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OBDII Throttle Positions (Confused about scanner output) / Both banks rich

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Old 03-09-14, 08:20 AM   #1
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Question Fuel trims both bank rich by approx. same amount.

I have been trying to diagnose a low negative fuel trim problem that exists equally on both banks of my 06 GS300 for a while now. For the most part my MAF readings seem OK. I am unsure exactly what my fuel pressure should be, they are all over the place - the only reported value is the fuel rail pressure... but that is another topic.

Right now I am trying to figure out if the throttle position sensor (or something else) is out to lunch. The car doesn't have any drivability problems, it isn't at the point that it is throwing any codes, but I do have negative fuel trims that seem the worst at idle/low rpm/low engine load up to -20 on the long term for both banks.

There are two throttle positions that the scanner is giving me.

1) Throttle Position
2) Absolute Throttle Position B

My throttle position seems to read what I expect (for the most part) starting as a low percent with foot off the accelerator idling and increasing with accelerator pressure.

It is the Absolute Throttle Position B which I wonder about. Hot idle, it starts off at like 50% (!).. and slowly creeps down to something around 45% when fully warmed up. I highly doubt that the throttle plate is actually 50% open at 600 rpm. I have yet to try to visually inspect it but....

First of all, does this sound wrong... does this almost definitely point to a bad TPS? unfortunately on this car TPS is not a separate part it comes all or nothing with the rest of the throttle body so "replacing it" means replacing the whole throttle body.

Second of all, does this reading support the fuel trim issue? Logically...

1) MAF reads X amount of air which is correct (assume) and feeds to ECU.
2) Throttle place opened the correct amount for accelerator position closed, letting correct amount of air in for engine speed.
3) TPS sends wrong signal to ECU saying the throttle place is opened maybe 30%+ more than it really is.
4) ECU uses TPS input to do (what exactly?) something that results in a richer fuel mixture.
5) AF ratio sensors see too little oxygen in exhaust, and lean out the fuel injection to compensate (thus far within the very wide "acceptable" range).

It makes sense in my head, but what I can't reconcile is the conflict between the MAF and the TPS in the above scenario. Shouldn't throttle position almost always be directly related to the MAF reading? The larger the throttle position opening, the higher the mass air flow...

If somebody could point out where my logic has gone wrong please do. I have no problem ordering a junker throttle body and installing it myself but I don't want to just make wild guesses.

Car only has around 70,000 miles on it, throttle body has been cleaned by a mechanic a year ago or so though I have no idea what method they used.

I am hoping to rule out causes of the negative fuel trim being the actual AF ratio sensors or fuel injector problems just because it is on both banks pretty equally. Could be too high of fuel pressure, but I really want to rule out the easier fixes like MAF and throttle body first.

P.S. I have access to the diagnostic range specifications for this engine and one mysterious thing in the table is:

Throttle Position No. 2
Min.: 0 V, Max.: 5 V
2.0 to 2.9 V: Throttle fully closed
3.2 to 4.8 V: Throttle fully open

It does not specify what PID this is supposed to be, but if it does happen to be the PID that is coming out of Torque as "Absolute Throttle Position B" then... the reading in percentage does make sense. 2.0 to 2.9V does indeed fall into the range of "fully closed" out of the 0-5V range. Of course, at idle, the throttle body would not be fully closed... and 50% (2.5V) is right in the middle of what they refer to as fully closed so again I am confused.

I am less and less inclined to think there is a TPS problem because there are no codes, and it seems that there is a good failsafe for detecting problems with it built in:

"This sensor, which is located in the throttle body, detects the position of the throttle valve. By using a Hall element, in the same way as the
accelerator position sensor, this electronic position sensor enables accurate control and ensures permanent reliability. In addition, this sensor
consists of a dual system having different output characteristics to ensure reliability."

LOL, you know sometimes you just need to say something out loud or type it out to figure it out yourself... I think I understand now... one value reported by the scanner is "VTA1" which is one of the "dual system outputs" mentioned above and the other is "VTA2" which are DESIGNED to have different outputs as a sanity check for eachother. It appears VTA2 is DESIGNED to output a higher voltage than the main one and the two relative values are compared by the ECU. My scanner (Torque/Bluetooth adapter) is not smart enough to know what the voltage being output is, so it just reports it as a percentage leading to confusion.

Well, saved myself from buying a throttle body... but still have no idea why the car is running rich. Maybe I'm back to square one on the MAF even though cleaning with contact cleaner did nothing.

Last edited by BinaryJay; 03-21-14 at 01:27 PM.
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Old 03-09-14, 05:47 PM   #2
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I'd have to do some research into the throttle body, but I might be able to address the rich condition...

first off, is the car 100% stock?

secondly, what has made you look into this to start? you said it runs ok, - any symptoms whatsoever?
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Old 03-09-14, 06:15 PM   #3
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Originally Posted by mitsuguy View Post
I'd have to do some research into the throttle body, but I might be able to address the rich condition...

first off, is the car 100% stock?

secondly, what has made you look into this to start? you said it runs ok, - any symptoms whatsoever?
Thanks. Yes, 100% stock car. I began to look into it because I get an intermittent rear O2 sensor high voltage code on one bank which I think is unrelated as it seems to get set shortly after going through car washes though I can't figure out where the water is getting that is causing a short in that case. So that's what got me to finally just buy a working BT OBDII adapter/torque because I wanted to start logging everything to see if I could catch the voltage either creeping up or just suddenly spiking during a water event. I just happened to notice the high negative trims and started to try to diagnose it since it seems they are too low to be "normal" while not quite low enough yet to actually set rich codes.

As for symptoms, no, driving around is fine except for maybe a slight delay that occurs if I quickly put the pedal to the floor between the pedal going WOT and the car actually kicking into a lower gear and going. Normal driving around is fine as I would expect since the ECU is still able to lean out the mixture enough.

The fuel trims do normalize out depending on engine speed it seems, where at high engine speed the long term trims go back closer to zero but then shoot right back into low negatives when dropping back to idle. This does make me start thinking about causes that could be linked to vacuum but it's a returnless fuel system so there is no vacuum controlled fuel pressure regulator (it appears pressure is cam controlled where the cam directly runs a secondary pump faster or slower when the engine speed changes), and a vacuum leak would cause positive fuel trims and not negative ones unless I am thinking completely illogically about this.

Only other thing would be that my wife complains about the smell of gas from the exhaust particularly on cold starts but it is winter and I'm not sure if it is related to the trim or just normal. Leaking fuel injectors? Would it not be very unlucky to have leaking on both banks of approximately equal amounts? Maybe a badly leaking cold start injector could do it since it feeds into surge tank but I have never heard of a cold start injector going bad.

Any thoughts to bounce around would be appreciated.

I never really thought about the cold start injector until I wrote this post -- it does make sense in a lot of ways to fit into the puzzle. At high engine speed the amount of fuel coming from the cold start injector even if it is leaking or stuck open would be less of an issue for the air/fuel ratio than at idle. Fuel leaking from the cold start injector should affect all cylinders (obviously). It is also fuel that the ECU does not anticipate or monitor, so it would cause the air fuel sensors to see a rich condition and lean things out to compensate. Maybe I am just fooling myself though since this part is easy to get to and doesn't cost a lot.

So an additional two things to consider are:

1) Is it (relatively...) safe to remove the injector just to see if it is dribbling? I assume yes, if the engine is off - but I'm not sure if the fuel system is pressurized in the key on engine off condition - well I have the starter button. If the fuel system isn't pressurized before ignition is actually started then, well, removing it at that point would introduce a huge vacuum leak I guess I could have a rag handy to jam into the hole it leaves.

I imagine I could even remove it with everything off and just smell it or smell inside the hole, if it is leaking fuel I imagine it's going to stink like gas where it normally wouldn't.

2) If it comes to replacing it, am I better off *shudder* taking it to a shop to do so... I have not had to work on the fuel system on any car ever and I'm not even sure where to start when it comes to depressurizing things ahead of time and making sure that things go back together again properly to avoid some fire hazard type of DIY blunders.

-- With engine off after my wife got home from a drive, I watched the fuel rail pressure from the scanner readings... it was dropping 20Kpa every 15 or so seconds. I'm pretty sure in a returnless system that the pressure in the rail should stay there when the engine is off unless going through an injector.

I'm ordering a cold start injector cheap from a junker just to rule it out, no point in going through the trouble of disconnecting everything to test it just to have to do it again if I find it's bad.

Last edited by BinaryJay; 03-10-14 at 02:45 PM.
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Old 03-21-14, 01:27 PM   #4
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Well, I replaced my cold start injector with one from a lower mileage junked car. When I disconnected the fuel line from the injector there was like no fuel in it - it had been sitting undriven for about a day. I expected at least more than just some dribble as I didn't bother depressurizing anything ahead of time.

Car didn't start on first crank, had to crank it again and it was off...

Unfortunately after tying everything back together again and reconnecting the negative to the battery, I looked at how my fuel trims were behaving and the numbers remained largely the same... I guess I will drive around for a while to get the car out of learning and check again but still... around -15 fuel trim on both banks are being read.

So I've cleaned the MAF with no change, replaced cold start injector with no change... lack of fuel in the line from the rail to the cold start injector after sitting overnight still makes me think something is/was leaking but jeez what are the odds of injectors on both banks leaking the same amount? And wouldn't I be greeted by a big poof of black smoke when starting the car if the injectors in the cylinder were leaking?

Oil smells a little bit like fuel on the dipstick. Possibly the injector was leaking down when the car was off and over a long enough period of time leaked down into cylinders and passed the rings into the oil and the fuel trims are due to PCV feeding fuel fumes constantly back into the intake? Will just get an oil change I'm due for anyway and see again if there is any difference... and if there is, monitor it over time to see if it slowly creeps up again on me.

But I am out of easy things to do myself I think, and since the car isn't throwing errors at me I'm not inclined to go much deeper into this myself. Still if anybody has any suggestions... Guess I was hoping to see fuel trims go right back to normal after spending $60 and 15 minutes changing the cold start injector but no such silver bullet - at least no instant gratification.
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