Hi all,

I have some minor, self-taught, mechanical skills that and have been limited to replacing alternators, starters, blower motors, batteries, oil changes, hoses, belts, thermostats, coolant flushes, swapping out air intakes... basic work like that. I do my best to work on my own vehicles whenever possible. I am reaching out to the community here for some assistance because I have a lean condition on my 2012 Corvette Grand Sport (12,000 miles, Auto, LS3, NPP Exhaust and stock except for a catch-can and a Corsa Carbon Fiber Air Intake) that has been causing me a lot of stress. Here's some background:

The 'Vette is behaving normally...idle upon start up is around 700 RPM and then quickly drops and stays at 500 RPM. I've had 3 different experienced mechanics listen (but not actually work on it) and they all tell me that the car sounds fine - no discernable vacuum or exhaust leaks to their trained ears. The only noticeable symptom of anything being wrong is a lack of power. I really have very little to base that on as it seemed to be present when I purchased the vehicle and I remember thinking that the acceleration was a lot less than I expected compared to the 2012 Dodge Challenger SRT8 that I owned previously, The Challenger had around 40 more HP and Torque but was obviously quite a bit heavier...but was noticeably quicker off of red lights on open roads with no one in front of me. The only reason I discovered that the Vette was even running lean was because I purchased a DiabloSport Trinity T2 Tuner and custom tune from DiabLew (whom I know has helped out a number of folks on this forum with great tunes). When I sent my data back to him for a 93 Octane tune, he pointed out that my Long-term Fuel Trims were way up there at +14% and he can't custom tune until this is resolved. This is not enough to throw a code (I believe it has to be at +20% or higher for that) but it an indicator that something is definitely not as it should be - and probably explains the less-than-impressive acceleration compared to my old ride.

So, as I mentioned, I am only basically mechanically inclined but I'm really hoping to tackle this myself if at all possible. I've removed and cleaned the MAF Sensor, checked the air intake a bunch of times, sprayed water and MAF Sensor Cleaner around various places to check for a vacuum leak...reconnected the battery each time and logged around 200 miles in the hope that LTFT will drop to an acceptable number after recalibration. No luck. I've also hooked up a separate scanner, which also showed the same LTFT levels as the DiabloSport tuner did. I just purchased a smoke machine and a Bosch FIX 7803 Vacuum & Fuel Pressure Tester. I played around with the smoke machine just a bit...removed the air intake and fit a rubber glove onto the throttle body. Cut a hole in a finger of the glove and sent smoke in. No vacuum leaks detected that way.

My initial questions to you fine folk are:

1). Can someone help me out by letting me know to what vacuum hose I should connect the vacuum tester to? (Photo and any advice would be of great help as I'm a total noob when it comes to vacuum tests!) I also can't seem to find vacuum line diagrams for the LS3 anywhere on this forum or elsewhere.

2). From my understanding, vacuum pressure should read between 17-21, is that correct?

3). If the vacuum pressure reads normal, is it a waste of time to proceed with the smoke machine or will that still have some use in other potential diagnostics?

4). What should my LTFT levels be reading on a stock LS3 motor? (I know as close to 0 as possible is optimal but don't know what a standard stock reading is typically at...)

I have other questions about some other potential causes of the lean condition but probably best to wait and see if there's a vacuum issue or not before asking. I'm feeling like my best chance of an easy fix is to find a vacuum leak that won't involve tearing apart the motor to resolve!

Thanks for any assistance you can offer.

 

Long term fuel trim — Ricks Free Auto Repair Advice Ricks Free Auto Repair Advice | Automotive Repair Tips and How-To
I don't know if this will help but it's the best I can do.It's a start.

 

The best way to find a vacuum leak is to perform a smoke test. If that doesn't reveal any leaks, then it might be time to examine the other side of the lean condition coin, which is lack of fuel.

 

Thanks jamieo. That is an incredibly informative article that I hadn't come across yet!

 

Thank you. I will have some more questions an alternative ways to perform a smoke test aside from sending smoke through the throttle body. But first, hoping to get that vacuum pressure test out of the way when someone is able to help me out with answers to questions 1 thru 3.

 

Yes, 14% on your LTFT is excessive…plus or minus 10% is OK but I like to see not more than 5%…are BOTH banks at 14 ??…if so and I’m hoping you have a scan tool run the engine at 3000 RPM for 3-4 minutes and watch your long terms…if long terms slowly decrease you have a vacuum leak…this is engine diagnostics 101 !!…if long terms stay the same or increase it’s either fuel delivery or a MAF issue !!

 

Yessir, both banks are high. Just did as you recommended and LTFTs held steady over the course of 3 minutes at 3,000 - 3,100 RPMs. Bank 1 was a steady 12% and Bank 2 was a steady 11%. This is slightly lower than the 3 freeway cruising scans I sent out to DiabLew last summer - which were 3 minute or so logs at a lower but steady freeway RPM (around 1,600 - 1,800 RPM) and both banks were steady at around 14%. So is it not even worth me finding a vacuum line to hook up the Bosch Vacuum/Fuel Pressure tester that I bought? I previously removed and cleaned my MAF sensor, but at only 12,000 miles it didn't look too bad. I added a bottle of ISO-Heet last season shortly before storing for the winter but probably didn't drive it enough after for it to do any potential fuel injector cleaning...

Advice on next steps would be greatly appreciated!

 

I would take off the catch can and put the stock PCV line back on and see if the LTFT's are still high.

If you have the stock intake, I would put that back on and see if the condition is still there.

Another thing to check the exhaust. Are all the exhaust manifold bolts tight? Are there any exhaust leaks at the heads or any joints downstream? Pretty cheap to replace the exhaust manifold gaskets with a new set.

Are the oxygen sensors tight in their sockets? A loose sensor could cause this also.

 

Well if you ran the engine for that amount of time and didn’t see a decrease in long terms it’s NOT a vacuum leak…usually with a dirty MAF you’d see negative fuel trims at idle and that go positive under load…like I said may be a fuel delivery (clogged injectors on both banks or weak fuel pump)…I use a scope and without this diagnostic tool it’s only a guess !!

 

Thanks Spaceme1117 and C5 Diag. Yes, the thought of restoring the intake and PCV line to original did occur to me. I was hoping to avoid that, but I may put it back on my to-do list....although if a vacuum leak has basically been ruled out then I wouldn't think the catch-can and intake would be the cause of my lean condition.

For next steps I plan on smoking the exhaust system, checking the manifold bolts, and purchasing a new MAF Sensor (O'Reilly's says I can return it within a year if it isn't needed....so no risk there). After that, I'll be looking to find someone to check out the fuel delivery system as I don't think I'm able to do that myself. I just want this resolved so I can enjoy the car for the 5 or 6 months per year that I have with it!

 

If you are going to replace the MAF sensor I would HIGHLY recommend going with a factory sensor.

 

Thank you. Yes, I had planned on purchasing the AC Delco.

 

I wanted to update this post - both to potentially assist any future owners who run into a similar issue and to ask for a bit more advice from anyone with more expertise than I.

The first thing I did since my last post was to install a new AC Delco MAF sensor. I did not expect this to be me issue and it was not. I then removed the Catch Can and reinstalled the OEM "U-shape hose" (sorry, don't know the technical name for it). No change, still running very lean on LTFT (around 14% in both banks) - but not lean enough to throw a code. Then I removed the Corsa Carbon Fiber Air Intake and reinstalled the stock air intake. Problem solved! After both starting and idling and taking the car out for a ride, my LTFT's were almost always at 0%, fluctuating occasionally _+/- 1-2% either way. With the problem solved, I was now wondering if I had a $600+ paperweight with that Corsa intake or if I may have installed it wrong. Fast forward to today...

Despite having installed air intakes many times on previous vehicles without any mistakes, I am open-minded enough to consider the possibility that I may have done something wrong with this one. Never mind the fact that I've checked and reinstalled it 3 times while trying to resolve this lean issue (I just wish I would have reinstalled the stock intake sooner). I did see that I had the silicon sleeve a bit too close to the grommet that the oil breather hose fits into. This was causing the sleeve to slightly bunch up around the grommet. Despite having the 2 air filter clamps tight and secure, I thought this could have been the cause of my lean issue. Unfortunately, after reinstalling and moving the sleeve far enough from the grommet where it could not interfere, the lean condition got slightly worse, with LTFT's as high as 20%! (I didn't want to take the car out in the rain so this 20% LTFT was at idle and while revving the motor to 3,000 RPMs in the garage. When I did the same with the OEM intake, I was at or right around 0%.)

At this point, my next step is to contact Corsa to see what advice they may have. Unfortunately, I purchased the intake in January of 2021, installed it in June of 2021, and then spent the rest of the season trying to diagnose my lean issue after discovering it while sending a data log away for a custom tune. So even though it was only on the vehicle for less than 1,000 miles, I'm not feeling optimistic that they are going to be too willing to help me out. But time will tell on that and I will report back! I may need to make another post to warn purchasers of this intake to check their LTFTs. Had I not purchased a tuner, I probably wouldn't have known the car was running so lean as it wasn't enough to throw a code.

My questions for you all, just to cover all angles, is.... Have you ever heard of a Corsa Air Intake (or other aftermarket air intake) causing this kind of issue on an LS3 motor and is there a remedy for it? Or did a potentially get a bad intake? If it's not a bad part, I'm left wondering why my LS3 doesn't like this intake when so many others have purchased the same one and seem to love it.

Thanks!

 

 

That is helpful. Thank you. I called Corsa this morning but they didn't answer. Left them a message but haven't heard back yet.

 

unless the intake completely messes up airflow through maf.

 

very funny you say that. my very first thought was the CAI causing the issue. I would put the stocker on, see what happens.

 

Putting the stock intake on completely solved the issue and restored my LTFTs to 0% +/- 1-2%. So now I am trying to find out if I got a defective intake or if my LS3 just hates that intake. But, if the latter, it has me wondering how many people running the Corsa Carbon Fiber air intake have that same problem as I did but just don't realize it as the lean issue is bad...but not bad enough to throw a code.

 

My 04 was throwing lean codes after a Vararam install with the power duct. The Vararam screws up the air flow and the MAF does not report the correct airflow at low engine speeds. It was not an air leak. Your aftermarket air filter may be causing the same kind of issue. My issue was temporarily fixed by changing the fuel injectorslope to -10 and then increasing the power enrichment +10 with a Trinity handheld tuner. The fuel trims were still high at first like +10, but the PCM corrected it to around 0 and they stayed good. Before that change they were hitting +25 and throwing the codes. Since you have a Diablo tuner you can try making the same kind of change to see if it fixes your issue. There are complete instructions on how to do this on the diablo website. DiabLew should be able to help you with this. The car ran great but I felt it was too lean at part throttle so I just got a $150 tune at ECS. That fixed everything and the car runs great.

 

My thoughts are that if the intake is a slightly different size at the maf, that will mean the car is actually getting a different amount of air than it thinks it's getting. I would expect an aftermarket intake to be slightly bigger to flow a little more and cause this problem. I think it should be easily tuned out. I'm surprised diablew didn't just do that, but I'm sure he knows a lot more about it than I do. Maybe I'm wrong.
Actually, I think I remember seeing a video years ago about the halltech intake where he basically proved that it flows more air by checking ltft before and after installing it