2005 2.7 5 speed idle speed unstable as idle timing varies.

My 2005 2.7 5 speed manual develops an unstable idle speed about 20 seconds after truck slows to a stop. My OBD2 reader shows the ignition timing moving from 5 degrees to 11 degrees advance as the idle speed varies. The advance change is in sync with the idle speed change ( 500 -800 rpm ). When driving the truck feels fine and the ignition advance is stable, only changing with load and throttle position as you would expect. This seems to be an idle only problem. What inputs could be causing ignition timing to vary at idle? I do not see any vacuum changes that would cause timing to vary. I have owned a lot of Toyota cars and trucks and all have had stable idles at 700 rpm. This is my only Toyota that has behaved like this.

 

Have you cleaned the throttle body and MAF sensor?

 

The throttle body and MAF were cleaned and oily gunk cleaned out of intake manifold. New OEM brand knock sensor, upstream O2 sensor, spark plugs, and air cleaner. The truck otherwise runs fine, returns 23 mpg, uses very little oil now that the valve cover gasket has been replaced. I have owned lots of Toyotas and all idled smoothly and steadily. There must be a spark trigger sensor down near the crankshaft but I'm guessing it is OK because the truck has good steady power when accelerating or cruising, and when I first come to a stop. Then the timing starts drifting and it seems like the computer is doing it, but I have no codes so I'm thinking a computer input is causing the computer to vary the timing. .

 

Did you disconnect the battery when you cleaned them and let it relearn it's idle?

 

The battery was disconnected so the computer could relearn the engine tune. I also did this a few times before the recent work, thinking the computer was trying to determine an ignition advance that resulted in a steady idle. It did not help the situation then, and it is not helping the situation now. Before the recent work the idle would shudder down to 300 rpm then recover to 700 rpm then drift back down to 300. Now it goes down to 500 rpm then recovers to 800. So the idle has improved but not a steady 700, which is where my other Toyotas idle in neutral.

 

Just trying to rule out the simple things.
I believe the timing moving is an effect of engine speed changing rather than a cause, what are your Long and Short Term Fuel Trims, MAF, TPS, and Coolant Temp reading at idle?

 

I will have to get fresh data readings for you tomorrow while the engine is warmed up and idling. Thank you for your interest.

 

Is the a/c clutch engaging?

 

I'll try but issues like this can be tricky to diagnose even in person sometimes.

 

I don't usually watch timing on any modern engine, but even at idle I'm used to it bouncing all over the damn place.

The biggest influence would be detonation, so watch knock feedback while it's doing this.

I show more concern for the fuel trim and misfire counters at very first, then I would check throttle position sensor input and output next.

 

This is a good place to start!
you can easily see the AC Compressor on the 2.7. You could watch and see if the clutch is engaging at the same time the idle goes irradic.

if that is the case, and you don't have the $$ to fix the AC, you can bypass the compressor on the 2.7 by getting the "non AC" belt. I'm currently doing this myself since my AC clutch is squealing pretty bad. (Dayco 5070735 Serpentine belt is the one I'm currently using)

 

Regarding the a/c compressor, mine seized up 4 years ago. I pulled a/c fuse and found 5070735 belt too short for my truck . A 5070750 belt fit and my belt tensioner is near the end of its stroke because the rebuilt alternator had a smaller pulley than the original alternator. If this belt ever gets too loose I will go with a 5070745 belt.

Regarding the unsteady idle. I read the following this afternoon on my OBD2 reader with the engine idling:

Coolant temp 185 F, idle rpm 497-802, Long fuel trim -4.7%, Short fuel trim -1.6 to +1.6%, Spark advance 5-11 degrees btdc, MAF .274-.419lb/min, TPS 14.5-15.3%, A/F sensor 1 3.004-3.315 volts, Oxygen sensor 2 .075-.875 volts

The short fuel trim was mostly -0.8 to +0.8%, but occasionally it went to -1.6 to +1.6% as shown above. The SFT fuctuations seem to follow the rpm changes, the spark advance changes seem to lead the rpm changes. At a steady 2000 rpm the advance varied between 24-26 degrees.
There were no engine codes and no engine codes pending.

 

Everything looks normal there other than the TPS seems a little high for hot unloaded idle.

 

Is the 5-11 degree btdc ignition timing at idle fluctuation normal? I will reread the idle timing on a 2007 2.4 Camry. I recall it held steady.

 

My old 2007 2.4 Camry has the following OBD2 readouts at idle in neutral:
Coolant temp: 187, Idle rpm: 656-674, stft: 4.7, ltft: 4.7, spark advance: 15 degrees, maf: .28 lb/min, tps: 15.7 %, A/F sensor 1 : 3.219-3.266v, Oxygen sensor 2: .115v-.995-1.002v
Comparing these to the Tacoma OBD2 readouts:
Coolant temp 185 F, idle rpm 497-802, stft -1.6 to +1.6%, ltft -4.7%, Spark advance 5-11 degrees , MAF .274-.419lb/min, TPS 14.5-15.3%, A/F sensor 1: 3.004-3.315 volts, Oxygen sensor 2: .075-.875 volts

I appears the variable stft, idle rpm, maf, and tps are the differences between the two vehicles. Is the maf or tps causing the stft and timing advance change or vice versa? Is something mechanically causing the idle rpm to vary and the stft is just compensating and the maf just reading ? This cannot be the only 2005 4 cylinder Tacoma with this problem and someone must have already solved what is causing the idle speed to fluctuate. Can timing chain wear or timing chain tensioner problems cause the variable idle speed? Isn't there a camshaft position sensor and what do its outputs tell the computer to do?