# 17536 or p01128 Long Term Fuel trim - lean mixture



## emefef (May 12, 2014)

Long post - but I tried to cover everything I've done before people just post "it's this" or "get a that" - I've already done a lot of this and that!
We did a lot on this 2001 Mk4 2.0 AEG to get it going last summer, and it ran great after we got everything done, but lately we've been getting codes for a lean mixture.

The long term trim goes right to the maximum 25% and then we get the codes (usually both short term and long term). We replaced both oxygen sensors about 10,000 km ago when we did everything else (*new coil, plugs, wires, engine speed sensor, temperature sensor*). For the most part, it runs smoothly at idle.

With the lean code, my first search was for a vacuum leak. I couldn't hear one, or see obviously bad hoses, and couldn't find one with a propane torch.
Since* I couldn't find a vacuum leak*, and I had no idea how old the MAF was,* I installed a new MAF* because it was one thing we didn't do at the beginning. 

With the codes cleared, I've monitored and* graphed the O2 sensor voltage and fuel trim at idle and while driving*, and everything looks good. Short term trim varies usually between barely negative and only 3 to 4% positive, occasionally a little more but well within spec (-10% to +10%). I had it running for an hour, idling, then driving around the neighbourhood, then idling some more, then along the highway - no changes. 

My son drove it for a day and half and on the check engine light came back on. We checked it again, and sure enough the long term trim was bumped right up to 25%. At the time of the code, the coolant temperature was 78 degrees, fuel system was in closed loop and the load was 36.9%.

Everything I've read seems to lead to vacuum leaks or a bad MAF, but replacing the MAF with a new one made no difference and I can't find a vacuum leak. An intermittent vacuum leak makes it even worse.

I started to think it's got to be fuel, rather than air. The* fuel pump is is clearly working*, so I* checked pressure and it was all within spec* - almost 36psi at idle, jumps to 44psi with the fuel pressure regulator disconnected, holds 29psi for at least 10 minutes after shutting it off (*the regulator is new, too*). I decided I'd replace the fuel filter just for good measure, and found that one of the hoses wasn't fitted all the way on - it pulled off when I tugged on the filter. I figured if the pressure was okay, this probably didn't mean anything but after putting the new one in and ensuring the fuel lines were secure, the car ran way better. My son (who had spent the day driving my A4 Quattro for comparison) even commented that it drove better when he took it back.....but he came home last night with the CEL on and short and long term trim codes again!

I cleared the codes, started the car and the short term trim was still at 25%. I stabbed the brake a few times, and everything settled down. O2 sensor voltages were good, trim was up and down above and below zero....and idling smoothly.

So is it the changes in vacuum caused by stepping on the brakes that's making a difference, or a component of the brake system itself? I wonder because long before I checked the fuel pressure and replaced the filter, I was able to create the problem while driving, and catch it before it threw the code. 
On a graph, it's very clear: RPM coming down from fairly high, and the short term trim starts to climb, right up until it hits 25%. Seeing that, stabbing the brake pedal a few times corrected it, and the long term trim wasn't affected. I was able to do it a few times, but not consistently. Letting off the throttle to decelerate, braking lightly (braking may or may not have been part of it, but I think it was), the short term fuel trim climbs. So It doesn't happen at idle but it happens when vacuum is highest and the throttle is closed - then pumping the brake pedal a few times corrects it. Obviously if we're not monitoring it at the time we can't see it, then the long term trim will start to be affected.

There are no problems with vacuum for braking - they always have plenty of power. If the check valve in the booster hose was intermittently bad, it shouldn't make a difference, should it? That's the direction it's supposed to work - the engine can draw vacuum, but the check valve will hold it when engine vacuum is low. It seems to be working. My other thought is the seal between the brake booster and master cylinder - perhaps it's a weak point and when there is too much vacuum, air gets drawn in. 

Am I missing something obvious?


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## Anony00GT (Mar 6, 2002)

This is easier diagnosed with VCDS, as VW doesn't handle fuel trims the same way domestic makes do. On VW (and most other German cars), LTFT is trim under load (multiplicative), STFT is trim at idle (additive). On generic OBDII, they will bounce around because of the generic translation from the scanner, which can (and it seems like it is) throwing your diagnosis off. VCDS will give you a solid adapted number for both in block 032, which is what you need to see.

17536 is a code for lean under load, not at idle. This is not a vacuum leak. 99.9% of the time, it's caused by a bad MAF. Is the new MAF an OE one from the dealer, or is it aftermarket/online purchase, etc? VW's are very sensitive to aftermarket MAF's (and O2 sensors), the only way to be 100% sure that the new one is good is to get it from the dealer (you definitely get what you pay for). O2 sensors should be direct-fit genuine Bosch from a reputable supplier (not online) or OE from the dealer only.

I'm thinking this is a MAF issue though. Graph the MAF reading (g/s) and go for a ride. What is the max g/s achieved during a WOT blast to redline in 2nd or 3rd gear?

The other thing I have seen cause this fault is an exhaust leak. Even a pinhole leak in front of the rear sensor can cause this (yes, the rear sensor does affect fuel trimming). Carefully check the exhaust system as well.


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## emefef (May 12, 2014)

Anony00GT said:


> This is easier diagnosed with VCDS, as VW doesn't handle fuel trims the same way domestic makes do. On VW (and most other German cars), LTFT is trim under load (multiplicative), STFT is trim at idle (additive). On generic OBDII, they will bounce around because of the generic translation from the scanner, which can (and it seems like it is) throwing your diagnosis off. VCDS will give you a solid adapted number for both in block 032, which is what you need to see.
> 
> 17536 is a code for lean under load, not at idle. This is not a vacuum leak. 99.9% of the time, it's caused by a bad MAF. Is the new MAF an OE one from the dealer, or is it aftermarket/online purchase, etc? VW's are very sensitive to aftermarket MAF's (and O2 sensors), the only way to be 100% sure that the new one is good is to get it from the dealer (you definitely get what you pay for). O2 sensors should be direct-fit genuine Bosch from a reputable supplier (not online) or OE from the dealer only.
> 
> ...


Thanks! I was monitoring blocks 1, 32, 33, which is what I was graphing. I know I already read about the differences you've described here (multiplicative and additive) but forgot totally about it during my troubleshooting. The MAF was an online purchase, so I'll check the numbers and I'll graph the g/s on a blast up the highway. O2 sensors are both Bosch direct replacement. I don't think there's an exhaust leak before the rear sensor, but I'll check that as well.
Any idea why pumping the brake pedal would correct the fuel trim?


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## Anony00GT (Mar 6, 2002)

I doubt that pumping the pedal has any real effect on trim, unless there's a booster problem. You can watch the upstream O2 sensor voltage at idle, operating temp. If it goes lean (low) when the pedal is pressed and switches normally without the pedal pressed, suspect a leak in the booster.

That wouldn't cause a lean multi code though. I suspect exhaust leak or MAF.

Since you're using VCDS, post up a full auto-scan and values in 032.


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## emefef (May 12, 2014)

Anony00GT said:


> I doubt that pumping the pedal has any real effect on trim, unless there's a booster problem. You can watch the upstream O2 sensor voltage at idle, operating temp. If it goes lean (low) when the pedal is pressed and switches normally without the pedal pressed, suspect a leak in the booster.
> 
> That wouldn't cause a lean multi code though. I suspect exhaust leak or MAF.
> 
> Since you're using VCDS, post up a full auto-scan and values in 032.


I did check the MAF, and the replacement is the exact Bosch sensor that was on there. I didn't get a chance to drive it last night but I'll post a scan when I can. Pumping the pedal does change it - but it may be irrelevant if I'm looking at the wrong number! It's the first field in block 032 or 033, but I wouldn't swear to which one until I try it again. After I've cleared all DTC and started the car, and the value is still at 25% - then pumping the brake pedal resets it. 

Thanks for the feedback. I'll get better details.


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## Anony00GT (Mar 6, 2002)

I'd be very suspect of any MAF that didn't come from the dealer. Just because it says Bosch on it, does not mean it IS a Bosch. I can't tell you how many times I've run into "Bosch" sensors from unreliable suppliers that were really just reman'd Bosch housings with junk internals.

To watch O2 sensor voltage, just click OBDII mode and select Bank 1 Sensor 1 from the drop down menu.


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## emefef (May 12, 2014)

*Updates*

Having already checked the fuel pressure I knew I still wouldn't be satisfied until I rule out a fuel pump problem - so I rigged up a switch and tested the volume. I didn't even need to worry about the battery vs pump voltage on the graph - it easily squirted out between 650 to 700 ml in 30 seconds.

I get your point about the MAF and genuine replacement parts. After I checked the fuel volume I took it out for a spin last night up to 6k rpm in a couple of times. I was graphing it so I wouldn't need to watch while I was driving, but I did see numbers in the high 80's g/s in the measuring block field. Good thing too, because when I sat down after, I grabbed the corner of the graph to stretch it and make it bigger - and (everyone else probably knows this already) the graph window gets bigger, but refreshes and there's no data.

Intermittent problems are the WORST!

The kid bought an '07 323i that he picks up tomorrow. I'll have to decide how much I really care now about fixing the Golf. I was really hoping to have it fixed, because it was last safetied and emission tested in August - only about 15,000 km since then. I may have been able to sell it that way if I could fix this.


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## Anony00GT (Mar 6, 2002)

I'm thinking your MAF is no good. 2.0's usually hit mid-90s with a good MAF. You won't see this in any service manual, I say this from years of experience diagnosing bazillions of MAF/lean issues on VW (and verifying associated repairs). That little bit is just enough to throw a code.

Verify that there are no exhaust leaks first....not even a tiny pinhole.

Where is the auto-scan? What are the readings in 032 after all the monitors are set and the car has been driven awhile?


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## emefef (May 12, 2014)

He picks up the new BMW today so I'll have more access to the Golf tomorrow night for testing. I work days, teach some weekends and evenings, and my son is often out when I'm home - so it's not that I'm not motivated to get the right data, I just have to get to the car! I really appreciate the feedback.

This data is from another post before I got my VCDS (right when this trouble started). The data is saved from a generic scanner when the CEL came on as I was driving:
_Description, Value and Units, Min, Mean, Max.
Fuel Rate, 7.62 litres/hr, min 1.11, mean 3.66, max 15.51
Instant fuel economy, 7.05 litres/hr, min 0, mean 11.01, max 1.95
Mass air flow rate, 22.9 g/s, min 3.26, mean 10.63, max 46.65
vehicle speed, 108 km/hr, min 0, mean 28.54, max 111
Calculated load value, 25.1%, min 3.14, mean 11.55, max 50.2
Engine coolant temp, 86 C, min 81C, mean 82.67C, max 86C
Short term fuel % trim, 25%, min 0, mean 12.98, max 25
Long term fuel % trim, 25%, min 0, max 6.68, max 25
Engine RPM, 3248 RPM, min 779, mean 1577, max 3765
Ignition timing advance #1 cylinder, 28.5, min -3.5, mean 13.34, max 30
Absolute throttle position, 19.61%, min 1.96%, mean 9.66%, max 52.94%
Commanded secondary air status, Atmosphere/off
Oxygen Sensors present, Bank 1: Sensor 1, Sensor 2
O2 Voltage (Bank 1, Sensor 1), 0V, min 0V, mean 0.03V, max 0.66V
Short Teerm fuel trim (bank 1 sensor 1), 25%, min 0%, mean 13.89%, max 25%
02 Voltage (Bank 1, Sensor 2), 0.04V, min 0v, mean 0.35V, max 0.86V
Short term fuel trim (Bank 1, Sensor 2) 99.22%, min 99.22%, max 99.22%, max 99.22%_

At that point, the MAF sensor was reading something, but based on a lot of the posts about MAF readings, mine seemed low, and that's why I replaced it. But when you look at the RPM, on that drive I only got up to 3765 and the MAF rate got up to 46.65. I was also worried here about the O2 sensors, but once I got the VCDS and started tracking voltage there, it seems much more consistent. Both O2 sensors only have about 10,000 km on them.

I'll do a full scan on the weekend, and check for exhaust leaks - you mean in the exhaust manifold or more accurately anywhere upstream of either sensor, right?


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## Anony00GT (Mar 6, 2002)

If Sensor 1 really is maxed at 0.66v, either the sensor is bad or there's a vacuum leak.


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