LBZ low boost? 18psi.

[thunder550]

Reading the ECM with a OBDLink MX bluetooth dongle connected to my phone, running Torque Pro app.

App tells me I'm peaking around 17.5-18 psi. From a quick google search, it seems like I should be making around 22psi stock. Truck is 100% stock, down to the intake and exhaust. No black smoke on acceleration, maybe a very slight haze but nothing I would notice if I wasn't looking for it. Truck has 255k miles on it. Mileage seems ok....recently got 17.5 mpg on an unloaded road trip from Phoenix to ABQ, on the interstate running around 78mph. I get about 10mpg towing my 12k lb 5th wheel, which seems about normal from what I've heard.

I bought a boost leak tester and found that the gasket between the EGR bridge and y-bridge inlet pipe was leaking a tiny bit. Had to spray with soapy water to find it. Replaced all the EGR-to-intake gaskets with new ones...no more leak as far as I can tell. Also no change to peak boost numbers.

When I boost leak test the truck, I can hear air noise from the oval port on the bottom of the EGR valve once I get above 10psi. I can cover it with my finger and the noise goes away, but the boost tester gauge still drops at the same rate. I've found a handful of threads with similar symptoms...people have replaced the valve with new and had the same symptoms, so I'm not sure that's actually an issue.

I have not been able to find any other leaks in the intake/charge air system. Rail pressure peaks around 26k according to the app, so I think that's ok.

Here's a short video of the boost tester gauge dropping...I'm not sure what an acceptable rate is.

Source: https://youtu.be/pUEezEVdVDc


Am I on a wild goose chase? Is 18psi actually low, or ok? Beyond that, any ideas on where I can go from here? Any advice would be appreciated.

 

[clrussell]

Do the vanes fully cycle? You can see it in data

 

[thunder550]

Appears that I'll need a real scan tool for that, Torque Pro doesn't have a PID for vane position that I can find.

I've read varying things about stock boost levels though....seen some places say 22 and others say 18-19. If the latter is true, I'm chasing my tail. I guess first step is to confirm what I should be seeing.

 

[rcr1978]

Check the Baro sensor reading if you can, i've seen a couple go bad and the ecm was using the high altitude boost tables. Bad news it's not replaceable and it's on the ecm.

 

[Chevy1925]

based on what i see in the factory tune, 18 psi seems about right. stock is limited to 90mm3 at WOT or there abouts. what are you seeing for either max torque or max mm3 at wot in the 2500-3k rpm range?

 

[thunder550]

Online weather tells me my location is at 29.91 inHg, and my home weather station says 29.80 inHg, which would be 14.6 to 14.7 psi. I just went and connected my scan tool with engine off and it's showing 13.8 Baro and 11.9 intake pressure...why would I be showing vacuum with the engine off? Wondering if the scan tool is converting the values incorrectly, or maybe there's a sensor issue going on? This seems strange.

 

[thunder550]

based on what i see in the factory tune, 18 psi seems about right. stock is limited to 90mm3 at WOT or there abouts. what are you seeing for either max torque or max mm3 at wot in the 2500-3k rpm range?

I'll go for a drive in a bit and get torque numbers. My crappy scan tool app doesn't have mm3.

 

[Bdsankey]

Online weather tells me my location is at 29.91 inHg, and my home weather station says 29.80 inHg, which would be 14.6 to 14.7 psi. I just went and connected my scan tool with engine off and it's showing 13.8 Baro and 11.9 intake pressure...why would I be showing vacuum with the engine off? Wondering if the scan tool is converting the values incorrectly, or maybe there's a sensor issue going on? This seems strange.

View attachment 105283

What makes you think you're showing vacuum? The sensors for intake pressure (MAP) and barometric pressure report PSIA not PSIG. PSIA is pressure absolute (no offset for atmospheric, it reads it as well) while PSIG is pressure gauge and is "zeroed" at STP (standard temperature and pressure) so PSIG readings would show 0psi when no additional pressure above atmospheric pressure is added.

 

[thunder550]

What makes you think you're showing vacuum? The sensors for intake pressure (MAP) and barometric pressure report PSIA not PSIG. PSIA is pressure absolute (no offset for atmospheric, it reads it as well) while PSIG is pressure gauge and is "zeroed" at STP (standard temperature and pressure) so PSIG readings would show 0psi when no additional pressure above atmospheric pressure is added.

Baro is 13.8 in the screenshot above, and intake pressure is 11.9. If I'm understanding correctly, intake pressure is showing almost 2psi lower than barometric, which would indicate a slight vacumm in the intake, with the engine off. Unless I'm missing something?

 

[Bdsankey]

Baro is 13.8 in the screenshot above, and intake pressure is 11.9. If I'm understanding correctly, intake pressure is showing almost 2psi lower than barometric, which would indicate a slight vacumm in the intake, with the engine off. Unless I'm missing something?

Could also be an error in the sensor side. With the engine off you will not have any latent pressure in the intake manifold and it will normalize to ambient pressure.

 

[thunder550]

Could also be an error in the sensor side. With the engine off you will not have any latent pressure in the intake manifold and it will normalize to ambient pressure.

That's what I was getting at, should have been more clear...I'm wondering if there's a problem with the MAP sensor, it should show the same as barometric pressure with the engine off.

 

[Chevy1925]

Baro is 13.8 in the screenshot above, and intake pressure is 11.9. If I'm understanding correctly, intake pressure is showing almost 2psi lower than barometric, which would indicate a slight vacumm in the intake, with the engine off. Unless I'm missing something?

if you start the truck and let it idle, what does it read?

 

[thunder550]

if you start the truck and let it idle, what does it read?

It drops to 11.6 at idle, then goes back to 11.9 when I shut it off.

 

[Chevy1925]

so its 2 psi off what i should be.

swap the baro sensor with the map sensor and see if the reading follow with key on engine off, iirc they will plug in the same.

 

[thunder550]

so its 2 psi off what i should be.

swap the baro sensor with the map sensor and see if the reading follow with key on engine off, iirc they will plug in the same.

LBZ baros are built into the ECM case from what I have read and what @rcr1978 posted above. But it does add up like you're saying...I took a look at a stock LBZ tune file and it looks like I should be around 20 psi at 90 mm3, so that 2 psi low from the MAP sensor matches the 2psi off I'm seeing. I may just go get a new MAP sensor.

 

[thunder550]

Going to pick up a MAP sensor from Courtesy Chevrolet here in a bit, they have them in stock.

 

[Chevy1925]

LBZ baros are built into the ECM case from what I have read and what @rcr1978 posted above. But it does add up like you're saying...I took a look at a stock LBZ tune file and it looks like I should be around 20 psi at 90 mm3, so that 2 psi low from the MAP sensor matches the 2psi off I'm seeing. I may just go get a new MAP sensor.

ah, im thinking lly/lb7. forgot about that.

yes but thats why i wanted to see what you max fueling/torque is under wot. there are limiters in play that affect it. assuming all perfect, you should net 20ish psi

 

[thunder550]

ah, im thinking lly/lb7. forgot about that.

yes but thats why i wanted to see what you max fueling/torque is under wot. there are limiters in play that affect it. assuming all perfect, you should net 20ish psi

Well the new MAP sensor didn't change anything. Blah. Without a good scan tool I think I'm shooting in the dark here. The app on my phone just doesn't have what's needed to get good diagnostic type data...no turbo vane %, no fueling numbers...it shows torque, but the refresh rate is so bad that it's hard to get the needed info. Best I was able to get out of it was 665 ft-lbs at 2820 rpm with the original sensor. I tried to do a few wot pulls with the new one and closest I was able to see was 616 ft-lbs at 25xx rpm. I did see 18.4 psi boost at one point, but the new MAP still shows 11.9 psi with the engine off, while the baro reading is still 13.8-13.9, which makes no sense. Now with the engine warm though, it goes from 11.9 with engine off to 12.2 with it idling, so that's a little different, but I'm guessing it has more to do with the engine temp rather than anything to do with swapping in a new MAP sensor.

 

[thunder550]

Can one of you guys check to see if your MAP sensor value matches your barometer value with the engine off? Just want to validate that this is or is not a normal thing.

 

[rcr1978]

I can check a LMM here in a bit basically it's same ecm arrangement= Baro sensor on the ECM. Check your 5v supply to Map the sensor to if it's low the MAP sensor will also read low.