boost gauge

[newbie]

Great site been on all night reading about hesitation and fault codes. Anyhow i'll try and keep it short. I've got hesitation and back firing, splutering when i boot it, 2nd gear ok but soon as it's in 3rd foot to the floor it goes wrong until i back off but by then it's all over.
From what i've read i think it might be plugs. What i did notice but thought nothing of it was my boost gauge when flat out it goes up to just short of 1bar about .8 but when decelerating it goes way below 0(zero) is that right or am i loosing boost somewhere.
Thanks

[legacy92ej22t]

It's totally normal for the boost gauge to read vacuum when off throttle.

[newbie]

Thanks for replying
I put the same gauge on my turbo diesel VW Type2 and it never went below zero, this is what led me post this question

[555BCTurbo]

Hey...there is a reason that your gauge didn't go below zero on your diesel...because diesels don't have any manifold vacuum

[THAWA]

why is that?

[vrg3]

Because they're not throttled.

[azn2nr]

yep and 1 bar gets pretty close to fuel cut

[THAWA]

so how does it work then? what controls the air going in?

[vrg3]

Spark ignition engines always have to run at more or less stoichiometric air/fuel ratios. The throttle is a driver-controlled restriction of air into the engine, which controls how much air gets in. The carburetor or fuel injection then meters an appropriate amount of fuel.

Whenever you have flow past a restriction, you have a pressure drop. This is what creates a vacuum downstream of the throttle body of an SI engine.

Diesel engines can run at a wide range of air/fuel ratios. They have done away with the barbaric intentional suffocation of the engine and instead allow as much air as possible into the engine at all times.

The power output is controlled by the quantity of fuel injected. The accelerator pedal is either connected to a physical linkage that regulates the quantity of injected fuel, or a potentiometer that serves as the input to a computer that regulates the quantity of injected fuel. The fuel is always injected into a cylinder that is completely full of pressurized air, and the pressure starts the burn.

So at idle the air/fuel ratio might be as lean as 75:1. Lots of air, very little fuel. Of course, that makes for very little power, which is why the engine remains at idle. The more fuel you inject, the more power the engine produces.

When you got on it, mixtures still should stay leaner than stoichiometric. Diesels work best when there's a nice excess of air. Without that excess, they tend to smoke some and and otherwise don't perform very well.

I know of at least one guy who slapped a turbo onto his diesel Benz and was sorely disappointed that the performance stayed exactly the same despite almost a bar of boost.

Incidentally, this is what makes diesel engines sometimes scary when they're in machines that work on natural gas pipelines. If you break one of the pipes and the engine starts ingesting the gas, it can start accelerating uncontrolledly until you somehow plug the intake. Bad blowby can also be problematic, since a diesel engine can run on its blowby fumes.

[THAWA]

That's awesome!

[vrg3]

Yes. Rudolf Diesel was an awesome man.

[THAWA]

Wow no joke, a solar powered air engine! his first working diesel ran at a 75% effeciency! That's just amazing. If he could have continued developing engines, think of where we could be right now.

[BAC5.2]

Everytime Vikash says something, I get smarter. I had no idea that was true of diesels.

I guess I'll have to inspect my neighbors cars and poke around.

So Diesels have no throttle bodies? You are just controlling the amount of fuel, and that's it? Sick.

[vrg3]

Yup. Throttles are for spark-ignition. Compression-ignition motors can run very lean because what we SI folk call "knock" is SOP on a diesel.

On my friend's 98 Jetta TDI, fuel injection is controlled by the ECU. Under certain failure conditions, the fail-safe limp-home mode is to fix engine speed at a certain value (I wanna say 2000 RPM) at all times. So you just have to use the clutch and brake pedal to get you where you're going. It's funny.

The car will also happily idle up a 30-degree incline.

[Kelly]

Ya. In fact Diesels have vacum pumps, to run the brake booster, and other gismos that require vacum.

The VW motors pump is run by the intermediate shaft, which normally turns the ignition rotor. So in place of a distributor, theres a pump there. Pretty funny lookin.

Diesels rawk.

[Innovative Tuning]

Ya. In fact Diesels have vacum pumps, to run the brake booster, and other gismos that require vacum.

On a related note, those vacuum pumps are great for providing the brake booster with vacuum on turbocharged gasoline burning vehicles when you're left foot breaking.

Our shop car runs with no brake booster at all so the pedal feel and response are consistent under all conditions, but the vacuum pump is a more streetable solution.

-Mike

[vrg3]

I wonder if they'd work well to evacuate the crankcase instead of using PCV.

[BAC5.2]

Why do Diesel's have MAF's though?

My neighbor has a 2002 or 2003 VW Jetta TDI, and not only was the TDI MAF recalled, but he said that when it wen't bad, the car drove like shit.

What were the certain failure scenarios? Do you remember?

[vrg3]

TDI ECUs use the airflow measurement to determine how much fuel they can safely inject without causing smoking or other bad things.

When the TDI MAF sensor goes bad, you can actually replace it with a simple voltage divider that sends a constant airflow signal to the ECU and get it to perform okay again.

I don't remember the failure scenarios that triggered that failsafe mode. Certainly a damaged "throttle" position sensor would be one, but I don't know what else.