The Pitstop - 2015-08-03

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3:05 AM

@Paulster2 x2 !!

3 hours later…

6:16 AM

Questions like these: mechanics.stackexchange.com/questions/8052/…

I don't know how to say this without sounding like a jerk, but I don't know if those are useful.

2 hours later…

8:32 AM

Yesterday was a very quiet day. We're down to 9.7 QPD. Time to break out some questions from the sandbox, methinks

On a more positive note, Area 51 indicates 30k+ visitors every day. We're growing

1 hour later…

9:50 AM

wow most car do that when ignition key is turned on not when the door is opened. Wierd volkswagen

0

Q: What is that sound?

I have a 2004 VW Golf Variant. There's no problem, I'm just curious. Just after I have opened the driver's side door, a small motor runs for about one second, then stops. This also happens if I stay seated after stopping the car, and then open the door after some time has passed. I am certain t...

so whenever you get something out of the car your Fuel pump works?

non sense.

@Anarach Yes it does

what made vw come up with that design i wonder

It might seem strange, but then VW appears to have a different design philosophy

Their fuel rail pressure is much higher than other manufacturers, for example

hmmm

@Zaid ... what is VW fuel rail pressure normally?

9:57 AM

@Paulster2 : Trying to find where I read it

@Zaid - I'm not readily finding it.

I have a couple of books at my desk here.. trying to rifle through them but I'm struggling to recollect

@Zaid - And are you talking about the common rail or the high pressure fuel rail?

Or am I just talking out my butt?

Umm, is there a difference?

Probably not.

And assuming you just filed that under "who cares"?

lol

10:07 AM

Lol, because the fuel rail is high pressure, right?

I presume the 'common' is because it's shared by all cylinders

You are right. I'm not awake, so I'll claim ignorance on that bit alone.

Do most VAG fuel systems have two fuel pumps?

low/high pressure?

Don't think so

Grunt. Can't find it

@Paulster2 Although I'm seeing a lot of HPFP acronyms, so you may be onto something

@Paulster2 : There's a YT video which shows 58 psi

VW touareg Audi Q7 fuel pressure and fuel pump flow test DIY by Edge Motors

10:30 AM

@Zaid - I think most of the GM vehicles have been running around 60psi for a while now.

@Paulster2 : Why come to think of it, yes, you're right

I'm not sure if the new direct injected GM engines have a fuel pump which does the HP needs, or if it's done through the injector itself ...

Ze Germans operate a little lower

Around 36 - 45 psi

@Paulster2 : Questions like those are easy to answer with the Bosch Automotive Handbook in front of you :)

lol

I would imagine so ... cheater

Shall I quote it here?

"Compared with injecting fuel into the intake manifold, there is only a limitied time window available for injecting fuel directly into the combustion chamber. Mixture formation is therefore more important and in the case of direct injection, fuel must be injected at a pressure up to 50 times greater than is the case with manifold injection."

That's just a teaser for you :)

But basically in direct-injected setups you have two pumps. One pump bumps up pressure to regular fuel rail pressures (3-6 bar). The other high-pressure pump is where things get whacky (you get both continuous and demand-based flavors, up to 110 bar & 200 bar respectively)

200 bar! That's nuts!

10:44 AM

Consider that you have to overcome cylinder pressure near TDC on the compression stroke, then throw in turbo pressure on top of that ... doesn't seem so whacky now, does it?

My real question was, some DI systems allow the injector to pressurize the fuel, while others use the injectors in a more conventional manner of just allowing the fuel to flow (a HPFP provides the pressure). I'm pretty sure GM's method is the latter, but I'm not sure.

@Paulster2 Nothing in the Bosch book indicates that the injectors pressurize the fuel

That's what pumps are for

But let me see if there is something about DI-style injectors

Nope, a cursory scan does not contradict my last statement. They care more about things like swirl and spray pattern

11:00 AM

@Zaid - that is important.

They are trying to control the burn and avoid detonation. I think most of the direct injection models using the injector to do the final pressurization was mainly the diesels. There would be a "third" cam lobe which would actuate the plunger on the injector, which would squirt the diesel into the combustion chamber.

I think in modern gas (petrol) DI engines, they are all using a secondary HPFP to pressurize, then the injectors just meter the amount going into the engine. 200bar ~= 2940psi ... yah, that's pretty large.

@Paulster2 Yes, that's what the book indicates. The regular pulsed injector is just a butler. A DI injector is still a butler, just a beefier one that can pirouette the fuel

And if I'm not mistaken, it is the ability to swirl the fuel that makes DI worth all this hassle

Oh, great fun.

11:26 AM

@Paulster2 : Any info on TB cleaner?

I just posted something a couple of minutes ago.

3 hours later…

2:29 PM

for some reason i thought compression test figures were only applicable when compared to the readings from the other cylinders?

is that true? or is there an upper limit related to the volume of the compression chamber (or something) ?

@DavidWinslow Every engine has its own figure, but all cylinders should be in the same ballpark

Not like what was shown in the Cobalt forum. If 220 psi is where it should be, poor guy's going to have a tough time

@DavidWinslow - Like @Zaid alluded to, it's both figures you need: % difference between all cylinders; pressure for individual cylinders.

@Paulster2 : Is there a place where we can independently verify what the compression figures should be?

@Zaid - The only thing which may screw with your find a little is which engine does the OP have? If it is the 2.0 turbo version, the pressures could be lower on each cylinder.

I'd have to look on the interwebz to find something.

2:45 PM

@Paulster2 I left a comment for the OP to clarify

I have to say that I'm very pleased with the level of detail provided in his question

Oh, absolutely, which I upvoted and would suggest all do the same.

Looks like you and David have done the same :D

:)

I'm very surprised the mechanic only did the one cylinder, really.

Doesn't lead me to have a lot of confidence in him.

How much does a compression testing kit cost in the US?

The prices I've encountered in my neck of the woods are ~~slightly~~ rather discouraging

If you can do it to one cylinder, you can do it to all cylinders ... if the mech has the compression tester (under $50, I'm sure) he can do it to the rest of the cylinders. Takes at most 15 min tops per, probably more like 10 minutes.

All it costs is a little time. Only thing I can figure is if the OP told him to do one cylinder only ... the #4

2:59 PM

i think i got mine pressure gauge on amazon for like $40

They aren't expensive in the grand scheme of things.

I need a book about car engines. I'd like to know if there is a way to figure out what the numbers should be

when i tested my cadillac they were all between 135-160, i thought it was probably ok

Compression ratio

The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines. In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. For example, a cylinder and its combustion chamber with the piston at the bottom of its stroke...

3:28 PM

@DavidWinslow - Each engine model is going to be different. Even within models you are going to see differences. Take for instance the small block Chevrolet engine. Engines ranged from 263ci to 400ci stock. Every variation (size) might have a slight difference in what psi would be considered "normal".

4:08 PM

@DavidWinslow : What's your Cadillac again?

I remember it's an '81

4:29 PM

@Zaid - It's an '81 Eldorado.

This is the V8-6-4, right?. What's the compression ratio?

@Zaid - According to this page it's 8.2:1 ... really low.

4:50 PM

books.google.com.qa/…

Just confirmed that number myself

That web page is kind of funny ... it has all of the option prices, but doesn't have the MSRP listed, lol.

Well then I wonder how @DavidWinslow is getting 160 psi

Why?

@Paulster2 I'm speculating here but hear me out

If air at ambient pressure (14.7 psi) comes in to the combustion inlet

And volume is reduced by 8.2x

Pressure should go up by 8.2 * 14.7

Which is 120+

I may be grossly over-simplifying here

Remember, 8.2:1 is the static compression ratio ... dynamic is going to be less than that. And ... 14.7psi would be at 100% VE ... I'm sure there are other things in the equation you are not accounting for.

4:58 PM

Both points you make would lower the pressure readout, not increase it, no?

Exactly ... which is why I was suggesting there is more too it than that.

Dynamic is definitely going to be less than static. I think I need to check with another engine

I'm probably wrong. Just not sure why

This is why: en.wikipedia.org/wiki/…

8.2:1 compression ratio, would be more like 20:1 pressure ratio

hey oooo – im back

So, if the engine was at 50% VE @ 20:1, it would be more like 160 psi, right?

With only 140hp, the VE is not going to be great.

A good running engine is around 70-80% VE.

It says the reason is: The reason for this difference is that compression ratio is defined via the volume reduction, while pressure ratio is defined as the pressure increase.

5:07 PM

VE – volumetric efficiency?

Hey @DavidWinslow - We were just talking about your engine. Yes: VE=Volumetric Efficiency

Doh! @Paulster2 I forgot about that! Thanks for restoring my sanity

sounds like you guys have almost worked this out

I didn't know about that (or hadn't realized) until I read that Wiki article.

just finished reading

5:10 PM

It makes sense ... but don't you do that stuff for a living, @Zaid?

This tells me if we know the SCR, VE, and mean air pressure, we could tell what the cylinder pressures should be for a given engine and whether or not they are in good shape?

im having trouble following where cylinder pressure fits in?

We were looking at the question about the guy's cylinder pressure. According to one source it should have been ~220psi, but his reading was 125psi (I believe) for the one cylinder. This gives him, in effect, a very dead cylinder.

@Zaid - There is only one thing which bothers me about the cylinder pressure question: Why would the computer be reading a misfire for it?

how does the computer determine the misfire condition?

Good question.

i haven't owned a car that new >.<

5:17 PM

I'm not sure if it reads it through the knock sensor or some other method.

i would assume there are several paths

Pray, do tell.

im reading that for GM vehicles it follows the crank sensor pulses to determine which cylinder is misfiring

Oh: magnaflow.com/07techtips/TechBulletin/TB80011.pdf

Nice find!

And that would make sense, especially the flashing check engine light.

If the pressure in the cylinder is down by half, I'd suggest we could assume the power output is probably down by 1/2 as well ... (haha, probably not, but it sounds good)

I would think it would be enough to cause the crank to slow down, though.

5:33 PM

Need to tend to family here folks. Will get back to you in a bit @Paulster2

On the S62 misfires are detected throught he knock sensors

Which are essentially acoustic sensors (pick up vibrations)

@Zaid - I'd suggest you read the Magnaflow link which David provided. Seems it may be a little different under OBD-II ... as in a requirement via crank rotation. Not saying it couldn't be done both ways, though.

5:50 PM

it may be a combination on newer engines

the knock sensors on a gm engine are also acoustic sensors

Knock sensors are definitely acoustic, but how misfire is determined is the real question.

@Zaid - If BMW is such a great vehicle, why on earth did they decide they needed to pump a synthesized version of the exhaust note through the speakers on the F10 M5? That has CHEESE written all over it.

6:11 PM

im almost completely certain id prefer road noise over pretend exhaust

As would I, but I don't think I'd turn down 550hp ... that's quite a bit of go for any vehicle right off the assembly line.

I'd find a way to disable the "piped in sound"

I'm watching Top Gear right now ... must be a rerun from several years ago as the M5 was new in 2011.

Which is what Hammond was talking about in this episode.

WAIT! It has Slash in it!

@Paulster2 They lost the plot a while back. Purists aren't happy with the suits at ///M

Haha, that's funny

No worries. Like I said, with 550+hp, I could find a way to disable the feed ;-)

I'm headed out the door, so I'll be back later.

@Paulster2 My lapse is inexcusable, I made an ideal gas assumption

Oh, no worries. I have lapses all the time. I hope you are just as forgiving of me :P

7:03 PM

The Death of BMW's M Brand

The Number of M Logos on the 2015 BMW M3 Is Staggering

7:21 PM

So let's crunch some numbers for the Chevy Cobalt. CR is 10:1. 10 ^1.4 = 25.

14.7 * 25 = 367.5 psi (absolute), so roughly 350 psi on the gauge

220 / 350 is roughly 60% . 125 / 350 is 35%. Not an exact science, but that Cyl#4 pressure is in the wrong galaxy :(

1 hour later…

8:28 PM

sounds like 60% is the golden number for VE

my service book states 135-160 for my engine

@DavidWinslow Take it with a pinch of salt

Don't swear by it

^^

zaid, what do you do for a living? where do you live?

ive been curious for a few days now

You can check my profile

I'm a mechanical engineer

Turbomachinery design

But in reality I'm a ninja

2

so.. almost batman?

im stepping out for a while - see ya

Nearly. The only thing I'm missing is the costume :D

9:04 PM

0

Q: hyundai tucson died while driving

I have a Hyundai Tucson 2011 directed injected diesel which has just broken down. What happened is that while driving the engine suddenly stopped and wont start there was no check engine light on. the engine just cranks but won't start at all. one mechanic told me its the diesel high pressure p...

hyundai fuel-injection

^Funny how we were talking about DI engines and this one turns up

9:17 PM

For those of you already on Windows 10: Sadly, Windows 10 Is Stealing Your Bandwidth 'By Default' — Disable It Immediately

9:44 PM

@Zaid - Thanks for the article on W10 ... glad to see MSoft is keeping up with their usual build schedule (release in Beta so the user can debug, then issue SP1 to patch all the found issues).

9:57 PM

@Zaid - For your equation, you should look at the difference between the two cylinders, not the overall % after your math. Percentage difference between 220 & 125 ... which is a whopping 57% difference between the two. In most cars you are looking at no more than 5-10% difference. I'd definitely say there is an issue here.

10:17 PM

Windows 10 is what pushed me to go full-time arch linux

what kind of answer do you guys think i would get for a question about diagnosing a possible lean condition?

is that too broad?

Not too broad as long as you give specifics. "Possible" lean condition ... put why you believe you have a lean condition ... flesh the question out over on the sandbox

good idea. ill write it up a bit later

10:50 PM

@DavidWinslow - I'm not doing it at my current position, but my favorite job I've done in the past is being a UNIX SysAd. I've dealt with SunOS, Irix, DecAlpha (whatever DEC's version of UNIX is, lol, I don't remember). If I get my wish and I'm able to retire from the Government next year, I'll probably go back to being a SysAd again if I can find the right work.

ive hated windows since the first time i saw it

which is a mean thing to say

I don't mind using Windows ... I hate trying to do admin on it though.

but i stand by it because there is always a reason i have to be locked in to using it

if i wasnt forced to use it, i wouldnt have strong feelings about it

If you're a good Windows admin, you've always got a job.

IMO bad ones too ^^

IME**

10:51 PM

Same goes for UNIX or Linux as well (on the job front)

yah, I've seen a lot of bad (or not good at all) Windows admins ... we don't call it the unhelpful desk for nothing.

hehe yeah

Must be time to go put a bedroom back together. TTYL and have a great evening.

See ya!

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