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Discussion Starter · #1 ·
Hi,

a week ago I changed on my S80 V8 valve cover gaskets. A bit complicated but I was able to do it. When I was finished, connected VIDA (after very long time) and checked if everything is ok.
But live data wasn't good - especially STFT and LTFT. Vida shows from 85% to 115% on both banks.
I checked fuel pressure - it was OK. Also checked for any intake leaks, but nothing (btw. if I press accelerator and do higher revs (1500, 2000, 2500) there is almost no change. And absolutelly no change in way of lower fuel trims. Engine reacts when I open oil cap... So I think there won't be any intake leaks.
Idle rpm are normal - about 650-670.
Also tried some new MAF, but numbers are still the same.
Also checked exhaust manifolds, after front cats there are new gaskets, no leak there. But above I don't know (between cylinder and manifold). But I think, it would be possible to hear such leak so close to the cylinders... Engine is quiet.

What else to check? Any ideas?
Thanks for advice.
 

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I had the same issue, and it turned out that one of the 2 downstream o2 sensors was giving an incorrect reading. Have a look at the o2 sensor read-outs, and graph the output. One bank would cycle every second but the other one would cycle every 3 (slow to read). I ended up just leaving it as I didn't have an CEL and no miss-fires.
Have a look at your and let me know what you get. I'm currently at 178K miles
 

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Discussion Starter · #4 ·
I had some free time today so I tried to spray carb cleaner around intake. But without any changes. Engine still runs normally, without any rpm drop or increase. So I think my intake system is not leaking. As I thought thanks to fuel trim reading on VIDA. We know, that on N/A engines is vacuum biggest on idle and on higher RPMs are getting lower. Thanks to this fact it should be also numbers of fuel trims lower on higher RPMs ( higher revs -> smaller vacuum leak -> richer mixture = better fuel trim numbers). But this not happened.

I tried to change also upstream o2 sensors. Numbers were almost the same, maybe even little higher. With those new sensors there were also CEL and code P017100 and P017400 - system too lean, banks 1 and 2. With old sensors no CEL. I think it is caused by the fact, that older sensors are not so sensitive anymore.
I know about downstream o2 sensors that they are pretty old as well. Readings are no as I expect. But still no DTC. But who knows, can be one of possibilities.
Do you have improved fuel trim numbers now? I mean, when you replaced your downstream sensors...?

My spark plugs are white - like new. And I'm afraid of big heat in cylinder when it runs on lean mixture (especially on higher revs, which I don't use now, because I want to save my engine).
 

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I watch a lot of this guy's videos. He's pretty funny, but also pretty smart. Watch this, it explains a lot on how the 02 sensors would work, and goes through the charts and logging data, etc...


https://www.youtube.com/watch?v=CflIvd5VOoA

This makes me think though, my downstream o2s go up and down, and apparently they aren't supposed to. I hope I don't need new converters.
 

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You might want to drive it for a few days so fuel trims can adapt to work with the new 02 sensor before making any additional hardware changes. You might have had bad fuel trims that had been a result of slow reacting 02 sensors or trims resulting from faulty readings. Now with the new sensor you might have to let the ecu respond to more accurate readings to have adaptation walk back the old fuel trims and get them where they need to be. You might throw some CELs along the way but give it a few days and log/ scan again. Come to think of it, rather than having to adapt from the current state of tune, I’d also unplug the car battery for a while to clear the old state of adaptation and then start from scratch with adaptation working with your current fresh hardware.
 

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You can also reset the engine adaptations in Vida


Sent from my iPhone using Tapatalk
 

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Discussion Starter · #8 ·
Yes, I have done it, of course. But... even after reseting adaptations (and disconecting battery as well), the new values were too high. When I checked them, everyone of that adaptations (after reset, with new o2 sensors) were just to high. And as VIDA says - mixture too lean.
And I think, that even with new o2 sensors and high long term fuel trim... There should be normal values for short term fuel trim immediately, I think...
Tomorrow I'm going to do final vacuum leak test with propane. Just to be 101% sure.
There is one thing I realised, when I last time scaning with VIDA - there are some misfires. Does not feel it on idle, but between 1400-1900rpm it is pretty significant. And misfire counter in VIDA also shows some numbers for EVERY cylinder. Spark plugs are new. Coils aren't. I'm thinking, if this high fuel trim is not caused by these misfires. Because unburned mixture means more oxygen in exhaust as it should be. Or maybe misfires are caused by that lean mixture... Who knows.
 

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Clean maf sensor. I had problems on my 3.2 with system to lean on of the banks and after cleaning it no more codes. After replacing something you have to give time to adapt.
 

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Discussion Starter · #10 ·
After long time I can tell you the news. I have cleaned MAF sensor as you adviced. Then checked diferences between real and calculated values in VIDA. Differences are lower than before. But fuel trims stay still pretty high. About +80, +90%. Also tried disconnect battery.
I think it cannot be vacuum leak - such high fuel trim numbers would caused in really bad engine run and I also think it would be massive air leak - pretty noticable hissing noise. And there is not such a noise. And engine runs good (except some rpm range I mentioned about)
So what else to check? Take a look on those misfires? Try new ignition coils?
 

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After long time I can tell you the news. I have cleaned MAF sensor as you adviced. Then checked diferences between real and calculated values in VIDA. Differences are lower than before. But fuel trims stay still pretty high. About +80, +90%. Also tried disconnect battery.
I think it cannot be vacuum leak - such high fuel trim numbers would caused in really bad engine run and I also think it would be massive air leak - pretty noticable hissing noise. And there is not such a noise. And engine runs good (except some rpm range I mentioned about)
So what else to check? Take a look on those misfires? Try new ignition coils?
Misfires can definitely be caused by a fuel delivery issue or anything else that causes it to run excessively lean. If it was a coil pack issue you’d likely have very violent misfires, not the subtle ones that would register as a mild hiccup or which might trigger a CEL but be undetectable to the driver. If you can log, I’d try to measure requested fuel pressure vs. actual fuel pressure. If you can’t log, replace the fuel filter and consider that your fuel pump may be on its way out, especially if the car has upwards of 120k miles on it...that’s when fuel pumps start to get tired. It might take a while for it to fail completely but during that lead up time you may notice a subtle decrease in power and the occasional fuel cut during gentle driving, and possible misfire CELs. At least those are the subtle symptoms I experienced when the in-tank pump started going on my VW GTI after about 130k miles. I replaced a lot of other parts trying to fix what ended up being the fuel pump, because there was no way to know for sure the exact cause of my issues, other than symptoms of low pressure, slightly reduced power at WOT, and subtle cuts periodically, but nothing major enough that it was noticeable 99% of the time.

How many miles on your car?
 

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Discussion Starter · #12 ·
I checked fuel pressure as well. VIDA says, that values about 480 kPa is OK. I have almost the same (+ - 2 kPa). So fuel pressure is not problem. Btw. car has LPG system, and problems appears when running on LPG and petrol as well. So we can exclude any "fuel issues".

I have put my old upstream O2 sensors back to the car. And you know what? No more P0171 and P0174. This shows only with new sensors (probably because they are more sensitive than olds..?) It shows only one DTC - P0420 and P0430 - efficiency below treshold. That was the original reason why I decided to buy new upstrem O2 sensors.
And one more thing I noticed: if I do kick down and keeping the throttle on floor, on very high RPMs (more than 5000) CEL comes. Misfiring on cylinder 5 and 8. Sometimes cylinder 6... But during normal drive I have no such CEL.
 

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Hi,

Does anybody know what is the difference between the two fuel pressure sensors PN 31272732 and PN 31272730 ?
In different sources i found different reference for V8 engine.
Thank you in advance !
 
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