ok so i just built a 383 for my 93 z28..see sig for mods. it still has the EEPROM/MEMCAL setup in it. I had Mad Z make me a chip for my build. He said it would be close but would need additional tuning and it will probably run RICH. I just recently got my engine in and started. It runs great when cold. But as soon as it gets above 140 deg. it starts missing slightly and wont stay running when you attemp to move the car. I hooked my Wideband o2 sensor up last night and my A/F ratio is at 16-17.5(very lean) at idle(850-900rpm). It bounces all over the place. Now whenever the car is given a little bit of throttle(like 1300-1400rpm steady idle) it levels out to a steady reading of 13.5-15.0(fluxuates). I have an Adj FPR. would incorrect fuel psi cause an issue like this? Im considering getting some higher octane fuel (like 108 or 112) to see if it is the pump gas(93) im running. I just dont want to run this engine at all being that lean because im afraid of burning a piston up. Any help would be great and if you need additional info, just pm me or leave a post. Thanks

 

Id start with some new factory o2's. Sounds like the pcm may be getting a skewed O2 reading and casuing it to pull fuel. An adj. fpr will definitely change things because the tune is based on factory preset press. Any deviation will change the way the eng. runs.

 

Probably a very large cam with the 12:1 SCR. With high overlap, you're going to get lean readings at idle, since the incoming fresh charge is mixiing with the exhaust during overlap and blowing out through the exhaust valve. The O2 sensor sees the air and reads "lean".

As the tuner indicated, its probably actually running rich. The stock O2 sensors see the excess air from the overlap, and start to richen it up. But the air blowing out the exhaust due to the high overlap keeps telling both the stock O2's and your wideband that its running lean. Remember, the O2 sensors - narrow or wide band - can only "see" oxygen. They can not tell that there is a mix if fresh air and fuel that is blowing into the exhaust during overlap.


Not sure why you feel running higher octane fuel would change the fact that it appears to be running lean at idle. What is the dynamic compression ratio?

 

Im not sure on how to find my DCR..i know i used a more specific calculator to find my SCR that uses your Head Gasket bore size as well and it comes out right around 12.3:1. The reason i had considered running some higher octane fuel was because i am afraid of detonation w/ such lean conditions. What is the highest CR you can run on an LT1 w/ 93 pump gas? I figured maybe it was actually running lean since it only misfires and gives me trouble running after the engine is warmed up. If the 02 readings were false, how would i correct this issue? im going to hook my FP(fuel psi) gauge up tomorrow and see what for fp is actually going through the rails.

 

What octane fuel is required is determined by the dynamic compression ratio, not the static. You need the cam card, and a good online calculator. I'd guess you don't want to be over 9:1 DCR.

Its not going to detonate is idle. What is the A/F ratio under actual driving conditions? Until you find that out, you have no idea whether you're getting bad O2 readings because of the overlap, or because of the tune. Start to drive it, and watch the wide-band as you gradually push it harder.

Yes, you could buy yourself a bit of headroom, in case its also lean at max torque. Just make sure its unleaded. 112 is almost always leaded, and will damage the O2 sensors. The wide-band will die an early death with leaded fuel, even faster than the narrow band sensors.

 

ok, i used a DCR calculator courtesy of wallaceracing.com. According to my cam card, my Intake closes at 42 deg. ABDC. Now this is @.050 and i dont think this would be accurate as the cylinder isnt actually sealed at this recording. Am i correct? Anyways, i used that recording in the calc and i come up with a DCR of 11.05:1!!!! Which is like extremely high. I will get a chance to drive the car Thursday, if it will stay running. I am going to get 2 new o2 sensors since they are only $19 each and they havent ever been replaced that i know of. What should my fuel psi be at idle, no vacuum?i think it is 41-46 psi but i cant really remember.

 

$19 O2 sensor? I hope they work for you, seem's cheap to me?

 

1993 lt1= single wire non-heated o2 sensor

 

What did you tell the tuner the fuel pressure would be? He had to know the injector size, the pressure at which the injector was flow rated, and the pressure you are operating your system at, in order to program the injector size correctly.

Normally, lacking input from the owner, the tuner would probably assume stock fuel pressure, which is 43.5psi with the vacuum line disconnected.

 

Ion never asked me what fuel psi i would be running, so i am assuming he just programmed it for 43.5 like you said..i took my vacuum line off last night w/ the car running and installed a manual gauge. My fuel pressure is 50 psi at idle. it goes to 55 psi w/ vacuum. What would high Fuel Psi do? I am going to call Ion(mad z) thursday to find out what fp the tune is for and ask a couple questions. I appreciate you guys who are taking the time to help.

 

Unless you are running a forced induction setup, the fuel pressure should DROP when you reattach the vacuum compensation line. The purpose of the vacuum compensation line is to maintain the DIFFERENTIAL pressure between the fuel rail and the intake manifold. The pressure in the rail is pushing the fuel through the injector. The vacuum in the manifold is sucking the fuel through the injector. You want the difference between the rail pressure and the intake manifold pressure (negative of vacuum) to equal 43.5psi.

Do you by any chance have an Aeromotive AFPR?

Running at 50psi, instead of 43.5psi would increase the flow through the injectors by 7.2%. Your 42 #/HR injectors would flow 45.0 #/HR. Next question is what brand are your iunjectors? If they are Ford injectors, they are usually flow rated at 39.15psi, so running them at 50psi would cause them to flow 13.0% extra fuel, or about 47.5 #/HR. That would force the long term fuel corrections to pretty much bottom out at 108 in order to prevent the engine from running excessively rich in closed loop. Unfotunately when you go to WOT, the PCM would be over fueling the engine by 13.0%, since it ignores negative long term corrections at WOT (power enrichment, or PE mode).

 

I am running Racetronix Injectors. Believe it or not, my AFPR is indeed an Aeromotive. So if the injectors are putting out 43-45 lb/hr., would this cause the car to stumble/miss and shut off on me?

 

ok, so i talked to ION today. I imformed him that i was running open headers and my o2's were about 12 inches before the end. He said i should unplug them and run the car at idle w/o them hooked up. The car still doesnt want to stay running on its own and my A/F is still at 16.5/17.0 at idle. I think it is just because of being so close to the end of the pipe and is sucking fresh air back through during overlap. I hooked my FP gauge up and cycled the key. The fuel pump didnt purge when i cycled the key. Out of 5 key cycles, the fuel pump came on once and only for like 1 sec. Batt. Voltage is 12.6 volts. Hooked a batt. charger to the car and turned the key on and the fuel pump purged first cycle. My gauge is only reading 22 psi when it purges(engine off). Then the 22 psi bleeds down to about 10 psi in a matter of 25-30 seconds. Bad check valve? Im just getting very frustrated w/ this 93(OBD1). Unfortunately, everyone in my area says that they are not capable of tuning it. So i have to record datalogs and send them to ION. Also, i hooked my scan tool(Snap-on MT2500) to the car. No fault codes. Both Left and Right BLM's are 128. Is this because of the o2's being unhooked?

 

If your O2's aren't connected, the BLM's will default to 128.

When your fuel pump "primes", you should see at least 40psi. If you don't get that high, its not the check valve. That only affects how fast the pressure bleeds off after the pump stops running. If you only have 20psi, your pump is weak, the line at the pump is leaking badly or the fuel pressure regulator is bad. Even a plugged fuel filter will allow the system produce decent pressure when it primes without the engine running.

The Aeromotive LT1 AFPR's are not reliable at all. Hard to understand, when most of there stuff it very high quality, but the LT1 part is a problem.

 

I have the same fuel regulator and it primes to 46-47 ,i have it set to 50 at WOT
with the key on prime is 47 then it bleeds back down because there is no flaper valve to hold the pressure like the factory [A EPA requierment }one due to this unti being high flow one well this is what Aero tech said to me on the phone when i called about it bleeding down though i had a bad check valve to .
I to have a high lift cam .567 at idel low MV readings but when i drive it it where it should be switching from 190-840 mv