Author Topic: Fuel economy  (Read 2346 times)

Offline NCali

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Fuel economy
« on: August 07, 2015, 05:10:38 PM »
Has anyone heard of this:
an 87 305 tbi in a 1/2 ton 2 wd
I have read somewhere that the engine has to come up to temp in order for the tbi to achive the proper fuel/air ratio.  If it doesnt the computer will remain in a loop condition and adjust for a richer mixture.
  I cant find where I read this, I only found that the temp should be 195 degrees. I have a 4 core radiator with a flex fan and even out here in cali I am not getting 195 degrees
  I would like to confirm that 195 degrees. With the cap off on a hot day I am getting 180 deg
at best with a thermometer stuck in the radiator
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Offline enaberif

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Re: Fuel economy
« Reply #1 on: August 07, 2015, 05:12:36 PM »
Has anyone heard of this:
an 87 305 tbi in a 1/2 ton 2 wd
I have read somewhere that the engine has to come up to temp in order for the tbi to achive the proper fuel/air ratio.  If it doesnt the computer will remain in a loop condition and adjust for a richer mixture.
  I cant find where I read this, I only found that the temp should be 195 degrees. I have a 4 core radiator with a flex fan and even out here in cali I am not getting 195 degrees
  I would like to confirm that 195 degrees. With the cap off on a hot day I am getting 180 deg
at best with a thermometer stuck in the radiator

This is the function of the O2 sensor not the engine.

Offline NCali

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Re: Fuel economy
« Reply #2 on: August 07, 2015, 05:21:39 PM »

the way I understand it (and correct me if I am wrong)

the fuel ratio is the function of the ox after reaching temp right? If the water temp is below 195 the comp adjusts for a richer mixture  (like having the choke closed on a carb vehicle)
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Offline zieg85

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Re: Fuel economy
« Reply #3 on: August 07, 2015, 05:23:40 PM »
Thermostat needs to be the right one in order for it to run as efficient as possible
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Offline NCali

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Re: Fuel economy
« Reply #4 on: August 07, 2015, 05:29:32 PM »
yes , thank you
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Offline Irish_Alley

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Re: Fuel economy
« Reply #5 on: August 07, 2015, 05:59:18 PM »
When the engine is first started, and engine speed is above 400 rpm, the system goes into OPEN LOOP operation. In OPEN LOOP, the ECM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and Manifold Absolute Pressure (MAP) sensors. After water temp reaches 105° or what ever temp the ecm is looking for O2 sensors are warm enough to read properly and some time limit has been reached then the ecm will go into closed loop as long as all "requirements" are reached


the hotter the engine runs the better the efficiency. but if you run too hot you can have predestination or cause damage to the engine. the ecm will deliver more fuel if the sensors are told its cooler
« Last Edit: August 07, 2015, 11:55:49 PM by Irish_Alley »
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Offline NCali

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Re: Fuel economy
« Reply #6 on: August 07, 2015, 10:41:19 PM »
Thank you. Thats what I read and couldn't find that info again. I didnt realize the temp was 105 degrees. The truck runs great @ 70+  daily   @ 10 miles to the gallon . so with the water temp at 180 I thought that could have been the reason. I usually have the cruze control on so I know it is not my foot causing the problem
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Online bd

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Re: Fuel economy
« Reply #7 on: August 07, 2015, 11:44:17 PM »
Actually, the engine temperature and time delay after starting, both cold and hot, to enter closed loop is unique to the emission calibration of the particular engine/drivetrain combination.  My engine for instance crosses its closed loop temperature threshold at 77° F (not 105°), with a 45-second cold startup delay, and 10-second warm startup delay. 

Yet, the biggest hurdle to entering closed loop on factory calibrations is O2 sensor temperature.  An O2 sensor must surpass 600° F and alternately fluctuate across its median voltage (referred to as the cross-count value or the number of times per second that O2 voltage output swings rich-lean-rich-lean...) before the feedback system will enter closed loop. 

Additional influences on open vs closed loop operation are road speed and TPS opening - again, both unique to the emission calibration, and both weighted toward open loop at idle.  So, open or closed loop operation is not a fixed or simple black or white proposition.
Rich
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Offline LTZ C20

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Re: Fuel economy
« Reply #8 on: August 08, 2015, 12:56:19 AM »
BD nailed it on the head. My truck very similar in reaching closed loop status as his does. Even on cold mornings, the truck will go into closed loop with the first min or 2 of driving, less than a mile from my house. But my system also uses a heated O2 sensor for faster closed loop status when cold.
« Last Edit: August 08, 2015, 01:01:37 AM by LTZ C20 »
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Offline Captain Swampy

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Re: Fuel economy
« Reply #9 on: August 11, 2015, 03:12:25 PM »
GM designed the exhaust system to have high back pressure to create more heat so the O2 sensor would work. If you've gotten rid of the stock Y pipe you need a 3 wire heated O2 sensor. I don't know if any are available on this site, but tbichips.com has a kit. They have a lot of info on making TBI perform well.
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http://forum.73-87chevytrucks.com/smforum/index.php?topic=32209.0