The 71 and 72 F engine emissions manual states idle speed is 600 rpm. In 73 it changed to 650 and stayed there.
But I think it's only for emissions in the US, not the optimum for the engine (when it was new). As posted before the 1971 F engine FSM states 500rpm except for the US (probably for the emissions).
I know this 1971 FSM has a reprinted and updated version in 1974 to cover up to 1974 but I never saw a scan of this version... Would be curious to see what it says for the F.5... Because otherwise it's always a guess game between the 1971 F and the 1975 2F to guess which one is the closer in spec.
Some Toyota specs are spot-on, some are flexible.
Over space and time, some specs mean less and less. You learn where your engine likes it by fiddling with it.
I can get mine to idle very slowly. I can get a pretty smooth idle at 500 and 400 and 300ish, but there's a rough spot just below 600, and the lower the idle the longer the hesitation. My engine seems more comfortable just over 600, so that's where i keep it.
Maybe Toyota knew about this hesitation, couldn't fix it so just changed the specs, maybe i just suck at tuning.
I got a mechanical oil pressure gauge on my engine and do not understand your answer.
Fiddling with my engine today, i thought "let's see". As i had the dizzy loose and a timing light on it, i simply retarded and watched the rpms. Got a decent idle in the low 300s and walked around to view the oil pressure gauge. 28psi.
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Yes while tuning up my engine I had it as low as 410rpm at some point and it was idling smooth (but did not tried to drive it or checked the oil pressure).
Currently at 530rpm the idle is smooth, acceleration is linear, engine feels lively on the road, engine has never been that silent.
Oil pressure stays just bellows the 2nd lines (~3/4) on regular roads and on idle, and goes juste over the line at freeway speed. Even if I don't have a mechanical gauge I tried to check if my OEM gauge was consistent. I ran the engine on starter for 30 sec, after 10 sec the pressure slowly started to climb and then on a linear manner up to the first line after ~30 sec. It didn't jumped to the 2nd line or something like that.
I did 500 miles since my idle is set at 530rpm and my engine just seems happy like that.
I recently beat my best fuel consumption ever with a whopping 15.9mpg that I wasn't even thinking possible...
The engine braking is awesome offroad but it's actually really nice on road too, particularly to handle all the roundabouts that we have here x)
A note, that I just rebuilt my carb, my engine has less then 100k original miles since factory, valves were changed 4000 miles ago, it's the F.5 without the oil bypass issue, and I use a heavy 20W50. So yeah the story may be different with another engine.
I really see only 2 downsides with the low idle:
- Alternator not energized: should not be a big issue in normal driving scenarios but if you idle for a long time and charge a lot things on the cig lighter you may see the voltage getting low.
- Cooling: As those engines have a huge thermal inertia, if you go to idle just after you pushed the engine for some time, the temp raises a bit. As I have a very steep 1km driveway to get home, that I tend to drive like a rally special (
), I need to high idle for 30sec-1min for the engine temp to regulate before to shutoff.
Still thinking about using my TP (if my vacuum valve is still functional) connected to a button to force a high idle when wished.
