4.9 random misfire
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Senior User
Joined: Aug 2010
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From: Port Washington, Wi
4.9 random misfire
ok so ive kinda got my 4.9 narrowed down, since ive got it its had a bad random misfire. ive replaced tps and set it to .96 volts, replaced egr sensor, coolant temp sensor, air charge sensor, o2 sensor, ignition module, plugs, wires, coil, cap and rotor. i just replaced the intake gaskets and looked at all of the nozzels and they all looked clean. truck has a new fuel filter, also a new filter on the fuel pump. the only thing left to replace would be the pickup in the distributor of the ignition condensor by the coil. anyways there is only one code in the computer at this time and its 85, purge failure or something. ok now on to the problem..... the truck runs great, lots of power, and it drives really good until the temp gauge hits the first mark and then it starts missfiring. its like as soon as it comes out of cold start mode it goes downhill from there. thanks
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Joined: Sep 2010
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From: Oregon
We had a guy on here a couple weeks ago, same kinda symptoms.
Somehow I remembered an old farmer trick.
Use a hairdryer to heat up things one at a time.
While it's running predictably good, like before it warms up and craps out and hit the distributor with the hairdryer and see if it dies.
I think he also blew compressed air on the dist to cool it down and did that a few times to confirm that's what it was.
You can also heat the ignition module on the inner fender wall, coil etc
I just wonder also, if a ground is going bad when the temp 'switch' goes up?
Everyone also says that it's easier to get a full distributor than to replace the sensor inside and even better to get a known working OEM unit from a junkyard than generic replacement.
Just do NOT drop the distributor drive shaft into the oil pan.
Keep us posted.
Somehow I remembered an old farmer trick.
Use a hairdryer to heat up things one at a time.
While it's running predictably good, like before it warms up and craps out and hit the distributor with the hairdryer and see if it dies.
I think he also blew compressed air on the dist to cool it down and did that a few times to confirm that's what it was.
You can also heat the ignition module on the inner fender wall, coil etc
I just wonder also, if a ground is going bad when the temp 'switch' goes up?
Everyone also says that it's easier to get a full distributor than to replace the sensor inside and even better to get a known working OEM unit from a junkyard than generic replacement.
Just do NOT drop the distributor drive shaft into the oil pan.
Keep us posted.
Thread Starter
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Senior User
Joined: Aug 2010
Posts: 189
Likes: 1
From: Port Washington, Wi
is the stator the thing that ignition module plugs into? i just orderd the pickup for the distibutor today, it will be here monday. one of my buddies that used to work at autozone said that little condensor thing by the coil could have some thing to do with it but i dont know. also i should mention that by flooring the truck the misfire goes away for as long as i hold it wide open.
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Joined: Sep 2010
Posts: 385
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From: Oregon
is the stator the thing that ignition module plugs into? i just orderd the pickup for the distibutor today, it will be here monday. one of my buddies that used to work at autozone said that little condensor thing by the coil could have some thing to do with it but i dont know. also i should mention that by flooring the truck the misfire goes away for as long as i hold it wide open.
There are so many more people on here that know waay more than I do.
Wide open under load? or free revving?
I've heard that flooring it while cranking kills the injectors to help clear flooding. but the more I learn, the less I know.
That little condensor by the coil may be a noise suppression for the radio noise.
I never changed the distrib parts (yet) but they say it ( the dist) has to come out to replace the sensor.
so I was looking into getting a spare distributor for when it's my turn.
Someone will chime in when they get home.
or
you could try the search key and get TONS of info instantly.
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Senior User
Joined: Aug 2010
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From: Port Washington, Wi
ok so i was bored and went outside, loosend the distributor and turned it a little with it running. i could notice a differnce in the way it ran when i would advance and retard the timing but it would still miss the whole time. im wondering if there is a way to adjust the tps to make the fuel mixture richer. i have it set at .96 because its optimal voltage for these trucks. i think if i turn it more to 1 volt it will ad more fuel. i hope someone will have a better idea about this. ive been battling this truck since july!
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Senior User
Joined: Aug 2010
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From: Port Washington, Wi
ok i tried the tps adjustment and i set it to 1.12 volts and it still misfires so its back down to .96. thinking i have a bad injector, maybe more than one, or maybe they are dirty. how would i be able to diagnose what ones are bad? thanks
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As you found out adjusting the TPS does nothing to lean/richen up the mixture. The PCM uses a ratchet function to detect the lowest TPS voltage then adjusts the idle speed, shift points accordingly.
You could pull the injector plug on each cylinder to see if the missing increases or stays the same. If you suspect any injector is plugged/faulty have them all cleaned.
You could pull the injector plug on each cylinder to see if the missing increases or stays the same. If you suspect any injector is plugged/faulty have them all cleaned.
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Senior User
Joined: Aug 2010
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From: Port Washington, Wi
on an inline six you cant get to all the injectors to unplug them, only like two of them. im gunna pull a set from a junk truck at my buddies that i know ran good. hopefully autozone has enough new o rings and pintle caps and ill clean up the filter baskets and put em in
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From: Oregon
Man, I miss the good old days when we'd put these things on a scope and you could SEE which one was misfiring.
What I did was, Some dielectric grease on the plug boots and a pair of insulated plug boot pliers and pull one at a time to see which one makes a difference.
I'm pretty sure voltage to the plugs goes up when you pull the wires (off) back a little. An old farm trick to clear fouled plugs.
Not sure how that relates to full throttle and NO miss.
Mine was dead from July to last month with an intermittent EEC problem so I know the frustration.
Thinking I'll look for a scope for sale since no one uses them anymore.
Maybe all the smart people will chime in after ice fishin' and skiing and plowing snow.
What I did was, Some dielectric grease on the plug boots and a pair of insulated plug boot pliers and pull one at a time to see which one makes a difference.
I'm pretty sure voltage to the plugs goes up when you pull the wires (off) back a little. An old farm trick to clear fouled plugs.
Not sure how that relates to full throttle and NO miss.
Mine was dead from July to last month with an intermittent EEC problem so I know the frustration.
Thinking I'll look for a scope for sale since no one uses them anymore.
Maybe all the smart people will chime in after ice fishin' and skiing and plowing snow.
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