[cwebster]

Engine has been intermittently cutting out on the big end. This is the 6th Gen ZL1 LT4 motor. I've had the car for about 13 months now. It's got just over 5K miles, nearly all of which is to/from and at the drag strip. First, watch this half-minute video montage. The first clip was pass #4 on 27 Oct. The rest were from my 3 most recent trips. I've edited out all but the last few seconds from each of 5 passes (3rd and 4th gear). Then read the details below and tell me what you think.



Update 20180123: See Post #5 for solution.

[Details]

Feels like engine cuts out completely one or more times for a split second while at WOT but only while pushing hard in 3rd and 4th gear. When this happens it's always around 5500 RPM. The worst time it happened, when I first really took notice, was my 27 Oct drag strip visit after I'd been using Torco fuel additive for several weeks. I thought at the time that it could be from too rich a mixture - ECM couldn't compensate for the high 107 octane.

It was pass #3 of the 27 Oct trip when it occurred in both 3rd and 4th gear, starting at about 5400 RPM in 3rd, then cycling the engine off and on about every 1/2 sec. After short-shifting to 4th it was okay 'til it reached about 5400 RPM when it started again. I stopped using Torco after that trip. Unfortunately, I forgot to turn on my GoPro for that pass and the microSD card in the PDR for that visit was in my pants pocket and accidentally got lost in the wash. So there's no video evidence of the worst case.

Looking back at previous videos I found other blips in engine output at 5500 in 4th as far back as Aug/Sep 2017 but not as noticeable as the ones in these clips.

I've been running nothing but E10 93 octane pump gas since the 27 Oct trip (3 track visits - 21 passes) but I'm still seeing these issues. I feel like there might be deposits on the plugs, injectors, and/or valves. Since this is a DI engine I don't see an easy way to clean the valves, though. Fuel additives can only potentially clean the injectors.

I'll be changing the spark plugs before I go to the track again this Sunday. Anyone recommend a good fuel additive? Besides tearing apart the engine, any ideas on cleaning the valves?

After reading the WEAPON-X thread "Guide to fueling for higher horsepower LT4 ZL1s" I'm thinking that low fuel pressure in the lines or on the rail could be part of this, possibly brought on by clogged jets affecting fuel flow or dirty valves affecting combustion chamber pressure.

Any insights would be welcomed. Thanks!

--Cal

[grampa_ss]

I may be wrong but I think there's a thread regarding this issue. It has to do with a wiring harness rubbing up against a belt.
maybe start here http://www.camaro6.com/forums/showth...=490819&page=5

[cwebster]

Originally Posted by grampa_ss

I may be wrong but I think there's a thread regarding this issue. It has to do with a wiring harness rubbing up against a belt.
maybe start here http://www.camaro6.com/forums/showth...=490819&page=5

Yeah, I've seen that thread before. Thanks for the idea but my wiring harness is clear.

[cwebster]

Okay, I changed my spark plugs on Friday. I used exact replacements [AC Delco 41-128 Iridium]. Last time was about 11 months ago right after break-in. I did not expect to see them so dirty. As you can see, there is a reddish-brown coating on both electrodes and center insulator. There's also a small amount of evenly distributed carbon deposits around the rim and inside the well. All 8 plugs looked identical.



[Possible Cause - Torco Fuel Additive]

These kind of deposits are usually caused by octane boosters, from what I've read. This might explain why I first noticed the anomaly after using the Torco for a few months. Still, I'm not sure that's the cause of the engine cut-out. After all, I added the Torco because I felt like the engine was producing less power at the time. It could have been caused by a latent issue and just happened to appear at that time.

I suppose this could have been a factor in the engine cutting out but why only at 5500-6000 RPM in 4th (once in 3rd) gear? I would think that fouled plugs would manifest at a range of RPMs in all gears.

[Alternative cause #1 - Fuel Pump Supply]

This still feels like a fuel flow or ignition system issue, though. From the Weapon-X detailed explanation of the fuel system I wonder...

Could the low-side pump be failing to supply the HPFP at high load?

AFM issues that Weapon-X describes in his excellent thread could account for intermittent failures in 3rd & 4th gear at 5500-6000 RPM.

Originally Posted by WEAPON-X

The Dangers:
...
"If the line pressure isn't force feeding the HPFP, the entire system will falter."

Originally Posted by WEAPON-X

Followup post:
...
"A lot of the mishaps are simply inadequate fueling. Once the rail pressure falls, it cannot recover until a shift and then is depleted again as the gearing is smaller."

[Alternative cause #2 - Dirty Valves]

Could my problems be the result of "dirty valves"?

Direct injection (DI) lacks valve cleaning benefits of port injection... Even though the ZL1 LT4 employs a clean side catch can, the dirty side still recycles combustion vapors and vaporized oil from the crankcase into the intercooler where they can coat cooling fins, compressor blades, and valves.

Originally Posted by WEAPON-X

How it works:
"Another drawback [of DI] is the lack of the fuel cleaning the valves, which on a parasitic PCV system like the GM OHV engines use, they use the engine to draw the crank case pressure though it to maintain seal life and longevity, at the expense of future performance if proper care isn't taken."

If it were a simple matter of inserting a catch can in the PCV line I might consider doing that. However, for the LT4 motor it would require a warranty-breaking mod. I'm just not ready to void my warranty yet. I don't see another way to clean the valves other than taking the engine apart.

[Alternative cause #2 - Malfunction of NLS]

Could this be a malfunctioning No-Lift Shift (NLS) system?

I got to thinking about how it was completely loosing power off and on. Normally NLS cuts fuel and ignition when you depress the clutch at WOT. If this system were activated intermittently without pressing the clutch, this is what I would expect to happen. Even though I almost always run with the nannies off, such a malfunction could still occur.


Does anyone have any thoughts on these ideas?

--Cal

P.S. The car feels like it's got more power after changing the plugs. If this engine cut-out issue doesn't rear its ugly head, I'm hoping to see some improved times at the drag strip tomorrow. We've got a weather window and I'm trying out a different track.

[cwebster]

I believe I've found the culprit!

While conducting pre-race checks in the engine bay I discovered a vacuum line has been rubbing on the supercharger's upper pulley. It had worn through the foam sleeve and half-way through the plastic vacuum line. This is apparently the vacuum line supplying negative pressure to the clean-side PCV catch can.

I performed a temporary repair so I could still go racing:
- Cut the surrounding foam sleeve around its circumference for access.
- Wrapped electrical tape around the damaged portion of the plastic vacuum line.
- Secured the vacuum line to its neighboring PCV line to reposition it above the pulley. The heat and pressure had apparently reset the bends in the plastic line.

Here is the damage to the plastic vacuum line:


Here's the temporary repair:


A zip-tie to push the line back where it belongs:


Back to normal:



The engine seems to have regained all its power now. I noticed an immediate improvement in responsiveness and there were no engine cut-outs at the track on Sunday.

I wondered why it was positioned so close to the pulley in the first place, though. At first I thought it must have just relaxed over time because there's really no solid mounted stand-off bracket for these lines. They're loosely bound with plastic loops to adjacent lines. Looking back at older photos of my engine bay, it became apparent that the coolant recovery tank line had been improperly routed, probably when I changed plugs Mar 2017. It was putting pressure on the vacuum line pulling it toward the pulley.

Here's how the lines were routed when I took delivery 30 Dec 2016:
(Compare lines within green box to next photo)


While changing plugs last Friday I did notice that the recovery tank line was out of place but it didn't dawn on me at the time that it was pulling on the vacuum line. I went ahead and re-routed it when I put everything back together.

I found this photo in a set I took on 27 Jul 2017:
Only thing that happened between delivery and July was change of spark plugs.



Now I just have to order a replacement line and install it.

--Cal

[JeffFox]

Thanks for sharing your experience with us.

[oldman]

I can't see how the line is the problem it is a filter side breather there is a slight vacuum at high intake flow in front of the throttle body. If cracked, it can cause low speed stumble as it will leak air into the already metered air (post MAF). I have a LT1 but the system is similar.

[cwebster]

Originally Posted by oldman

I can't see how the line is the problem it is a filter side breather there is a slight vacuum at high intake flow in front of the throttle body. If cracked, it can cause low speed stumble as it will leak air into the already metered air (post MAF). I have a LT1 but the system is similar.

Sorry, I didn't see your post 'til now. It doesn't seem like it would cause such a big issue but I've seen vacuum leaks cause some crazy symptoms over the years. Maybe it wasn't the only problem but it has to be a major contributor. As I indicated earlier, I was still seeing the issue long after I stopped using Torco so that pretty much rules that out. The only other thing I changed were the spark plugs so that may have been contributing too. I'm back to full-power now with no hint of the problem on the big-end. That's the important thing.

--Cal

[oldman]

Yep, it just not vacuum side, it is on the other side of the TB, but yeah at idle I can see it bleeding enough air to confuse the MAS.

[Boost or bust]

Torco will foul your plugs every time. As long as you don’t mind changing your plugs frequently....also, realize you ain’t getting the octane numbers you think by using it. Use high octane fuel to be certain....

[cwebster]

Originally Posted by Boost or bust

Torco will foul your plugs every time. As long as you don’t mind changing your plugs frequently....also, realize you ain’t getting the octane numbers you think by using it. Use high octane fuel to be certain....

As I indicated earlier in this post, I stopped using the Torco months ago. The Torco was an experiment, along with other things I was trying out last year to get faster. I believe the Torco didn't have the desired effect mainly because the ECM is tuned specifically for E10 pump gas between 89-93 octane. Basically the same reason the E00 gas didn't help. Apparently, it can't compensate for higher octane fuels or different blends without a tune. I'm not willing to void the warranty yet so I moved on. Now my best hope for improvement is in the new Hoosier DOT slicks I got coming (see Help selecting replacement DRs).

There's no shortage of opinions when it comes to performance. I value the opinions and suggestions I get in the forum. However, the best way learn is to try things for myself. You can read a summary of my results from previous experiments in this thread: Back To The ZL1 Bone Stock Basics

--Cal

[DirectTo]

Thanks for sharing, that’s an interesting malfunction. Glad you’re back on the track!