Simplify IQ plz? I read a lot, nothing makes sense!?!

guillaumeber

Vendor
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Aug 26, 2012
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Canada
TDI
Jetta 2000 TDI
Hello,

I'm trying to setup my car for best economy, I don't care about performance.
The 2 basic settings I read about were IQ and injection timing.

I got VCDS, managed to adjust timing in the middle of the top half of the graph. All this is fine.

My problem is with the IQ. I read a lot, especially on the well reputed ''myturbodiesel'''s how-to.

I REALLY don't get it. They insist that increasing the IQ will give less fuel. Lowering IQ would increase fuel delivery. Well...I get confused because the IQ is in mg/stroke, so my perception tends to think that more milligrams/stroke means more fuel injected. I finally decided not to try to understand why and go like they say.

But,

While adjusting the IQ to a higher number (near 4.4), I saw in a measuring block on the VCDS that increasing the IQ does, in fact, RAISE the consumption in ''litres / hour'' in one of the box. So my questionning came back... how the hell more L/hour consumed results in less fuel injected...!?


Suddenly, I got hit by my old physics classes, and I got confused more than before!

To run at a steady RPM, any rotating mechanism needs a fixed amount of ''added'' energy to overcome the forces of friction sustained by the different parts.

With this in mind, the engine should need a fixed amount of fuel to keep it's steady 903 RPM idle speed. Adding fuel should result in more RPM, and vice-versa.

All this mesmerizes me, and I can't untangle that puzzle. I tried reading but the only explanations I can find are either very unclear, or on the other side, requires to be an astrophysicist, which I am not.

If anyone can manage to sort this out, I'd be moreeee than happy!

Thanks a lot!

G.b.
 

KLXD

Top Post Dawg
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Lompoc, CA
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'98, '2 Jettas
I'll try it again.

You're right; 5.0 IQ means more fuel than 4.5. Turn the AC on at idle and the IQ goes up. More load, more fuel required, higher IQ.

The lower IQ = more fuel relationship only applies when comparing the fueling at idle before the hammer mod to the after condition.

As you said. Idle requires a certain amount of fuel, let's say 5.0. If you hammer the QA so it reads 4.0 at idle you've made a mechanical recalibration and programmed in a 1.0 offset. It's still idling at 5.0 since the load is the same but the ECU sees 4.0.

Now off idle when the ECU wants a certain IQ the pump provides 1.0 more.
 
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guillaumeber

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Canada
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Jetta 2000 TDI
Thanks for the explanation, I might start getting it, still a bit confusing but well...!

I'll just trust you more saavy people, so putting my IQ up, should reduce the effective injection quantity while cruising, and thus, saving a bit of fuel, am right?
 

mk3

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Sep 13, 2005
Location
Wisconsin, USA
TDI
03 Jetta GLS 5-speed
The reason it doesn't make sense is because everyone explains it differently and most of us are making educated guesses.

One thing you should know is that this system cannot control how much fuel is injected (accurately). The pump and nozzles were designed with certain mechanisms (cams, springs, holes) and then it was characterized (measured) by the engineers and they mapped out how much fuel comes out under certain mechanical conditions.

Another thing to know is that there must be a certain amount of air to burn the fuel (or extra air, which doesn't do the same harm it would do on a gas engine).

The amount of fuel at idle will be precisely what is required to keep the engine running at idle speed, no more no less because this is the control loop that is operating, more rpm == less fuel, less rpm == more fuel.

Meanwhile the pump is also giving feedback to the engine computer and this computer is tell us how much fuel it * thinks * is being sent in. this is not a measurement of actual fuel it is an indication of the maps and tables that the engineers figured out a long time ago.

If the computer * thinks * more fuel is going in, then it will put more air in to match this. Because of this if the IQ reads 6.0 at idle, the ECU will push in more air. if the IQ reads 4.0 at idle then the ECU will push in less air. Meanwhile under both conditions the amount of fuel going in is just what is required to turn the engine and it's light load. (maybe it is actually 5.0 mg ??) but that does not matter.

You cannot really trick your engine into using less fuel than it needs for a load but you can make sure that it gets sufficient air.

It gets more complicated when we put different (usually larger) nozzles in the engine because the measurements and maps are not correct anymore. We have to trick the engine into sending more air by tricking it that more fuel is going in. (Hence, my IQ at idle was raised up by hammer mod when I installed larger nozzles)
 

jmodge

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2001 alh Jetta, RC2 w/.205's 5speed daily summer commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/.216's winter cruiser, 1996 Tacoma ALh
I always thought the hammer mod was to center the IQ range in relation to the default number due to wear or something. More to it than that I guess, so after replacing seals, marking the casting, coming up with a lowIQ at idle (below 2), I set the default number and modded it to about 6. What is my ECU seeing while accelerating or cruising? The need for more air?
 

mk3

Veteran Member
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03 Jetta GLS 5-speed
... I set the default number and modded it to about 6. What is my ECU seeing while accelerating or cruising? The need for more air?
yes, relatively because I think the 'standard' number is lower than that. however for your car it's possibly correct : ) You've got non-standard nozzles?
The standard expected IQ reading at idle is only valid for stock pumps, injectors and nozzles at nominal conditions. Also, if you have a tune then imo the IQ reading should be coming out in the correct nominal range (not high or low) assuming the tune was done correctly.
 

jmodge

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2001 alh Jetta, RC2 w/.205's 5speed daily summer commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/.216's winter cruiser, 1996 Tacoma ALh
Had 502's but were having issues, while swapping from cylinder to cylinder to see if readings followed injectors or stayed with cylinder I noticed the pump leaking. Also noticed injectors dry if removed immediately and wet if set for awhile (suspect they leak, but that is for another thread). I threw a set of stock MT's in it by the way until I get around to checking the other's out.

So I changed IP seals the other day which brought me to hammer modding to bring the IQ back up because of where the QA lined up upon reassembly. I would have to check my notes, but previous to all this IQ was raised from the original tune because of smoke. Which I think was due to injector issues.

But at any rate, I had to hammer mod originally to get the engine to "lean out" and reduce smoke under acceleration. VCDS showed injector imbalance, much better with stock nozzles, but less power of course.

Maybe I am getting to the point I am off subject and belong under a different thread now
 

BobnOH

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May 29, 2004
Location
central Ohio
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New Beetle 2003 manual
Leave it at 4.4. You're over thinking it. If you must know, search up the white paper for the VE Injection Pump. It's not like a mixture screw on a carburetor, once the motor is motoring the ECU will change it dynamically
 

jmodge

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yes, relatively because I think the 'standard' number is lower than that. however for your car it's possibly correct : ) You've got non-standard nozzles?
The standard expected IQ reading at idle is only valid for stock pumps, injectors and nozzles at nominal conditions. Also, if you have a tune then imo the IQ reading should be coming out in the correct nominal range (not high or low) assuming the tune was done correctly.
Ok, if I am understanding this correctly, I should contact my tuner to see what the IQ should be reading at the default setting?
 

AndyBees

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(I agree with the OP. This subject has baffled me too.)

So, to stir the pot a bit .............

A cold engine just fired up will show a 5 to 8 IQ depending on where it is set and temperatures (ambient, fuel and coolant). As the engine begins to warm-up the IQ number will drop until full operating temp is achieved. So, as the IQ number (as observed in VCDS) falls, isn't fueling actually leaning-out?

Alluding to Mk3's comments about adding air.... Adding more air can only be done by the Turbo's response to the ECU, at idle or otherwise. I believe we all agree on that. So, at idle (903 RPMs), one has to assume for the ECU to manage more (or less) air then it tweaks the Turbo ever so slightly. Interestingly, the Turbo Pressure Gauge I have mounted for the ALH in my Vanagon also shows vacuum. At idle, I see a very slight vacuum. But, I do realize the measurement of that gauge is not ideal. So, it may or may not show those slight tweaks of the Turbo pressure in maintaining the 903 RPMs at idle...
Make sense?
 
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jmodge

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Actually Andy, you stating you were baffled in another thread, is what got me thinking deeper. I figured if I guy who swaps an alh into something it didn't come from and drives it all over the country is baffled, I should be also.
 

jmodge

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I am going to go sit in the woods for a few days, then come back to see what is cooking. This is getting interesting!
 

AndyBees

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Jmodge, I need to sit in the woods too....

Right now, the ALH in my Vanagon has me baffled worse than anytime previously. Not to high-jack the thread. But, I had to have it towed about a month ago. The Accelerator pedal went dead..... but, there are other things amuck too. It's back to the hard-starts. And, on top of that, I've not had time to dig in and see what's going on. I'm thinking grounding as the symptoms are very, very similar to what I experienced back from day one to about 18 months out.

Yeah, there seems to be nothing logical about the IQ setting for warmed-up idle. I understand that fueling is leaning out as the engine warms-up. There's no doubt in my mind about that. Also, the consumption in Liters Per Hour begins to drop as the engine warms-up. But, why is a 2.2 or lower on IQ considered over-fueling. Way back when my 2000 Jetta begin to develop the slow-down shudders (whatever it's called), I never saw any smoke. I did up the IQ to about 3.2 from 2.0....

Oh, and now, my 2000 Jetta, at about 370k miles, belongs to my son... new TB, filters, tranny oil change, etc.

I now have an 03 Silver at 252k miles with an 01 engine at 302k miles. I know the history on both the car and engine...

To the OP..
Well, I guess, just set the IQ where it's recommended by VW and drive conservatively for better fuel economy ... Oh, and don't forget to a WOT two or three times per tank of fuel to exercise the VNT mechanism on the Turbo and blown-out the Inter Cooler.
 

mk3

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Wisconsin, USA
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03 Jetta GLS 5-speed
(I agree with the OP. This subject has baffled me too.)
.....
A cold engine just fired up will show a 5 to 8 IQ depending on where it is set and temperatures (ambient, fuel and coolant). As the engine begins to warm-up the IQ number will drop until full operating temp is achieved. ......
This makes perfect sense. A cold engine likely has more load on it for a variety of reasons including the fact that the oil has higher viscosity. The IQ reading is high at first due to the higher load and then reduces. I think the instructions for checking idle IQ specify a warmed up engine, accessories off and ambient air 70F to try to normalize all of this.
 

guillaumeber

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Jetta 2000 TDI
Well, this thread took a bit of time to get some replies but it's starting to be very interesting!
Mk3, can you clarify just how can the ECU adjust the entering air? Only way I can imagine is the N15 valve, adjusting the turbo. Is that it ?
 

AndyBees

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Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Help me out...

I've looked high & low and cannot find an N15 Valve.
 

mk3

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03 Jetta GLS 5-speed
Well, this thread took a bit of time to get some replies but it's starting to be very interesting!
Mk3, can you clarify just how can the ECU adjust the entering air? Only way I can imagine is the N15 valve, adjusting the turbo. Is that it ?
yes, the amount of air is adjusted by the turbo control actuator. I don't imagine there is any adjustment at idle but under load it is.
 

eddieleephd

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Battle Ground, Wa
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2002 jetta Wagon
IQ as we set it is solely to tell the ECU how much fuel is being injected by a specific nozzle set.
The ECU uses this value to adjust the quantity that it injects while running.
We have to increase the IQ with larger nozzles to decrease smoke.
If the ECU thinks it has smaller injectors that are only injecting 3mg/stroke and they actually inject 5mg/stroke it'll smoke like crazy! So we adjust the IQ by trial and error until the smoke is within tolerable quantities.
The hammer mod is physically changing the IQ and works better. Often people do hammer mod first then computer set to fine tune it.
IQ means Injectcted Quantity, as said earlier the computer is dumb and needs you to tell it the base line to start. With the proper base line the maps should work well, however, improper I will cause either too much smoke, or not enough power.

"why is a 2.2 or lower on IQ considered over-fueling."
Because the ECU thinks the injectors are only capable of injecting 2mg of fuel per stroke at the specific pressure and adds more than it should.
Let's think about it this way. You think you have 2 and need ten, so you add 8. In reality you have 5, you add 8 and now you have 13. You're over by 3mg/St and causing excess fuel to be injected.
It's all about fluid dynamics. At 5psi let's say 4 litres can pass through a 3mm orifice, however, you can get 8 litres through a 6mm orifice. If you run the same psi and have a larger orifice, however, don't change the pressure you end up with double the Quantity desired. Thus you're over fueling
 
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