Unstable idle: Down the rabbit hole
 

I recently bought an FD, which ran great for three days

Then I noticed the idle started pulsing a bit when warm. It doesn't do it by much, but just enough to be noticeable.

I apologize as it's hard to hear the engine here, I assume my phone's mic thinks it's background noise. This also isn't quite as bad as it has been, but it should give you a general idea of the magnitude and frequency.


So after doing a quick eyeball of the engine, I notice an uncapped nipple on the intake manifold (Looks like it goes to the boost gauge in later model cars). I capped that, and now the car idles very low when warm, to the point where it sound like it may be at risk of stalling. (On the "minor" tick just above the first major tick on the tach, after the gap.

And it still pulses.



So I've only managed to make things worse by fixing it. The CEL isn't on, but this is OBD-I so I'm not expecting much help on that front anyway.

I'm about to go rip out the entire vacuum system and replace all of the lines, but I'm hoping maybe someone here knows some proper tests I could do before I take the plunge. (My enthusiasm for this sharply dropped when I looked at the helpfully provided diagram under the hood)

Ok, update #1:

The air pump connector is shot. It's corroded, half pulled out and basically crumbled in my hand when I tried to inspect it. I'll have to hunt down a replacement, but this is likely the cause for the rather rank exhaust. Then again how the hell did this thing pass emissions two months ago? :scratch:

I was going to comment earlier that the exhaust smelled very rich, but couldn't get a particularly clear consensus when searching whether or not this was normal (I don't have any benchmark for what a rotary should smell like, but this has all of its emissions gear sans the precat (I think)

Once I get the connector fixed I'll do another run and see if it still smells rich, that may or may not be another useful data point. If it helps, the tank has premix which may also affect things.

Since you fixed a vacuum leak, your engine is now not getting as much air to keep it idling higher.
I would start by adjusting the idle air bleed screw under the throttle body to put back some of the air you took away when you fixed the leak.
The previous owner may have set the idle with the vacuum leak...you never know
It could also be a heat soaked IAT sensor if it happens after warm startup and goes away after some driving

The idle bleed screw is certainly on my radar, but right now I'm more focused on making sure nothing else is wrong before I start fiddling with it.

New connector for the air pump arrives tomorrow, so I'll wire that up and give it a drive when the weather clears up to see what kind of overall effect that has on things. at a glance it appears to affect idle stability, but I'm not entirely sure if it's a coincidence or not.

I'm halfway through tearing the intake off to do a general inspection and check the air filter, and noticed the lower intake hose isn't clamped on correctly; so I'll need to go back a bit further and see if the secondary intake needs cleaned.

I also need to check the compressed side for any signs of oil. Might be worth checking the MAP and TPS while I have everything apart. I still suspect it's running rich, but don't have any hard data to back this up yet.

So now things get weird.

Installed a new connector on the air pump, then after about an hour of head scratching discovered the air pump is fused with the A/C (WHY?) and that had blown during the install, likely because I had no idea it was live even with the ignition off and as such did not take proper precautions.

Anyway, took it for a quick spin to make sure the fuse didn't blow again but now the idle problems are magically solved?

Either I'm missing some secondary function of the air pump, or I have everyone's favorite intermittent problem.

Will update this thread again if the idle starts acting up again, else :scratch:

the air pump does help idle by pushing air to the exhaust ports during idle, on a proper setup car without an ACV delete when the pump isn't running the idle will surge lightly.