03 Tahoe

[biggsteve97]

I have an 03 Z71 Tahoe 5.3L flex fuel and I am trying to find the intake manifold vacuum spec. My current map sensor reading at hot idle in park is 10.0 in/hg. I have tried google and it keeps giving me fuel pressure specs. I even contacted two local dealers,twice, and they have never returned my calls. Even went to one of them and I was told to bring my vehicle in and they would look at it. I just want to confirm what the reading is suppose to be so I know where best to go next on my diag. I have found no vacuum leaks, no ses light, just a driveability issue that has been confusing to figure out. The vacuum reading was the only reading that seemed wrong.

[clinebarger]

Your readings fall within specs, Hard specs are hard to find because it depends on Altitude.

5 Hg to 14 Hg is a "loose" spec at idle.

Your also reading the Data wrong, Those are Barometric Pressure readings, Not Vacuum gauge readings. 10 Hg of BARO pressure is roughly equal to 18 Hg of manifold vacuum.

[biggsteve97]

Possibly, i was just going by what the scanner data reader said...Map: 10.0 in/hg. There is another reading labled Baro but i dont remember what it said the data was on that one. I am used to the older motors where the map sensor only read vacuum and that reading of 10 was way to low to me based off all my other motors readings if that reading was vacuum and nothing else. I couldnt get anywhere with my local gm dealer, they just kept blowing me off. thanks.

[clinebarger]

What issue's are you having?

[biggsteve97]

I had intermittent no to very low power output, bad miss and intermittent stumble at idle yet wouldnt always throw a code, my mileage has dropped over the last 2 years slowly from 18-19 down to 10-11. Ive changed plugs, wires, converters, exhaust manifolds, intake gaskets, a rusted out muffler, fuel pump, fuel pressure regulator, swapped back an forth between the stock air box and a k&n cold airkit, changed MAF sensor. I finally just recently was able to get ahold of a live data scanner, saw my fuel trims were off slightly and that map reading. I changed the MAF sensor again under warranty thinkin it might be bad, and I found the MAP sensor loose in the manifold with a torn seal. Though I couldnt verify a leak there, I changed the MAP sensor anyway cause of the torn seal. The scanner showed a constant miss on cylinder 8 but wouldnt throw a code. I pulled the plug and it wasnt sealing on the head all the way around so I changed it and swapped the wire to another cylinder. These three things have helped power and the miss but mileage is still down, so far. The miss is still there but nowhere near as bad. I need to get the scanner again and see if it moved cylinders after swappin the wire around.

[clinebarger]

Keep us updated.

[biggsteve97]

Update: I also swapped the coil to a different cyl. I was finally able to get the scanner again today and the following is what I found. Map: hot idle in park 10.6, hot idle in D 12.4 in/hg, Baro: 28.3, Ign voltage 13.3 volts. The miss did not change cylinders, it still misses on number 8. The higher the rpm the less it misses though so I am going to swap the Injector out with a brand new one I got for free and see if that helps. I am still curious about the map sensor reading. I guess I will just need to hook the scanner up to the 02 Silverado and see what it shows for a comparison. I have power again though it is still a pig below 3000 rpm and mileage has come back up but not much, from 10-11 to right at 14 so far.

[clinebarger]

Originally Posted by biggsteve97

Update: I also swapped the coil to a different cyl. I was finally able to get the scanner again today and the following is what I found. Map: hot idle in park 10.6, hot idle in D 12.4 in/hg, Baro: 28.3, Ign voltage 13.3 volts. The miss did not change cylinders, it still misses on number 8. The higher the rpm the less it misses though so I am going to swap the Injector out with a brand new one I got for free and see if that helps. I am still curious about the map sensor reading. I guess I will just need to hook the scanner up to the 02 Silverado and see what it shows for a comparison. I have power again though it is still a pig below 3000 rpm and mileage has come back up but not much, from 10-11 to right at 14 so far.

I didn't do a very good job of explaining the MAP readings.

MAP sensors read on a Hg Pressure Scale not a PSI/Vacuum scale, That's why they are called Manifold Absolute Pressure Sensors.

Your BARO reading is the Pressure reading inside the intake Before starting the engine....This gives the ECM an idea of the Altitude that the engine is operating in. 28.3 hg would put you at around 1500 ft above sea level.

[buddy_1]

What kind of scanner are you using?

[biggsteve97]

I believe it is a Snap On scanner. Im not 100% on that. It belongs to a tech at my dealership.

[buddy_1]

To start with, turn the key on with engine off and see what the MAP reads. Then compare that to an Altitude vs Barometric pressure chart. See this

Altitude Versus Barometric Pressure

Determine your altitude by contacting a local weather station or by using another reference source.

Altitude Measured in Meters (m)
Altitude Measured in Feet (ft)
Barometric Pressure Measured in Kilopascals (kPa)
Barometric Pressure Measured in Pounds Per Square Inch (PSI)


4,267
14,000
56–64
8.1–9.3

3,962
13,000
58–66
8.4–9.6

3,658
12,000
61–69
8.8–10.0

3,353
11,000
64–72
9.3–10.4

3,048
10,000
66–74
9.6–10.7

2,743
9,000
69–77
10.0–11.2

2,438
8,000
71–79
10.3–11.5

2,134
7,000
74–82
10.7–11.9

1,829
6,000
77–85
11.2–12.3

1,524
5,000
80–88
11.6–12.8

1,219
4,000
83–91
12.0–13.2

914
3,000
87–95
12.6–13.8

610
2,000
90–98
13.1–14.2

305
1,000
94–102
13.6–14.8

0
0 Sea Level
96–104
13.9–15.1

−305
−1,000
101–105
14.6–15.2

Sorry about how the chart displays when I copy and paste out of the manual.

[biggsteve97]

Update: I tried the to use the new free injector I have but it was different despite have the same connector. It was shorter than my injectors so the oring looked like it didn't quite go into the intake far enough to make a good enough seal so I didn't chance it. So I just swapped the injector to a different cylinder. Power is back up almost to where it should be and the miss at idle is intermittent now. I finally got to use the scanner today and I still have a cylinder 8 misfire so the injector was good after all. I am going to do a compression test this weekend. Odd thing though around 900 rpm the miss almost completely goes away according to the scanner. anything above or below that and it comes back. at 1300-1500 it misses just like at idle according to the scanner but cant feel it driving down the road. still no light. I am suspecting a burnt valve now only due to last year the spark plug backed halfway out of cylinder 8 and started causing a neck jerking miss while cruising. When I figured that out i installed new plugs just to make sure all were good. Fuel mileage has even come up into the mid-high 14's so its better a tolerable at least. A guy at my shop offered me a set of 6.0L heads if it turns out my head is bad or needs work. So maybe Ill be able to use to 6.0 hot cam Ive had for a few years afterall.