XK8 / XKR ( X100 ) 1996 - 2006

Lean codes but negative fuel trim

 
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Old 03-12-2019, 07:39 PM
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Default Lean codes but negative fuel trim

I am completely confused by the symptoms I have. Rather than theorize about it, and in so doing possibly pollute people's thinking, I want to just lay out the facts and see if anyone has an idea about it . Any suggestions much appreciated.

- Occasionally I will get a restricted performance message, after which Codes P171 And P174 are found . This seems to happen never under load ... but rather usually while coasting or braking.
- Monitoring fuel trim shows negative(!) numbers, as much as negative 17. This is seen primarily around idle; more normal trims, mostly negative low single digits, appear at higher RPM .

Has anyone ever seen a condition like this? Any ideas about it?

TIA,
 
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Old 03-13-2019, 06:47 AM
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Those two codes are sometimes triggered by failing VVT solenoid O-ring seals. They sit there and bake in the engine bay, get hard and brittle, then begin to allow oil to seep out and air to seep in. In my wife's 2006 XK8, they only last two to three years before they need to be replaced again....
 
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Old 03-13-2019, 07:03 AM
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When the codes flag a lot of data is stored - known as freeze frame data - so read it and you'll see what was happening at the time the codes flagged.
 
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Old 03-13-2019, 07:28 AM
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Thanks guys,

I've cleared the codes and will watch for the next event. I've never tried to retrieve freeze frame data so I'll have to learn about that.

The codes, and when they trigger, seem to me like classic symptoms of an induction air leak. I had started to look at the usual suspects for that. But the fuel trim data says the computer is trying to correct an over-rich condition. Curious, ain't it?
 

Last edited by Dennis07; 03-13-2019 at 07:30 AM.
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Old 03-13-2019, 10:03 AM
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Keep in mind the fuel trims are really a table of many values, indexed by rpm and load. It is possible some trims are positive somewhere in the table, while being negative somewhere else in the same table (say, different rpm), at the same time. For an '03, the air flow meters are dirt cheap (same model used in MANY cars), so I would start there. The catch is to make sure the o-ring that seals it up is also functional (replaced or somehow seals). Once you have reliable sensor, best is to do a smoke test to identify all possible leaks. There are so many opportunities for leaks, it is hard to go around and repair them all.

Best of luck, keep us posted.
 
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Old 03-13-2019, 10:29 AM
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The MAFS can also cause this. Try some MAFS cleaner.
 
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Old 03-13-2019, 01:33 PM
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I have cleaned the MAF. New one on order.
Also looked over the prime air leak suspects, but seems like there must be something more than air leak going on; it would not account for negative fuel trim.
 
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Old 03-18-2019, 06:50 AM
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Installed new MAF. Both this one and the one it replaced are Denso units, and the old one was in place only about 3 years, 20k miles.

But ... they read differently. My interpretation is that the new one senses less air going by; i.e. it seems that it is calling for less fuel, so high negative fuel trims are gone. But now we're working with LTFT values built up using the old MAF, mixed with readings from the new MAF, so hard to reckon what the new STFT values mean. I'll drive it some and see where all the FT values go.

Still don't see how old MAF could have led to P0171 / 0174, but it may be I have a second problem that has not reappeared yet.
 
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Old 03-22-2019, 07:22 AM
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Post #3 if you want that info.
 
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Old 03-22-2019, 08:23 AM
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Originally Posted by rj237 View Post
the mafs can also cause this. Try some mafs cleaner.
or replace it, these do cause erroneous lean conditions. Just make sure that youre not going lean under off idle conditions as you seem to. So may people including techs llok for vacuum leaks totally overloooking things like dirty poorly spraying injectors that cause lean conditions under different cruise and load conditions. When a ecu sees this kind of issue it will pop restricted performance much faster than a idle issue because lean under load can melt pistons caused by very high cylinder temps. Remember too that the crankcase has to be sealed tight and you will not see this unless you smoke the complete induction system and crankcase. Oil fill cap orings, dipstick, vvt, cam covers etc can all add up to a issue creating lean conditions. Ie may not be 1 thing as the car i just fixed last week for this showed.
 
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Old 04-21-2019, 08:17 AM
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I’ve had my 03 XKR for a few weeks now, and had it out driving during the day, even tried to use the torque app to get some 0-60 times, but need to learn how to use the app better. Anyway I just replaced a HID ballast last week, so I hadn’t been driving at night. But last night I took the car out to dinner, driving there all was good. I even accidentally squawked the tires a bit coming off a light, gota watch that. Anyway, the motor has been running great, but coming home last night after having the car driving through town for a few minutes warming up, I was gently accelerating to about 50 when I got the “restricted performance” warning, and a cel. I kept driving and increased speed to about 60 and really didn’t feel the restriction, but I didn’t gun it either. After a few minutes the text went away, but the cel stayed. I pulled over before I got home, shut it down, restarted, drove up to speed again, no issues, but of coarse the cel was still on. I got home, through on the OBDII reader, and got a P174. It cleared fine. Now the temp had probably gotten a bit cooler then I’ve been driving in, about 44F, and since it was night, probably a bit denser. But I was curious if this is normal, or something I need to keep more of an eye on?

I’ve run about a 1/4 tank of 91 octane through the car now. I’m not sure the previous owner, used that or less. I was thinking about adding a bottle of Techron just as a good measure anyway. Just after I switched over I noticed when idling in the garage it did smell a bit richer then I would have expected, but I thought the computer might have still been self adjusting.

Anyway, thanks for the insights.
 
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Old 04-21-2019, 09:26 AM
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You mean a P0174 code, right? Could be an air leak. Check to see if your two VVT O-ring seals are weeping. Do you see any oil residue on the plastic covers directly below them?

Also, check the accordion-pleated section of your plastic air intake duct between the air filter box and the throttle body. That area tends to split and crack after baking in the engine bay for years....
 
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Old 04-21-2019, 12:14 PM
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Yes I did mean P0174, I was trying to use the same nomenclature as the first post, and thought the "0" might just get dropped. Jon, thank you for the suggestions. So, while I'm mechanically inclined, I'm not a mechanic, so I had to go look up where the VVT was located on our cars. I found a few pictures of the 4.0, and took a guess that it would be similar on the 4.2. But I think that is a bad assumption, as there is no sensor wires coming out of that area. Here is a picture of my 4.2, with no leakage.


I even popped the beauty plate off, to make sure nothing was nothing leaking down there, but I didn't see anything (that was harder then I thought to line back up). But to be honest this seems more like a PPV, or something along those lines. Especially since the other end is just connected to the intake near the throttle body. But here is what it looks like on the inside just because I took a look.


There is a bit of sediment in there, and I don't know if that is normal or not. But it seems like it might be normal.

The air intake plastic seemed fine, I had taken a look at this when I first got the car, but I took a closer look now, and with out doing a smoke test, I don't see any cracks.

For now, I'm just going to keep an eye on things. If I get the error again, I may try looking harder. And maybe investigate the MAF sensor like the original poster did. But so far, I don't see it as being anything to be too worried about.
 
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Old 04-22-2019, 07:23 AM
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John,

There are two "lean" codes that often trigger together, one for each cylinder bank: p0171 and p0174. If you get a recurrence of your problem and again you find p0174, but not p0171, this suggests an air leak effecting one bank but not the other. Seems to me the seal around the VVT connector would be a prime suspect in that case. A comparison of the fuel trims for each bank might shed some light.

Good luck.
 
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Old 04-22-2019, 08:06 AM
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Originally Posted by RJ237 View Post
The MAFS can also cause this. Try some MAFS cleaner.
^^^^^^^ +1

I had to clean my MAF sensor twice before my rich code throwing went away. It certainly looked clean to me after the 1st cleaning, but apparently was still not 100%. I think the cleaning process difficulty is compounded by the fact that one cannot touch the exposed "hot" wire(s) in the MAF, and must rely on the jet stream of cleaner to do the job.

Z
 
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Old 04-22-2019, 08:57 AM
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Originally Posted by John Nevets View Post
Yes I did mean P0174, I was trying to use the same nomenclature as the first post, and thought the "0" might just get dropped. Jon, thank you for the suggestions. So, while I'm mechanically inclined, I'm not a mechanic, so I had to go look up where the VVT was located on our cars. I found a few pictures of the 4.0, and took a guess that it would be similar on the 4.2. But I think that is a bad assumption, as there is no sensor wires coming out of that area. Here is a picture of my 4.2, with no leakage.
Somebody else will correct me if I am wrong, but I thought that the supercharged engines don't have VVT? For the X150 they introduced VVT for the supercharged engines, but not on our cars.
 
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Old 04-22-2019, 09:09 AM
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Originally Posted by dibbit View Post
Somebody else will correct me if I am wrong, but I thought that the supercharged engines don't have VVT? For the X150 they introduced VVT for the supercharged engines, but not on our cars.
that is my understanding.

Z
 
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Old 05-11-2019, 03:29 PM
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So yesterday I went for a bit of a drive, and about half way through got both the P0171 and P0174 error codes this time, along with the check engine cel and for a little while the reduced performance warning, although that had gone away by the time I got home. So I started digging into the fuel trims. This is something new to me, so I wasn't quite sure what to look for at first. Couple that with me having a harder time then usual figuring out the Torque app, and it took me a few tries to figure out that it does look like I have a lean problem of some kind. I'm guessing vacuum leak, but I will try cleaning the MAF sensor first.

I still haven't figured out how to just go for a short trip with torque and have it give me the data at the end. And I don't like looking at the app too much while actually driving. But here is what I was seeing. I'm able to see the Long term Fuel Trim for both Banks, and The Short Term, and the second sensor on each bank, but not the first. The second sensor reports things normal, bounces between -.8% and 1.5%. And when I checked at idle this was matching Short Term, so I thought they were the same (found out a bit later they were not). The Long Term trim for both banks is at 3.1% when Idling, or even when raving it up while stopped. Took it on one trip (about 15 miles) this morning, and it did get into 13% or so for short bursts, but seemed about 3-5% most of the time. Left it outside in partially sunny and 60 degree F weather for a few hours. And then this afternoon I took it on another trip (about the same length), and it seemed to live above 13% much of the time I was driving, and the max was 19.5% (both banks). I finally added the Short Term gauge to what I was monitoring on the afternoon trip, and it showed a whole different picture then the second sensor, it would live at about -7% most of the time, but when I checked the min/max at the end of the trip, it had gotten up to 25% at one point, and -19% at another. I know some of this has to do with what the supercharger is doing, but this does seem to be all over the map a bit. While it never sent up any error codes on the dash on either of these trips, there was both the P0171 and P0174 pending when I looked them up in the app. When I got back to the garage, and just Idled the long term would settle in to about 3.1% again and the short term would be about 0%.

So like I said at the begging, I'll try cleaning the MAF since I can buy a can of that locally. And then if that doesn't do anything I'll probably invest in one of those $100 smoke testers on Amazon and see if I can find any other leaks. But any other suggestions are welcomed, including how to better utilize the Torque app. During all of this the actual performance of the car seems fine, maybe a bit sluggish off the line when cold, but nothing drastic.
 
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Old 05-12-2019, 06:45 AM
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As you might know, you can use various types of gauge displays.

One of them gives a live indication, but also records the lowest and highest value of each reading.
That allows you to keep your attention on the road, but also get a better indication of range without the process of logging (which is rather limited in Torque).

For your further info, both my XJ8 (4L N/A) and XK8 (4.2L N/A) run Short Term -5% ~ +10%, most of time, Long Term -3% ~ 0%, and a balance within 2%.

My set up of the Short and Long Term readings (one of the various pages I have).

 
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Old 05-12-2019, 09:36 AM
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Thanks Eric. I did find those gauges eventually. That was how I got the final numbers on my last trip, but it took me a while to figure that out. I think after digging in the settings a bit, figured out how to possibly log some of this info, but I haven't tested that out yet. We will see. Will try the MAFS cleaner in a little bit, and then go from there.
 

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