Hey everyone, hoping to get some experienced eyes on this before I authorize a major repair. Long-time lurker, first big issue on my C7. Trying to be data-driven before spending big.

THE CAR
- 2016 Corvette C7 Stingray, LT1 6.2L, A8 (8L90)
- Recent purchase, took on this issue after I bought it
- Located in Central Florida (Orlando area)

SYMPTOMS
- Persistent misfire under load (hills, sudden acceleration)
- Worse on AFM V4→V8 transitions
- Shaking at idle once warm RPM drops from 1000→600
- Cold start is fine
- ~30% reduction in misfires when driven in manual mode (higher RPMs)
- Light dark smoke at startup, faint fuel smell at exhaust
- Light "tic-tic" sound (told this is normal GDI noise)

WHAT'S BEEN DONE (didn't fix it)
- Spark plugs replaced (cyls 2 and 7 came out WET with liquid fuel + dark)
- Coils replaced
- Thermostat replaced (had P0128)
- PCV valve replaced (the external one)
- No improvement from any of these

CRITICAL FINDINGS
1) Oil level was ABOVE max, smelled strongly of gasoline
2) Mechanic opened a valve cover - about 1 quart of oil drained out that had pooled there. Oil visible in intake. Excessive blowby confirmed.
3) Compression "slightly low" (no specific numbers given, no method specified)
4) Oil fill cap was CLEAN (no coolant emulsion)

MODE 6 DATA (2 reports, ~5 days apart, stable pattern):
- Cyl 1 (AFM): 2 → 2
- Cyl 2 (non-AFM): 41 → 38 ← WORST, by far
- Cyl 3: 0 → 1
- Cyl 4 (AFM): 3 → 7
- Cyl 5: 1 → 2
- Cyl 6 (AFM): 3 → 1
- Cyl 7 (AFM): 2 → 4
- Cyl 8: 2 → 4

HISTORICAL CODES (not active now)
- P0128 (thermostat - already replaced)
- P0300 (random misfire)
- P2098 (Post Cat Fuel Trim Low Limit Bank 2 - "TOO LEAN" per spec)

BLUEDRIVER LOGS (6 total, 2Hz sample rate, March-April 2026):
- LTFT B1: -6.7% to -9.8% / B2: -7.1% to -9.7% (consistently rich)
- B2 always ~3-4% richer than B1
- One April snapshot showed LTFT saturated at -24.2% B1 / -28.1% B2 (key-on, 11s)
- Fuel pressure (low-side) generally stable around 46-48 PSI, std dev 1.4-2.9
- One drive log had erratic fuel pressure (24-88 PSI, std 6.6) but median normal 47.4 - probably sensor glitch, not real HPFP behavior
- 749 lean spikes (>+20%) across logs

THE QUOTE I GOT
A well-known local Corvette shop quoted me $9,348 for:
- Cam swap
- LS7 lifters
- AFM delete
- Oil pump
- Head gaskets
- VLOM (lifter oil manifold)

But here's what bothers me:
- No leak-down test (or no numbers documented)
- No borescope of cyl 2
- No injector balance test
- No data log with Tech 2 / HPTuners
- No Mode 6 review apparently
- Worst cyl in Mode 6 is cyl 2 (NON-AFM) - if AFM lifter failure, why would cyl 2 be the worst by 10x?
- Oil fill cap clean = head gasket failure unlikely
- No metal in oil filter mentioned
- No aggressive ticking

MY HYPOTHESIS
I think this is a leaking GDI injector on cylinder 2, possibly cyl 7 marginal too. Reasoning:
- WET plugs on those exact cylinders
- Mode 6 stable on cyl 2 (consistent failure pattern, not random)
- B2 consistently richer than B1 (cyl 2 is on bank 2)
- Fuel dilution in oil (over-fueling somewhere)
- PCV overwhelmed by fuel vapor and blowby
- Could explain misfires under load + manual mode improvement

QUESTIONS FOR THE FORUM
1. Has anyone seen exactly this pattern (WET plugs cyl 2, fuel dilution, B2 richer) turn out to be a leaking GDI injector? Other diagnoses I'm missing?

2. For diagnostic confirmation, what would you do first?
- Injector swap (cyl 2 ↔ cyl 1) and recheck Mode 6?
- Send injectors out for ultrasonic flow test?
- Borescope cyl 2 first?
- Tech 2 fuel system data log?

3. If injectors confirmed, are you guys replacing all 8 or just the bad ones? The labor is the same to access them, but cost diff is significant.

4. Anyone in the Orlando / Central Florida area can recommend an INDEPENDENT shop with experience on LT1 GDI issues? I've gotten skeptical of the local "Corvette specialist" after this quote without documented diagnostics. (I know about Florida High Performance in Pompano - too far. Looking for something closer.)

5. Anything I should be checking that I'm missing? VLOM solenoid contamination from fuel-diluted oil? AFM lifter follow-on damage if I delay too long?

I have all 6 BlueDriver CSV logs and both Mode 6 PDFs - happy to share if anyone wants to look closer.

In the meantime: changing the oil immediately (gas-diluted oil destroys bearings), and driving in manual mode until I sort this out.

Thanks in advance - this community has been a goldmine of info while I've been researching.

 

I replaced 2 injectors on my new to me Z06 that had not been driven much. 6800 miles in 10 years. The same running issues as you are just fine now. I did not have your other issues.

 

I might just replace all the injectors to be safe, then see where you are.

 

Please name the shop that did the above so we can avoid them. They sound like part changers, not mechanics that diagnose the problems first and then fix what is really broke.

 

First I'd borescope the bad cylinders.

Then do a leak down test.

If all looks good then.

I'd change all injectors with new OEM and go from there.

Don't keep running it with gas diluted oil.

If no luck or you want a real professional I believe DiagC5 Is in your state. He's your best bet. Have the car taken to him and you'll get the correct answer.

 

I am with you on the leaking injectors. Seems like that could cause all of your symptoms. I don't see how any of the items in the shop estimate would cause your symptoms. I like Tinkertech's idea for repair. My only Q is just because I have never worked on an LT1 so not sure of the difficulty of replacing injectors. Can just 2 be replaced reasonably or is the engine so opened up to do 2, that you might as well do all of them?

 

Hi Flame. I’d rather not mention the shop’s name for now, because they might be right and I might just be having a hard time accepting the situation.

They’re pretty well known around here when it comes to Corvettes, and I don’t want to hurt anyone’s reputation. If I eventually find out their diagnosis was wrong, I’ll post the name here.

 

Hi Flame. I’d rather not mention the shop’s name for now, because they might be right and I might just be having a hard time accepting the situation.

They’re pretty well known around here when it comes to Corvettes, and I don’t want to hurt anyone’s reputation. If I eventually find out their diagnosis was wrong, I’ll post the name here.

 

Would Lyle at C&C be a good choice?
IDK, other than viewing his YT videos, his oufit appear to know their stuff.
Not parts cannon outfit, just proper diagnostics.

 

I picked up the car from the shop today and have a few updates.

The shop said they tested the injectors, coils, plugs/wires, compression, and performed a cylinder leak-down test.

I asked for the numbers, and this is what they wrote on the estimate:

CYL 1: 175
CYL 2: 175
CYL 3: 170
CYL 4: 175
CYL 5: 175
CYL 6: 178
CYL 7: 178
CYL 8: 178

These numbers look pretty normal and balanced to me, right?

Leak-down test: all cylinders less than 10%. From what I’ve researched, that also seems to be within the normal range.

I also installed an AFM disabler, and the misfire counts dropped quite a bit. Cylinder 2 was the worst one and went from 41 misfires down to 10. It seems like the misfires happen mostly under load and below 2,500 RPM.

For context, this is now the third mechanic who has looked at the car.

The first mechanic found cylinders 2 and 7 wet with fuel. He checked injector pulse, fuel pressure, fuel trims, air volume/pressure/temp, the air filter, O2 sensor readings, and EVAP pressures. His suspicion was either stuck injectors or an EVAP issue bleeding fuel into the intake and causing the random misfire.

The second shop said there were no codes, confirmed the rough idle, removed the valve covers, and said the valves and springs looked good. They said compression was slightly low, the injectors tested okay, and that excessive blowby was causing a valve to stick and creating a slight misfire. They replaced the PCV valve and suggested an oil catch can.

At this point, I’m wondering if it might be time to take it to a GM dealer and see what they say. I’ve already been through three mechanics. The first one seemed very good, but he’s a mobile mechanic, so he’s limited in what he can do. The second shop was highly rated, but they had the car for about 40 days and never really reached a solid conclusion.

My impression is that both shops may have been too busy to really spend enough time diagnosing the car carefully, but I’m trying to be fair because I can’t say that for sure. To give you an idea, I never received a call from either shop with updates. I was always the one calling or stopping by to ask how the diagnosis was going.

With the last shop, it honestly felt like the estimate came together only after I called again for an update. They answered the phone, asked for a few minutes, and about five minutes later came back with the estimate and the lifter diagnosis.

Based on the symptoms, the compression numbers, and the leak-down results, I’m starting to question whether this is really a lifter issue. To me, the compression and leak-down numbers seem pretty normal.

Sorry for the long post. I guess I’m also using the forum a little bit to vent my frustration, but I’m trying to be fair and figure out the right next step before throwing a lot of money at the wrong repair.

 

Yes, they seem like a good option. I’ve watched several of their videos. I’m considering taking it to them as well.

Thanks for your reply.

 

C&S Corvette ???

 

Yup C&S.

Beeen watching too much of Clint's C&C Equip channel.

 

Moving this over to C7 Tech for more discussion. And best of luck finding out just what is going on with your car - hope it is not a $9K repair bill.

 

I may be a little far !!...I'll recommend a good diagnostic shop just SE on him in Melbourne, Florida.

 

C&S really doesn't do diagnostic work.

 

C&S really doesn't do deep diagnostic work like this but you can ask them about your concern...go see Brin Kline at Assured Auto in Melbourne...SHARP diagnostician NOT mechanic !!...I'll post one of his YT videos below...he uses advanced diag equipment like me...a leaking high pressure fuel pump shaft seal can also cause fuel dilution of your oil...if you have a scan tool that can look at the fuel rail pressure the fuel pressure should raise slightly when you shut the engine down as the trapped hot fuel expands and raises the fuel pressure...if it decreases it is going somewhere !!...you said you changed the coil packs...did you check the wiring to them ??...I scope the ignition system looking at the current (Amps) and KV output looking at the unique secondary ignition waveform...also a leaking EVAP purge valve can also cause the car to run rich at startup especially after filling your tank up and then run LEAN (now a vacuum leak) after all that fuel vapor is burned but no oil dilution like you have...get that oil changed ASAP !!...and you DON'T take your car to a dealership for a diagnosis...they're idiots !!...you are not going to find an expert on an LT1 GDI issues...a good diagnostician can diagnose ANY car and any engine and get rid of that Blue Driver which is useless when it comes to diagnosing...get a good factory level aftermarket bidirectional scan tool for $500 !!...articulating borescopes are cheap on Amazon too...mine is a Teslong costing $120.

 

Thanks C5D, didn't realize it, simply trying to be helpful.
FWIW you always offer sage advice; especially, suggesting owners w/ electronic/al issues seek out qualified shops.
Must admit I smile whenever you've provided schimatics/EWDs/PNIDs to laypeople.
Akin to asking laypeople to interpret hierogliphs, no?
Will checkout site you provided.

OT, acquainted w/ YTuber 'WatchWesWork'?
Man's amazing.
Increadible skill reading all manner of vehicle, truck, farm equipmen schematics.
Blown away w/ Wes' diagnostic troubleshooting ability. Uncanny.
Check him out.

 

No problem !!...I visited C&S a few years ago and talked to Lyle and Steve the owner for about an hour...he told me they don't get into heavy diagnostics using advanced test equipment...it would help their business tremendously if they hired a diagnostician...I'm too far away but would love to work there part time...never seen that YT channel but I follow Scanner Danner, Pine Hollow Auto Diagnostics and Bernie Thompson "the master" at Automotive Test Solutions YT channel.