Thread: High positive fuel trims at idle but no vacuum leak?

Hi all,

A few days ago I felt some misfires at idle, so I plugged in my scan tool and am indeed showing some misfires on Cyl 2 and 6, but only when completely unloaded (at idle and in park/neutral). No DTC's, and everything looks normal on the scan - except that LTFTs are between 11 and 16.4% (which is the max for positive correction, if I'm not mistaken?). The value occasonally drops, and the more the load on the engine, the lower the LTFTs. They're 0 at WOT. The STFTs are usually near zero, so it looks like it's not catastrophically bad, just weird.

Obviously this suggests vacuum leak.

But I redid all the vacuum lines some time ago with nice silicone lines. I started checking hoses by unplugging them (and manually plugging the ports), and also by spraying around the S/C and all vacuum ports and hoses with carb cleaner (carefully) to check for changes in engine revs.

Everything seems fine. I can't find any leaks anywhere.

Also - the scan tool shows vacuum at 20.2 inHg. That's quite high, IIRC, and it's also normal for my car at idle and matches exactly what it normally shows, from before this problem developed.

So then I suspected MAF or other input air sensor failure. I swapped the MAF with a friend's known working one - no effect. Also the MAF shown at idle (~4.7) is correct, as is the IAT.

It's not the upstream O2 sensor. That was due for a replacement anyway and I put in a new (OEM) one, to no effect.

So I'm sort of at a loss here. I tried searching for other threads with similar issues but couldn't find anything that wasn't either a vac leak or the MAF.

Has anyone seen something like this before, or has any suggestions for what to try next?

Thank you in advance!

Do you have the ability to use a smoke machine or a fogger?

That would let you fill the whole motor up with smoke and see a leak from a maybe not so common area.

Other places to look are the EGR pipes, they can get a hairline crack in them and allow extra air into the motor.

Thank you!! I hadn't thought of that. I'll call around and see if I can get access to one.

Wouldn't any leak show up as a drop in vacuum pressure though? I know the drop can be very slight sometimes, but surely if it's reading a steady 20.2 inHg there isn't a leak?

Many times a smaller vacuum leak will not change the inHGs that a gauge/scanner will show. It can still pull that 19-20in of vacuum and still run like normal and act normal.

My old car always idled around -20.2ish like yours is.

Noted. Thanks! I'll check the EGR tubes and look for a shop with a smoke machine.

SOLVED. Just wanted to update for anyone who might be browsing and have this problem in the future.

Had a shop do a smoke test. They said EGR tube and fuel injector O-rings looked leaky. Couldn't tell whether LIM was too.

So I replaced the EGR tube. Fuel trims went down, but only slightly.

So I replaced all 6 fuel injector O-rings. Problem solved! Looks like the LIM is fine, the misfires are gone, and fuel trims are back around 0. It was leaky fuel injector O-rings.

Thanks again for all the help!

Very nice to follow up with the cure! So many who ask for help, don't!

Where IS that "like" button...? Lol