Hi Scotty. I've got a 2005 Corolla with 300K miles and I'm trying to identify the cause of a rough idle. I'm the original owner of the car, which has been trouble-free for over 15 years, so I've not done any engine repairs except for regular spark plug, motor oil, and ATF changes. Since the car is so old now, this year I've replaced injectors, fuel pump, vacuum hoses, throttle-body gasket, intake-manifold gasket, valve-cover gasket, and PVC valve in hopes of eliminating the rough idle which started last year. I've also replaced all four engine mounts, which were badly worn. Most of these repairs had some improvement on the rough idle, but it's not entirely gone. I suspect the cause of the rough idle is making the ECM trigger a P0171 code (too lean), but it doesn't make sense because the ECM is also adjusting the short-term fuel trim to –21% as if it were idling too rich.
I noticed that there is a loud hiss in the engine compartment that lasts about one or two seconds at startup and then it quickly goes away. It doesn't always occur, but I'm guessing this might be related to my idling problem. Any idea what's going on? I can provide OBD data if needed.
A hiss, bad idle and weird fuel trim? Sure sounds like it might be a air intake system leak. Here's some info on that.
The hiss is only momentary and does not occur at every start up. I don't think a smoke machine can be used to find this. BTW, all vacuum lines are new as well as intake gaskets. What part of the engine occasionally opens and shuts in two seconds?
I should mention that I did a compression check before I started repairs this year and got 180 to 200 PSI on all four cylinders.
In what shape is the air filter? Probably issue can also be the ECM.
https://tuningpro.co/the-3-most-common-toyota-1zz-fe-engine-problems/
How can I find out if the faulty ECM has been replaced? If it hasn't been replaced, will Toyota still honor the recall?
If you live in USA, you can find it out there
https://www.nhtsa.gov/recalls