Possible Vacuum Leak, With Graphs

[2001LSHoe]

Thinking I have a vacuum leak. I get the really high STFT only on startup after leaving it sit overnight. It causes random misfires. It doesn’t always pull a code. If anything the check engine light flashes but it doesn’t register a misfire on a particular cylinder. Although cylinder three seems to be one that misfires more than others. Cleaned MAF and spark plugs and switched spark plug wires on cylinder 3.

Just wanted to see what others thought. On my phone and will try to add more detail when I can get to a pc

 

[Scottydoggs]

cats could be on their way out. only two things make the cel flash, needing a tune up like super bad, or the cats are clogged up. flashing light = multiple misfires. aka all of em or most of em are misfiring.

 

[Fless]

Any change to the fuel trims at idle after it warms up a bit?

Any exhaust leaks?

 

[B-train]

Watch this video and the second. It will help alot, the guy is a good teacher.

 

[2001LSHoe]

Any change to the fuel trims at idle after it warms up a bit?

Any exhaust leaks?

Yes, after it warms up all the fuel trims are +/- within 10% and very rarely outside of 5%. In fact, the only time I see the fuel trim at these levels is after an overnight start up. I've even tried running it in the morning working through lunch and trying in the evening after it's sat for 10 hours and I still cannot replicate the fuel trims after an overnight cold start.

No exhaust leaks that I notice.

 

[2001LSHoe]

cats could be on their way out. only two things make the cel flash, needing a tune up like super bad, or the cats are clogged up. flashing light = multiple misfires. aka all of em or most of em are misfiring.

I did new plugs and wires, fuel filter and cleaned MAF all within the last year. I initially had this issue last winter on a morning when it got down to -15F. In the winter time the symptoms are much worse. The car had 90,000 miles and was a one owner when I purchased it in 2022. It now has 150,000 on the clock. It was garage kept its entire life and was very well maintained.

 

[2001LSHoe]

Below are graphs of the rear o2 sensors… is this an indication that the cats are going out?

 

[2001LSHoe]

Fuel trims from cold start overnight with 30 Minute drive at near constant speed and incredibly flat.

 

[Scottydoggs]

looks ok to me really. if you had a large vac leak the ltft would be pegged at +20

keep in mine, the plus side is adding fuel, so its lean and the pcm is making it richer. all normal.

on a cold start the engine is running in open loop aka the pcm is running it till it warms up some, then the o2's take over and its then in closed loop.

 

[B-train]

I was going to say the same after looking at it. My 2008 looks similar to that - about STFT of 5 or so. 190k on the clock. I was concerned too, but after watching those videos, I don't really see anything to worry about.

If it's a 2001 like your name, then I would run some good injector cleaner through it. 1/2 quart of Dexron 3 in a full tank of fuel works well too - done this for years.