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Vac leak LTFT?

Sabrewings

Sabrewings

New member
When I first started my top swapped L36, I was running a stock L67 file with adjustments for my different gear ratio and the 42.5# injectors. My idle LTFT is +7, which I don't feel is all that bad since I have a ported TB, blower, LIM, and 1.9 rockers.

My question is would I see a much higher LTFT if I had a vac leak? The only reason I suspect one (other than all the connections I just put together that might be leaking) is that at idle once it's warmed if I blurp the throttle the RPMs don't fall as fast as they used to. Could be the supercharger making the difference, I dunno.

So, question stands. What sort of LTFT would I see with a vac leak?
 


The answer to your question is yes. If you have a vacuum leak your your LTFTs will be higher and possibly through the roof. However, seeing as how you did the topswap, your LTFTs will be off as well, and I would think most likely in the positive (lean) range, possible due to the increase in air flow.
 
Yeah, I'm only +7 to +10 to start off with from the factory L67 tables. And I've got a ported TB and rockers, plus the fact it's top swapped.

Like I said, what concerns me is how the RPMs don't want to fall very fast when I give it a light rev at idle (after it's warmed up). Also, while looking around the t-fitting that goes on the FPR doesn't seem to fit too snug. I'm almost convinced it would blow off under boost. It's definitely nowhere near as snug as the t-fitting on the LIM or the fitting on the MAP sensor.
 
Bio248 said:
a vacuum leak LTFT is going to be +16.4. been there, done that.
Click to expand...

I think the severity of the vacuum leak would dictate how high the LTFTs would go.

Sabre, isn't there tables that you can mess with concerning IAC? Maybe there is adjustments that need to be made in that area....just a thought.

My opinion on the "T" fitting not holding as snug as may be required....Heck just get a new one, that way you can set your mind at ease.
 
well, if youre stuck in the positive then it is probably a vacuum leak. reset your fuel trims, drive around, and scan it. if the LTFTs just keep going up and dont ever even out then youve got a leak somewhere.
 


Blown97 said:
I think the severity of the vacuum leak would dictate how high the LTFTs would go.
Click to expand...


Not as much as you think it would. Any of the many vac leaks I have had resulted in +16.4


But your thinking is accurate and it makes sense. Just does not seem to work that way.

I have had a very small one and got the same thing.
 
Blown97 said:
Sabre, isn't there tables that you can mess with concerning IAC? Maybe there is adjustments that need to be made in that area....just a thought.
Click to expand...



Unfortunatly we don't have acess to those tables yet.
 
With tuning my LTFTs are pretty close to zero. They're about in line now. Could be the zip tie I added, but it seems to be working. I'm slowly getting more and more aggressive with it as I feel it is capable of handling it.
 
Bio248 said:
well, if youre stuck in the positive then it is probably a vacuum leak. reset your fuel trims, drive around, and scan it. if the LTFTs just keep going up and dont ever even out then youve got a leak somewhere.
Click to expand...

When you mean stuck in positive... do you mean for each Fuel Cell or just idle ?

The reason why I ask is that I had a +16.4 experience for awhile (01+ Model "Full Rich Issue" ) However, when I put a 98 file on it, it did not climb to +16.4, rather stayed around +9 going - on accel.
 


dont get cheap on the vacuum lines, check the injector O-rings, everything. a vac leak is what caused me to chip #4. i ditched all the factory GM crappy plastic hoses and got nice rubber ones with tiny zip ties to secure everything.....good luck
 
Lawngoose said:
When you mean stuck in positive... do you mean for each Fuel Cell or just idle ?

The reason why I ask is that I had a +16.4 experience for awhile (01+ Model "Full Rich Issue" ) However, when I put a 98 file on it, it did not climb to +16.4, rather stayed around +9 going - on accel.
Click to expand...

you should be able to get it to stay around -3 to +3, but if it is always stuck in the positive numbers then you have a vacuum leak or your tune is WAY off.
 
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