Hi All,

I've asked this before but have gotten nowhere fast in diagnosis. We have an boosted (9psi) LS4 with a misfire on cylinder 5. This has been scanned with HPT and shows as cyl 5. It only happens above 5500 rpm or so. So far, we've moved components around to see if the misfire moves to another cylinder. We've swapped injector, spark plug, valve springs (stiffer ones), plug wire, plug coil. The misfire never moves. The car pulls very hard until the miss starts, and then it's obvious and happens as if someone turned on a switch. We've tried turning up the dwell but that didn't help either. AFR is ok, everything seems fine (no other cylinders are misfiring!?!!!)

Any ideas? Has anyone battled this before? Thanks in advance.

The only thing I can think of is that it's reporting cyl 5, but it might be cyl 4 or 6.

 

Have you tried leaking down the engine yet?

What is the setup of the car?

 

setup is single turbo T-66 LS4 (5.3L FWD all alum). Completely stock motor except valve springs. Intercooled. Alky. We've tested the misfire with alky off, no change at all. I guess I could do a leakdown/compression... but the car pulls so well until the misfire starts... and it's not like it gradually gets worse.... the best discription would be to say that someone is under the hood unplugging the ign coil 4-way connector and just shutting off the coil when it happens. Doesn't feel like a compression issue. Right when the float happens, it's making about 470whp through an auto, and climbing for about 520 if the misfire wasn't there. One cyl becomes lazy. I'm wondering if it's the crank sensor.... but how? This is a new GM car with pretty low mileage.

 

I can't add much, but at 5500rpm, I would NOT trust the PCM to accurately report the proper cylinder. However, if you have access to a dyno, a quick infrared thermometer scan of the exhaust manifold should show which one (or more) is misfiring. If you have access to a complete ******* idiot, I guess you could take off the hood and do the same test on the road with them in the engine compartment.

 

definately a good laugh!!! I guess you see what I see... HPT repeatedly reports cyl 5, but I suspect it could be cyl 4 or 6 as it's calculations could be off at that RPM as well. Is it commong for the pcm to report the wrong cyl? I'm pretty sure the box was recently case learned.

 

Check out the wiring to the crank sensor. Been down that road before.

Also, when datalogging does anything odd happen up top in the pull?

 

I guess we'll look at the wiring. Nothing odd happens at all. IPW is constant, timing is consistant, 0 KR, etc. AFR is very solid also. I've turned off all limiters, fuel/spark/etc. I'll look into crank wiring... so odd.

 

Wait a minute. AFR should NOT be solid under a miss condition. It should go lean from the unburned air. This sounds unusual.

 

that's possible, we haven't had the WB on it in a while. I'm watching IPW like a hawk though, and it doesn't budge when the issue happens.

 

I just realized you wrote "stock motor except for valvesprings". Please tell me you aren't using stock pushrods. If you are I would be surprised if any of them are still straight.

 

stock rods. The springs are LS6, 90lb. Just slightly stiffer then she stock LS4's. Do you still think the rods are an issue?

 

When we were on the dyno this is the section of the graph where the car falls flat on it's face, We are at WOT all the way to 6200rpm during this run.


This is a log of that dyno run, it's a bit hard to see but if you follow the RPM line it goes all the way up to 6200rpm and then it drops off only when we let off the throttle. You don't really notice anything out of the ordinary that would make it lose 200whp.



FULL SIZE PICTURE OF THE SCAN
http://i37.photobucket.com/albums/e5...h9425tmp-1.jpg

 

Anyone else have any ideas?

 

Not the rods, the pushrods. I think Gametech may have found the problem for you. Sounds like the #5 pushrod is bent. Quick, cheap fix if so. Cross your fingers.

 

Well, actually I just noticed that you have LS6 springs. So you more than likely have two problems. Number one, the LS6 springs won't handle the boost. I know this for sure. And two, you've probably still got a bent pushrod or two.

I had actually responded to GXPJay's post about this problem a while back on ClubGP. At the time, I think it just had stock valvesprings? I thought the springs were already being swapped. Didn't realize you were only going to use LS6's.

FWIW, the Patriot Gold's are inexpensive and work well up until about 650rwhp (or fwhp in this case).

 

We had some new LS6 springs lying around so we swapped to those. I mean, there are several people with LS2's etc running stock valvetrain with over 700whp so I figured we would be ok at 500whp.

As for the bent pushrod, easy enough to check it out since it's on the front bank so very easy access.

We're trying to keep things as stock as possible, I don't mind getting the Patriot Golds if that's the problem, I'll look into the Pushrods.

 

just change all of ur plugs and then see whats up... the computer wont pick up the different cylinder misfires that quickly

 

As noted, LS6 springs are inadequate, I would run them only on lower HP combos. You need more spring than stock for sure.

 

ok ok ok I know I need new springs for sure, I am just looking to see if it's the cause of this problem would it only happen on one cylinder?

On the HPT log, at WOTfrom 3000rpm to 5900rpm you'll see about 4 misfire counts on cyl4, 100 on cyl 5 and 6 on cyl 6

This stayed constant no matter what we changed/swapped. I would imagine if it was the valve springs it would not only be on one cyl.

It looks like it's isolated to Cyl 5 and it's not the coil, plug, wire, injector and it's still doing it on cyl 5 even after we swapped to the LS6 springs. Maybe it's Pushrods as we have not checked them and they are stock.

 

Maybe valvefloat.