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TBI Throttle Body Injection discussion and questions. L03/CFI tech and other performance enhancements.

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Old 06-22-2005, 10:45 AM   #1
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Question on MAP sensor...

Does the MAP sensor use any other inputs to calculate its reading aside from engine Vacuum?

Reason I ask is that mine is reading 40-50kpa at idle, instead of the normal 10-20ish. This started happening after I swapped my 454 tbi on, so I put the stock tbi back on and the MAP was still reading high.

If I know that no other sensors affect the reading, only vacuum, I would think a vacuum leak is the only thing that would be causing this? * edit - I did swap a known working MAP sensor on and it was reading the same thing, so I dont think the MAP sensor itself is bad.

Thanks for any help, as always.
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Old 06-22-2005, 01:54 PM   #2
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mine at idle is 35-40 KPA.
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Old 06-22-2005, 05:04 PM   #3
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The MAP sensor actually measures "pressure" not "vacuum". There have been some GM vehicles with vacuum sensors. But, I think these were largely used in computer q-jet apps. The sensor will produce the same reading for a 40 kPa manifold pressure whether the barometric pressure is 100 kPa or 60 kPa.

Your "new" values seem a lot more sensible than the "old" values. (This is assuming a pretty much stock engine...?) I've generally only seen 10-20 kPa values during decel/overrun conditions. My guess would be that your original setup had a vacuum leak giving you the lower sensor readings which you fixed by installing the new TBI........ Did you notice any change in engine operation when you swapped back to your original TBI? (I.e. IAC counts change at idle, idle RPM stability/magnitude, etc.)
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Old 06-22-2005, 05:25 PM   #4
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Like mentioned before, kPa is a measurement of pressure with 0 kPa representing a perfect vacuum (29.9" HG). 35kPa is about 20" HG, typicaly considered the max for a good running engine. You are unlikely to see below 35 kPa except on decel, so I don't know what number you were seeing. A typical engine can idle between 15" and 20" HG, so to convert that to kPa, gives you a range of 35-50kPa. I don't see a problem.
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Old 06-23-2005, 07:34 AM   #5
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There's not much stock left on the engine - Vortec heads, LT1 cam, Performer RPM intake manifold. I'll have to go though my old logs, but I do remember it being a lot lower with the old TBI. I did not realize that running that high at idle was normal, because the car is running stupid rich and previously it was running OK. Must be something else causing it then... back to the drawing board.

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Old 06-23-2005, 12:26 PM   #6
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your vacuum is between around 14 and 17 inHg. Could be better. Probably due to a tuning issue or due to the rich condition. Did you change injectors or the fuel pressure at all?
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Old 06-27-2005, 08:42 AM   #7
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I had 80 pph injectors on there when I put the 454 tbi on, now I have the stock ~45 pph injectors on there with the stock tbi. Fuel pressure is about 14psi with the regulator allowing max fuel pressure now. The car idles fine, but as soon as I give it any gas it blows black smoke out the tailpipe.
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Old 06-27-2005, 08:42 AM
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2003, 350, change, chevy, intake, kpa, ls6, map, normal, readings, sensor, tbi, vortec, xl, yukon
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