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High fuel trims. Bad ECU or what?

Help, Advice, Owners' Discussion and DIY Tutorials on S40 and V40. In this forum you'll find S40/V40-specific owners asking and answering questions on maintenance, ownership, repairs, tutorials and almost every do-it-yourself thing you can do to save money owning these Volvos.

1996 - 2004 S40
1996 - 2004 V40

jakeb123
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High fuel trims. Bad ECU or what?

Post by jakeb123 » Mon Jun 10, 2019 4:54 am

I want to preface this by saying I've posted this problem many times on many sites, and nobody has managed to help me.
I really wish to get the bottom of this.

My volvo is S40 2002, it's petrol non-turbo variant. I have high Long Term Fuel Trim %. It's 10% but has jumped to 13%. It doesn't matter if I drive longer or shorter distances.
I know what you must be thinking. It's the O2 sensor. I'm not sure, the car still has the original one, so it has seen over 200k miles on it.

I have an OBD2 scanner and managed to graph it. Can anyone confirm whether this is a normal operation of the upstream oxygen sensor?

From what I've seen, the values should be continous and vary between 0.1 and 0.8v, but you can see on the graph, that is not the case. It turns on, works for a bit and turns off.

Is there anyone with a S40 between 2001 and 2003 that can graph his voltage so I can compare?

What are the symptoms? Well I have no rough idle, and I can't notice any particularly bad fuel consumption, but if the ECU is adding more fuel, it's definitely affecting gas mileage.

Now, I do have oil burning. My PCV is clogged and it's causing really excessive vacuum and whistling, the vacuum is so strong that trying to take off the oil cap is difficult. I was wondering if this oil burning could be the reason for the fuel trims? I have taken off the O2 sensor and cleaned it with carb cleaner, the next day I took it off and saw that it was pitch black again. However the graph of the voltage is really weird, could it also be a bad ECU? I have studied O2 sensor a bit, and they generally have two categories, Zirconia and Titania. The difference is, that Zirconia-type oxygen sensors produce voltage, whereas Titania-type need voltage from the ECU and have a variable resistance based on oxygen content. My volvo uses the Titania-type.

So I have a few potential suspects. PCV system clogged, vacuum leak, bad ECU, bad oxygen sensor.

Some websites have suggested dirty fuel injectors, so I bought and used two additives but to no avail, they were Liqui Moly Jectron and Bardahl - Fuel Treatment Gasoline. They did nothing.



jakeb123
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Re: High fuel trims. Bad ECU or what?

Post by jakeb123 » Thu Jun 13, 2019 5:12 pm

Self-bump, but after replacing my tires with brand new ones and cleaning the MAF sensor with a special spray, my LTFT dropped to 5%, now this isn't still 0 or even 1-2%.
However, I think the rest is caused by the clogged PCV. As excessive vacuum builds up, air is sucked in from the cam seals or anywhere it can from, bypassing the MAF, and since it's unmetered air, the O2 sensor compensates.



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alschnertz
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Re: High fuel trims. Bad ECU or what?

Post by alschnertz » Fri Jun 14, 2019 2:04 pm

I'm confused.
You state "My PCV is clogged and it's causing really excessive vacuum and whistling, the vacuum is so strong that trying to take off the oil cap is difficult."
Usually a failed / clogged PCV system results in excessive crankcase pressure .

If your PCV system needs repair, do that first.


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Re: High fuel trims. Bad ECU or what?

Post by jimmy57 » Fri Jun 14, 2019 3:28 pm

On an engine with wear the fuel trim is not going to be near 0. I work on high mile cars all the time with fuel trim values of 10% and the crankcase vent and other stuff is working properly. The emissions warning will alert if it gets near 25%.



jakeb123
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Re: High fuel trims. Bad ECU or what?

Post by jakeb123 » Sat Jun 15, 2019 4:00 am

alschnertz wrote:
Fri Jun 14, 2019 2:04 pm
I'm confused.
You state "My PCV is clogged and it's causing really excessive vacuum and whistling, the vacuum is so strong that trying to take off the oil cap is difficult."
Usually a failed / clogged PCV system results in excessive crankcase pressure .

If your PCV system needs repair, do that first.
I've booked a day for the mechanic. But yes, that is exactly what is happening. Very strong vacuum occurs, accompanied by whistling. Presumably the whistling is from air entering from anywhere it can, bypassing the MAF. Nothing else should cause this but a failed PCV system/valve.

Just yesterday when the whistling occurred, I had my OBD2 dongle plugged in, and saw that my airflow had decreased a bit, and STFT was nearly 50%. However as the O2 sensor was compensating, my engine idle was fine at 750 RPM. I can stop this whistling and strong vacuum by just moving the car forward a bit.

Also, my LTFT had dropped to 2.45% but then increased to 3.13%. Still better than 13% that it was before.



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