# Random "Underboost" Issue



## Flapjack (Nov 20, 2013)

I've had my car to the dealership a few times over this issue. However, though they never gave me the actual OBD code, so I don't know if it's the same issue.

The first time I got a CEL for intake/boost-related issues was due to a quite obvious situation where the clamp on the turbo cold side came loose. They put it back on and tightened it. No CELs for several months.

On two other occasions, the CEL came on and the tech stated it was a "underboost" code. We looked everywhere for an obvious hose loose, but couldn't find anything. Both times, they cleared the code, which solves it for months at a time.

I'm starting to be concerned, as I also have an issue where my mpg dropped significantly over the course of one winter. Where I used to average 45mpg almost religiously, I now get roughly 35mpg, with no changes to trip, driving style, gas station, etc.

Has anyone else experienced an underboost code issue?


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## LulzT1 (Aug 24, 2015)

I had the same issue with the cold side of the turbo coming loose. Brought it to the dealer and was ok for a while until I started getting a lot of emission sensor failures (may or may not be related). Not sure if I wrote the code(s) down, will check. 

Does your mileage drop with the CEL only? Or just in general?

Sounds like you should invest in a scan tool! I have the Scangauge in additiom to torque and dashcommanf for my phone/ipod.


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## obermd (Mar 3, 2012)

Does anyone know if the CTD has the same waste gate configuration as the 1.4T? If it does it could be the problem. I suspect your underboost and your fuel economy are related. Fix the underboost and your fuel economy should return to the mid 40s you initially saw.


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## Flapjack (Nov 20, 2013)

LulzT1 said:


> I had the same issue with the cold side of the turbo coming loose. Brought it to the dealer and was ok for a while until I started getting a lot of emission sensor failures (may or may not be related). Not sure if I wrote the code(s) down, will check.


Thanks. I would appreciate it.



> Does your mileage drop with the CEL only? Or just in general?


It dropped prior to that... after I believe 5 months of owning it. It was right after a nasty cold spell (-25 below, IIRC) and the car wouldn't start. They blamed it on fuel. It happened again, and they ended up towing it and were able to replicate the problem. They said GM was aware of the issue with some cars, but didn't have an answer yet. They gave me a letter saying I could use additives. 

The loss in mpg happened right after that. I chalked it up to additives, since winter was still in full swing. The problem is that even during the summer, with no additives, the mpg is essentially the same. It doesn't seem to go up or down when using additives, which is something I sometimes hear with others troubleshooting mpg issues.



> Sounds like you should invest in a scan tool! I have the Scangauge in additiom to torque and dashcommanf for my phone/ipod.


I have several scan tools, but they don't seem to work reliably in my GMs. Both are tuners for my 1000 rwhp Mustang (Diablo and SCT X3). I have used them to pull codes from my wife's Traverse, but it will not clear them. 

I'll have to try it on the Cruze next time I get the light.



obermd said:


> Does anyone know if the CTD has the same waste gate configuration as the 1.4T? If it does it could be the problem. I suspect your underboost and your fuel economy are related. Fix the underboost and your fuel economy should return to the mid 40s you initially saw.


I have not heard of that problem, so that is a bit more than what I started today with.


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## diesel (Jun 8, 2013)

Best guess is there is an air (boost) leak somewhere. I remember reading about a "smoke test" but I am not sure what that is or if that could help in your situation, but IIRC, it had something to do with tracing down a boost leak. You would need to go to a shop that has a mechanic who can do more than just hook the car up to a computer.


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## Flapjack (Nov 20, 2013)

Normally for vacuum leaks, I use a can of carb or MAF cleaner around all sealing surfaces, tubing, etc. Smoke tests are good for exhaust leaks, but elsewhere around the engine bay, you'd be surprised how much air is moving around. My only concern with using carb/MAF cleaner is that this is a diesel. I've started old diesel engines with either many a time, but those were the older kinds you'd find on a generator, light cart, etc...


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## diesel (Jun 8, 2013)

I don't know what you can get away with, but I have heard that the combination of ether and glow plugs and compression ignition can literally blow the head off. I think the main ingredient is the glow plugs. I have used starting fluid to start an old diesel Benz without the glow plugs and it actually worked quite well.


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## Flapjack (Nov 20, 2013)

So does that mean I pretty much have no other option than the smoke test? I was thinking that the minute amount of carb/maf cleaner I'd be spraying around the engine to check for a change in rpm (and an indication of a leak) wouldn't be enough to hurt the head gasket. The second I heard even the slightest rev I would stop spraying. 

The real answer is probably to bring it to the Chevy dealer and say I think there might be a vacuum leak based on the engine codes I'm seeing. 

I still haven't had a chance to see if there are any long-term/ongoing codes saved. I'll try the two tuners I have tomorrow to see if either will pull a code.


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## diesel (Jun 8, 2013)

Flapjack said:


> So does that mean I pretty much have no other option than the smoke test? I was thinking that the minute amount of carb/maf cleaner I'd be spraying around the engine to check for a change in rpm (and an indication of a leak) wouldn't be enough to hurt the head gasket. The second I heard even the slightest rev I would stop spraying.
> 
> The real answer is probably to bring it to the Chevy dealer and say I think there might be a vacuum leak based on the engine codes I'm seeing.
> 
> I still haven't had a chance to see if there are any long-term/ongoing codes saved. I'll try the two tuners I have tomorrow to see if either will pull a code.


You could try tightening all the clamps. There's another thread on there (not sure if you saw it) in which several people have found loose intake clamps. Other than that, a smoke test would be the most effective way to find the leak.


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