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bucking at highway speed and rough idle 4.0 SOHC

kburley

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Hi guys

123000km
4.0 4x4 automatic

At idle, my 4.0 sohc will fluctuate RPMs. idles fine, drops suddenly by a few hundred rpm then bounces right back to normal for a few second (up to 10 seconds between jolts)

additionally, at highway speeds, the truck will buck under very specific conditions. let off the gas, transmission drops to low gear, then gently press the gas so that the truck doesnt downshift. trying to accelerate the truck at low rpms will cause a sudden "buck".

Initially, I thought i was having a spark issue. I replaced plugs and wires and the problem got better for about a week. then it started up again. the truck has only bucked once or twice since replacing plugs and wires, but idle is still just as rough as before.

I have ordered a coil pack, I am reasonably sure that is my problem. I have a co-worker who described his problem the same way, and a coil pack fixed his problem. I do not hear a vaccum leak, but the more i research the more i find other potential causes to this problem.

If anyone can offer advice before I spend money unnecessarily, that would be great.
Thanks in advance.
 


adsm08

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Don't do anything until you get that coil pack on.

You can also check the rubber elbow coming off the intake just behind the throttle body on the passenger side. It probably isn't your problem, but they do rub through and cause an air leak.
 

kburley

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Don't do anything until you get that coil pack on.

You can also check the rubber elbow coming off the intake just behind the throttle body on the passenger side. It probably isn't your problem, but they do rub through and cause an air leak.
I've already ordered the coil pack, its going on either way. I love to overthink, and im running through all the other stuff that could be wrong with it,

I can see a vacum leak causing a rough idle, but I find it hard to believe a vacuum would cause bucking at 120 km/h. plus I dont hear a vacuum leak when the truck is at idle. Either way thanks for the input, im really hoping someone can confirm what I already assumed, that coils are likely the problem.
 

Craig0320

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I had the same issue on my 98 4.0 it would buck and miss. I unplugged the vacuum line off the egr valve and put a bolt in it and it ran great ever since\. Turns out the vacuum purge valve went bad in turn was letting un-metered hot engine gas into the intake all the time causing a extreme lean condition. By the way my fuel mileage went from 18 to 14 when this happened. Give it a shot it might be your problem. Plus this solution is free.
 

kburley

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I had the same issue on my 98 4.0 it would buck and miss. I unplugged the vacuum line off the egr valve and put a bolt in it and it ran great ever since\. Turns out the vacuum purge valve went bad in turn was letting un-metered hot engine gas into the intake all the time causing a extreme lean condition. By the way my fuel mileage went from 18 to 14 when this happened. Give it a shot it might be your problem. Plus this solution is free.
I will certainly give that a shot. is the egr hole threaded? or did you just jam a bolt in there? i drive all highway and my last tank of fuel was 15l/100km which is slightly more than normal, but I could easily chock that up to extreme cold.
 

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He was referring to putting a bolt in the vacuum hose off the EGR after it was removed so no vacuum leak.

Yes, coil pack could be only issue, but spark plugs and wires don't last forever so good normal service parts

Clean MAF sensor, should be done every 1 or 2 years depending on driving conditions, i.e. dusty roads, easy to do

Not your problem just a heads up because of mileage
Upstream O2 sensors have recommended service life of 100k miles, but I think 150k myself :)
Downstream O2 can be well over 300k, it "sees" cleaner exhaust after Cats

O2 sensors are like car batteries in that they use a chemical reaction to generate electricity, and the chemicals get used up.
As the O2 runs low on chemicals the computer will run engine Richer than needed based on O2 voltage, just slightly, no big drop in MPG, but it is a drop, so money out of your wallet.
New upstream O2s can pay for themselves in better MPG over the next 150k miles
 
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Craig0320

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Sorry I should have been more clear. You put a bolt in the end of the vacuum line that connects to the egr valve. Make sure it fits in there snug. You do not have to worry about plugging the egr valve vacuum port.
 

kburley

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He was referring to putting a bolt in the vacuum hose off the EGR after it was removed so no vacuum leak.

Yes, coil pack could be only issue, but spark plugs and wires don't last forever so good normal service parts

Clean MAF sensor, should be done every 1 or 2 years depending on driving conditions, i.e. dusty roads, easy to do

Not your problem just a heads up because of mileage
Upstream O2 sensors have recommended service life of 100k miles, but I think 150k myself :)
Downstream O2 can be well over 300k, it "sees" cleaner exhaust after Cats

O2 sensors are like car batteries in that they use a chemical reaction to generate electricity, and the chemicals get used up.
As the O2 runs low on chemicals the computer will run engine Richer than needed based on O2 voltage, just slightly, no big drop in MPG, but it is a drop, so money out of your wallet.
New upstream O2s can pay for themselves in better MPG over the next 150k miles
So you are saying you figure it definitely is coils then? the fact that there is no engine light makes me think its nothing to do with emissions (i.e. EGR)
the truck is in kilometers so the 123000 you see is actually 75k miles. Duly noted though, I will replace the sensors at 160000km.
 

kburley

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Sorry I should have been more clear. You put a bolt in the end of the vacuum line that connects to the egr valve. Make sure it fits in there snug. You do not have to worry about plugging the egr valve vacuum port.
just looked at EGR and didnt see a vacuum line, looks to me like its just bolted directly to the manifold. any chance you could snap a quick pic of what you're talking about? i want to try it if its easy and free.
Thanks in advance brother
 

adsm08

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The issue and conditions you have described are consistent with a misfire caused by a weak coil. That 55-60 MPH range while cruising on the highway is a key to it. 120 KPH is right up the middle of that.

The EGR valve should have a green vacuum line on the top of it. But I really doubt that is the issue here. I seriously would do nothing until you get that coil and get it on and see if that fixes it.
 

kburley

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The issue and conditions you have described are consistent with a misfire caused by a weak coil. That 55-60 MPH range while cruising on the highway is a key to it. 120 KPH is right up the middle of that.

The EGR valve should have a green vacuum line on the top of it. But I really doubt that is the issue here. I seriously would do nothing until you get that coil and get it on and see if that fixes it.
Oh, I know the line you are talking about, makes sense.

Thanks for that input, I think you are right, I will leave it for now. none of this work is getting done until saturday anyway.

120km/h is 75 mph, but now that i think about it, it usually happens closer to 55 to 60mph where the RMPs are a bit lower (for example, when I let off the gas due to a slow driver ahead of me)

Thanks for the help, I will replace coils and get back to you. hopefully this diatribe can help someone diagnose this problem in the future.
 

RonD

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And WATCH firing order on coil pack, I still do it wrong when I KNOW it is RIGHT
3 4
2 6
1 5
front

5 6 4 side gets me on the 4.0l and 3.0l
 

Craig0320

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Hope you get it fixed brother. Keep us up to date.
 

kburley

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And WATCH firing order on coil pack, I still do it wrong when I KNOW it is RIGHT
3 4
2 6
1 5
front

5 6 4 side gets me on the 4.0l and 3.0l
Im going to hold the coil packs side by side and move one wire at a time. hopefully that leaves me enough room to mount the wires. alternatively, I might label them. my coil pack runs side to side, not front to back like you showed in your post
i.e:

x x x
x x x
front

but I will definitely look up diagrams before I start lol


everyone else, thanks for the info. I will be posting back here around noon on saturday to let everyone know how it whent. Cheers!
 

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looking at it that way it's

4 6 5
3 2 1
front
 

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