Need some help finding this misfire problem I am having so please chime in with any suggestions. Mods are in my sig and my tune has been checked 3 times on a dyno all seems good. The issue is at exactly 2300rpms up to 2800rpms im having severe misfires during moderate to hard acceleration. No codes being thrown at all...... even during deceleration evidence of popping is very pronounced through the exhaust in that rpm range. Really pulling my hair out with this so any help would be appreciated. If you need more info please ask.

Thanks
Phil

 

What is the history? When did it start (sequence of mods added)? No codes, are you getting a flashing check engine light? Did the tuner log the misfires, and are they isolated to a cylinder?

 

Well the history was after my intial tune the car felt rich so i returned and it was adjusted. Following that adjustment it seemed to really get bad i'm assuming that it was leaned out at that time which is making more noticable. Mods were all done at once heads,cam,intake,full exhaust then tuned. No codes or check engine lights, my tuner di not mention any log or any particular cylinder. He was very confident that it wasnt tuned related and suggested i begin with new plugs and wires? The car only has 9k on the clock still running stock wires with tr-55 plugs.

Thanks for any help.

 

I bet your problem is your plug wires, do yourself a favor and get some Taylor spark plug wires and problem solved.

 

I agreee with the above suggestion....could be a cracked plug. A tech II can easily monitor the cylinders in real time to discover a misfire. You did not mention the check engine light. Is it flashing?

Also, are you saying the intial tune did not precipitate a misfire?

 

Lucky,
it could have been missing from the start but since it was running rich i think it was less noticable. There was definatly something going on from the start which is the reason i returned. Sorry no codes and no check engine light.

I have already picked up a new set of plugs and will order some wires, i really hope that would be the problem. I'm just hung up on the fact its between that specific rpm range. If it were a bad plug wouldnt it be through out all rpms? And if i floor it when it starts to miss it is obviously really loud backfiring why isnt it throwing a miss code?

 

I had a misfire issue a while back that drove me up a wall. It was caused by unmetered air coming in through the intake gaskets which were deformed. After that was fixed the same problem came back and nearly drove me insane as I checked all sources of potential unmetered air entry. I eventually decided to check the plug wires. I had one badly scorched plug wire that was causing the problem the entire time. Moral of my story: it takes 5 mins to pull your wires and inspect them, do it before anything else just to rule it out.

 

If the plugs and wires dont fix the issue, closely monitor all the sensors that effect timing & fueling. See if there are any spikes from thoses sensors that could cause the issue. (i.e. A MAP sensor that is not analog in its output or a Throttle Position sensor that gets noisey at a specific throttle angle)

Dont get confused with changes from a sensor that result from the problem happening. (ie When it stumbles at 2300rpms up to 2800rpms the 02 Sensors will react. )

Bill

 

Thanks guys, i will start with changing the plugs tonight and inspecting the wires. I will order the new wires either way just to be on the safe side, and report back hopefully with good news.

 

Not necessarily. Cracked plugs can yield odd results. Many guys crack plugs when doing a header install. You won't always get a code with a miss. Like I said, if there is a small miss, a tech II will pick it up by reading the cylinders in real time. Try to get it isolated before throwing parts at it.

 

Lucky I wish i did but i have no access to a tech 2.

Well a little update i change all the plugs and inspected all wires everything looked good. Took it out for a test run and same results however it did clean up some if that makes sense? It still will miss but for some reason seems like a smaller rpm range now.....?

Lost for words now...... double checked all codes and still nothing..... racking my brain over this......

 

I'm really wondering how you know it's missing, as opposed to pulling timing. I'm also at a loss why the tuner could not log the issue. You get no flashing check engine light, and no miss codes, but you need someone to read all the sensors in real time. Just curious, how do you have the O2's hooked up?

 

My tuner was the one that told me it was a miss, i have no experience with tuning software so i'm going by what he explained to me yesterday.
While the car was on the dyno he ran through a quick explanation with me looking at the software. He explained and demonstrated how the afr was constant through out the rpms, also pointed out i guess the fuel trims and showed me exactly were the miss was and how the mass air sensor was reacting to it. I hope that makes sense i'm doing my best to trouble shoot, i really appreciate your time.

Quick history on the O2's, since the work was completed and tuned my car was getting horrible mileage and my exhaust tips were black. The car was even spitting carbon out the tips during idle in my garage. This was what originally had me question the tune being rich. Last weekend i wanted to double check the o2's to make sure i didnt cross a wire when i extended them. I removed the front o2's checked my wiring which was correct. I did notice the passenger side o2 was darker than the drivers side. So i installed the rear o2's which were not being used and not installed, in the front and not crossed. Drove the car about 100 miles this week and back to averaging 25-28 mpgs on the highway and clean air out the exhaust with no more carbon spitting. I'm assuming i may have damaged the passenger side trying to have the cpu learn an idle during initial startup.

 

Take your car to Corvettes of Westchester. I took mine there yesterday. WOW! Chuck is AMAZING. If he cant fix and tune your car, NO ONE CAN!

If you dont figure it out or your tune is hoses up, he can help you sort it out!

Check out this post. Start at #24

http://forums.corvetteforum.com/c5-t...-of-my-c5.html

BC

 

How did you use the rear o2's in the front positions without adapters?

 

I cut and crimped them to the flat connectors matching colors/letter designation.

 

CRIMPED SPLICED should be avoided in an area that is exposed to the elements and heat. A more preferred method is to use a soldered splice and cover the splice with heat shrink. You can get some water tight heat shrink. The signal wire voltage is small (0-1 VDC) and corrosion on the crimp splice will eventually degrade that signal.

 

Thanks Bill for the suggestion, I wrote cut and crimped to be short. I'm very experienced in wiring so they were actually spliced, soldered, gel filled butt connectors, and taped then covered in the original mesh loom.

 

Guys hate to bump an old thread but I’m at a total loss here.

The car is still doing the same problem loss of power, totally breaks up under hard throttle at 2300 - almost 3k.
Spent 4 hours at a tuner last week scanning and looking into the tune with no luck, except he said he can see the AFR intermittingly going lean under hard throttle.

So before I tear this entire build down any last minute suggestions can’t hurt. Here is what was checked:

Plugs & Wires replaced - No Change
Intake swapped back to stock to rule out cracks. - No Change
Pushrod length checked - Correct
O2 sensors are disabled so can't be them
MAF swapped - no change
Injectors swapped - no change
Fuel pressure checked and verified good
Valve train new and everything checked good
Leakdown and Compression checks both good all 8 cylinders
Car only has 10k on the clock
And as always NO CODES?

Any help would be greatly appreciated.

Phil

 

Desperation Bump .....