Why am i running lean on boost???

Gannon · Post by **Gannon** » Tue Dec 11, 2007 5:26 pm

I installed an "Air Fuel Ratio" gauge on the weekend to try to find why im surging as i rev up to 5k rpm.

I found that under 3k rpm nearing full boost, the gauge shows full rich, (orange LEDs) (which is normal for a turbo car)
But as i rev (on boost) it leans out.

By about 3000rpm, its showing about stoich (middle green LEDs)
By 4000rpm its lower than stoich (lower green LEDs)
And near 5000rpm its in the lean section (red LEDs)

So....

I hooked up a fuel pressure gauge to see if its my fuel pressure or my pump is running out of capacity

On idle it shows (-20mmHg) it shows 38psi

At cruise (-10mmHg) it shows about 42psi

At at full boost (7psi) it shows about 46psi

On boost, fuel pressure stays a constant 46psi even when it leans out (just before 5000rpm) and i stop revving in fear of breaking something. So the pump is not running out of capacity.

Now i notice that there is only a 4psi increase in fuel pressure for a 7psi increase in manifold pressure.

ISNT IT SUPPOSED TO INCREASE BY 1:1 ???

ie. 49psi fuel pressure at 7psi boost?

Help me.

Gannon · Post by **Gannon** » Tue Dec 11, 2007 6:54 pm

According to the 88 service manual, fuel pressure should always be 36 psi above atmospheric pressure.

My fuel pressure is above that.

Surely that cant be right?

BrennyV · Post by **BrennyV** » Wed Dec 12, 2007 9:52 am

time to relax and just start driving you car!
gauges are for space ships not family cars

discopotato03 · Post by **discopotato03** » Wed Dec 12, 2007 1:00 pm

What you really need is a fuel pressure gauge that reads in psi/or kpa for a start . Inches of mercury is a confusing measure to work off .

Pressure , yes the fuel rail pressure SHOULD be rising in parallel with manifold pressure - the regulators spring load making for the constant head of pressure over atmospheric . I'm not at home ATM so cant look up WSM to see if they show what the figures need to be for a std car .

If for whatever reason your manifold pressure under boost climbs faster (out of sync) with fuel rail pressure then all else std and being equal you would expect it to go lean .

With most engine related things try the easy things first , fuel filter needs to be as new and a 15-20 yr old std EFI pump would have to be suspect .
An easy replacement could be a VL turbo one but shop around for best price . It won't have the pulse damper like some early Nissan and Subaru ones did but thats just a noise damping thing really .

If you have a spare MPFI-T fuel pressure reg try that too , they don't often die but once in a blue moon .

Another thing to check is the TPS particularly on 3 plug cars . They have 2 internal micro switches , one which opens just off idle and the other that closes ~ half throttle on the turbo EA82 . If the open throttle contacts don't work the computer thinks the engine is in part throttle cruize mode and wont use its richer wide open throttle mapping .

Cheers A .

Wilbur · Post by **Wilbur** » Wed Dec 12, 2007 1:36 pm

I wonder what ratios the little LED things relate to.

Gannon · Post by **Gannon** » Wed Dec 12, 2007 3:20 pm

My fuel pressure gauge does read in PSI, its the boost gauge i was comparing it to that reads mmHg in the vac section.

Fuel filter is near new, fuel pressure in the FSM says 36psi above atmospheric, but i think my gauge was out, im getting 42psi on the gauge

Matatak · Post by **Matatak** » Wed Dec 12, 2007 7:11 pm

is it a stock ECU?

get it an aftermarket one and tune it to 10PSI then go cruizing

Subafury · Post by **Subafury** » Wed Dec 12, 2007 7:29 pm

do u run extra boost?

Matatak · Post by **Matatak** » Wed Dec 12, 2007 7:33 pm

says hes on full boost at 7 PSi Subafury

Gannon · Post by **Gannon** » Thu Dec 13, 2007 4:17 pm

i have a spair fuel press reg lying on my spare manifold.

Gonna swap that on and see what happens