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Rttoys

Goblin Guru
Ya. Evap doesn’t matter and the HO2S doesn’t either in this instance. Plus they were for I/M monitoring status, not trouble codes.
 

Gtstorey

Goblin Guru
That’s red/pink and white.

I guess I left out the bypass valve in my previous post. It looks like you have a problem there based upon it sort of working when the vacuum hose was disconnected. It could be someone screwed the tune up badly also and the ECM isn’t calling for boost.
 

Dale E

Well-Known Member
On your solenoid with blue plug is that an open hole on top left side? May be the picture, but it looks like there is something broken there?
 

Rttoys

Goblin Guru
It’s a casting spot. Mine functions properly. I had to replace it after i finished mine. It’s a ac Delco.
 

Gtstorey

Goblin Guru
Check the part number on the MAP sensor and we can try and see if they are different from stock and might not be compensated for correctly in the tune. That appears to be the map sensor in your 5th picture. I don’t know if the SC cars have more than 1 or not. The TC cars have 2 map sensors.
 

Chubbs

Well-Known Member
The SC cars have 2 MAP sensors....one in the supercharger inlet (MAP) and one to ambient air, just behind the supercharger, behind the SC body and the valve cover (called the BARO sensor, for barometric pressure). It also has a TMAP sensor, which is on the intake manifold near the supercharger snout (pulley side). I am still not completely certain if the MAP and BARO sensors are the same part number or not. Some websites list interchangeable part numbers, and some do not. They are cosmetically identical. The Tmap sensor is different and can't be confused with the others.

The plugs to the MAP and BARO sensors are identical, but the wire harness is designed such that you can't really mix them up without seriously stretching wires. Also, if you mix them up, you'll get a code for a pressure mis-match, or something like that.

My car exhibited very similar symptoms to yours, and was throwing a code for the BARO sensor. I have been unable to solve it after 6 months of troubleshooting, and replacing every part in the system.

The good news in all this, is you can just block off the vacuum solenoid, and tune around the problem. That's what I have done. It's not ideal, as it eliminates the PCMs ability to limit boost in a trouble situation. But if you're desperate, as I was, it's an option. My car works great. Until I push it too hard and blow it up.
 

Gtstorey

Goblin Guru
Barometric sensor may be the normal atmospheric pressure? Does it plug into something that would see pressure/vacuum while running?

I sort of remember that GM stage kits had 2 pressure sensors that are maybe 3 bar vs stock 2bar? I’m thinking someone may have either upgraded the sensors without changing the tune or some installed stock sensors on a tuned car?
 

Chubbs

Well-Known Member
Barometric sensor may be the normal atmospheric pressure? Does it plug into something that would see pressure/vacuum while running?
It just mounts to a bracket that is vented to atmosphere. So it's just measuring ambient pressure, and using that as a baseline to compare intake pressure to.

Since there are no codes, I have no idea if this is part of the op's issue or not. It's just a good example of how the PCM might decide to pull boost based on a whole variety of issues.
 

Rttoys

Goblin Guru
There is also the possibility that all or some of the codes have been turned of with HP Tuner.
I was thinking about that last night. If they were trying to pass emissions, but couldn’t fix the problem, it’s a very good possibility.
 

Gtstorey

Goblin Guru
On some cars their are legitimate reasons to turn off some of the "pressure" related codes as they get tuned also. Some will have checks to see if the pressures seen are within bounds and if the car has the boost increased it will throw a code. Don't know about the LSJ.

I think I would probably start with trouble shooting the boost control/Bypass solenoid(s) and try and determine if they are working and getting signal. Are they Pulse Width Modulated like on the LNF or are they more a straight on/off system?
 

Sluggonaut

Goblin Guru
Because you were able to build boost with the lower actuator hose disconnected, we know the SC works. This means that with everything plugged in, the actuator is getting vacuum as it should and holding the bypass valve open so I don't think there is an issue with the vacuum lines.

My understanding is that when engine load increases and vacuum decreases, the actuator spring is able to overcome vacuum and close the bypass valve. This is done through the upper hose.

The lower hose is how the boost control solenoid gets involved. It is used to control boost by the ECM. It is normally open, which lets vacuum pressure dictate what the actuator is doing. From my research, this is done by the ECM commanding the solenoid to run at/near 100% duty cycle. When the ECM wants to control boost, it commands 0% duty cycle on the solenoid allowing manifold pressure to pass through on the lower hose and counteract the spring. This is why unplugging the lower hose is allowing boost to build and means the problem is likely somewhere along the lower hose tract. Suspects include boost control solenoid, ECM, or anything that sends signals to the ECM (MAP sensor, IC pump, etc.).

If the solenoid is bad it would be allowing the pressure to constantly keep the bypass valve open and you would just always be recirculating boost.

If the solenoid is working, a bad MAP sensor could be falsely indicating an over-boost condition causing the ECM to tell the solenoid to open.

I've read some examples of the ECM being an aftermarket or wrong unit and not working as it should

Other reasons the ECM would command reduce boost include:
When reverse gear is selected
Excessively high engine coolant temperature
Intercooler pump failure
Excessively high IAT2 temps

If any of these are occurring/falsely detected, they could cause this issue even if the solenoid/actuator/ECM are all healthy. Additionally, these safeguards are the only risk to just bypassing the solenoid altogether. That would fix the issue but you are removing the safety features that caused the engineers to design it in the first place.
 

Joebob

Goblin Guru
I will be the first to admit that I know nothing about the LSJ and boost issues, but had a lot of funky things going on with my engine ('08 SS/NA). I got the following scanner:


And used it for data logging while idling and driving. You can look at your long term and short term trims to see if you are running rich or lean at certain times as well as spark. Instead of guessing that it is pulling fuel or spark, you can tell and only change the sensors/wires/hoses that could affect the issue. I was running pretty lean (+15-20% LTFT) without any codes and was looking at the MAF, O2 sensors, TB, ect. While newer than 14 year old sensors helped increase the accuracy of the readings, the final change was the air filter pushed too far on the inlet pipe choking off perceived air to the MAF sensor. After stretching the filter to the end of the intake tube, I now run +/-5% on my long term trims.

Short story is get the data to tell you where to look and what function is outside normal parameters and you can narrow down the reason for the issue. If someone killed the CEL light in HPtuners, the data will still show something is up even if it won't trip the light.

Hope that helps.

Joe
 

Gtstorey

Goblin Guru
The ECM can be read and compared to a stock tune with HPTUNER and rewritten if things are screwed up. A dealer could reflash it to stock. A Tech2 clone might be able to read whether the ECM is telling the boost solenoid to make boost. Have you checked the diameter of the SC pulley to see if it’s stock as check to see how much the car may have been modified in the past?

what would it take to check a Pulse Width Modulated signal from the ECM? Would a regular digital voltage meter tell you anything? Maybe wire a light in with the boost solenoid to see if it try’s to light up?
 

Gtstorey

Goblin Guru
So a little research shows that some/most digital voltage meters will probably tell you something about whether the signal is coming from the ECM. I would run wires to where I could connect a DVM while driving and see if it registered any voltage.
 

Desert Sasqwatch

Goblin Guru
Cruise is electronic - tied into the electronic throttle. GM dealer may be the best bet to get a 100% stock reflash, unless someone on the forum pulled one for your specific Cobalt and wants to share. ;)
 

Rauq

Goblin Guru
If you want to do your own tuning (which I wholly endorse), you might as well go ahead and buy HP Tuners. They have all the stock tunes in their repository.
 

Gtstorey

Goblin Guru
If you want to do your own tuning (which I wholly endorse), you might as well go ahead and buy HP Tuners. They have all the stock tunes in their repository.
And anyone with HPTUNER should be able to read your tune and compare it to a stock tune. It doesn’t take any HP credits to do this and will only take a few minutes. I would want to do that vs just flashing back to stock.

If you are close to ZZPerformance I’m sure they could figure out what is wrong in just a few minutes.
 
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