My swap is complete and tuned. Since Ive been logging more and more miles (3000) the swap is running progressively worse. Its surging and backfiring the longer it waits at stop lights and now its gotten into my accelleration

Scanner shows good readings on MAF, TPS, O2s. Fuel trims stay around 0 then slowly work up to +12-16 where it backfires and stalls out

Ive swapped the O2s with several others and gotten the same results. Ive swapped MAF as well just to be sure. Same thing

Plugs looked fine both times I pulled them out. TR5 @ .040". I opened them up to .050" a few weeks ago and it wouldnt run at all, so back to .040" they went

Something is giving a false lean condition and its adding fuel. Any ideas where to look? It runs ok when its warming up, and Ive tried unplugging the O2s to force OL and it still runs like poo once warm

Fuel system is a brand new Walbro 255 with a 99 corvette regulator. Stock rails with stock injectors matching my tune

 

Post your tune, logs, and your cars specific setup....we need more info. That will help us help you get this figured out!

 

i´m having similuar problems.....

can´t find out where the problem is.

Baezi

 

Same here

 

What is your wideband reading when it starts to pop and backfire. Like the OP said, post up your scan and tune. Sounds like you need to have the car re-tuned, but with a wideband this time.

 

Car was tuned with a wideband to begin with. Ran awesome for about a week then slowly got worse

Only real logs I can get are from an OBDII code reader live data feature. Nothing I can print off/save and post up

Logical approach would suggest some for of exhaust leak is causing the O2s to detect a lean condition. Ive been over the whole exhaust system and found zero leaks, even replaced a few gaskets for GP and saw no black soot tracers

Strange watching the fuel trims start at -4 or -5 each bank and slowly increase to +10 or more as the car progressively runs worse (3-5 mins idling in gear). Then if it doesnt stall it will reset to -4 or -5 and slowly work up to rich again. Generally bank 1 will make it the highest by 5% or so

Retested with the O2s unplugged and it reacts the same (idles fine then progressively worse), just no feedback from the scanner. Same in SD mode with the MAF un-hooked

Thoughts? Im not looking for an answer like "its a broken X", instead just a starting point

 

well since it's adding fuel then it's lean for sure. Did you check for air leaks? Around intake throttle body or any place in front of the maf.

 

I have several times with carb cleaner. No change in idle

I even pulled the vaccum and PCV lines to cap in order to make sure there was no leak in the extremities. No change

 

Im thinking its a fuel pressure problem

Running, shut the car off it takes approx 3 sec to bleed the pressure from the schrader valve to nothing. Ive never been around LSx fuel systems before but this seems like there isnt much pressure in the line. TPIs and LT1s generally take 10-15 sec and they operate at a much lower pressure, but they are return style so I dont know if thats a valid comparision

Tested the injectors, none leaked. Removed the intake for a thorough inspection for cracks or leaks. Reseated every line in the engine bay

No dice

During an extensive search of tech last night for similar problems the overwhelming majority was with an exhaust leak screwing with the O2s. #2 problem was low fuel pressure from a dying pump. I can count the O2s out because it is repeatable with them unplugged

 

Do you have a test gauge for fuel pressure? That would be a good place to start to verify or eliminate pressure problems.

 

ltft's can also be a clue as to whats going on. stft when it happens?

 

LTFTs stayed at 0. I ran the scanner for over 100 miles and those never moved

STFTs jumped around depending on how I was driving and how bad it was running. Only had a steady climb at idle in gear with it running progressively worse

Ordered a new pump with a warranty today, see how that works

 

As stated above - a fuel pressure gage at the rail will tell you if you have a pump problem...

 

What are you running for a cam? What are your O2 sensors reading when the fuel trims are climbing?

 

Stock LM7 cam

O2s jump around quite a bit, but seem on average higher than when its running smooth

 

O2 voltages are supposed to jump from around .1-.8V when they are working properly. If they hang high or low then the fuel trims start correcting to try and get them back in line.

If you have reset the LTFTs and tried it without the O2s plugged in, then it would seem that the fuel trims shouldn't be the problem.

Was it not doing this when you first got it tuned? What does Mike say about it?

 

Ive been talking to Mike trying to diagnose this since it began. I really dont think there is anything wrong with his tune

New pump didnt solve the issue. I was sure I had it nailed though with the warm problems and stumbling under load. Exact problems of a bad walbro not putting enough pressure out especially with the problem being more prevalent with the car warmed up

I watched the idle timing advance from 20* to 36* as the car ran progressively worse. Mike said that was expected

Ive nearly run out of key words to punch in the search feature. Im wondering how deep to go looking for the problem before swapping in a new long block

Ive read a few threads about dying cam/crank sensors causing problems at certain RPMs. Other say they work or they dont. My tach sometimes will go to zero for a split second then back to the proper RPM so it may be a crank sensor. Swapped Firebirds have screwy tachs and the engine runs perfectly whenever this happens so I ignore it. Its pretty rare and always at high speed mid RPMs low throttle cruise, exactly where the engine runs the best

Most crank sensor threads go over rechecking the wiring esp if its not factory. I checked mine 3 times before looming it and installing it in the car. Others cooked the pigtail on the exhaust. Mine hugs the block tightly and isnt anywhere near the manifolds. I really dont think this is a wiring problem, but its not too much of a problem to remove the entire harness and ohm it out again

Grounds should be acceptable. I have 3 from the main harness. Two are the main going to a single stud on the pass head, and a redundant 3rd crossing to the drivers head. A ground strap connects the block to the K-member and the K-member to the chassis along with the standard battery to block and chassis grounds. Resistance is less than .5 ohms

Some had problems with an old battery and/or bad connections. The battery does have some age, but such a specific problem area suggests more than just a battery

I found a disheartening thread about fubared thrust bearings causing the converter to pull away from the pump and kill trannys. The reluctor ring also moves out of view of the crank sensor and mucks up performance. I did hammer the balancer on there the first time until I got a few threads to engage with the old bolt, then used the threaded rod tool the second balancer install, so its not totally out there. I didnt drive it all the way on there, so I doubt I hit it hard enough to damage anything. Oil is spotless though, even sifting through it with a white paper towel. Last oil filter looked good inside

Another guy had my symptoms to a T, but it ended up being a clogged cat. I have no cats. Not helpful

Any ideas?

 

What PCM is this? Also, are you putting the tune in Open Loop? Don't understand how you are keeping LTFT's at 0 for 100 miles, but you have STFT action during that timeframe... That is not in Open Loop - your STFTs are still effecting fueling if you are seeing them change. You need to force it into Open Loop with the tune, clear fuel trims, and then see what happens.

 

LTFTs are probably disabled by the sounds of it. But that has nothing to do with the cause of the problem.

Without a scan log to look at, and the tune, not much to help with.

 

The fuel recently changed around metro areas in NC. It has screwed us around a little bit already