Need Diagnostic help...

[3vs197]

So my car developed an interesting problem last week. Long story short, see sig for mods...When I put it back together a couple weeks ago (mildly ported heads, 04 cobra injectors, 2 valve timing tensioners) it ran great for like a week. Now under very light throttle, it feels like its missing. RPM's don't matter, it will do it at 1k or 5k rpm's, just under very light throttle. Its worse on the highway with the cruise set.

Hooked up the Ford IDS, show's no missfires, fuel trims and good, power balance was good. Only thing I haven't checked yet is compression. It almost backfires out of the exhaust sometimes. There are no vacuum leaks. No codes. I have no idea whats going on and don't even know where to go next. The car idles fine and when you floor it, it works great, just under very light throttle, it feels like its missing and you can hear it in the exhaust.

Ideas?

Im also ruling out the tune because, like i said the car ran awesome for over a week before this happened.

 

[Swarzkopf]

Since there seems to be an absense of better responses so far...

No clue what's wrong- but maybe start by scanning every relevant sensor and make sure that it's reading correctly across all load values- MAF, IAT, ECT, O2, etc.

Try reloading the tune or resetting the VCM by unplugging the battery just for the heck of it?

 

[irishpwr46]

check the oil pressure, it may not be running he vct

 

[3vs197]

i have data logged the vct angle and the vct angle error and there is nothing wrong there. The error is as low as it ever has been and I would think if oil pressure was suspect, the car wouldn't run fine @ 7000 rpms.

 

[3vs197]

ok just made a good logging run and here are some results. Bank 2 o2 sensor appears to be dead, never saw more than 200mv, and was just about flat lined the whole time. It did not do this in previous logs. I do not know if this is a symptom or a cause. Another thing i noticed, not sure if it matters is that the LTFT's and Spark advance were in direct correlation. The higher the advance, the higher the LTFT's. Bank 1 was consistently higher than bank 2, about 15 at times. Bank 2 hovered at pretty close to 0 the whole time.

 

[Alfiedagr8]

Do a relative compression and see if one bank has more compression than the other. Also 1 or 2% could mean a problem if one bank shows more then the next. That's all that comes to mind. Maybe something happen to the chain timing.

 

[Swarzkopf]

ok just made a good logging run and here are some results. Bank 2 o2 sensor appears to be dead, never saw more than 200mv, and was just about flat lined the whole time.

Really stupid question here- as I'm still learning 3Vs- does "Bank 2 O2 sensor" on our cars refer to a front O2 sensor in the exhaust stream for one of the cylinder banks, or does "Bank 2 O2 sensor" refer to a downstream, post-cat sensor functioning to make sure the cat is cleaning the exhaust? If the former...I think you found your problem. Replace it. If the latter, it's probably functioning the way it should.

 

[3vs197]

Bank refers to the side of the engine. Sensor indicates where on that bank it is. Examble B1S1 B1S2 B2S1 B2S2. Typically sensor 1 means pre cat and sensor 2 means post cat.

I think im just going to put a o2 sensor in it and see what happens.

 

[fdjizm]

I would go with changing the o2 sensor.
I believe the cars go into open loop at WOT and that would explain why it may be fine during WOT, just a stab.

 

[3vs197]

O2 sensor did not fix the problem. Not any better. Next step is to reload the tune. If that doesn't work I'm really stumped.

 

[TexasBlownV8]

You've found some sort of problem by the O2 sensor being flat. Under part throttle, it should be oscillating back and forth between lean and rich, like the other side.

When you say the o2 sensor was "dead", if it's staying that low in voltage, either that side is running extremely rich (low volts mean rich, higher volts mean lean). Per what you're seeing on bank 1, running +15% ltft, that's pretty rich.

Check the wiring/connection to the o2 sensor that is flat-lined, make sure it's not open nor shorted anywhere in the harness, make sure continuity to the pcm connector is there, etc. ...make sure you didnt accidentally plug in the IMRC connector to the O2 sensor, too!

I would think if there is some kind of engine problem, like a stuck valve, it wouldn't run as good and/or it'd be making more noise. Doing a compression check on that side of the engine would not be a bad idea, too.

The correlation between timing and ltft is interesting, but probably just a side effect of other issues.

(bank 1 is passenger side, bank 2 is driver side, sensor 1 is front, sensor 2 is rear after-cat).

 

[3vs197]

low voltage is lean, high is rich. But it wasn't completely flat lined but it never got over 200mv. And of course when I said its not the tune, insert foot in mouth lol. I put a new o2 sensor in it and added 3% fuel across the board for the hell of it and the problem is 90% fixed. So think adding about 2% more will fix it all together. I just don't get it because I could pinpoint the day when it started happening and I didn't change anything with the tune/engine. So I think everything is ok for now.

Edit: I also used a wideband to check the tune when I got it all together and it was reading high 14's at cruise and high 12's, low 13's @ WOT like it always has but obviously something changed.