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Discussion Starter #21
Cool - thanks for the detailed explanation.

On the LE5, removing the intake manifold is a 15 minute process. Not sure about the LNF. If the rain ever stops, I'll go ahead and yank it off. I found a video from one of the Cobalt SS guys showing the cleaning process, and it looks pretty straightforward.

In the mean time, I ordered a cheap bore scope (really "needed" one, anyway). I'll check the accessible valves through the vacuum port on the manifold. With 100k miles, I'm sure it's going to look pretty terrible.

The real question is if this will impact the misfire issue. The TSB implies that it will, and it's worth taking care of in any case. Fingers crossed.
 

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Thanks for posting that service bulletin. It’s not clear to me if that stuff can be sprayed through the throttle (with the plate open) or if the intake manifold needs to come off. ??

Why would the carbon buildup issue only present at higher boost? I can create the misfire at 3500-4000 RPM (under load) if the boost is high.

When the weather was a little warmer, the GMPP tune was making closer to 18PSI, and the 15# number is in 2nd gear. I haven’t tested on the highway yet, so getting up to higher RPMs, under load, out of 1st gear hasn't happened.

I *can* get 18psi-20psi in neutral (sitting in the driveway) if I floor it, at which point it misfires even on GMPP.

Putting the car back to stock won’t really help here since the problem only happens at boost levels higher than those allowed by the stock tune.

I may try copying the air load tables and wastegate duty cycle table over from the other tune to GMPP just to up the boost and see what I get. Which hopefully won’t include detonation and holes in pistons. :)
Thats interesting
 

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the 'carbon plating' of the intake runners is insignificant compared to the buildup on the valves and is of little consequence unless it liquifies or shrouds the valves.

the CRC product is less than 1/2 the price of the GM, and there seems to be other similar products
Bill
 

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If you had significant carbon build up it would likely not only effect the intake tract but also the value seats. If it effected the value seats you would see that in the compression test you did. If you are getting that bad of a misfire under load at high boost my guess would be fuel starvation or too much timing leading to knock. Where did you get your tune?
 

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For what it's worth, I've had significant carbon buildup on my intake valves twice due to oil getting into the intake. Once from a bad PCV valve and once from a turbo that was on its way out. I never had much luck with solvents, I ended up walnut shell blasting the valves.

Misfire at boost was never an issue for me even when they were badly gunked up. The only symptom I had was a random cold start misfire CEL. The CEL would set between 10 and 40 seconds after starting the car. The best way to check for buildup is to pull the intake manifold. It's a pretty easy job and the one way you'll know for sure if that's your issue.
 

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I'm surprised noone has asked this yet, but....If your Gmpp tune is only giving you 14psi.... What amount of boost were you seeing stock? Also, it could be possible at the 15psi boost level that you are blowing out spark slightly. What do you have your spark plugs gapped at?
 

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Discussion Starter #29
I'll flash it back to stock this weekend and compare boost levels vs GMPP. What sort of boost should I expect from the GMPP tune? I also need to get it out on the highway and log some WOT pulls.

Spark plugs are the Delco 41-108s, which come pre-gapped.

Also, I previously had a CEL due to multiple random misfires, even before the new tune. There's something else going on here. I replaced the HPFP recently, but as far as I can tell the new one is working fine.

It's still a balmy 40° F here; supposed to be back up close to 70° in the next day or two. New Year's resolution: clean out the garage so that my wrenching time isn't restricted to good weather days.
 

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Discussion Starter #30
the CRC product is less than 1/2 the price of the GM, and there seems to be other similar products
Bill
What is "CRC?" I'm thinking error detection & correction, but that can't be right. :)
 

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My friend used CRC Intake Valve Cleaner that is safe for direct injected engines and turbos. He pulled the sensor on the cold side of the intercooler pipe (to bypass the turbo). He used a heat resistant putty to hold the solvent straw in place (and to seal the rest of the hole) and sprayed in the cleaner. He followed all the instructions on the can. Right after that, he changed the oil (as it was dirty with carbon) and the car ran fine.




 

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I'll flash it back to stock this weekend and compare boost levels vs GMPP. What sort of boost should I expect from the GMPP tune? I also need to get it out on the highway and log some WOT pulls.

Spark plugs are the Delco 41-108s, which come pre-gapped.

Also, I previously had a CEL due to multiple random misfires, even before the new tune. There's something else going on here. I replaced the HPFP recently, but as far as I can tell the new one is working fine.

It's still a balmy 40° F here; supposed to be back up close to 70° in the next day or two. New Year's resolution: clean out the garage so that my wrenching time isn't restricted to good weather days.
What are they pregapped at? Just because they are pregapped doesn't mean they didn't get knocked around in the boxes messing up the gaps. Also, the gaps may still be too far for the strength of spark your coils are putting to them (thus blowing spark out).

The Gmpp should be the same (fairly close) amount of boost as a trifecta tune which should be around 20 to 24psi. 14psi is flat out stock. I'm saying this because you are only 800 ft above sea level so it won't have that much of an effect on your boost levels either. Regardless you should be seeing around 20+ psi with the GMPP.
 

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Discussion Starter #35
They're at 0.035, I think? They were all identical, with the little plastic cylinders protecting the electrode. I'll pull them and check again, but I've been told by more than one tuner not to mess with the plug gap if it's at spec.

My handy-dandy endoscope showed up today, so I pulled the brake booster hose & nipple off of the manifold and had a look. There's so much crap in the thing that I'm not sure how it runs *at all*.

If I can get through the honey-do list tomorrow, I'll go ahead and pull the manifold and clean things up.
 

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Discussion Starter #36 (Edited)
Solved, finally.

After walnut blasting the (REALLY gross) intake valves, replacing the bypass valve that I broke, replacing the cam actuator solenoids - I wasn't getting a code, but they looked pretty rough and I had a set in a box from the other car - everything is working very well.

These are from Virtual Dyno, based on some HPT logs. Obviously that's not as accurate as a real dyno, but the buzz online is that this should be really close. Having owned & driven a number of other cars in that power range, I don't have trouble believing that the numbers are close to reality. :)

3rd gear is a pretty graph,



But 4th is the 1:1 ratio gear, so it's probably more accurate:

 

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Discussion Starter #38 (Edited)
@Saveaux Yeah... the more I think about it, I think I'm just not handling the virtual dyno software correctly. Getting the right data out of HP Tuners is a bit of a trick. If you include data from part of the log that was in a different gear, the charts go nuts. I need a button on the steering wheel that will tell HPT to stop and start logging. I could probably find some sort of USB or BT "button" and map that to a key macro... (Thought for the day: over-engineering is under-rated.)

Once I get it on the real dyno on March, we'll see.

But, fun fact: if you floor it in second, the traction control turns on.

I had another few misfires after logging some hard 3rd gear pulls, so something else is still going on. Hopefully the next revision of the tune will address that.
 
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