# Negative boost readings at idle



## MRO1791 (Sep 2, 2016)

BodhiBenz1987 said:


> I've noticed recently when I finish a drive (usually a bunch of highway) and get home, while idling in the driveway in neutral the boost reading (via Scangauge II) is usually around -1.8 to -2.0 psi (ie vacuum). I occasionally saw negative readings in the past but it was usually around 0 at idle. It seems to do this after any long drive ever since I had the EGR/cooler replaced. When I first start it up (cold or warm) it's still around 0, or if I just do a short trip or two (which I don't do much of). Just curious if anyone else has seen boost readings drop below 0 psi? I have Gretio as well, which gives a MAP reading for boost (so ~14.5 psi at idle would be about 0 boost). Is it normal to see it below 0 on the SGII or below 14 on the others? My boost readings seem normal relative to what I saw in the past when I'm actually calling for boost.


Yes, seen same, and it is normal. The lower boost at idle seems to be associated with keeping EGTs up for the health of the DPF. Gen 1 car does it much more than my Gen 2s. I would not worry about it. Has your Regen frequency become more typical post EGR and recall?


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## Barry Allen (Apr 18, 2018)

Doesn't boost have a lot to do with how much EGR is being used and how much the throttle is closed to draw that EGR into the intake?

My understanding is that a diesel engine, especially with a VNT, can easily make boost at idle. But given that there is so much EGR used at idle for emissions compliance, the EGR valve is open to draw a lot of exhaust gas into the intake stream and the throttle valve is correspondingly closed to choke off intake air.

I can't think of circumstances where the engine would make boost at idle unless something was going wrong with the engine and/or ECU. The engine is really choked/clogged with EGR at ldle when running "properly" (according to the EPA) and it means that requires a lot of exhaust gas flowing through the engine.


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## BodhiBenz1987 (Jan 13, 2018)

MRO1791 said:


> Yes, seen same, and it is normal. The lower boost at idle seems to be associated with keeping EGTs up for the health of the DPF. Gen 1 car does it much more than my Gen 2s. I would not worry about it. Has your Regen frequency become more typical post EGR and recall?


Thanks, that is reassuring. It does seem like a more controlled action based on the consistency, I just couldn't figure out what the purpose would be. Maybe since I'm coming off the highway with higher EGTs, it tries to keep them that way. @Barry Allen I wondered if it is EGR-related too since it seems to have changed since I got the new EGR ... maybe the clogged EGR resulted in it allowing more intake air and a different throttle position. 
I've had mixed results since the replacement as far as regens. At first it went about 180 miles before a regen, but after that one it went 600 miles, which is awesome and got my hopes up. After doing a regen today it seems to be building very fast again (61% after 39 miles) but I'm still hoping it will level out on the next drive. It seemed like more than a coincidence that it had it's first normal length between regens so soon after the repair but it's so erratic I'll have to just wait and see. My fuel economy seems down this spring, compared to the last two springs, though it's hard to read into that with so many variables (especially this year).


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## Snipesy (Dec 7, 2015)

Barry Allen said:


> Doesn't boost have a lot to do with how much EGR is being used and how much the throttle is closed to draw that EGR into the intake?
> 
> My understanding is that a diesel engine, especially with a VNT, can easily make boost at idle. But given that there is so much EGR used at idle for emissions compliance, the EGR valve is open to draw a lot of exhaust gas into the intake stream and the throttle valve is correspondingly closed to choke off intake air.
> 
> I can't think of circumstances where the engine would make boost at idle unless something was going wrong with the engine and/or ECU. The engine is really choked/clogged with EGR at ldle when running "properly" (according to the EPA) and it means that requires a lot of exhaust gas flowing through the engine.


EGR is made for controlling temperatures, and AFR to a degree. The air from the EGR in the cyllinders isn't that much. At most half but most of the time it is less than 10%.

Throttle is rarely used except under certain conditions during a regen. And in those cases the EGR is almost always closed. Ideally we wouldn't want a throttle valve but it's too difficult to control the turbo in edge cases.


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## Blasirl (Mar 31, 2015)

BodhiBenz1987 said:


> I've noticed recently when I finish a drive (usually a bunch of highway) and get home, while idling in the driveway in neutral the boost reading (via Scangauge II) is usually around -1.8 to -2.0 psi (ie vacuum). I occasionally saw negative readings in the past but it was usually around 0 at idle. It seems to do this after any long drive ever since I had the EGR/cooler replaced. When I first start it up (cold or warm) it's still around 0, or if I just do a short trip or two (which I don't do much of). Just curious if anyone else has seen boost readings drop below 0 psi? I have Gretio as well, which gives a MAP reading for boost (so ~14.5 psi at idle would be about 0 boost). Is it normal to see it below 0 on the SGII or below 14 on the others? My boost readings seem normal relative to what I saw in the past when I'm actually calling for boost.


Most engines are in vacuum at idle. Boost is only associated with forced induction. (I believe that) the turbo isn't being utilized at idle at all. It needs to be spun up to be useful. As for the 14.7 vs 0, it depends on what you are reading. PSI or PSIg.


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## C4r7m4n (Apr 10, 2019)

Diesel engines do not produce vacuum.


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## vwgtiglx (Jun 13, 2013)

My Gen. 1 Cruze diesel always shows a neg. reading on my Scan Guage II right before it goes into a regen. That's what I use as a clue not to shut off the engine until after it goes into the regen or after it completes it.


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## BodhiBenz1987 (Jan 13, 2018)

Blasirl said:


> Most engines are in vacuum at idle. Boost is only associated with forced induction. (I believe that) the turbo isn't being utilized at idle at all. It needs to be spun up to be useful. As for the 14.7 vs 0, it depends on what you are reading. PSI or PSIg.


I know there should not be boost at idle, so it is not the 0 I am worried about, but the negatives (vacuum). My understanding is the diesel engines do not normally produce vacuum (hence the need for a vacuum pump). But I know a lot of my knowledge on diesel engines doesn't apply to this one because new ones have so many more features that my old ones didn't. I know unlike my old ones these do have a throttle valve, though seldom used, so my worry was if there is something malfunctioning with the valve, or anything else.

@vwgtiglx I have heard that about Gen 1s but haven't seen negative boost before a regen or hear others report it on a Gen 2. The last few times it's done it it has not been near a regen, it's just any time I come off the highway and stop and idle. Though it would make sense if it is related to keeping EGTs up like @MRO1791 said ... in the Gen 1 case it's trying to raise them before starting the regen, in the case I am observing maybe it's more of a DPF maintenance thing. My car's bizarre regen behavior adds another layer of concern for me which is another reason I wanted to know if this particular observation is unique to my car, or just something they all do. I.e., I don't want to add an unnecessary worry, but don't want to miss a potential piece of the puzzle.


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## Blasirl (Mar 31, 2015)

BodhiBenz1987 said:


> I know there should not be boost at idle, so it is not the 0 I am worried about, but the negatives (vacuum). My understanding is the diesel engines do not normally produce vacuum (hence the need for a vacuum pump). But I know a lot of my knowledge on diesel engines doesn't apply to this one because new ones have so many more features that my old ones didn't. I know unlike my old ones these do have a throttle valve, though seldom used, so my worry was if there is something malfunctioning with the valve, or anything else.


Hmm. Interesting. I am unfortunately for you, am not well versed on the diesels although I do remember the Brayton cycle, the Otto Cycle, the Diesel cycle and I think the joule cycle from when I had to study diesel generators. Fortunately for me I decided to become an electrician.


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