having trouble with cyliinder 7 missing after reving the engine up. car will run fine for weeks as long as you keep the rpms below 4000 or so. once you rev it up and drop back to idle it starts missing. after a while it fixes itself till you rev it up again.

changed:
plugs, wires, fuel filter

tried:
headers (had on order before problem started), swapped around coils, swapped around coil packs, switched sides on o2 sensors, swapped around injectors, flushed oil twice and have checked plugs several times.

nothing has changed which cylinder misses...always #7

today i passed someone and reved it up a bit too much, and it started missing. hooked the scanner to it and it recorded 1000+ misses on 7 within 2 or 3 minutes, and a about 100 on cylinder 5 which happens usually too.

i tried blocking the throttle open a little. held it at 1800rpms for a little while and for 2 mins it never missed again according to the scanner. drop back to idle and it misses again right away.

is it possible that with the lower oil pressure at idle the lifters are not getting pumped up enough? and when you get to a higher pressure due to a certain rpm they start functioning as normal?

HELP
this has been a recurring problem for a whole YEAR now...still trying to figure it out!

 

Sounds like it might be a lifter. In the old days if a lifter stuck after a high rpm excursion, a can of Rislone was popped in to try to unstick it. Don't know if they even make that stuff anymore, or if it's recommended for use in modern engines.

If the hyd lifter taps, it's collapsing. If it doesn't tap, it can be that the pushrod cup rode up when the lifter pumped up at high rpm and stayed there. So the lifter keeps the valve off its seat.

 

I think that's the best possible scenario so far Jeff. If that's the case you should be able to take off the valve cover when it's having it's little episode and check the rockers on #7. If it's sticking open it'll be real tight even when the valve is closed. I would suspect the exhaust valve since it's not backfiring.

 

hmmm...

having coil packs makes this kindda tricky to check...since the wire harness doesn't reach far and the plug wires are short too.

i'll see what i can figure out.

what exactly am i looking for if the valve is sticking?

 

Have you ever tried doing a compression test on the motor to see if #7 is different from the rest?? Let's say there is a lifter problem (or maybe a bent pushrod, or a bent valve or something like that) it might show up in a compression test? If a valve sticks open, that cylinder will have little to no pressure in it. And you know that LS1's have a common problem of bending pushrods when over-revved. It's just a *thought*, but it goes along the lines of your bad lifter theory.

 

well i've tried "oil flush" two separate times and it did nothing for the problem.


i have not gotten a compression check as of yet. i think i'm gonna drop it off at the shop this week and have them do one.

the reason i don't think it's a bend valve or pushrod is that it runs fine for long periods of time...untill you rev it up once. then it misses.

something bent would happen more continuously wouldn't it?

we'll see what the shop says.

bad lifter gives me a good excuse to do a cam!!!
yeah baby

 

You wouldn't check the rockers with it running knucklehead!

Normally when the cam lobe is not compressing the rocker you can wiggle the rocker or get the pushrod to spin a bit by hand. They shouldn't be sloppy loose, just not too tight.

So get somebody to watch the rockers as you crank the motor by hand with a big ratchet or something. Spark plug removal makes this a lot easier but isn't completely necessary. Turn the motor over until it gets to a position where the #7 intake rocker is pushing the valve open. Once it allows the valve to close again, you are starting your compression stroke. Turn it another 1/2 turn on the crank and you'll be at the top of your compression stroke for that piston. Both valves should be closed so your rockers should not be applying pressure to either of the valves (other than the normal amount of preload - compare to others that are closed too). If your exhaust valve is being held open just a bit from a sticky lifter the rocker will be real tight and the pushrod won't have any give at all (again compare to others to get a feel for what's a good amount of pressure on the rockers).

I suspect a overinflated, sticking exhaust lifter on #7. It would do exactly as you have described.

***edit*** You would definately need to rev it up and get it missing again before you perform the above test.

 

cyl 3 - 180#
cyl 5 - ~160#
cyl 7 - 0

guess i know something is wrong with the valve train....


what? whats that i hear????

HEADS/CAM TIME?!!!!!!

 

Seriously?! Damn, and I was just taking a shot in the dark! Glad that a simple *suggestion* could lead you to the problem. The only thing that made me think of that is we had a bent valve in my Dad's old truck, and it was running like crap. When you starting wondering about a bad lifter, it made me start thinking about the entire valvetrain. My guess is that the motor was over-revved at some point? (you'll likely run into a bent pushrod (or 2, or 3, or 16!) while you're in there.

Sorry to hear you're into some serious work now, but glad you've finally found the problem.

 

thanks for all the help....

so what cam should i go with?

 

I was reading about a head/cam test the other day in HotRod. It was on an LS2 but it sounded like they had an awesome combination. They pretty much were just taking their design for what they had done to an LS1 and putting it in the LS2. Stock tuning and nothing but their heads and cam (and maybe rockers) and they made 499/462. I would think that an LS1 should top out at around 400 hp more in comparison. I'll go look it up and get you more info.