1999 5.7 350 vortec P0304 cylinder 4 misfire

[socal k1500]

Good news update frens, the broken fan was the primary source of my rough idle. I replaced the fan with a new one like the picture above with the 11 blades and the engine no longer shakes like it did before. its not a smooth as brand new but it definitely doesn't move like it did before.

kind of funny and sad after all this I never noticed the broken fan blade.

I checked the waterpump for play and it had none but i already know you guys will be on me to replace it and the fan clutch ASAP. I will try to get them replaced as soon as i can.

 

[Road Trip]

Good news update frens, the broken fan was the primary source of my rough idle. I replaced the fan with a new one like the picture above with the 11 blades and the engine no longer shakes like it did before. its not a smooth as brand new but it definitely doesn't move like it did before.

kind of funny and sad after all this I never noticed the broken fan blade.

I checked the waterpump for play and it had none but i already know you guys will be on me to replace it and the fan clutch ASAP. I will try to get them replaced as soon as i can.

Hello socalK1500,

I really appreciate you coming back and closing the loop with the final solution to your rough idle.

This is one of those gotchas in remote troubleshooting that would have been avoided if we were working
shoulder to shoulder under the hood. With 1 set of eyes most of the problems are found short term, and
all problems are eventually found when we dig a little deeper after the first few theories don't pan out.
(Or just serendipity when you are focused on something else and *then* you find the root cause of the first issue. BTDT.)

On the other hand, 2 or more sets of eyes in the engine bay at the same time usually finds the problem in the first outing or
two, for it's just the way these things pan out when a couple of mechanics are bouncing ideas off of each other...and there's the
unspoken race to be the one to ferret it out first. This tends to bring that extra layer of focus to the troubleshooting event. :0)

FWIW, all this is *not* wasted effort, for your documented troubleshooting session reinforces how a shake from a 'misfiring engine'
may instead be a shake from an ancillary part driven by the serpentine belt superimposed onto a engine hitting on all 8. (more or less)

****

Listen, I know life is busy, and you aren't here in order to provide me with troubleshooting entertainment, but
I for one would be really interested in what kind of misfires (if any) are still being logged after fixing your fan.
I really want to see all zeros across the board, but if we can't have that, then documenting what you've got will at least give us
a reference to better judge future changes against.

Best of luck keeping your GMT400 on the road.

 

[socal k1500]

Hello socalK1500,

I really appreciate you coming back and closing the loop with the final solution to your rough idle.

This is one of those gotchas in remote troubleshooting that would have been avoided if we were working
shoulder to shoulder under the hood. With 1 set of eyes most of the problems are found short term, and
all problems are eventually found when we did a little deeper after the first few theories don't pan out.
(Or just serendipity when you are focused on something else and *then* you find the root cause of the first issue. BTDT.)

On the other hand, 2 or more sets of eyes in the engine bay at the same time usually finds the problem in the first outing or
two, for it's just the way these things pan out when a couple of mechanics are bouncing ideas off of each other...and there's the
unspoken race to be the one to ferret it out first. This tends to bring that extra layer of focus to the troubleshooting event. :0)

FWIW, all this is *not* wasted effort, for your documented troubleshooting session reinforces how a shake from a 'misfiring engine'
may instead be a shake superimposed onto a engine hitting on all 8. (more or less)

****

Listen, I know life is busy, and you aren't here in order to provide me with troubleshooting entertainment, but
I for one would be really interested in what kind of misfires (if any) are still being logged after fixing your fan.
I really want to see all zeros across the board, but if we can't have that, then documenting what you've got will at least give us
a reference to better judge future changes against.

Best of luck keeping your GMT400 on the road.

going up a long a grade with the engine trying to hold about 70mph at 30-40% throttle i am still getting Cylinder 4 misfires and cylinder 3

However as previously stated the cylinder 4 misfire counts are down significantly, previously under the above circumstance id log about 500-1000 misfires on #4 now that count is always around 100 or less. This improvement happened after i did the lower intake manifold gasket .

Cylinder 3 misfire is not consistent every time like #4 and the scanner shows a misfire count of 15-30 when it does pop up from this circumstance.

The actual misfiring that you could feel when it was happening before is also improved.

My best guess at this point is that my truck does have the previously posted valve guide issue that the GM TSB goes over. I'm going to drive it how it is because its running better than ever despite that and in a few years ill end up buying new heads for it as long as it doesnt get any worse.

 

[Road Trip]

going up a long a grade with the engine trying to hold about 70mph at 30-40% throttle i am still getting Cylinder 4 misfires and cylinder 3

However as previously stated the cylinder 4 misfire counts are down significantly, previously under the above circumstance id log about 500-1000 misfires on #4 now that count is always around 100 or less. This improvement happened after i did the lower intake manifold gasket .

Cylinder 3 misfire is not consistent every time like #4 and the scanner shows a misfire count of 15-30 when it does pop up from this circumstance.

The actual misfiring that you could feel when it was happening before is also improved.

My best guess at this point is that my truck does have the previously posted valve guide issue that the GM TSB goes over. I'm going to drive it how it is because its running better than ever despite that and in a few years ill end up buying new heads for it as long as it doesnt get any worse.

socal k1500,

Sounds like a good plan of attack. You have made the best of what you have to work with,
you know how to stimulate the fault, and you also know how to avoid stimulating the fault
if need be. By minimizing the misfiring you will extend the lifespan of the cats.

And finally this approach allows you to wait for a good deal to pop up on a new/new condition
set of heads, instead of driving to complete failure and then being forced by time constraints to
pay the long dollar in order to get your DD back up online asap.

Over the years I've been in a similar situation where my DD had a mechanical weakness, but by
driving it sympathetically I was able to nurse it along until I was in a better position to make it better.
(Or retire it.)

Good stuff. I enjoyed the collaboration with all involved.

Safe travels --

 

[socal k1500]

You guys around here dont get enough credit for helping people out. I really appreciate the time yall have put into this and i hope this thread helps others out.