I have an E320 year 2000, I was trying to get it smoged but the check engine light came on. The code was PO170 and PO173 It said Fuel Trim Malfunction. I read somewhere it was most likely the mass flow sensor, or it could be a vacuum leak. My question is what do you guys think it is and how do I find out for sure what it is so I can fix the problem. Thanks for the help by the way.

 

look for intake leak.

make sure tank cap is tightly closed.

car must be running lean.
how much is the engine mileage? worn out engine also get this.

if fuel pressure is low, this also cause this problem. your fuel filter is good?

it is better to reset the adaptation data and set enrichment of fuel to stage 1. this will cure the problem.

 

I had the same thing happen to me recently (a few months ago) after I disconnected the battery and did some work. I had the same code read from my check engine light. I was advised by a technician to disconnect the battery negative cable and wait about 30 seconds before reconnecting it. The problem went away and has not been back since.

 

The mileage is 78000. How do I look for intake leak. I got the fuel filter changed about 15000 miles ago. Also what does it mean to reset the adaptation data and set enrichment of fuel to stage 1?

I tried the battery disconnecting thing, and that didnt work. So maybe it must be something serious. The only thing that frustrates me is that the code could be so many different things. I want to find out exactly what it is so i can fix the one problem instead of experimenting with different ones, and wasting a lot of money.
How would you guys approach it?

 

I had the same problem on a Ford Windstar van last year. I found a hose off on the air intake plenum. The engine was getting air that was not being seen by the MAF sensor. reconnected the hose & reset the light & it never returned. good luck.

 

After you've made sure there aren't any obvious leaks like hoses disconnected, try this:
1. Have your car idling
2. Spray some intake cleaner (carb cleaner) on the areas where any two pieces of the intake come together.
3. Listen for an increase in RPM
If there is a leak at any of the connections, the engine will be able to inhale the carb cleaner through the leak which will increase the fuel/air ratio and the RPMs will go up momentarily.

 

Thanks guys, I will see if it works. Thanks again.

 

Hi, I've been away.
I agree that a possible cause is the MAF particularly at the mileage you have covered although as others have suggested any number of things could cause this problem.
My suggestion is to take full advantage of the on board diagnostics. When the technician accessed the fault codes only part of the data stored in the ECU was accessed. There is also data known as LTFT (Long Term Fuel Trim) If this is running circa 10 it is unlikely to be the MAF. If it is running circa 30 is is likely the MAF.
In addition their is a "page" of data which shows exactly what conditions were prevailing when the Yellow MIL (Malfunction Indicator) or CEL (Check Engine) light came on. get the dealer/garage to give you a printed copy of the results.
If the LTFT is high it can sometimes be corrected without replacement by cleaning the hot film sensor with Toluene or SOME carburettor cleaners. Only some cleaners work dependent upon chemicals used.
First step is to RESET (turn off) the CEL. Run the car till the light comes on again (assuming it does) and then access the ECU and get all the data.
Once you have done this (and the other suggestions made e.g. air leak) come back and I will provide additional details.
Stuart.

 

Thanks guys for your help. I changed the MAF sensor for $108.00. and I finally passed smog. thanks to you guys, I saved around 3-400 dollars not taking it to the dealer. Thanks again. The sensor takes about 5 minutes to change, thats it.