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Mystery Leak? Lean condition

wstefan20

wstefan20

New member
Alright, so I'm getting to the point where I might have to break my rule and take my beauty in to the shop.... hopefully someone can help and save me the hassle!

To preface, the car runs "ok" and has since I got everything rebuilt. However, I've always noticed that it struggles going up hills and doesn't have much "get up and go" which has bothered me.

I finally had enough time to hook up my scan tool and check things out in detail this weekend. There are NO check engine lights, but after bringing everything up to temperature, I get around 15% lean long term fuel trim at idle and when my foot is off the accelerator, which immediately makes me think vacuum leak.

The bad is that there is no auditory leak either vacuum or exhaust. The MAP pressure is normal vacuum and responds with throttle accordingly. The fuel pressure is around 55-60 psi and increases to 70 when regulator vacuum is removed or throttle is snapped. MAF sensor is in range and responds as it should (and I just cleaned it). IAT sensor is at correct readings. EGR seems to be functioning correctly. O2 sensors are operating normally. Idle rpm is around 750-800 (I think that's normal).

I used the propane test EVERYWHERE and no leaks detected. Unfortunately, I don't have access to a smoke machine.

There isn't excessive crankcase pressure, and no detectable blow-by by removing the oil fill cap.

Basically, I'm lost... help! lol
 


It's an off brand scanner, but it's never been wrong on me yet. If anyone is curious, it's by Launch and it's the EasyDiag2.0 (don't think they sell it in the US anymore but it's fantastic). The LTFT read around +15-16% at idle and drop to between -3-+3% when any amount of pedal is applied (I think our cars won't throw P0171 until it's over +-20%) and immediately returns to 15% after pedal is released both at idle and at town and highway speeds, same result.

I have done some basic mods, but for the most part the engine is stock, so it shouldn't be out of wack. :(
 
just so we're clear trim read neg as rich, pos is lean. so a -15 is kinda normal at idle stock tune. -3 cruising is also normal.

last tune up was? cat still on the car? miles? being na ill lean towards the cat being pooched if over 100,00 miles.
 


Scottydoggs said:
just so we're clear trim read neg as rich, pos is lean. so a -15 is kinda normal at idle stock tune. -3 cruising is also normal.

last tune up was? cat still on the car? miles? being na ill lean towards the cat being pooched if over 100,00 miles.
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Engine rebuilt and running perfect. Catalytic converter brand new along with headers and catback exhaust. It is a high flow cat so it does throw P0420, but this won't affect fuel trim.

Body mileage is 120,000 but the engine has around 7,000 since the rebuild.

Name a part, I've probably replaced it. There is literally not a single thing I haven't laid my hands on... lol That's why I'm so stumped.... :(
 
Swiggles said:
What was rebuilt on the engine? Top end or pistons etc?
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EVERYTHING! LOL no but really.... every bearing replaced, all clearances back to factory both heads and block. Yes, I replaced all tty bolts and main line honed. 20 thou overbore. New pistons, rings, wrist pins, balanced, all new gaskets. Basically, my engine rebuild is not the problem.
 


I was chasing a +16% ltft, did a smoke test and still had no leaks. Ended up being my tune. Where you are mostly stock it could still be the stock tune.

Also rebuilding these motors usually doesn’t end well. If you are staying close to stock with mods you’ll be fine.
 
What about a stock tune would cause a lean condition? Fuel injectors?

And I think people's problems and fears with rebuilding these engines is merely they cut corners on the rebuild. I haven't seen a single properly rebuilt engine failure to date, they all admit skipping something along the way. I'm at over 7000 miles with no issues. I'm hoping for many more
 
If it's tune related it's because you technically increased the size of the engine without telling the computer.

Ideally you'd add in a couple % to the airflow tables like the MAF curve or Primary VE table. The additional % is how you tell the computer to add more fuel. The more air the engine see the more fuel it will give in a simple terms.
 
Darn. That's what I was afraid of... I'm guessing this would need a dyno tune for this? I'm still confused why if it's the piston size it decreases when I accelerate? Wouldn't this be across the board?
 


street tune would be better. if you can find a local with a tuner that would be best, most dyno shops dont tune 3800's.
 
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