single cylinder misfire on TT setup

[icicleboy]

Hey all,

just thought I'd check in if anyone might have any ideas. I'm tuning a 2005 C5 with a 427ci motor/AFR heads. Car runs a TTix turbo kit with 62mm turbo's. Method of tuning is a newer style maf card with everything /2 as necessary and the tune seems to be working out well. 8 - 9 psi on a 90f summer day netted about 700whp. Although there is a lot of room to turn up the wick on power, we have a misfire issue which is yet to be solved and am hoping some of the experience on this board might help solve the issue.

Misfire only happens under heavy load (3rd gear and up) with boost. Only on cylinder 6. A hard run could produce up to 60 misfires (meaning it's not each and every ignition that misfires, but enough as it should be zero). We have checked compression and leakdown numerous times. We have checked cam lift on that cylinder, plug gap, plug, wire, coil, injector, low imp inj driver box, harness, etc (each of those components have been eliminated by removing and substituting another part). I finally got down to feeling it is a lean misfire on that cylinder. Motor uses a fast92. We took off the fast92 and installed a stock LS2 intake manifold and the misfire left cylinder 6 completely and started on #1. That is where I am currently at.

Any suggestions?

Much appreciated.

[IDRIVEAG8GT]

Is your timing erratic in the area it misfires?

[icicleboy]

steady 18 dg. Thanks for chiming in

[IDRIVEAG8GT]

Weird.... Alot of the LS motors that run Prochargers just flat out start lacking ignition power, but it's on multiple cylinders, not just one. Possibility though, the vacuum line from the intake to valve cover on either side could be pulling oil from the cylinder heads and back into the intake, thus traveling into a designated cylinder at WOT causing a misfire.

[DSteck]

Swap injectors around, and see if the misfire follows the injector. That's the only thing I could think of since you said changing intakes (which meant the injectors were removed and reinstalled) moved the misfire.

[icicleboy]

thanks Dsteck,

we've swapped inj 3 times now with no change. I'm actually wondering if it's possible that there is a charge pipe leak causing high backpressure. Any more suggestions are welcome.

[DSteck]

You could try changing the gaskets for the intake manifold around. Swap the misfiring cylinder with a good one. That's the best thing you can do... just try swapping parts with known good ones, one at a time, and see if it follows a specific part.

[IDRIVEAG8GT]

My suggestion: This sucks. ok let's get back to basics. What on an LS engine detects misfire?

[IDRIVEAG8GT]

Also you said you're running AFR heads. Not quite sure what that means, but are they shaved? Say the heads sit differently on the block than stock. Should the actual geometry be changed of how the heads/block/intake sit, than it's highly possible that you have a leak on the underneath side of the intake ports from the intake manifold as the gasket wouldn't mesh correctly. It's a long shot, but I've seen it happen before, and since the actual intake manifold changed which cylinder is misfiring than you have to account for variable different flaws in design and structure.

[DSteck]

Also you said you're running AFR heads. Not quite sure what that means, but are they shaved? Say the heads sit differently on the block than stock. Should the actual geometry be changed of how the heads/block/intake sit, than it's highly possible that you have a leak on the underneath side of the intake ports from the intake manifold as the gasket wouldn't mesh correctly. It's a long shot, but I've seen it happen before, and since the actual intake manifold changed which cylinder is misfiring than you have to account for variable different flaws in design and structure.

The gaskets will take up a lot of slop.

[IDRIVEAG8GT]

I Understand that, but is it not a possibility? I said it because it only happens under heavy load (maximum boost, and cylinder heat)

[Street Legal]

I Understand that, but is it not a possibility? I said it because it only happens under heavy load (maximum boost, and cylinder heat)

what's the sprak plugs gap ? it could be a spring issue.not a tune issue. any leak or gasket issue will be felt under regular driving conditions.either trims or map readings will show you that.and does that car backfire when cyl 6 missfires ? if it does then you aren't getting any spark there.

you could remove the discharge pipe and try to run the car N/A and see if it missfires.

[Street Legal]

Weird.... Alot of the LS motors that run Prochargers just flat out start lacking ignition power, but it's on multiple cylinders, not just one. Possibility though, the vacuum line from the intake to valve cover on either side could be pulling oil from the cylinder heads and back into the intake, thus traveling into a designated cylinder at WOT causing a misfire.

if it sucks oil from the valve covers to the intake the car would smoke it and it would reach most of the cylinders not possible to get in one cyl only.missfires under a boost is mostly related with ignition systems , coils,wires,plugs ,otherwise a bad valve spring

[icicleboy]

street legal, interesting you mention that. The customer has the odd backfire up top at high rpm. I'd bet that happens after a high misfire count. This has always been my guess. I'm betting on a lean misfire but have a feeling on what it could be. Hopefully will have some info tomorrow.

[Mez]

I helped find a misfire on a 7.0L S/C that eventually dyno'd at 809 rwhp. It was traced it to a broken wire to the fuel injector. Actually, the wire was broken in two spots near the connector at the injector. The insulation on one of the breaks looked fine.

Could be anything on that cylinder. Make sure the compression is fine then look at wires, plugs, etc. Swap between cylinders to test.

[IDRIVEAG8GT]

if it sucks oil from the valve covers to the intake the car would smoke it and it would reach most of the cylinders not possible to get in one cyl only.missfires under a boost is mostly related with ignition systems , coils,wires,plugs ,otherwise a bad valve spring

This is true. I'm just suggesting. But per smoke and what not, this all depends on the volume of oil to enter the intake. A minute enough amount of oil would barely show smoke, but could cause a Misfire depending on Weight/Temperature/Synthetic/Conventional/etc.

[IDRIVEAG8GT]

I have an idea now because of a past event that happened to me. What kind of spark plugs are you running? Please tell me they are NOT E3's. If they are, yank them all out and shake them and watch to see if the porcelain rattles up and down inside the plug. E3 went through several lawsuits for defective plugs in which the porcelain separates from the body and would eventually break at the end and the pieces destroy your engine. I've been there and done that and have the proof to back it.

[EC_Tune]

thanks Dsteck,

we've swapped inj 3 times now with no change. I'm actually wondering if it's possible that there is a charge pipe leak causing high backpressure. Any more suggestions are welcome.

How about turning up the dwell on the coils? Stock dwell settings will give those coils 300,000 miles of service. You can run up to 9 msec on them but the longevity goes down quickly above 6 msec. At 9 msec you will get about 20K out of them.

[icicleboy]

TR6's gapped to 0.030. My gut is telling me that it's a bad boost leak causing extremely high backpressure in the exhaust manifold which could cause a misfire.

Springs on that cylinder have been checked. I'm also going to do as advised and swap some injector harness stuff around to see if that cylinder has a bad wire to the inj that only shows up at load.

Thanks so far everyone!

[icicleboy]

EC_Tune,

thanks for the suggestion, that was my first course of action and it made no change at all. I incremented everything from 12 - 16V by 1ms per reflash until I was just over 6. No change.