine for the first 30-45 minutes and then gets touchy to exactly the accelerator is applied, you have to listen to the engine and how it reacts to accelerate with out issue.......Does anyone have any suggestions that might help with this misfiring issue

BernieB · 10-25-2009, 03:26 PM

The check engine light coming on when this happens? Any codes?

It sounds to me like something is getting hot after about 45 minutes, and that could be anything from a bad coil or ignition module to a bad pressure regulator.

kcotruck · 10-25-2009, 05:04 PM

I just had the ignition coil and ignition module tested at a local auto parts store and they said they were fine......I also had the dist. rotor switched out..... not sure what is next, maybe replace the dist entirely....a guy up there said his dad has a suburban that was doing the same thing and also had the transmission recently rebuilt just like me and they thought it was something to do with the transmission, they couldn't identify the problem though so that wasn;t much help............

BernieB · 10-25-2009, 05:35 PM

An ignition module or a coil can test just fine on a bench and then immediately fail under load or when hot. Unless they're sticking it in an oven and heating it up while they test it there's no way they'd find such a fault.

leolkfrm · 10-25-2009, 06:37 PM

if i understand this.....as long as you drive under 70 it runs fine?

Mrcrowley · 10-26-2009, 03:29 AM

Quote:

Originally Posted by BernieB

An ignition module or a coil can test just fine on a bench and then immediately fail under load or when hot. Unless they're sticking it in an oven and heating it up while they test it there's no way they'd find such a fault.

Yup I have a scanner I set it active reading and drove it would misfire and the Service engine soon light came on and scanner read it then it went out which is weird came up a a number 5 cylinder misfire. So big question is do you get a Service engine soon light when it misfires?

kcotruck · 10-26-2009, 05:34 PM

The service engine light does NOT come on....any way it could be the distributor shaft or even something electronic with the transmission?

kcotruck · 10-26-2009, 05:36 PM

The driving speed has nothing to do with the problem, it happens at many different speeds.

BernieB · 10-26-2009, 06:40 PM

Okay, so no SES (but you might still have a code set)...

My gut instinct is that it's electrical, and that it's something starting to fail after it's been heat soaked awhile. The two best candidates for this are the ignition module and the coil.

But, I just thought of one more possibility... It's an oddball, but I've seen it happen...

How's your gas tank vent?

Back in the days of vented caps you would sometimes get a car in the shop with symptoms just like yours, and later find out they had put a non-vented cap on, which was then causing a vacuum to build up in the tank as fuel was pumped out, until the pump couldn't overcome the vacuum any more. The engine would die a bit, which was sometimes just enough to let a little air seep into the tank, and then the pump would get a little more gas and the engine would surge.

Like I said, it's oddball and a bit off the wall, but if your tank vent were blocked or crimped that could be what's happening.

leolkfrm · 10-26-2009, 07:54 PM

are you still using fuses for the injector rails? or changed to breakers? you want to use fuses

10-25-2009, 01:38 PM	#1
kcotruck Full Member Join Date: Jul 2009 Posts: 38	Misfiring after 45 minutes of driving.......... 1995 GMC 1500 4.3L V6 2-3 Months ago I changed all the plugs, wires, dist. rotor, dist. cap, ignition coil and ignition module. Truck has been running fine, no misfiring on regular 20-30 minute drive to work. I have now recently been driving to school an hour away and the vehicle has lately been starting to misfire after about 45 minutes of driving. Seems to only be misfiring when cruising at 70 mph and giving it gas and sometimes even holding steady. I is like a surge when the accelerator is applied when cruising. Could this be a BAD ignition module or coil that is not working properly when it gets warm. Could this be a BAD distributor shaft (main part) I don't think the distributor is the problem but I'm not sure. The truck runs fine for the first 30-45 minutes and then gets touchy to exactly the accelerator is applied, you have to listen to the engine and how it reacts to accelerate with out issue.......Does anyone have any suggestions that might help with this misfiring issue

10-25-2009, 03:26 PM	#2
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 1,902	The check engine light coming on when this happens? Any codes? It sounds to me like something is getting hot after about 45 minutes, and that could be anything from a bad coil or ignition module to a bad pressure regulator. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs.

10-25-2009, 05:35 PM	#4
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 1,902	An ignition module or a coil can test just fine on a bench and then immediately fail under load or when hot. Unless they're sticking it in an oven and heating it up while they test it there's no way they'd find such a fault. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs.

10-25-2009, 06:37 PM	#5
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 4,300	if i understand this.....as long as you drive under 70 it runs fine? __________________ 4 wheels move the body 2 wheels move the soul B.W.A.I

10-26-2009, 06:40 PM	#9
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 1,902	Okay, so no SES (but you might still have a code set)... My gut instinct is that it's electrical, and that it's something starting to fail after it's been heat soaked awhile. The two best candidates for this are the ignition module and the coil. But, I just thought of one more possibility... It's an oddball, but I've seen it happen... How's your gas tank vent? Back in the days of vented caps you would sometimes get a car in the shop with symptoms just like yours, and later find out they had put a non-vented cap on, which was then causing a vacuum to build up in the tank as fuel was pumped out, until the pump couldn't overcome the vacuum any more. The engine would die a bit, which was sometimes just enough to let a little air seep into the tank, and then the pump would get a little more gas and the engine would surge. Like I said, it's oddball and a bit off the wall, but if your tank vent were blocked or crimped that could be what's happening. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs.

10-25-2009, 05:04 PM	#3
kcotruck Full Member Join Date: Jul 2009 Posts: 38	I just had the ignition coil and ignition module tested at a local auto parts store and they said they were fine......I also had the dist. rotor switched out..... not sure what is next, maybe replace the dist entirely....a guy up there said his dad has a suburban that was doing the same thing and also had the transmission recently rebuilt just like me and they thought it was something to do with the transmission, they couldn't identify the problem though so that wasn;t much help............

10-26-2009, 05:34 PM	#7
kcotruck Full Member Join Date: Jul 2009 Posts: 38	The service engine light does NOT come on....any way it could be the distributor shaft or even something electronic with the transmission?

10-26-2009, 05:36 PM	#8
kcotruck Full Member Join Date: Jul 2009 Posts: 38	The driving speed has nothing to do with the problem, it happens at many different speeds.

10-26-2009, 07:54 PM	#10
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 4,300	are you still using fuses for the injector rails? or changed to breakers? you want to use fuses __________________ 4 wheels move the body 2 wheels move the soul B.W.A.I

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