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George H.
George H., ASE Certified Technician
Category: Chevy
Satisfied Customers: 18668
Experience:  GM Tech ASE Master Tech 15+ years
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1994 Chevy truck with a 4.3.Where does the computer get

Customer Question

1994 Chevy truck with a 4.3.Where does the computer get its Barometic Pressure reading from?
Submitted: 1 year ago.
Category: Chevy
Expert:  George H. replied 1 year ago.
Hello I will help you with your question, The baro reading is taken in the time the key is in the run position until you turn it to start and the engine starts turning. This is done from the manifold absolute pressure sensor signal. The MAP is rectangular and mounted on a bracket above the EGR valve at the rear of the engine. Let me know how I can help Thank you
Customer: replied 1 year ago.
So once the reading is determined it will not change until the key is cycled on and off again? When the truck is running BARO(V) is reading 4.98 and the BARO(inHg) is at 30.1
Customer: replied 1 year ago.
And it never changes.
Expert:  George H. replied 1 year ago.
Correct, once it knows you are not parked on the side of a mountain at 10,000 ft the baro reading is not used until the RPMs of the engine = 0. The 30.1 may be a value from the lookup table in the computer, you could try changing the pressure vacuum on the sensing port of the MAP and see. Should not take more than a plastic tube and sucking or boloing on the tube you put on the opening of the nipple connected to the vacuum line.
Customer: replied 1 year ago.
Ok next question.MAP (inHg) reading on my monitor says 11.2 and MAP(V) says 1.48, but my vacuum gauge says 20(inHg). Explain why the difference?
Expert:  George H. replied 1 year ago.
The sensor is for absolute pressure. You probably have 14.7 lbs of air pressure (30 inHg) so the difference between that and the 20 inHg is what you are seeing. Not the most accurate with that reading. Your vacuum gauge starts at 0 even though you have 30 inches of pressure in reality.It is like accounting for gravity when you lift, do you lift the weight in relation to everyday gravity or do you add the force of gravity? The sensor is working properly.
Customer: replied 1 year ago.
I figured it was due to the voltage being correct.
Expert:  George H. replied 1 year ago.
You posted that you have 4.98 key on engine off and 1.48 volts at 20" vacuum. Those readings are correct.Please let me know if you have further questions Thank you
Customer: replied 1 year ago.
Those where engine running readings.
Expert:  George H. replied 1 year ago.
The first one was? You said you saw 4.98 @ 30.1 inHg? That is not a running engine.
Customer: replied 1 year ago.
Correct on the first reading. However it stays the same when running as well.
Expert:  George H. replied 1 year ago.
You posted that you were seeing 11.2 inHg, 1.48 volts and your vacuum gauge was at 20"? I am confused now. The first was key on engine off and is correct the second is an engine running at idle and is correct. Where do you not see a change between those?
Expert:  George H. replied 1 year ago.
If you are concerned about the Baro PID not changing once the engine is running it is that the computer only look at it when the key is on and the RPMs are 0. That value is stored and once it determines that you are not parked on a mountain it does not get used again.
Customer: replied 1 year ago.
Sorry for the confusion.4.98v and 30.1 inHg is KOEO or KOER.11.2 inHg and 1.48 and vacuum gauge at 20 is KOER.
Expert:  George H. replied 1 year ago.
The only time the computer senses and sets the Baro PID is when the engine is not running. After that it does not change until you start the engine again. If your altitude has not changed you will see the same reading all the time. If you started the engine and drove up a mountain the next Baro pid would be about 4 volts at 10,000 feet. This is a normally operating system
Customer: replied 1 year ago.
Ok
Customer: replied 1 year ago.
I will be asking a few more questions possibly here shortly.
Expert:  George H. replied 1 year ago.
Great, please let me know how I can help Thank you
Customer: replied 1 year ago.
Ok this truck runs great the first time of the day for about 5 minutes or so. After that it runs but has no power at all. It acts like a stopped up converter. (Which I have already confirmed it is not). Any suggestions?
Expert:  George H. replied 1 year ago.
Sounds like something is getting hot. Might be a coil, the module in the distributor (I think yours still had one) or the computer itself. Could be a worn fuel pump losing flow as it heats up. You would have to test to see what is breaking down when it is acting up.
Customer: replied 1 year ago.
Usually when a module gets hot it cuts out. This is smooth.
Expert:  George H. replied 1 year ago.
Testing is the only way to know.
Customer: replied 1 year ago.
Ok I'll let you know what I find.
Expert:  George H. replied 1 year ago.
OK, let me know how I can help Thanks