Help diagnosing my rough running 2013 GT500

[JPs2013GT500]

(Sorry in advance for the long post)

Here's what it's doing:
Marginally rough idle (not terribly bad, but noticeable).
Surging when running normal highway speeds. Seems most noticeable in the 1800 to 2600 RPM range.
Extreme popping on deceleration. I mean like 4th of July fireworks going off.
Loss of power, especially on the high end. Feels like it just quits pulling after 5000 RPM.
Hesitation when starting out in 1st gear sometimes. This mainly seems to happen before the car is warmed up completely.

This behavior started a month or two ago. Around the time the first blast of warm/humid weather hit. At first I thought I may just have a bad tank of fuel and/or it's the summer heat & humidity. However, I've been through a couple tanks of gas (Sunoco/Shell/BP) and had some cool evenings to test. It still ran rough with 'fresh' fuel and in the cooler, less humid conditions.

Here's what I've tried:
Data logging. I've sent data files to my current tuner (Pro-Dyno) and was told nothing looks to be out of normal ranges.
I've tried a tune from a different vendor and the car behaves similar with it as well. The exception being the exhaust was louder and the popping issue was even more pronounced.

Reload tune/KAM reset/Throttle relearn - No difference.

I changed the spark plugs. I removed the TR7IX plugs (.030 gap) and replaced with some new BR7EF's I had on hand. I think the TR7IX plugs looked ok, but I'm no expert on reading plugs. I did notice the plugs in cylinders #2 and #3 seemed to have some extra build up (?) on the ground electrode. For reference, here are some photos:
http://jeffpatton.net/downloads/gt500_plugs.jpg
http://jeffpatton.net/downloads/gt500_plugs_passengerside.jpg
http://jeffpatton.net/downloads/gt500_plugs_driverside.jpg

On the BR7EF replacement plugs I wanted to try a slightly larger .034 gap to see if it made any difference. I didn't really notice any difference.

Installed a new air filter (old one wasn't really dirty though).
I checked the vacuum lines to make sure they looked ok and were connected (they were).
Charged the battery overnight to make sure it was 100%. When I connected the charger it showed the battery (original battery) at 55%.

Mods:
Dual 65mm throttle body
JLT 127mm intake
BPS oversized ported plenum
2.39" pulley with 90mm idler & tensioner
Plugs: NGK BR7EF (.034 gap)
JLT oil separators
Off-road x-pipe
Shelby SS mufflers
170 deg. Thermostat

There are no stored codes or CELs.

The car ran great after the last tuning session (June 2015). The only changes I've made since having the car tuned last year are the 170* thermostat and SS axle-back mufflers.

In looking through new & old data logs the main difference was the coolant temp because of the 170 deg. t-stat. So, today I put the original t-stat back in. Did a KAM reset, reload tune, and throttle relearn process. <Poof> it ran better! Figured I had solved this issue. However, I took the car out tonight (82 degrees / 42% RH) and the hesitation/surging is back. :bored:

On the upside, the exhaust is not popping now as it was with the 170* thermostat and it pulls fine but 1800 - 2600 RPM still surges.

Maybe I just need to reduce the gap on the new spark plugs at this point but thought I'd ask if there's anything you folks recommend I look for/check next?

 

[Catmonkey]

I'd pull plugs and run a compression test, just for peace of mind.

 

[Beercules]

When my 2010 GT started sputtering it was because the plugs were burned down and needed replacing. ..but you've looked into that.

Coil pack? Do these fade out or completely go at once? I've also heard these are really good oem.

Crud under a valve preventing full seating?
Plugged injector?
For both of these. .. stick some BG 44k in the fuel tank and give it time to clean.
Though like Catmonkey said, a compression test will tell you if a valve isn't closing right, unless it's just sticking.

Actually, quicklane has a service where they stick something (BG) in the fuel rails and 44k in the tank. Might try that. ..you'd be out around $100 and a cleaner top end.


I'm interested to know what it turns out to be. ..hopefully something simple.

 

[JPs2013GT500]

Thanks for the feedback. Kudos for making it through my wall of text. :rockon:

Great ideas, very much appreciated. I'm scheduled to have headers installed next week but I think I'll change that to have them run a compression test & diagnosis session instead.

Side note: This morning I removed the super snake axle back and put the stock mufflers back on. I did so because this puts the car back to the exact configuration when it was tuned last year. It runs great again. Not sure if that will change again tonight like it did with the thermostat swap, but I'll see this evening.

If it doesn't change and runs as well as it does right now, then it would seem as though my engine is hyper sensitive to the slightest changes. I wouldn't have expected to have the tune updated for a simple axle-back swap.

 

[Softballer77]

Exhaust swap like that shouldn't make the vehicle do that. Especially if no O2 sensors have been messed with. I'd assume the 170 degree t-stat wouldn't affect anything either. At least not to that caliber.

Something else isn't right. Sometimes you can get surges if you have a leak in the intake system somewhere. Around where the CAI goes into your TB, or possibly somewhere around the plenum. Check your MAF sensor in the intake and make sure it isn't covered in anything. If it is, that can limit performance noticeably as well. I've cleaned ones on vehicles before with a little brake cleaner or carb cleaner with a q-tip and instantly runs better.

Just a few suggestions. Hope you get it figured out.

 

[myke hunt]

sounds like a vacuum leak. do a smoke test

 

[JPs2013GT500]

Thanks for the additional ideas. I went through the intake system just now. I checked the MAF sensor and with a magnifying glass I noticed a dark spot of something on one side of the resistor(?) wire. I took a q-tip to it (dry) and was able to wipe it clean. Then hit it with some compressed air (camera lens pump, non-aerosol).

Moving up the line, the air intake tube and throttle body looked fine. Moving around to the ported plenum I discovered a connector on the side that when touched would easily move. Photos:
http://jeffpatton.net/downloads/ported-plenum_01.jpg
http://jeffpatton.net/downloads/ported-plenum_02.jpg
http://jeffpatton.net/downloads/ported-plenum_03.jpg

Seems like the black 'gunk' that sealed this has deteriorated and is gone now. The connector would easily fall out of the plenum on its own if I turned it towards the ground. That alone may not have anything to do with the issues I've been fighting. However, it didn't inspire confidence so I removed this plenum and replaced it with the stock unit.

Found a small pool of oil in the back of the ported plenum, even though the air separator on the intake was bone dry:
http://jeffpatton.net/downloads/sc_inlet01.jpg

Just for kicks, a couple of shots inside the super charger:
http://jeffpatton.net/downloads/sc_inlet02.jpg
http://jeffpatton.net/downloads/sc_inlet03.jpg

Going back to the stock plenum also allowed the air filter to finally fit in the center of the housing. Previously the air filter was pushing against the fender and touched the rear of the headlight assembly.
http://jeffpatton.net/downloads/intake_01.jpg

Took it for another spin, it ran great again. The idle was really smooth. In fact, I'm so accustomed to the loud exhaust and rough idle that at a stoplight I had to look at the RPM gauge to make sure it was still running. I'm anxious to see what it does later tonight when the air cools off again. Usually it's a Jekyll & Hyde type of deal, runs one way during the day and another at night.

 

[JPs2013GT500]

Well, it ran nicely tonight. No noticeable hesitation or surging. The throttle was crisp & power level felt right. First time its ran decently back to back in quite a while.

Will try to drive it some more this week to see how it goes. Will pull the plugs later in the week and depending on how they look, figure out what steps to take next.

 

[tomshep]

One of the reasons I have a few simple bolt-ons and appearance mods. I simply don't want to sacrifice the driveability of this car like I have others. I enjoy getting in it and enjoying the drive.

Tom

 

[Softballer77]

That piece falling out of the plenum looks suspect to me. If I had to bet, that is your highest probable % item to create your problems. Poke a pin hole in your intake (well not really, but...) and take it for a drive. Surging is almost always the first side effect. Once things warm up, cool down, etc. the contracting can make it come and go. I'd fix that piece, slap it back on and see what you have. Bet you'd be good to go.

Glad it's driving better.