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Discussion Starter #1
Hey there all fellow enthusiasts.

I joined this because i need a little help with a project of mine.

I recently converted a rebuilt 305 from carb to TBI. The 305 has had a mild RV cam installed as well as an edelbrock Performer intake and Comp roller tip Rocker arms. All else is stock. I currently have the engine running very well with the TBI system and it definatly makes more power over the entire RPM range. However it seems to suck more fuel than it should. While there could be a few differnt problems im wondering if the cam is producing a lower than normal vaccume reading for the MAP sensor and as a result the ECM is feeding more fuel than nessisary. There are several other possabilities such as a cold O2 sensor as well as a faulty pressure regulator that i have to look at as well but the vaccume question poses more of a concern in that i am planning to build a 350 with a bit more of an agressive cam in it. ie. even lower vaccume.

I will provide photos soon.
 

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You need to be reading what the O2 is doing at all ranges to determine which way to go. the voltage on the o2 will go up to 1 volt on rich and down to .1 lean. should be steady around the mid range at a steady cruise, go way up on accel and way down on deaccel. Vacuum around 17-21 should be ok for the MAP and mixture on steady no load. You need to hook up voltmeters or scanner and see whats going on.
 

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Discussion Starter #3
Airflow at High RPM

Thanx for that info. I have been looking for a chart to tell me what the Vout from the sensor vs Rich/Lean comparison is but havent found one. .1 to 1 will help.

Ive recently discoverd that the reason my enging starts crapping out at higher RPM is that the TBI simlpy doesent allow enough Air flow for what the engine needs. Essentialy it is starving for air at high RPM. Now short of forced induction of any sort are there any tricks i can use to get more and cleaner airflow? I have already started to build a cold air intake but i dont think it will be enough.

As well my cam is not designed for a TBI setup. Will this have a serious effect on how the ECM behaves? ie. Timing, Advance etc...
 

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Vacuum will have a strong effect on the TBI unit. I have 2 91 5.0 tbi's in my "fleet", as well as a 5.0 pfi. Lowering the vacuum will result in more fuel injected. So, in order for the pcm to work with the cam, you may need to get a hypertech chip to reprogram the prom (operating stratagy)

I also have a 2.5 iron duke with tbi and one weak cylinder. It loads up constantly at idle because the lumpy vacuum signal is fooling the map sensor.

psi for a tbi should be 11-14, and should NOT hold after shut down.
 

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Discussion Starter #5
My vaccume gauge is reading between 14 and 15 psi at idle so im assuming this is satisfactory. Im going to take a closer look at my header seals as there may be to much "clean" air entering the system before the O2 sensor. However i dont think this is whats causing the rich mixture as i just replaces the header seals. Ive yet to read the O2 sensor with my DMM and will post results as soon as i have them.
 

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The pcm is looking for 18-20 inches of vacuum. I think you just described the cause of your running rich (14-15 inches)

Did you know that a DMM will average the voltake you are measuring? Iif you look at the o2 sensor with a regular dmm, it willl average the voltage it sees. 02 sensors by design are supposed to fluctuate between rich (above .45 volts) to lean (below .45 volts)

What model year is the engine and pcm you are using?
 

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Discussion Starter #7
Im running a mid 70s 305 block ( Dipstick on Drivers Side) that i had built for a carburated fuel system. It is in my 85 chev half ton. Ive adapted a TBI setup from an 87 grand sierra which was the same body style. I replaced the entire engine harness but didnt replace the cab harness so i had to do some custom wiring, but it works. As well i didnt put in the in/tank fuel pumps but am running a holley inline high pressure fuel pump.

I am also concerned that my vaccume lines are not all hooked up in the correct way which might also contribute to a rich mixture.

I had my laptop hooked up to the truck running the WINDAL software and i seem to recall the O2 voltage as hovering at .45v or above. But im a little hazy on that. I cant use it at the moment as my power adaper was stolen and i havent got another yet.

Damn theives. :evil:
 

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Discussion Starter #8
Ive made Some headway.

Seems i had some problems with how my Vaccume lines for the EGR and PVC systems were routed. I changed them and it has helped a bit.

Im still not satisfied though so i think i will be getting into programing a Chip for the ECM. Tunercat seems to be the most popular but any other info would be appriciated
 
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