Fuel Trim - That Elusive Mistress -- Codes P0170 & P0173 - Bank 1 & 2 Malfunction

[plik]

Recently I received this letter from a fan in Scottsdale:

Dear Plik,

I have a 1998 740iL with 99,000 miles.
Check Engine light is on: codes 0170, 0173.
I can't pass local emissions test.
I have changed the OSV.
I have changed all vacuum hoses.
I have cleaned the MAF.

I notice you have been a fantastic source of info regarding this problem. I have concluded my problem is likely a vacuum leak somewhere. My idle is absolutely horrible. The car feels like a motorboat; however, there is no hesitation on acceleration. It's just a crappy idle. Any suggestions or help would be greatly appreciated.

Sincerely,
Sleepless in Scottsdale

Dear Sleepless,

You have a classic condition of Fuel Trim - That Elusive Mistress!



Fuel Trim Bank Malfunction is a bad fuel/air mixture caused by either a leaky manifold gasket (which introduces too much cold air into the manifold) and/or bad O2 sensors and/or a bad MAF. Or, it might be the combination of the three, or a combination of just two.

Either way, with a car near 100,000 miles, the solution is to replace all of the manifold gaskets (included the one between the manifold and the throttle body, and the one between the manifold and the rear cover plate -- aka, OSV, *see illustration below), the O2 sensors, and MAF (just cleaning the MAF isn't a permanent solution).

Now, if you don't want to fork out a ton of money at once, then

1. - start with the manifold gaskets. Then,
2. - replace the O2 sensors, those (start with the pre-cat sensors, and then the post-cat sensors). Then,
3. - replace the MAF. And finally, after those have been changed, you'll probably need to
4. - replace the spark plugs because they have been fouled from all of the fuel trim issues. That will help pace the cost of parts to just a few hundred dollars per step.

However, you should take this opportunity to replace the

- valley pan and
- water pump and
- valve cover gaskets,

if you haven't had those replaced already.

Important: if you remove the valve covers or the valley pan, then you absolutely need to purchase new bolts. Don't re-use those old bolts because it's very likely that they will break when you try to tighten them back on.

Here is a link to a list of parts you'll need: http://forums.bimmerforums.com/forum...d.php?t=905568

Here is a link to a very good write-up: http://www.bimmerboard.com/forums/posts/344734 (except don't paint the valve covers).

[01sport]

I've heard rumors... That nasty Mistress can sometimes be found hanging out with bad cats.

[stritarja]

Cleaning the MAF is only a meaningful test if it makes the fuel trim codes go away. If cleaning the MAF doesn't make the codes go away it may be because the MAF is too far gone - that's what happened to my car. I'd replace the MAF before going after the intake manifold gaskets.

Bad O2 sensors will set specific O2 sensor codes before they set a fuel trim code - I wouldn't chase fuel trim codes by replacing O2 sensors if there's no O2 sensor code.

[plik]

Ugh ...

You should be able to delete replies from a thread that you create.

[nah2323]

This mistress has been hanging around my car too. I mean, I know it's a poon magnet, but jeez, I don't need some nasty FTMb1 STD from the ho. I'm going to try a MAF cleaning. I had similar problems in my Toyota 4Runner, and cleaning the MAF worked GREAT! My car runs fine, it doesn't stutter, or anything, so I'm hoping that its a quick cleaning that will do it! :cross fingers:

I do need to address my valley pan gasket anyway, so I'll pop the new manifold gaskets in when that goes on, should the MAF cleaning not work, and force me to take the top end apart to pass the inspection.

I do on occasion get EVAP something or other.

[CarlfromBoston]

I had 170 and 173, and it was a bad MAF and an intake leak. All good now, no check engine light and passes OBD inspection.

[rm0310]

So I have a 98 740i, the check engine light came on right before I needed to smog it ( pass emission inspection)... so I just cleared the codes and drove it around for a few days. It passed smog, but two days afterward the Check Engine light came back on...

Only one code (P0173) - Fuel Trim

What could this be? (Since it passed smog and it seems to be running well)


Thank you in advance for any help.

[vgo8888]

I have solved this problem .....

I did read a lot of treads - some usless and a few good ones that helped me. Thus I feel obligated to post my experience for others to share.

I had "check engine light" (CEL) and codes P0170 and P0173.
I did have MAF code and I did change MAF (I found just an insert, not Bosh, on ebay and my MAF code went away).

Thus I was concerned about vacuum leak and spent a lot of time on it.
This is after about two years and half a dozzen mechanics changing whatever "computer told them" ringing the cost close to several thousands.
So, I took things into my own hands - never trust a mechanic! this is what I have learned...

To make a story short - let me give you a summary how to go about if you have those codes:

(a) first check for vacuum leaks. you can listen with stetoscope, you can visually inspect etc. Check for those notorious hoses - this would be my first suspect - there is one that goes from the botton of your deepstick to EGR or someplace. This one has caused a lot of pain to many people and costed many $$$$ - in the end it is just $10 piece of hose to be replaced.
Check the back of the manifold - this is also notorious.
It could be EGR - rare.
If you have someone who can do "smoke - test" that would pretty much check your vacum leak and rule it out if it is not that.

(b) second, check your MAF: you may not have MAF code - as I did not and did not suspect it.... It is a $500 item (which you can find on ebay for less - never pay that price).
I obseved the voltage on my MAF at idle and 1000rpm and 2000rpm.
Fortunately my friend (mechanic) had a few MAFs which I could try.
One was obviously bad - stalling the engine (this is what bad MAF usually do), the second one had different voltages:
0.8V at idle and about 1.4 or so at 2000rpm. (see if you can find someplace a table with those voltages).
(mine was giving 0.6V at idle and about 1.1V at 2000rpm - and I knew imediately - this is it!).

(c) check your oxigen sensors (two on the pipes under the car). I would do that last. Also - do not waste money replacing them ($300 a piece) as a good "machine" can read their voltage output and you can tell by that if it is oxigen sensor or not.

Pertty much those three things: (a)-(c) is what causes those codes.

"Fuel Trim" - exceeding or not being able to adjust came in my case because MAF was giving too low voltage (out of the range) and it could not adjust it, however, it did not detect it as bad MAF (BMW?).

Never trust what computer tells you .... there is ony one worse thing than a compuer - a mechanic that belives computer.

Hope this will help some poor sole who spent hours trying to catch this elusive mistress.

Well - I got this mistress .... we are gettin' married....

[Route 66]

I had the same problem turned out to be the MAF.

[Xephius]

[IMG]He looks like a fag, probably asking her for a midol.

Hey Bub, comments like that will result in me introducing you to a bar of soap....

Ugh ...

You should be able to delete replies from a thread that you create.

Indeed.... I love the write up, I can't believe that I have never seen this tread before.

John

EDIT: Holy resurrected threads Batman..... How did a year old thread come back to life a second time!

[Irocwagon]

I dont like this lady...97 740il...cranks and idles perfect always..acceleration is ok for first few mins then none ,bogs out under any acceleration...i can turn the car off then on then can acceelerate for about the first rmin..normally enough time to get to highway speed..itll normally hold highway speed but no acceleration..only the 2 codes 173&170...new plugs, checked coils, cleaned maf..no difference...from first crank itll smoke the tires but after few mins nothing..had cats checked, flow fine...is this prob ever resolved by tps...gonna try new aftermarket maf sense that seems cheapest...ohyeah I did check for vaccum leaks visually ,by ear and brake cleaner test..found nothing..gomna check mak voltage...should be around 2 volts?

[tac711]

Replace MAF!!

[Irocwagon]

Also new battery this ,week...its so weak even in 35 mph I cant out of the,way..coughs and bogs..no missfire codes .check engine light doesnt even come on ..i ride with the scanner plugged in looking for more codes...im tired of driving the e46

Ordering a maf now...hopefully this takes care of her...road trips in the e46 arent bad but sure isnt the,same..

[IcemanBHE]

Bosch only or youll hate yourself.

[Mayorchuck]

Welcome to the forum. As stated, Bosch only MAF or you will have a MAF designer paperweight soon.

[Irocwagon]

Dang so no ebay maf..bosch isnt cheap...hope thats the problem...today car drove well till home....started horrible idle in driveway...restarted and it went away...still no full power...why its out that I can turn it off, restart and gain drivability..drive with mag unplugged..hard shifts after awhile power loss though..like pullover can't go even 25...are theses maf symptoms

[Irocwagon]

Should the first throttle body in the intake always be in the open position...mines is...off during start up...revving in park or in drive holding bra
ke

[IcemanBHE]

The ASC plate? Yes.

[maddoggyusa]

I currently have P0173 and P0174 codes on my '97 e38. I was able to find a big leak on the first intake gasket (sprayed brakeleen and it almost stalled). There is a good video on it on how to check... you have to blast it on there pretty heavy to find leaks. First time I just sprayed lightly and didn't find anything.

Today I started to tear it apart to replace the intake gaskets and OSV. I'm on the fence about changing the valley pan.

Damn Bentley manual screwed me over... says to just loosen up the alternator to detach wire on back (to get the wiring moved to passenger side). No way I can get to it... I have to remove it... which requires removing the air duct on the bottom. And I felt like a real dummy trying to figure out how the idler pulley comes off... never had a clue there was a cover on there.

[Irocwagon]

New fuel filter...car still runs terrible...car only drivable if maf is unplugged. Almost drive normal. Tried eBay maf and new Bosch maf. Fuel pump is very winey. Was replaced couple months ago. Always been noisy. Said it would do that cause it's aftermarket. It's pretty loud. Guess I'll fill car up to see if a difference. These cars have no schrader valve for fuel pressure testing do they?

Also cranks perfect.....however after it sits for several days first crank is rough.

Maf plugged up. Filled with gas. Car ran great. Cruised around 120 for awhile. Off public roads of course. Then 1 mile from the house it starts to fall on its face. Pulled codes. 170. 173. 174. 175. Now 4 codes. Wtf

Maf plugged up. Filled with gas. Car ran great. Cruised around 120 for awhile. Off public roads of course. Then 1 mile from the house it starts to fall on its face. Pulled codes. 170. 173. 174. 1175. Now 4 codes. Wtf

[shotheway1]

I had the same problem,misfire on cylinder 6&7,and also random misfire.car would run like crap ordered new vcg and the day it got here woke up to no codes and car has been running fine for 4 days.smdh

[kouks]

Damn Bentley manual screwed me over... says to just loosen up the alternator to detach wire on back (to get the wiring moved to passenger side). No way I can get to it... I have to remove it... which requires removing the air duct on the bottom. And I felt like a real dummy trying to figure out how the idler pulley comes off... never had a clue there was a cover on there.

Use the BMW TIS online. It's searchable and an excellent source for technical work. It would have shown you the pulley thing.

[tim8dena]

ive been getting same codes...clear them and back by end of day..car idles and runs perfect..i was getting these 2 codes with one for cam sensor..replaced cam sensor now just these...funny thing is still ran fine with supposed bad sensor..i kept it because i think something else was the problem...im still getting 22-26 mpg..most of my driving is hwy and back roads so no stop and go...i noramlly keep it hooked to obd2 reader...what is the normal range for ST and Lt fuel...mine seems to be like 24% on one and 1% on the other...i did notice small leak before on one of my fuel injectors but replaced bad fuel injector...new plugs...all 4 o2 sensors..new manifold gaskets..intake gaskets...so im assuming this means the MAF or the osv

[shotheway1]

Ill try the maf,or at least try cleaning. Yours and see if it makes a difference.don't knw wut my problem was but car is fine now running like a scolded dog beginning to enjoy it again

[Irocwagon]

Shottheway1. If you can come to cola sc I'll let you try my mafs since I have 2 that work.......pulled fuel pump out and cleaned out about an ounce of rust out of area where fuel pump goes..strainer was completely covered in it. Wow the power is back and no codes. I changed the main fuel filter couple weeks ago. Where is the second one. Owever all fuel coming out of flow side was clean but I'll change it again after about a week. Any ideas on getting all of the rust out.