Help with P1128

[Mr_Eddy]

Here's the story.

2000 1.8T with about 125K miles.

MAF went bad last week (verified by unplugging).

Replaced MAF. Now I have the CEL and a P1128 code. Seems to run fine, but a little weak, which makes sense if it's running lean. No stumbling, hesitating, stalling or anything.

Checked all the usual suspects. Can't find any leaks in the intake system. Can't find any cracked hoses. All the vacuum lines (replaced them all last summer) are in good shape and connected.

So what else can I look for?

O2 sensor bad?
Low fuel pressure?

Is there a way to check these without replacing them?

Thanks
Ed

[Mr_Eddy]

Another bit of info.

It seems there is little, or no turbo boost.

[Mr_Eddy]

Anybody have any suggestions?

I need to get this figured out. I have a series of road trips coming up where I'll be driving from Cedar Rapids to Milwaukee starting 17 March and running the following four weeks going home on weekends.

I may have to resort to taking him in to get it diagnosed if nobody can help.

[Scarab 1]

MAy I ask what you replaced your MAF with? New or used? Brand? and were the codes cleared after you did it?

You can check the O2 with Vag-com but the Fuel Pressure someone else would have to chime in on....

[pinky]

http://wiki.ross-tech.com/index.php/17536/P1128/004392

that site is every dubbers' friend.

[Mr_Eddy]

Originally Posted by Scarab 1

MAy I ask what you replaced your MAF with? New or used? Brand? and were the codes cleared after you did it?

You can check the O2 with Vag-com but the Fuel Pressure someone else would have to chime in on....

Replaced with a rebuilt Bosch. The CEL did go out after replacing the MAF, but came back on after about a day.

[Mr_Eddy]

Originally Posted by pinky

http://wiki.ross-tech.com/index.php/17536/P1128/004392

that site is every dubbers' friend.

Why haven't I found this before now.

Thanks for the link.

[Mr_Eddy]

I was poking around a bit this morning to see if I missed something obvious.

I pulled the DV to check the hoses more carefully in that area. I gave the DV a once over and was able to blow air from the intake port through to the vacuum connection. This obviously isn't right. My local VW dealer happen to have one in stock. (only $60. Not too bad for a dealer)

With that replaced, I see no noticeable difference. Still runs OK, but no power. The CEL is still on showing the same code. It does stumble a bit under hard acceleration.

Put my vacuum gage on today and it shows 20 inches at idle. (what's normal?) Odd thing is, it doesn't drop when RPM's go up. Just stays steady at 20.

So I must have a vacuum leak somewhere. I'm going to look at it again in the morning to see if I can spot the leak. I'm thinking maybe in the brake booster area. I didn't replace any vacuum lines in that area when I did the engine top ones last summer.

Any other places I should be looking?

Thanks
Ed

[johnmichael]

Originally Posted by Mr_Eddy

Put my vacuum gage on today and it shows 20 inches at idle. (what's normal?) Odd thing is, it doesn't drop when RPM's go up. Just stays steady at 20.

20 ish inches is about normal. As far as it not going towards positive pressure, have you messed with your wastegate rod at all?

[Mr_Eddy]

Originally Posted by johnmichael

20 ish inches is about normal. As far as it not going towards positive pressure, have you messed with your wastegate rod at all?

Haven't messed with anything. The car is completely stock.

So if 20 in. is about normal, that would be holding the DV open constantly. That would account for the power loss.

[johnmichael]

I'm wondering if your wastegate is stuck open. That's the only way I can see you not building positive pressure with what you've said.

On second thought, are there any other weird noises? Like air getting out where it shouldn't? I'd pull your front passenger tire off and look at your intercooler/throttle body piping... Wish you were closer dude, I'd take a look at it.

[Mr_Eddy]

So today, I pulled the air filter box and really looked at all the vacuum hoses.

I found the hard plastic hose that connects to the brake booster is cracked at the booster, and the larger diameter formed hose right out of the intake that connects to the hard plastic booster hose was cracked and collapsing.

Replaced both of those with a temporary section of bulk hose. Power is now much improved and I can once again feel the turbo boost kick in. It does stumble a bit around 5K RPM and it seemed to "pop" a bit as well.

Still have the CEL with the P1128 code.

Another question. How does the Pressure Regulator Valve work and what is it suppose to do? I had it off today while poking around and noticed it was just wide open between the ports. This is probably the way it's supposed to be, but just want to be sure.

I think the problem of the vacuum not going positive I previously listed may have been due to where I was checking the vacuum. I had the gage on the hose that goes to the DV. I realized today that the small valve that hose connects to probably only shuts off the vacuum under load so the DV will either fully open or fully close. I put the gage back there today and was able to make it go to 0 by stabbing the throttle quick. Vacuum works as expected right out of the intake.

I don't think the intercooler piping is the problem. I had all that off last summer when I did the timing belt and water pump. I don't hear any leaky air sounds anywhere.

So I need to figure out why it's running lean. I guess I'm going to have find a VAG-COM around here and take some one for a ride.

Thanks
Ed

[Mr_Eddy]

Just a little update.

Changed the fuel filter yesterday. (is just me, or do you not trust those funky little clips?)

CEL is now off and for the most part, he's running pretty good. I do have a hesitation problem when accelerating hard (seems to be a common problem after doing a bit of searching here) around 3500RPM. At half throttle it pulls pretty good, beyond that it almost feels like the ASR is kicking in. (no dash light though)

I changed the plugs about 35,000 miles ago. Probably time to do that again. There's about 10,000 on the air filter. I don't know of anybody in the area with a VAG-Com, so I can't do that right now. Maybe I'll just create my own.

I'm a little suspicious of the fuel pump. I don't suppose there's an easy way to check that? What about coil packs? Any other thoughts? The car has 128,000 miles.

Getting there, but not quick enough.

Thanks
Ed

[Too Cold NB1.8T]

You may or may not have had a bad MAF sensor but unplugging it on these cars in order to diagnose them will not work, these vehicles have redundant systems and by unplugging say the MAF sensor, the ECU will trigger an EPC dash warning and maybe a CEL then use other sensors and sources to obtain required information, the drivetrain and systems will operate on a sub-level pending proper repaires.

It is not uncommon for the car to run differently when sensors like the MAF is unplugged, it may even seemingly run better, however, the diagnosis process is not that simple on these OBDII vehicles, part or all of your vehicle symptoms are more likely related to the damaged pressure or vacuum parts that you have described, sounds to me like there are some issues still yet to be uncovered.

This problem could be the result of a bad fuel pump, FPR, additional vacuum/pressure leaks, failed system components etc., I would do whatever it takes to get this resolved quickly, driving a vehicle (especially a forced induction vehicle) with a lean condition could end badly.

[Mr_Eddy]

Originally Posted by Too Cold NB1.8T

You may or may not have had a bad MAF sensor.......

Thanks. I was a little suspicious of the "Unplug to Check" procedure. But there are many references to it here so....

But, I figured with almost 130,000 miles on it, it may time to replace it anyway.

I think I'm ready to take it to local dealer here that specializes in European cars and has a good, reputable shop and have them do a full diagnosis to try and pin point the problem.

Ed

[Mr_Eddy]

So today I took the car to the local dealer/shop I mentioned. (By the way, I've got to give credit to European Motors in Cedar Rapids, Ia. A nice little dealer with nice little shop. They'll let you lurk around the shop while they work so you can see what's going on. Plus they only charged half hour of labor when they really worked about 45 minutes.) A test drive later with their diagnostic scanner attached showed the MAF is reading consistently low. They said they have occasionally seen a bad re-manufactured MAF.

So, back to parts store where I got it. They will exchange it on warranty. I'll have the replacement tomorrow. Hopefully that will be the end of it.

Originally Posted by Mr_Eddy

Thanks. I was a little suspicious of the "Unplug to Check" procedure.

Now, not to contradict myself, but see if this makes sense to anybody else. Note that I'm not pulling some of this info out of thin air. I'm a Test Engineer and I create programs that deal with sensors on a daily basis. Plus the company I work for also creates applications for ECU's. It's our primary business. Mostly offroad vehicles (tractors and construction) but the basics are all the same. While I'm far from an expert on the ECU applications, I do know the basic concepts from having to test them.

While I agree with Too Cold that unplugging the MAF is not a fail proof diagnostic step, I think it has its merits. Consider this. (caution, this might get wordy)

This basic principle is true for most sensors.

The MAF measures the amount of air that is passing by. This air flow is converted to either small voltage or current signal by the sensor. The level of this voltage or current varies depending on how much air is passing by the sensor. This signal is read by the ECU and in software, is converted back to it's representative units. We'll say this particular one gets converted back to CFM (Cubic Feet/Minute).

So let's say at idle, the MAF is outputting a voltage that corresponds to 2 CFM. (these values are just made up) At idle, you are normally pulling about 4 CFM. The ECU is being told that only 2 CFM are getting past the MAF, so it instructs the fuel system to inject enough fuel for 2 CFM of air. If you're actually pulling that 4 CFM, you now are running lean and that pesky P1128 code pops up.

Now let's consider what happens when we unplug the MAF.

The ECU is no longer getting the MAF signal, so reverts to using alternate signals and/or other information that is programmed into the ECU to provide backup functionality. Most likely, this involves reading things throttle position, ground speed, engine speed, gear, air and engine temperature. It uses these values to approximate what the air flow should be for the current state of the system. It then calculates how much fuel is needed and commands the injectors appropriately. While not as good as reading the actual air flow, and not as responsive, it will not leave you stranded.

So here's what I observed today.

The shop showed me the MAF reading is consistently below what it should be regardless of engine speed and load. Assuming the engine is pulling 4 CFM and the MAF is only saying it's 2 CFM, the engine is going to run lean. It does, and hesitates under a WOT acceleration. I unplugged the MAF again today just to see what happens. While acceleration isn't what it should be, I get a nice smooth acceleration without any hesitation or stumbling. This points to the MAF not outputting the correct signal.

Again, this is not a fool proof troubleshooting method. But I think it's a pretty good indication that the MAF may have problems. It probably merits getting it looked at with a tool that can show its actual output instead of just an error code.

Just my $.02
Ed

[Too Cold NB1.8T]

Your theory makes a lot of sense and is well constructed, the problem is that when the MAF sensor is unplugged which completely removes it from the equation, the ECU is forced to follow a secondary course in order to maintain some level operation, it is like a train headed to the same destination by way of the second most efficient route, it still makes it but not as efficiently.

With the MAF sensor unplugged the car will run but differently and less efficiently, even VW technicians don't unplug this sensor in order to diagnose them, man, I often wish that newer cars were as easy to work on as the very old school cars.

[green02turbo]

Actually your engine wont run lean after a little bit of driving because it is constantly changing it's fuel tables according to Oxygen sensor voltage also. It's when your fuel table gets to far away from the predetermined points for injector pulse width and RPM or load that the cel illuminates and you get your code 1128 saying "Hey! im way off my normal fuel map table!!" Anyway usually a code will set of the fuel trim gets more than 25% either way. I'd say either you have another bad maf or you fuel system is restricted (ie plugged injectors, low fuel pressure,) or you have an o2 sensor thats not reporting correctly ( very unlikly though because o2 sensors will flag codes of their own) good luck

Some MAFs will output a frequency instead of Voltage or current also.

[Mr_Eddy]

So far, (fingers crossed, knocking on wood) it looks like the new MAF has done the trick.

Installed it and cleared the codes. I've driven about 250 miles with the new one and no CEL. Power feels about right and no hesitation.

When it warns up a bit more, I'm going to spend a day searching for any remaining vacuum hoses that I haven't replaced. I also have an idea to replace the brake booster hose with something better. I don't trust the one there now not to kink.

Originally Posted by Too Cold NB1.8T
I often wish that newer cars were as easy to work on as the very old school cars

I'll second that. I miss my '70 Mustang Mach 1 and '86 GTI.

Ed

[billymade]

Here is a good page you might like to check out; it has allot of good info about the 1.8T: http://forums.vwvortex.com/zerothread?id=2666114
Here is a DIY to test for vacuum leaks: http://forums.vwvortex.com/zerothread?id=2692757
more testing:http://forums.vwvortex.com/zerothread?id=1605131

The first post has a ton of good info!