DanielRicany

Hey guys. I have a 1985 Corvette with a Miniram/AFR 195 setup. So I'm finally on the edge of finishing tuning. I did it myself this time and got it done in about a week. Only thing left to tune was WOT and the timing tables.

ANYWAY, when I go WOT throttle, I am trying to target between 12.1:1 - 12.8:1. At about 5500 RPMs, it's at 14.3:1.

The lower RPMs are good, under 3500 RPMs, low to mid 12s.

As soon as I get above 3500-4500 ish it starts to lean out progressively up to like 14.3:1 at 5500 RPMs.

I added 31.64% fuel from 56* C - 152* C, and then after I saw it lean out after I added more fuel, I added 19.92% more fuel from 4000-6400 RPMs on top of the 31.64%.

So I'm thinking my fuel pump isn't keeping up. It was a cheapo pump from the local parts store that I bought over a year ago.

So what are the other possibilities besides the fuel pump that could need upgrading? Also my fuel pressure is at 43.5 PSI.

Thank you!

MavsAK

fuel filter.
Fuel pump. Likely it's one of those two. I'd go ahead and get rid of your cheapy pump. You shouldn't have done anything but order the 255 lph racetronix conversion imo.

Cheapy pumps aren't made for performance cars.

whalepirot

Nice work. I can only aver that my setup uses a stock pump just fine, under hard load and all the way to fuel shut-off at 6300 RPM. I recall that a GM truck pump works fine, too, for le$$.

43.5 psi at idle.. no vacuum? How's it holding up under the conditions you describe as problematic?

I'm not a tuner but regardless of what the ECM demands in the tables, cannot fix starvation due to reduced pressure.

DanielRicany

How do I test the fuel filter? I just replaced it actually but it may not be designed for performance.

DanielRicany

Also does it matter that I'm only slightly above a 1/4 tank of fuel?

Benny42

Yes, this part matters a LOT! Top it off and rerun the tests, what do you
have to lose.

DanielRicany

Like fill it up or can I just throw 20 bucks in?

Benny42

$20 should do fine. Your car accelerates hard with the mods you have, fuel sloshing around
and away from the pick-up could be what's up.

MavsAK

Filter unless you have bad gas should be OK then.

And oh yes. By the way you might not be at a 1/4... I'd find out just how far off your gas gauge is off sometime. These cars are notoriously finicky once you get to a 1/4 or less as well because of where the pickup is.

ANTI VENOM

When my car runs out of fuel it's obvious. It falls on its face.

Since the pump and filter have already been covered.
What injectors? Whats the duty cycle at WOT?

DanielRicany

The injectors are 24 lb Bosch III from FIC direct replacement for '85. I don't know the duty cycle yet. I will have to test Monday.

DanielRicany

Alright guys. I put more fuel in the tank. Still went lean. Tested fuel pressure. I had set it to 43 PSI a month ago and today I checked the pressure today and it was at 35 PSI with the hose unplugged. 28 PSI with the hose plugged in. I took it out to test to see if the pressure dropped off at WOT. When I floored it, it was at about 2500. The gauge went to 34 PSI, then dropped down to 31 about half a second later. I readjusted the pressure to 50 with the hose unplugged. About 43-44 with the hose plugged in. Took it for a test drive and at WOT it went to 49 and half a second later went to 47.

So, is there something wrong here with these results other than the fuel pressure being low the first time around?

Thanks

Tom400CFI

If FP is dropping under WOT from set pressure, then yes, something is wrong. Either your filter or your pump, as has been stated above.

DanielRicany

Yep and I did and got the same results. This time I made sure to add way more fuel.

DanielRicany

So the difference of 2 PSI can cause a difference from 11:1 or 12:1 all the way to 14.3:1?

Tom400CFI

IDK.. I'm sure that would depend on many things. What I DO know is that if the pressure is falling, the pump is not meeting the demand. This is basic hydraulics.

DanielRicany

I guess a better way to differentiate this from an actual fuel supply problem or a vacuum problem would be to disconnect the vacuum hose from the fuel pressure regulator and retest. Would you agree?

Cliff Harris

The fuel pressure is the INVERSE of the vacuum. When you go to WOT the vacuum goes very low and the fuel pressure goes up. As the engine increases in RPM the vacuum also increases and the fuel pressure drops.

bjankuski

You need to watch the fuel pressure all the way through the run. If you are showing lean AFR at WOT high RPM, you need to see what the fuel pressure is at under that condition. As the RPM goes up the duty cycle keeps climbing and if you are going to see a problem it will be at the high RPM high load condition.

FYI if the fuel pressure holds at or above 43 PSI at WOT high RPM the problem is not the fuel pump. It could be the tune or the injectors out of capacity.

Tom400CFI

^He set his FP at 50, so saying that "iif the fuel pressure holds at or above 43 PSI at WOT high RPM the problem is not the fuel pump is not right. If he goes from 50 down to say, 44 PSI at WOT....there is a fuel delivery issue.


NO! THere is no meaningful amount of vacuum in the plenum at WOT. Certainly not enough to pull down fuel pressure. No need to disconnect the vacuum reference line (although you could, for yuk-yuk's).