1999 5.7 350 vortec P0304 cylinder 4 misfire

[socal k1500]

What O Ring did you purchase? I need it for mine as well as it doesn't seat properly and I'm having misfires on my cylinder 2.

Thanks

My deep dive on this subject yielded this as the best solution. I went to a local hydraulic shop and got a 4" OD 1/8" thick rubber O-ring thats rated for higher temperatures they were like 2$ each or something.

the 1/8 thickness is key i tried 3/16" and it was too thick for my liking you would have really had to smash the hat onto the throttle body to get it on.

So you take 2 of them, lightly coat them in some dielectric grease and then stack them in the lip you see on the throttle body hat. They fit perfectly in the 2 slots that the previous one gasket used. it goes on snug. It doesn't perfectly seal it but in my situation it was obviously way better than the old one.

 

[alvaroemg]

Thank you for your reply! and It's always good to hear that you got your truck running!
I have replaced my distributor with a BraveX brand off of Amazon. CS was good to me when I reached out to them.

Now I'm encountering a pesky cyl. 2 misfire that I can't get rid of. Going to start from the inside out and replace my fuel spider first as that has been on my todo list since I got this truck.

 

[socal k1500]

I received the blue cap and rotor and the new distributor hold down clamp, the more open one that allows for greater adjustment as opposed to the new style which you cannot rotate. I got my cam retard to 0.2-0.8 and truck runs as good as ever. The slight random misfire is still there at idle but its very light.

I checked all the gaskets and seals for leaks on everything i replaced so far so good. For now I'm just going to drive the truck as is.

 

[socal k1500]

Got some readings of my o2 sensors at idle. Both bank one upstream sensors are new and seem normal but the downstream are still original and read slow but those don’t matter right ?

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[Schurkey]

Bank 1, sensor 2 (downstream) seems to have very high voltage, and not much swing--hovering around .8 volts.

Are you getting codes for catalyst efficiency?

 

[socal k1500]

Bank 1, sensor 2 (downstream) seems to have very high voltage, and not much swing--hovering around .8 volts.

Are you getting codes for catalyst efficiency?

No just checking everything I can think of.

After driving it for a few days the biggest change so far is that the random slight misfire at idle seems to be intermittent. Sometimes sitting at a light I don’t feel it at all and other times after I park in a spot you can feel the misfires happening / engine shaking.

 

[socal k1500]

Video link

Do you guys think this is too much wobble in my crankshaft pulley? im wondering if this making my engine shake at idle

 

[Orpedcrow]

Eek. That’s a little concerning to me. My first thought is metal fatigue at the crank snout.

 

[socal k1500]

Eek. That’s a little concerning to me. My first thought is metal fatigue at the crank snout.

alright, Looks like ill take another break from driving it for a little until i get that fixed. Ive been thinking about this rough idle a lot it just doesnt make sense to me, and im starting to wonder if it really is due to the random misfire.

watching the live stream of ob2 data during idle i do get random misfires occasionally but the misfires dont at all line up with when the engine is shaking and there is zero change in the rpms when it happens.

Lets put it this way the random misses happen maybe 1 or 2 counts every few minutes. the engine shaking at idle is consistent. You can feel it when it first starts cold but does get worse as the engine warms up.

I also wanted to report on the original problem from this thread i drove the truck up the steep long hill near my house and no flashing check engine light, my scanner did show cylinder 4 misfires (about 100) but thats much smaller than before on the same hill (about 1000) so something ive done improved the clyinder 4 misfire

 

[Road Trip]

No just checking everything I can think of.

After driving it for a few days the biggest change so far is that the random slight misfire at idle seems to be intermittent. Sometimes sitting at a light I don’t feel it at all and other times after I park in a spot you can feel the misfires happening / engine shaking.

Hello socal K1500,

When I first purchased my '99 C2500 with the 454, the engine was mechanically quiet,
but the SES was on, there was a lot of misfiring at idle, a little chuggy at neighborhood speeds, but
seemed to run better the more throttle you gave it? And, confusingly, felt strong like bull on the Interstate?

Anyway, if you haven't seen this thread before, I show how it took me a total of (4) troubleshooting
passes to fix overlapping issues before I got the idle sorted out to my satisfaction. Now the idle
is smooth big block burble, and the gas pedal has that nice 'iron fist in a velvet glove' feel.
Whatever you want, smooth & sure. Always a little more available if you wish. :0)


Not saying that you have any/all of the issues in your engine bay that I had, but I'd ask you to give
your powerplant a critical eye and make sure that all the common sense stuff that can affect your idle
quality is known-good before we dig in and look for the most obscure/exotic rough idle reasons.
(The Hunt for a smooth 7.4 idle)

The good news? If you are after a smooth idle, I actually believe that the SBC can deliver a smoother
idle than the BBC, all things being equal. (It gets into boring combustion chamber stuff that we'll not
go into here -- but this is one of those places where smaller is better when making only a couple % of
rated power.) We've budget blueprinted a couple of SBCs in the past, and they can be such a pleasure
to be around when everything is optimized, even in 'stock' tune.

For what it's worth. And if you discover anything of interest in your hunt for a factory smooth idle,
be sure to post photos and share the wealth.

Best of luck --