Dyno Results - Lean fuel condition. Need Help

[Green Machine]

Just return from the dyno shop - car made 274.07 rwhp and 304.94 torque with an A/F ratio average of 15.20.

After the 1st pull we checked and cleaned the MAF sensor which was dirty. and that neted 2 more hp with torque staying the same.

So, I then stopped back by the "garage" and had the mechanic run a diag on it. The fuel tabs ? setting would drop to -3 and -6 from 0 each time he rev'd it.

So now the fun begins what is causing the fuel lost ?

The car is backfiring which goes along with the lean condition. No check engine light has ever displayed.
I have new 24lb injectors installed, the car does not stall out, so I don't think the fuel pump or injectors are the problem.

Could it be, I just need to raise the fuel pressure from 32 to 35 / 40 lbs ?
The timing is set @ 12* and wont go past that.

Or would be a good time to replace the computer / pcm with a new one ? I believe the computer contols the duration of how long the fuel injectors spray, correct ?

History - the car previous made 281.18 / 308.55 with the same fuel pump, 24lb injectors and long tubes and Pro Chamber o/r h-pipe.

I've taken the o/r h off and added my Bassani o/r x pipe back on, bumped the timing up to 12*, removed the smog pump, put new 9mm wires on with dist cap and new rotor. Add a "Plasma Booster" spark enhancer.

I had to scan the dyno chart, and it's giving me issues uploading it. I'll work on. Thanks Dewman

 

[zenboy99]

Yes that fuel pressure is low, you didn't try adjusting it at the dyno?

By the way, great numbers for a stock cam.

 

[Green Machine]

It was a Chevy shop, and the guy suggested I bring a tuner in to work on it.

What you you recommend going up to from 32 lbs ?

 

[final5-0]

My first Q for you would be if you are a GT or Cobra?

Just return from the dyno shop - car made 274.07 rwhp and 304.94 torque with an A/F ratio average of 15.20.

After the 1st pull we checked and cleaned the MAF sensor which was dirty. and that neted 2 more hp with torque staying the same.

Too Lean for sure!

So, I then stopped back by the "garage" and had the mechanic run a diag on it. The fuel tabs ? setting would drop to -3 and -6 from 0 each time he rev'd it.

No clue of what this might mean?

So now the fun begins what is causing the fuel lost ?

The car is backfiring which goes along with the lean condition. No check engine light has ever displayed.
I have new 24lb injectors installed, the car does not stall out, so I don't think the fuel pump or injectors are the problem.

Could it be, I just need to raise the fuel pressure from 32 to 35 / 40 lbs ?
The timing is set @ 12* and wont go past that.

Something mechanical is most likely causing the bf's.

IIRC, the fuel pressure is supposed to be at 39 for a stock setting and if that is so ......... I would think you should be at that setting for the very minimun.

Or would be a good time to replace the computer / pcm with a new one ? I believe the computer contols the duration of how long the fuel injectors spray, correct?

No reason to replace the pcm I can see.

History - the car previous made 281.18 / 308.55 with the same fuel pump, 24lb injectors and long tubes and Pro Chamber o/r h-pipe.

What kind of fuel pump are we talking about here?

I've taken the o/r h off and added my Bassani o/r x pipe back on, bumped the timing up to 12*, removed the smog pump, put new 9mm wires on with dist cap and new rotor. Add a "Plasma Booster" spark enhancer.

I had to scan the dyno chart, and it's giving me issues uploading it. I'll work on. Thanks Dewman

How long has the bf and lean condition been going on and when did it start?

Grady

 

[Green Machine]

Hey Grady, that would be a 1994 GT.

A 255 Walbro fuel pump


the back fire condition started about 3 weeks ago. Again no check engine light or anything.

Thanks

 

[zenboy99]

I was running 39psi for my setup without a tune, A/F seemed to be OK checking the plugs. I have an aftermarket cam though. But I would assume you'd need to be around 39.

Just checking but, did you measure fuel pressure with the vacuum out?

 

[final5-0]

Hey Grady, that would be a 1994 GT.

A 255 Walbro fuel pump

the back fire condition started about 3 weeks ago. Again no check engine light or anything.

Thanks

You ought to be getting enough voulme with that pump

Did you change anything 3 weeks ago?

Like the other guys above, I would get the pressure up and see if that don't kill the bf's.

If the bf's continue .....................

at least you can use them to help you find the prob

 

[Green Machine]

Nope, just remember what the guage was reading when @ motor is ideling.

 

[final5-0]

Nope, just remember what the guage was reading when @ motor is ideling.

Hey Chad

Ain't that about what he should see when he looks at it that way

Grady

 

[final5-0]

The timing is set @ 12* and wont go past that.

I just noticed this!

More info please

Grady

 

[Green Machine]

I have a Kirban ? AFPR and it displays 32lbs when the motor is at idel etc. Timing was originally set @ 10* but we were able to get that bumped to 12* and the car has run much better.

If I remember correctly to adj fuel pressure you need car running then un-do the vacumn hose from the fuel pressure regulator and then turn the adjustment allen screw clockwise and watch the dial move up ? correct

Thanks

 

[VibrantRedGT]

A bump in fuel pressure would have probably made more HP and got rid of those lean pops. I ran into the same issue on the dyno a few years ago. I got 8HP more out of it plus my lean pops disappeared. Its as easy as that.

 

[Green Machine]

Just finished road testing, and I can't get it to backfire anymore. hard downshift then left off gas

The pressure guage showed 31lb's with vac hose attached, and 39lbs with out.

I bumped the pressure up to 35lb and I it sure pulls harder, but the dyno will tell for sure.I'm going to drive it for a few days and then try bumbing it up to 38lbs.

I'll swing by the gurage tomorrow and have them check the fuel trim settings. I betcha they're not in the negative anymore. I'll see if I can schedule a trip to the dyno for next week. Thanks for everyones help.

 

[zenboy99]

bumped the pressure up to 35 with the hose on or off?

 

[Green Machine]

Bumped up with vac hose off = 45lbs. vac hose on = 35lbs

 

[moneypit94]

38-40 is what i have always run with the hose off. If your running lean with that much you have some other problems not due to fuel pressure. When was the last time you changed your fuel filter? Sounds like your getting pressure but not volume which leads to no pressure underthrottle.

But then again i don't ever remember having that low of pressure with the vacuum hose on.. But then again you are probably running a lot more for vacuum pressure then i was.

 

[bimmertech]

i remember some guy in the last thread saying you need more FP, boy was that guy smart.

seriously 40psi with the hose on. after adding the more fuel bump the timing. after that check for vacuum leaks--very thouroghly.

my main concern here is that you are using a stock gt puter with 24's and the only way you may have "calibrated" it is with a c&l sample tube change. this might work, i dunno, i have never tried it.

 

[zenboy99]

38-40 is what i have always run with the hose off. If your running lean with that much you have some other problems not due to fuel pressure. When was the last time you changed your fuel filter? Sounds like your getting pressure but not volume which leads to no pressure underthrottle.

But then again i don't ever remember having that low of pressure with the vacuum hose on.. But then again you are probably running a lot more for vacuum pressure then i was.

I agree

 

[zenboy99]

i remember some guy in the last thread saying you need more FP, boy was that guy smart.

Guess I made the assumption that he knew the standard way of checking fuel pressure.

 

[SMOKEDYA]

Just return from the dyno shop - car made 274.07 rwhp and 304.94 torque with an A/F ratio average of 15.20.

After the 1st pull we checked and cleaned the MAF sensor which was dirty. and that neted 2 more hp with torque staying the same.

So, I then stopped back by the "garage" and had the mechanic run a diag on it. The fuel tabs ? setting would drop to -3 and -6 from 0 each time he rev'd it.

So now the fun begins what is causing the fuel lost ?

The car is backfiring which goes along with the lean condition. No check engine light has ever displayed.
I have new 24lb injectors installed, the car does not stall out, so I don't think the fuel pump or injectors are the problem.

Could it be, I just need to raise the fuel pressure from 32 to 35 / 40 lbs ?
The timing is set @ 12* and wont go past that.

Or would be a good time to replace the computer / pcm with a new one ? I believe the computer contols the duration of how long the fuel injectors spray, correct ?

History - the car previous made 281.18 / 308.55 with the same fuel pump, 24lb injectors and long tubes and Pro Chamber o/r h-pipe.

I've taken the o/r h off and added my Bassani o/r x pipe back on, bumped the timing up to 12*, removed the smog pump, put new 9mm wires on with dist cap and new rotor. Add a "Plasma Booster" spark enhancer.

I had to scan the dyno chart, and it's giving me issues uploading it. I'll work on. Thanks Dewman

You made these numbers with stock heads and cam???? I have 17* timming and no ping I'll check my fuel presire when i go out this morning to work. peace





john