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:
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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!
Is the clutch master Vacuum boosted? If so there might be a leak in the path the air takes in the clutch out position. Maybe when the clutch is in no leak occurs because a worn out component of the master cylinder is in another position? Just a thought.
Better run a level 3 diagnostic on the microfusion reactor.
Check the battery cables, as my svx dropped volts and hosed all the sensors to have 13.6v or so, but we’re getting 9ish at times. Low voltage sensors were low too as the cpu was low. Likely not your immediate issue but a possible one since corrosion happens to all old cars. A efi car seeing low voltage has every sensor lie to it.
Good point, I stored a mate’s Z32 300ZX for a while and the aftermarket alarm flattens the battery quickly. I kept charging the battery up but it would run weird at times.
Replacing the battery solved the issue as the electrical system was just slightly down on voltage below the normal range and it was freaking all the sensors out.
Good chance the Pao has some dinky little battery leads and earths that are now corroded after 30 years. If that and/or the battery is ancient as well it could be operating just outside what the electronics are meant to be getting.
I can see how a vacuum line could get damaged during a transmission swap.
However, assuming there is a clutch position sensor (I think there is), it should be super easy to eliminate the possibility of it being an electronic issue. Just open or close the switch without actually pushing the clutch pedal and if the idle changes you know it’s electric, if not, it’s the parasitic transmission drag revealing a mechanical nature to the problem. Viola’
Shouldn’t that be the top 99.86%? Just a wild guess when you put in the transmission did you make sure the clutch plate was not warped? Seems a warped plate might go from up against to apart causing this
Did you check the idle air controller for venison ingress?
Oh Deer….
…I bet that will take a couple Bucks to fix.
Might save a buck.
Congratulations. You are now one of the idle rich.
Possible causes other than a vacuum leak or IAC valve:
Bad or weak battery
Bad alternator
Dirty air filter
Fuel pump
MAF or TPS sensors
Deer guts
Love Bug or Christine syndrome
If it’s the last one, burn my email address.
The way I read it, you’ve answered all your questions. I mean a wee small engine at idle is a lot more sensitive to even minimal load then some V8.
(RRRrrrRRR, maybe that’s its deer call)
IAC – older EFI Fords had issues with these all the time. Clean the EGR as well.
A good soak in carb cleaner may fix it.
Clean the pintle valve with carb/brake clean. Make sure it actuates freely.
“Fixed” a ranger by unplugging the maf. My car that would kill it, but the ranger was fine with closed loop only.
It’s a vacuum leak. Pushing in the clutch unloads the engine, which lightens the vacuum pull from the intake. At that point the leak is probably not seeing enough vacuum to pull in extra air. Start hunting around with starter fluid, and concentrate on the vacuum hose connections, you’ll find it pretty quickly.
The oscillation is likely from the leak leaning out the car and naturally lowering the ability to pull a vacuum, which stops the leaking momentarily.
“Start hunting around with starter fluid,”
Wait, what?
This is something I’ve never heard of
carefully directed sprays of starter fluid will cause the engine to speed up when sprayed at the leak. the vacuum draws in the vaporized starter fluid.
OK, that makes sense. Thank you
Deer are attracted to starter fluid.
I got a cheap Honda Element from a coworker and it’s idle issue drove me crazy. During cold starts, it was normal. When it warmed up, though, it would pulse up and down rhythmically. A new throttle position sensor, throttle body, and some other random shit later, no change. The throttle body actually had coolant lines running to it, and the old one had some corrosion and a leak there, so I figured it was worthwhile even though it didn’t fix the problem – but it actually turned into a hint to the root cause….
Those Honda engines have their coolant temperature sensor mounted way high up. When the coolant level gets low (which happens when you have a leak) the coolant doesn’t cover the sensor. The engine keeps seeing temps jump up and down as it gets splashed with hot coolant. For a second it thinks it’s warmed up, and then the next second it thinks it’s cold and goes to high idle, and then back down, and back up.
20 hours of googling and reading forums, a bunch of unnecessary parts (including fouled O2 sensors from the super rich idle), and some lose sanity later, the stupid thing just needed a few ounces of coolant added.
That had to be fun
Wow! That’s kinda crazy!
Had a late 80s prelude do the same.
It was the EGR valve (and or the vacuum connector for the EGR valve)
If you press the clutch pedal down you are reducing the engine load (slightly) which increases engine vacuum (slightly). The EGR valve activates via engine vacuum.
The EGR valve diaphragm could have some small vacuum leak in it that with higher vacuum goes away, and lower vacuum causes the idle issue.
Could be the IAC as well, but whatever it is… vacuum is either the direct or indirect issue.
Start spraying brake cleaner (lightly) around certain areas to see if the idle issue gets worst or better. Bob’s your uncle.
This is my suggestion too, spray around the engine until it changes the RPMs and you’ve found the leak.
First. as part of the 0.14%, let me say that your acknowledgement has warmed the cockles of my heart..
My first thought (after the acknowledgement, which I acknowledge was the first thought) is that it would be even weirder if it stopped when you opened the door or sat down, so there’s that. Just to be safe, though. I suggest that you treat it as something possessed by an evil force, which, though I am not that familiar with Ashkenazi folklore, I think is called a gobot.
I’m not sure why everyone is going for IAC first. If this didn’t happen before the transmission swap and is happening now, post-transmission swap, plus it goes away with the clutch depressed… That to me says “transmission.”
Even with the transmission in neutral there is still some drag. You can see it happen with the car in the air and the transmission in neutral – the drive wheels will still spin.
So what’s the cause? I don’t know. Did you use non-spec gear oil? Are your CV shafts installed correctly? Is the “new” transmission in good internal condition? No clue – but I’m still saying it is a transmission issue, not an idle air control issue.
https://www.team-integra.net/threads/car-jacked-up-wheels-spin-in-neutral.126139/
The IAC valve is always a good thing to check because they’re usually cheap, and also usually very easy to change. I just googled for the price on IACVs for the Pao (found the Figaro, which is pretty much the same) and it’s $400, not the $20 I was expecting!
My lawn mower does this. I don’t mow much so I just hold the throttle down when I start it and hope for the best.
Gien the small size of the engine, the drag of the transmission is enough to load down the engine which cause the carb to dump more fuel to compensate, which overcomes the drag of the trans, causing the idle to drop, the drag to return etc; the clutch engagement disconnects the trans entirely like you said so the drag isn’t present to induce it.
Given that the carb is jsut a fancy toilet, I don’t have a better explanation than that, but I Agre the IACV is likely the culprit.
You need to plug in your OBD-II diagnostic tool and ask the car what’s wrong with it.
Oh, wait. Forget what I said.
RCR Has a video on tweaking Idle speed that may help. Even if IACV is the issue, sometimes tweaking idle up a hair can at least mask it.
Start with the Idle air control valve. If that doesn’t fix it, probably time to start hunting vacuum leaks.
Or if you’re cheap, hunt vacuum leaks first.
Torch is besties with David Tracy, I see vacuum leak hunting in my rusty crystal ball
My first thought was IAC. But if that has one of those Hitachi carbs they can do some weird things.
It’s similar behavior to what I’ve seen a LOT of posts about for NA Miatas, which are Fuel Injected, but have a similar IAC valve, and when they need adjustment, the idle is very similar. I can’t speak to adjusting or replacing a Pao IACV, but that’s a good place to start, especially if its a cheap and easy to replace part.
Yeah IAC valve is probably it