First, some background. I just replaced my ISV (ICV, whatever) and chased down some incorrect vacuum routing, and some self-created vacuum leaks. Thread https://rennlist.com/forums/928-forum/992389-isv-icv-replacement-86-5-a.html

I was doing that to fix a very high idle and a tendency for the RPMs to dive very low when clutch in, very occasionally resulting in a stall.

I now have mostly-correct vacuum routing (plugged off some emissions-related stuff due to not having an air pump and having a broken nipple on the thermoswitch) and my smoke test doesn't show any leaks, although I measure about 16 in-Hg at the EZF (20-21 at the brake booster) so I may have a small leak somewhere still.

Now, my idle is still a little high (1000-1100) but (ducks head in shame) it's always been that way since I owned it. Haven't messed with the idle adjustment screw yet, so I will see what that does. My idle switch itself passes the click test and I can see it operating via DMM at the LH connector). The thing is, if I have revs up 2500-3000, and push clutch (e.g. as I'm coming to a stop) the revs do the dive, but only down to about idle, then jump up to maybe 1600-1800, then slooowly settle back down to idle. This seems weird. Additionally, hard to explain but coming off throttle and coasting seems to have a delayed jerk, that wasn't there before. Tach doesn't really show this, but it's very perceptible. Almost like there's some lag before it REALLY comes off throttle.

Here is a vid of the tach, just sitting in my driveway, when I give it some gas and then release, to simulate the behavior:

 

I don't recall seeing that exact behavior on my early '86. But I can tell you that I had a weird power oscillation at low constant speeds (cruising in parking lots) that completely disappeared when I fixed my leaky cam covers. It had been driving me crazy, thinking something was wrong with my differential, or my transaxle mounts, or even the suspension.

 

The "dive" followed by the settling to your "normal" idle speed suggests the ISV loop is doing what it should...

So now is the time to adjust the idle speed screw.

 

A rich condition can make that symptom

 

Yes both to idle speed screw and rich issue. My car has cat delete and a programmable O2 plus AFR guage...I have the MAF Ohm setting quite high (rich). That being said I had issues you are describing above...plus difficulty starting (would catch fine..then rpms go up...then fall and car almost dies).

Opening the idle adjustment screw addressed the issue.

FYI a little goes a long way. My suggestion is go in quarter-turns counter-clockwise (allowing more air) and once you see improvement go in 1/8 turns back and forth to find the sweet spot.

You may find based on your MAF Ohms setting the car doesn't quite idle at 675 ish...don't be too worried about that...mine idles just under 800 most of the time again because of how I have other variables set up. Key is starting / de-celeration back to idle working the way it should.

Also I found that getting it so it is as closed (least amount of air being allowed but still behaving properly) makes the car perform better esp. on acceleration...I think on my car something to do with vacuum...my car makes about 18.5 - 19 @ idle @ EZK ...from anecdotal information mid-way between 17-22 ...perhaps with higher idle vacuum may not be as important...just some personal experience.

Idle set screw from my experience is part of the overall equation to the car behaving properly....a change in one of the other variables (ICV replacement, MAF Ohms adjustment, vacuum level changing etc) can mean adjustment is needed to regain balance...

 

I believe you are supposed to jumper the two pins on that test port to bypass the ISV, in order to use the adjustment screw, right? I couldn't find my jumper, but just for kicks tried to see what impact turning that screw would have....basically, it didn't seem to change anything. Is that normal, for no jumper? Doesn't seem like it should be...

 

If you don't disable your ICV, wouldn't it just open up to stabilize the idle as you close the throttle body screw?

 

idle circuit must be disabled before making idle screw adjustments

 

https://rennlist.com/forums/928-foru...maf+adjustment

https://rennlist.com/forums/928-foru...maf+adjustment

 

Well, that certainly explains why it wasn't working! Time to make up a new jumper.

Thanks

 

You could also turn in the Idle mix adjustment on the TB (the 7mm) in and count the number of turns until it seats, should be about 1/2 to 1 and 1/2 turns out, never full seated or unscrewed way out.

Once you know where its set at, remove it completely, it has a 'O' ring seal and a ball type (not needle) end/seat.
I've found most of them are gunk-ed up and can use a bit of carb cleaner, then re adjust as per the instructions

 

If it's not gunked up, it would be one of the few things that aren't :-) I'm making a Blink'r type thing with a switch, rather than trying to stuff a wire jumper in there...if it doesn't adjust as expected with that, I will do as you suggest. Thanks!

 

NOTE if the blinker switch will not shut off the ICV circuit,
OR the jumper wire inserted will not shut off the idle circuit.
then the computer has been damaged from a bad ICV

 

Fingers crossed...

 

OK, so back to this issue...if I warm the car up, then jumper B and C at the diagnostic port, I hear a change, but there is not a drastic change in idle speed. I actually think with no jumper, the idle speed varies slightly up and down every few seconds, but not enough to register on the tach, if that makes sense. I think that stopped with the jumper in place, but what I really noticed was a loud whistling noise, that I hadn't really processed before, went away (or at least is drastically reduced in pitch and volume).

Theory: Vacuum leak in the ISV or (more probably) the hose connections to the ISV. Does anyone know if, engine off, the ISV is in any particular configuration, which would cause a smoke test to pass, but open up a leak when the engine is on and ISV active? It makes sense in theory, but I don't know the state of this valve in various conditions.

Also of note: Idle speed control did nothing even with the jumper in place. Anyone know why that would be?

Can I change those ISV hoses without removing the whole intake? I really do not want to open up that can of worms...