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VW MKIII-A3/B4 TDIs This is a discussion about MKIII-A3/MkIII Jetta/Golf (<99.5) and B4 Passats (96,97) TDI's. Non TDI related postings will be moved or removed.

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Old September 14th, 2016, 06:32   #1
fortec
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Default Losing mpg on 1998 Jetta Tdi AHU

Hello,

I had recently replace the seal on my injection pump.
Have my intake manifold cleaned out and I also did the diesel purge.

I have seen my mpg going from 47mpg to about 38mpg.

I wanted to know if I should do the injection quantity IQ adjustment via VCDS (I have the VCDS Lite and an ebay blue plug).
My IQ is at 4.6 mg/str. I understand that the IQ should be between 3-5mg/str.

What is the limit that I can do as far as adjusting the IQ with VCDS?

Should I also do the hammer mod?

Would doing these help with my mpg?

Thanks to ALL for the help.
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Old September 14th, 2016, 07:23   #2
thechoochlyman
 
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Replacing the seal shouldn't have changed your MPG any. Are there any restrictions in the intake? Air filter? Are the brakes dragging?

I like running the IQ as low as I can get it before it starts loping, just because lower IQ = more fuel. 3 is absolutely safe, I run mine at 2.0 with no troubles.

VCDS will only adjust your IQ within a range of about .6 from what I recall.

You'll have to do the hammer mod to get your mg/str down to 3 if it's at 4.6 right now.

No fuel mods should affect your MPG unless you drive more aggressively with it - and of course, you will. But a hammer mod won't affect it enough to notice.
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2.5" Straight-pipe exhaust CCV/EGR Deleted Malone Stage 3, 2.0 IQ Autometer Gauges Billet Stuff
Billet Coolant Flanges - Idler Pulleys - CCV plates - Window Guide Clips
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Old September 14th, 2016, 07:52   #3
KLXD
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Did you set it to what it was before you had the pump apart?

A while back I had an increase in mileage. The IQ reset after the seal change was the only thing I could see that might have caused it in spite of the above post. No, it wasn't leaking that badly.

For some unknown reason I didn't think to get the IQ before I opened the pump. I've been meaning to mess with the setting to see if it was the cause but haven't gotten around to it.
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Current: 02 Jetta, Auto; 98 Jetta, 5 Spd; 98 Dodge, 5 Spd, SB, 4x4; 84 Grand Wagoneer with Nissan SD33T, NV4500, Dana 300, Reverse Cut Dana 44, Dana 60

The Black Sheep (Only gasser): 85 CJ, 4.2 w/4.0 Head and Mopar FI.

Past: 85 Mitsubishi PU, 4D55T; 81 Rabbit, 1.6; 80 Dasher, 1.5; 79 Rabbit, 1.5
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Old September 14th, 2016, 08:45   #4
fortec
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Quote:
Originally Posted by KLXD View Post
Did you set it to what it was before you had the pump apart?

A while back I had an increase in mileage. The IQ reset after the seal change was the only thing I could see that might have caused it in spite of the above post. No, it wasn't leaking that badly.

For some unknown reason I didn't think to get the IQ before I opened the pump. I've been meaning to mess with the setting to see if it was the cause but haven't gotten around to it.
Unfortunately, I didn't have access to VCDS prior to doing the ip seals.
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Old September 14th, 2016, 08:50   #5
fortec
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Quote:
Originally Posted by thechoochlyman View Post
Replacing the seal shouldn't have changed your MPG any. Are there any restrictions in the intake? Air filter? Are the brakes dragging?

I like running the IQ as low as I can get it before it starts loping, just because lower IQ = more fuel. 3 is absolutely safe, I run mine at 2.0 with no troubles.

VCDS will only adjust your IQ within a range of about .6 from what I recall.

You'll have to do the hammer mod to get your mg/str down to 3 if it's at 4.6 right now.

No fuel mods should affect your MPG unless you drive more aggressively with it - and of course, you will. But a hammer mod won't affect it enough to notice.
4.6 mg/str means 4.6 milligrams per stroke (I think). That means the injection amount into the engine per stroke.

So, how can lower IQ means more fuel. If I were to adjust this to 3 mg/str, it means it would be 3 milligrams of fuel per stroke. That means less fuel per stroke.

Am I right? Please correct me if I am wrong.
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Old September 14th, 2016, 08:58   #6
thechoochlyman
 
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Quote:
Originally Posted by fortec View Post
4.6 mg/str means 4.6 milligrams per stroke (I think). That means the injection amount into the engine per stroke.
So, how can lower IQ means more fuel. If I were to adjust this to 3 mg/str, it means it would be 3 milligrams of fuel per stroke. That means less fuel per stroke.
Am I right? Please correct me if I am wrong.
That would make sense, but for whatever reason a lower IQ value = more fuel. I'm not really sure why it's measured and programmed that way, but perhaps someone smarter than me can answer that question.
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1997 Passat 5 speed - No longer bi-tone
2.5" Straight-pipe exhaust CCV/EGR Deleted Malone Stage 3, 2.0 IQ Autometer Gauges Billet Stuff
Billet Coolant Flanges - Idler Pulleys - CCV plates - Window Guide Clips
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Old September 14th, 2016, 09:23   #7
fortec
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Quote:
Originally Posted by thechoochlyman View Post
That would make sense, but for whatever reason a lower IQ value = more fuel. I'm not really sure why it's measured and programmed that way, but perhaps someone smarter than me can answer that question.
If that's the case, then lower IQ (like me ) would translate into lower mpg right?
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Old September 14th, 2016, 20:23   #8
KLXD
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The ECU sets the IQ at idle to maintain the correct idle speed; doesn't matter what you set the IQ number to. It thinks it's some number a normally adjusted pump will be.

If you set the idle number to, say 3, instead of what the normal number should be, say 5, you've recalibrated the pump. ECU thinks the feedback it gets is for the normal 5. Now off idle the ECU is going to set the QA according to its programing but because of your recalibration the amount injected will actually be higher.

This inverse relationship only applies when a number before adjustment is compared to the same number after adjustement. Once an adjustment is made, from that point on a higher IQ does mean more fuel than a lower.
__________________
Saying no to gas for 25 years:

Current: 02 Jetta, Auto; 98 Jetta, 5 Spd; 98 Dodge, 5 Spd, SB, 4x4; 84 Grand Wagoneer with Nissan SD33T, NV4500, Dana 300, Reverse Cut Dana 44, Dana 60

The Black Sheep (Only gasser): 85 CJ, 4.2 w/4.0 Head and Mopar FI.

Past: 85 Mitsubishi PU, 4D55T; 81 Rabbit, 1.6; 80 Dasher, 1.5; 79 Rabbit, 1.5
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Old September 15th, 2016, 07:28   #9
fortec
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Quote:
Originally Posted by KLXD View Post
The ECU sets the IQ at idle to maintain the correct idle speed; doesn't matter what you set the IQ number to. It thinks it's some number a normally adjusted pump will be.

If you set the idle number to, say 3, instead of what the normal number should be, say 5, you've recalibrated the pump. ECU thinks the feedback it gets is for the normal 5. Now off idle the ECU is going to set the QA according to its programing but because of your recalibration the amount injected will actually be higher.

This inverse relationship only applies when a number before adjustment is compared to the same number after adjustement. Once an adjustment is made, from that point on a higher IQ does mean more fuel than a lower.
So, is my 1998 Jetta AHU Tdi IQ of 4.6 mpg/str correct? As should be within spec of the engine with economy?

Thanks,
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Old September 15th, 2016, 08:16   #10
thechoochlyman
 
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It's supposed to roll out of the factory between 3 and 5, so it's fine. If you're just wanting best fuel economy, you can leave it there. If the engine feels a little too anemic for you, bringing it down to 3 or slightly below is fine.
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1997 Passat 5 speed - No longer bi-tone
2.5" Straight-pipe exhaust CCV/EGR Deleted Malone Stage 3, 2.0 IQ Autometer Gauges Billet Stuff
Billet Coolant Flanges - Idler Pulleys - CCV plates - Window Guide Clips
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Old September 16th, 2016, 12:25   #11
tdidieselbobny
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Make sure you check all your wheels after driving to see if any are hot-you may have a caliper dragging or the parking brake not releasing fully.
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Old September 16th, 2016, 15:03   #12
fortec
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Quote:
Originally Posted by thechoochlyman View Post
It's supposed to roll out of the factory between 3 and 5, so it's fine. If you're just wanting best fuel economy, you can leave it there. If the engine feels a little too anemic for you, bringing it down to 3 or slightly below is fine.
I read my tdi with vag-com today and IQ is at 4.0 mg/str, sometimes 3.8.
I loosen the 4 bolts (including the triangle nut) to do the hammer mod.
For some reasons, I hammering (toward the battery) and the IQ did not budge. I did lose the nuts of the pump cover as below to the level of slight hand tighten only.

Am I missing something? I wanted to bring it down to 3 mg/str.

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Old September 16th, 2016, 15:47   #13
KLXD
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Only the triangle bolt on the top cover. The other three are on the lower flange. Two are visible in that pic at about 7 o'clock from the lower right and the upper left bolts.
__________________
Saying no to gas for 25 years:

Current: 02 Jetta, Auto; 98 Jetta, 5 Spd; 98 Dodge, 5 Spd, SB, 4x4; 84 Grand Wagoneer with Nissan SD33T, NV4500, Dana 300, Reverse Cut Dana 44, Dana 60

The Black Sheep (Only gasser): 85 CJ, 4.2 w/4.0 Head and Mopar FI.

Past: 85 Mitsubishi PU, 4D55T; 81 Rabbit, 1.6; 80 Dasher, 1.5; 79 Rabbit, 1.5
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Old September 17th, 2016, 13:49   #14
fortec
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Checking in...

I was able to play with the right torx bolts, which became stripped, to loosen the pump to reduce the IQ. I brought the IQ down to 2.4 mg/str, the car has no problem idling. I will have to drive it to see if it has any effects.

The stripped t-27 torx bolts were replaced with M6x25mm allen bolts of same metric threads which I have in my tool box. The new allen is a about 3mm shorter but it is holding very well since there is enough threads in the body.
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