so am I crazy?

this past summer I fixed an idle problem by cleaning the throttle body and mass air flow sensor on my 2008 Tacoma. It seemed to fix the problem and it ran smooth, started well and idled well.
Then just a few weeks ago when the temperatures had dropped, the origionally issue raised its head again.
Idling going up and down from 400rpm to 1200 rpm and back and forth, up and down, almost feeling like its stalling and maybe running rich out of the pipe.
This seems to happen when the air temps are below 45 degrees.

So im now looking at the fuel filter as the cause.... is this logical? could a fuel filter be affected by temp?

many thanks - jane

 

This sounds very similar to my issues with my previous 2006 with the V6. Not sure the exact fix for you but what I did in total was

1. Found a vaccuum leak which caused my rough idle. A mechanic actually found it, took him 10 minutes to find cause he knew Toyotas.

2. Changed fuel injectors cause one or more were leaky which caused a lean condition and rough start/bad idle. (most expensive and time consuming of these options).

3. Cleaned my mass airflow sensor which I see you've done.

4. I also changed the fuel pressure regulator which can slowly fail but I'm not sure this did anything for me. Could also check the hose that attaches to it since the regulator uses vaccuum from the intake to open and close. Could be clogged or cracked.

1, 2, and 3 had the biggest effect for me. Engine ran perfect after that. Wish I knew more about how to narrow down the issue based on the tempurature. Point is, it could be multiple things causing this which was the case for me.

 

Doubt it would be a fuel filter as temp wouldnt matter to a restricted filter. If its restricted, its restricted. Vacuum leak as the above person mentioned is where I would be looking. Does it throw any codes?

 

Usually a stuffed up fuel filter will result in extended crank times, it shouldn’t really affect idle. It could affect higher RPM in load, if it’s clogged enough.

As said above, a vacuum leak from a crack or broken hose is most likely the issue.
With the engine running, hopefully at a steady idle if you can get it too, you can take some brake clean or other similar ignitable spray, and give all the junctions between the air intake manifold and any hoses a quick spray.

this should hopefully reveal any leaks by sucking in the brake clean, and causing your idle to change for a second or two

just be careful about what spray you use, and what you spread on, as spray clean will dissolve some painted or anodized surfaces.

 

try this, turn key to ON position (i think, not ACC)
depress gas pedal slowly, to the floor, and release slowly to the top
and repeat 3 times, and it can trigger the ecu to remap the idle

otherwise disconnect negative battery for half an hr

 

At this age injectors and pump may not be a bad idea.

question is, is there a way to have them serviced with minimal downtime. I have sent out injectors to be rebuilt before but it took like a week. Way cheaper than new.

 

Possibly a bad water temp sensor.

 

forgot all about that. Common to fail on German cars probably no different. Could be a good idea to throw one at it as preventative maintenance in general.

Google shows it here as not looking too hard to get to or expensive.

 

Thanks for all the help. Im going to try see if I have a vac leak somewhere first, then go from there. Is MAF Sensor cleaner ok to check for vac leaks? Im not sure which chemicals are super corrosive vs ok to use.

I don't have a code reader, but maybe I should try find a simple inexpensive one.

Appreciate the comments on fuel filter not being the cause.

 

autozone/advance auto have code readers for free. If you dont have a Check Engine Light, it might be stored and perhaps not showing. Yes, MAF sensor cleaner will work although not as good as carb cleaner