The engine in my 2005 CTS-V is misfiring, and I've tried almost everything I could think of to try to figure it out.

Here are some basics of the engine
TSP LS1/LS6 short block, all forged, +30CC domed pistons.
317 Heads
Internet calculator: 11.3:1 compression
Some camshaft. Probably an LS6 cam. Only inspected it visually and it looked undamaged.

External
LS6 Intake
K&N CAI
Full exhaust w/ shorty headers
Unknown tune, could be stock but someone turned off the secondary O2 sensors.

Codes
Random Misfire P0300
Misfire history shows 3 + 7 misfiring

I've had this problem for over 2 years now.
The car feels like it's running great, except at high RPM in first, where there is a noticeable misfire @ 5500RPM. It will also throw a flashing CEL at 70+MPH or anytime RPM is held above about >2100RPM .

Here's what I've done so far to try to figure it out. I haven't been able to move or change the misfires in cylinders 3 + 7 with any of the changes nor identify the cause with any of the tests:

1) Swapped all the coils and fuel injectors from one bank to another. Cylinder misfires counts on 3+7 unchanged.
2) Leak down test all within 5%, warm compression test all 174-180
3) I changed the heads from some new heads I bought from GM to rebuilt units I bought from ebay, this fixed an oil consumption issue but the misfiring remained unchanged
4) I did a compression test, and leakdown test on the offending bank (drivers side bank), everything is good.
5) I checked fuel pressure, good
6) I checked engine backpressure, looking for a plugged cat or something, all good
7) I've been through a three sets of plugs and wires
8) Checked intake gaskets visually and with a spray to find leakage
9) New front O2 sensors
10) Cleaned MAF
11) Checked pushrods (old but straight), roller rockers (new)
12) CASE Learn

I think I've covered the obvious, here's what I'm thinking about next:
1) Use my oscilloscope and see if the signal coming to fuel injectors / coil packs are different on the misfiring cylinders during the misfire condition.
2) Ditto for the crankshaft position sensor, and MAF
3) Replace engine computer
4) Replace engine wiring harness - I can't seem to find one anywhere?

I do have a USB code reader and I can setup whatever dashboard ya'll think might be helpful.

I live in Lithia Springs GA, and I ain't opposed to professional help at this point either .

 

I can think of right off the bat..
#1. Fuel injector needs to be replaced, or cleaned/flow tested on those cylinders that are misfiring.

 

Unless you've overlooked something that you've already crossed off your list, you need to address these items in order.

 

Thanks all.
Kawa - as mentioned I swapped fuel injectors side to side (and coils) - with no change in which cylinders are misfiring (3&7)

My rockers (trunnion upgrade kit) are good. Or at least they all look good.

Here's something weird. My bank 1 Long term fuel trim was bank 1 11%, bank 2 16%. It also is obviously running rich (you can smell it and see it on the back bumper).

I was thinking that's really high, and on a whim I disconnected the MAF. I hit the highway for a few miles and boom - misfire counts well within normal. My Long term fuel trim (although it didn't have a was 6% bank 1 8% bank 2). In a steady cruise my STFT would actually go slightly negative yielding a total fuel trim around 0%.

When MAF's are dirty I'm told they underreport airflow, the computer compensates but removes too much fuel when you have a quick change in airflow. Overreporting airflow - again computer compensates but instead adds fuel when you have a quick reduction in airflow. That seems consistent with whats happening in my car as with the MAF plugged in I get a ton of backfires on decel, when it's not plugged in my backfires are gone. Apparently, it's possible to have a failing or dirty MAF but not throw a MAF code or a lean/rich code, sometimes it is just a random misfire code.

I ordered a new delphi MAF from Amazon. I'll reinstall it and see if this theory is correct. Crossed fingers!

(P.S. I'll for sure check the camshaft next is that isn't it and get a tune either way)

 

How do the plugs look when you change them, on these cylinders?
I’d start with a datalog, and see what that tells me. Then, I’d go to a good tuner, and tell him your story, and show him the datalog. Spend some money on a good tune.

 

broken valve spring/s.....outers of a dual set will give those symptoms

 

Valve springs are new and I just inspected / swapped from head to head but that's great advice.

Plugs are black / sooty on all cylinders. Not really any different on 3+7. Lots of carbon buildup on the piston heads for only 10K on the motor when I pulled the heads. No surprise, it's pig rich.

 

I'd replace ALL the springs before doing anything else and remove the injectors and have them flow tested. Another thing to check is the valve lift on the misfiring cylinders and compare it to the others. Also grab a ebay cheapo infrared temp scanner and check the exhaust temps. 3&7 are showing up as misfiring and that is going to screw with the o2 sensors which in turn is going screw with everything. Misfires can be caused by vacuum leaks, WEAK or WORN or IMPROPERLY SETUP VALVE springs<that's my guess>leaking or starving fuel injectors, broken rings and ring lands<another common problem. Number 7 cylinder rings break easily.

 

I'll check the exhaust temps, I do have an infrared temp scanner

Valve Springs are new GM springs and misfiring since day 1, and I'm hoping a broken ring / ring land would show up on either a compression test or a leakdown test I did.

Anyway here's what I did:
Misfiring went away with MAF removed
Cleaned K&N CAI, lightly oiled per instructions
Installed new MAF
Cleared codes
Misfiring continues even with new MAF
Unplugged new MAF - misfiring went away

My ECU is 'Tuner Locked' - it came to me this way - and without being able to read the tune I can only guess that the previous tuner messed with the MAF table to richen up the fuel mixture for god knows what engine configuration. This car has had 4 different engines in it, and the person who had the car before me had 0 clue what he was doing.

Since I'm throwing parts at it at this point anyway, my tuner said I needed a new (used) ECU due to the tuner lock and that's what I'm doing. Once I get that ECU installed I'll recheck the misfire situation and go from there.

 

Just saw your in Lithia Springs. LSXperts locks their tunes, which is an hour and a half away from you. Perhaps they have been in this car? I have a friend who had to pick up a new ECM because of their locked out “tuning”. Irrelevant...
As I posted previously, I feel you need a good dyno tune. Stinks that your gonna have to pick up a new ECM, but that’s the route I’d go if it were mine.

 

It wasn't tuned by LSXPerts - it was probably tuned somewhere in Michigan.
Tuner lock pisses me off for this exact reason.
Dyno tune makes sense to me, It's a weird grab-bag setup.

I'm guessing Vengeance Racing is the only where to go and get Dyno tuned?

New ECU programmed for the VIN coming Thursday. We will see.

 

Locking a tune, you know anyone that does anything like that doesn't have a clue. The only possible reason would be to stop someone from seeing how effed up the tune is. Sorry I didn't thoroughly read everything before posting, certainly the 1st place to look is the tune.

 

Feel free to shoot Mike our tuner an email. We are glad to help in any way we can.

We do not Lock our tunes

Mike@VengeancePCM.com

David @ Vengeance

 

Disconnecting maf and seeing an improvement is your indicator that it's a fuel issue. That and the fuel fouled plugs you found.

Get it tuned ASAP, hate to see you wash out the rings on that new motor. (this is why you ALWAYS get a new tune with a new setup)

 

Turns out my PCM was not tuner locked. There is (or was) a problem with my wiring harness preventing the download.

Misfiring 3+7, P0300 code, in CLOSED LOOP, 2000K RPM - no load or cruise same thing.
It's an intermittent misfire, happening about 1/3 of the time.

Doing a running cylinder balance load test, cranked up @ 2000RPM, I could hear any particular cylinder drop the RPM signficantly more than

Engine Specs: TSP 5.7L shortblock, +30cc domed pistons, 317 heads, 11.3:1 static CR. TR6 plugs @ 0.35. Shorty headers, K&N CAI, catback, LS6 intake, unknown camshaft probably LS1 or LS6. Machine shop installed camshaft and did timing dot-to-dot.

Summary of everything so far
1) Replaced PCM w/ stock
2) Did a CASE learn
3) New Plugs / Wires
4) Checked all power, negative, and low reference wires for voltage coils + injectors
5) Checked resistance to all signal wires for coils + injectors
6) Replaced plugs (again) on 3+7 w/ TR5 @ 0.38. Only because
7) Double checked compression in 3+7, 175, 180. TB wide open.
8) Leakdown test 2-8% across all cylinders, 3%-4% on 3-7. Cylinders @ TDC
9) Swapped injectors side to side
10) Swapped coils side to side
11) Checked fuel pressure @ idle - good. (going to redo @ 2K misfire event)
12) Checked fuel trim @ 2k (LTFT ~3% STFT ~+/- 3%)
13) @2K RPM, checked signal of crank sensor, cam sensor, all fuel injectors, all coil packs
14) Used IR gun @ 2K RPM, hard to get a consistent read but it looks like 400F across the board after letting it get really hot staying still @ 2K RPM for quite some time.
15) Used IR gun on cats. Not crazy hot and consistent from bank to bank.
16) For a different reason, replaced heads (Changing position of pushrods and springs randomly)
16) Visually inspected valvetrain movements @ idle as well as springs.

I have never been successful in even getting the misfire to move, let alone go away.

I'm calling in the professionals now. I'll keep this thread updated.