Fuel Trim Malfunction

[ricehunter]

So I got a CEL yesterday, Fuel Trim Malfunction, I looked it up and it looks like it could be related to either a bad MAF, vacuum leak or O2 error. I know I have a pretty bad exhaust leak, and the location (on the Down pipe) is after the leak, so I'm guessing its just running to rich. Any comments?

[Toybreaker]

So I got a CEL yesterday, Fuel Trim Malfunction, I looked it up and it looks like it could be related to either a bad MAF, vacuum leak or O2 error. I know I have a pretty bad exhaust leak, and the location (on the Down pipe) is after the leak, so I'm guessing its just running to rich. Any comments?

Ummmmm,

Fix the exhaust leak? ;)

Seriously, the o2 sensor is only as accurate as the exhaust gas stream that's reaching it. Diluuting it with ambient air may to confizzle it some.

Other than that, it's just looking for something could cause the measured air in to be different than the actual air ingested by the engine.

False air getting in after the airflow meter, or measured air getting out (boost leak) will get the trim out of bounds and flag that code.

Incorrect iat, baro, coolant temp, tps readings can also skew the ecu's interptratations of what's coming in/going out as well.

Which way are the trims reading?

Incorrect fuel pressure can also cause these kinds of issues.

[matthewdesigns]

Toybreaker covered pretty much all of it. Here's what the FSM says:

- Volume air flow sensor output frequency
- Injector
- Fuel pressure
- Intake air leaks
- Engine coolant temp sensor
- Intake air temp sensor
- Baro pressure sensor
- o2 sensor
- Exhaust mani cracks

[thiazole]

If your fuel trims are off, I'd think you'd see it when you logged them. What are they saying?

[ricehunter]

Haven't logged fuel trims yet, I will most likely do it on the way home, if the roads aren't to harry.

I'm going to fix the leak asap, but its kind of hard with a broken jack and snow in my work area, I think the closest time that I would be able to do that would be saturday =\

[Cloud]

Exhaust leaks mess up your fuel trims more than anything else except for maybe blowing a coupler out. I had a massive fight with my entire exhaust system because I warped some of the cast parts and it took a lot to get them leak free, even small leaks blew my trims way out of proportion. I have had leaks just past the o2 sensor still screw things up somehow... It has to be the most annoying thing in the world.

[Gimpin]

If you need a place to wrench some time let me know... you can use my garage ... its a little out of the way since its in falcon but a 15 min drive is better than laying in the snow

[thiazole]

Exhaust leaks mess up your fuel trims more than anything else except for maybe blowing a coupler out. I had a massive fight with my entire exhaust system because I warped some of the cast parts and it took a lot to get them leak free, even small leaks blew my trims way out of proportion. I have had leaks just past the o2 sensor still screw things up somehow... It has to be the most annoying thing in the world.

It makes sense when you think about it. If you are running stoichiometric a/f, then there are probably only ppm O2 in your exhaust - even the tiniest exhaust leak will allow more ppm of O2 in totally messing with the O2 sensor.

[ricehunter]

Yea, I did a log and my LTFTs were 12.5 so I'm pretty sure that the O2 is what's causing it. GimPin just PM me when is good I realized I went 110 on 6 gallons... so it would great if I could do it soon.