Too Lean

S

SpyderDiablo

Registered
I dyno'd my mark 8, and got some disturbing news. I was running extremely lean for my three pulls. I did have some good numbers, 210 hp, 203 ft/lbs. But I was shocked at the lean condition. I took it to class the next day, and did a fuel pressure test(off the fuel rails), and got a constant 30 psi. Mitchell on demand has my norm from 30-45 psi. What potential problem areas should I be looking at? Thanks in advance.
 
So you did test he fuel pressure under load? Because you can have good pressure at idle but under load a weak pump will fall off or a clogged filter could cause it too.
 
Stock fuel pressure is 39 psi with vacuum removed and maintaining that under load at WOT(vacuum attached of course).

Most Mark VIII dyno numbers I have seen have 10 more foot pounds of RWTQ than RWHP. Depending upon the dyno, your HP numbers seem about 10 HP low.

Do you have any mods? Have you checked for an codes? What is "extremely lean"? :confused:
 
Ok, first question...109,500 miles on the sensors.
2nd...well, if you consider racing the engine, under load, then yes. stayed 30 psi all the time.
3rd...The actual numbers are 207.13 hp, 221.42 ft/lbs, and my A/F was an average of 11. The A/F ran from almost 12, and just got worse, my lowest was 10.3.
4th...I was comparing my numbers with a classmates' TransAm.
5th...I have no mods that would affect the engine. Only extra driving lights, and stereo system.
6th...I have been told that I am running very lean, both by my ASE instructor and by the dyno owner.
 
Both your ASE instructor and the dyno owner are morons then if they said you were running lean. You are running rich which is what all Mark VIII do at WOT with the stock tune.
 
Cheapest repair first...fuel filter. If that don't do it, fuel pump. Your milage says you are about ready...maybe O2 sensors.
 
Don't just throw parts at it....hold on and someone that can troubleshoot better than I will be along shortly....
 
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SpyderDiablo said:
2nd...well, if you consider racing the engine, under load, then yes. stayed 30 psi all the time.
Click to expand...

No, that is not under load.

Best way is to buy an a fuel pressure test gauge setup with a long enough hose to allow the gauge to be duct-taped to the outside of the windshield so a passenger can monitor and record the fuel pressure under several WOT takeoffs in a safe area to drive. Again, the minimum should be 39 psi.
 
The Dyno was a "DynoJet 248x". I have the actual data logs from all three pulls, but it takes the "WinPep" software to view it, and I have no way to upload the data to here.
 
You are pig rich, not lean. I agree with who ever told you that you were lean needs to go back to the basics. Most completely stock Marks with high mileage will put out about 215 - 220 rwhp. A good custom program will take care of the rich condition but first you need to make sure everything else is working properly.
 
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