As I mentioned earlier this week, I finally got my Nissan Pao back after a prolonged absence due to having to locate, import, and install a new (by new I mean old, but un-borked) transmission. Delighted by my car-pal’s return, I’ve been driving it a lot. And, for the most part, it’s been great – the new transmission shifts nice and smooth, I’ve been whipping it around town and on the highway, giving all 52 of those tiny horses a good workout. But there is one weird thing going on, and I thought we should discuss it.
The issue I’m having isn’t all that severe, but it is puzzling. So, I’m hoping one of you out there who is smarter than me (our research indicates only the bottom 99.86% of our readers are clearly smarter than I am) can help me figure this out.
It’s a problem with the idle. It’s probably better shown than told, so I made this little video:
View this post on Instagram
See what’s going on there? At idle, the engine’s RPMs are pulsing, in a sort of hypnotic, wave-like pattern. It almost feels like what I imagine a sleeping wildebeest’s breathing would sound like. It’s almost soothing!
Now, why it’s doing this isn’t the mysterious part; there’s a lot of old hoses in there, so there could be a vacuum leak, and that could cause this to happen. It’s a strange computer-controlled feedback carburetor setup — one of those odd things that existed as the world transitioned from carburetors to near-exclusive fuel injection, so there’s plenty of causes for weirdness. It also could be a bad idle air control valve (IAC), which I think is this bit here, highlighted in yellow:
That’s a pretty likely culprit for the large-mammal-sleeping-breath-idle, but that’s not really what’s puzzling me. What’s puzzling me is why the idle evens out when I put in the clutch?
I mean, the clutch should have nothing to do with the engine’s air-fuel system at all! It’s just disconnecting the engine from the transmission mechanically. And when the engine is idling in neutral, how different is that, really?
When I drive, I’m adding more throttle and everything is engaged, so the problem goes away. But as soon as I go back into neutral, there it is – until I push the clutch pedal.
Why?
It might have something to do with what you see explained here:
See those gears and shafts that are always connected to the engine, even when in neutral (unless you’re on the clutch)? Normally, I think the drag of those are pretty negligible; in neutral, the engine just spins them like they’re no big thing, slushing a bit of oil. But I’m wondering if the equivalent of those shafts in my transmission may be providing just enough load so that it causes the idle to drop and then surge, and once that load is removed from the engine via putting the clutch in, the engine, now freed of even that tiny extra load, idles more easily and at a consistent RPM.
Could that be it? If so, that’s pretty bonkers, that something like that – the tiny load of the transmission when not even connected to the output, and really just spinning some gears and bearings in oil – should make such a difference. I mean, once I figure out why the idle is pulsing, be it a leaky vacuum hose or bad IAC valve or whatever, it should all stop, anyway.
But for now? I ‘m still a bit baffled by why the clutch should be smoothing out the idle in neutral! I’m open to hearing all your theories, whether they confirm my crude theory or not. It’s strange!
” bad IAC valve”
That would be my first thought.
I had a BMW 633i that would have similar idle issues. I replaced the IAC with an under sink water shut off valve. It not only fixed the problem, but it let me change the idle speed by turning the knob.
I experienced similar bizzare-o idle issues in my Mitsu that took me months to figure out. In addition to the vacuum leak checks other folks have mentioned, check every single ground point you can find. In my case, a poorly grounded AC control module was sending wild signals to the ECU causing really erratic idle behavior. It’s possible that something was disturbed when the transmission was swapped.
Good luck with the hunt 🙂
You’re gonna need a new Johnson rod.
Nah, I think it’s the turboencabulator. Good thing the Pao’s a hatchback, on sedans the back window would have to come out for access.
Could also be a torqued-out sludge pump…
As others have said, it’s almost certainly vacuum-related and the most likey explanation seems to be that the load of spinning the input shaft is enough to change the equation.
There is one other thing you should check though. Stand in front of the car with the hood up, and have someone press the clutch. Watch the firewall where the clutch mechanism passes through, see if you can see it flex when the clutch is depressed. In certain vehicles I’ve seen a surprising amount of movement of the firewall when the someone presses the clutch to the floor. If there are vacuum lines mounted to the firewall, this movement could be enough to affect a vacuum leak.