bureau13

After going through an intake refresh, I am back exactly where I started. My initial, nagging symptom was high idle (1200 or so) and sort of an odd behavior off-throttle where the RPMs would drop, dive and come back up, then slowly settle down to (high) idle. It was actually doing this back with the original ISV, only it was dropping so low it would occasionally stall. I replaced it with a new one, but the idle got slightly higher, and it would dive down only to maybe 600, then bounce up to 1500 or more and then settle back to idle. Weird. I decided maybe one of the ISV hoses had a leak, although smoke didn't find it. So I did an intake refresh minus the ISV ('cause it was new!) and then I had a serious high idle...over 2K. Pinching off the ISV hose slowed it right down, so I took the ISV off and sure enough, it was frozen wide open. Meanwhile, testing the original that I still had, it was frozen but completely closed.

I now have a new ISV. I bench tested it with a 9V battery (that should work, right? Or is that not enough voltage to drive it properly?) It moved once, but I could never get it to move back. I decided maybe a 9V was insufficient, blowing into it it was not closed but not nearly as wide open as before...so I put it on the car. Yay, it's driveable again! Really though...it's now exactly like it was before I started the intake refresh. Idling around 1200 once it warms up (a bit lower at cold start) and then the weird off-throttle thing with diving, jumping up to 1500+ and then slowly settling back down to 1200 or so. WTF.

I'm convinced it's ISV circuit-related. I am suspicious the ISV is still not right, based on the 9V test. Thoughts on that? Even if that's true, I'm also suspicious that I've killed the drivers in the LH. I haven't been able to find any description of how to test that, however. Obviously, the LH needs to be connected for the car to run. Maybe I can disconnect the ISV harness and measure at the connector? Although...I guess I need an o-scope for that? Am I making this too difficult? I'm determined to get this damn thing right...

928NOOBIE

This is interesting....I did an ICV a few years ago...SMTCapeCod and a few others were all having similar high-idle when hot issues..I think the answer for all of us was a new ICV...

I started having some high idle issues this year..only a few years after the last ICV install...my answer was to re-do the side plenum on passenger side...I used a combination of friction tape and some red loctite to ease the grommets on...I used a few layers so it would basically stick to itself....it paid off...and I plan on doing the drivers' side...but I still feel like I'm not where I was...

Thankfully, I am getting idle again...I jumpered B and C on the diag plug and screwed the idle bypass valve all the way closed....this was before I re-did the plenum connection...the engine was gasping but not dying...undoing the jumper immediately brought the ICV alive...but I'm wondering if it isn't closing all the way or quickly enough...I seem to have found a new resting spot with MAF at 250 Ohms and adjustable Zeitronix AFR / O2 sensor set at Lambda (14.7)....but it was a tough run.

How long ago did you do your ICV? I am struggling a lot with thinking the life span of these things is a few years...I haven't put that many miles on mine...and as far as I can tell when I got the car it had the original one..almost 30 years old....wonder if anyone else that did their ICV around the same time as me is starting to have issues?

I'm on the 2nd replacement ICV since I got the car in 2010...way too soon...will be following this thread closely....

Bureau in answer to your testing question I got a Radio Shack 12V box that allows me to test pushing 12V to the right hand connector, then the left...with middle as common/ground if I remember right...both ICV's moved...and so I saved the old one...and it appeared they moved all the way...maybe I was having vacuum issues after all and didn't need another ICV...hope this helps...

bureau13

I replaced my ISV/ICV/Whatever a year ago the first time, and then just last week.

Jumpering the two diagnostic port pins didn't seem to make a difference for me, but maybe I should try it with the idle screw all the way shut, like you did.

I need to test idle switch at the LH, as well as the ISV connections, and I need to test that the LH is actually doing things. I may borrow a friend's scope for that one, I'm not sure how else to effectively test it. And then I guess I should recheck the ISV itself with 12V instead of the 9V battery, just to rule that out.

FredR

Chasing down idle issues seems to be a real PITA. To analyse whether the ISV is operating correctly one needs a scope. Voltage drives the ISV open or closed- nothing more but at least if one knows that is happening then it is reasonable to assume the ISV will work or so I understand.

I suspect the problem you chaps will have is related to having an O2 sensor which assuming it operates at idle, will try to correct for false air and the ISV itself will drive to the closed position- after that false air is driving the idle. When power to the ISV fails completely the shuttle drives to a position where there is an aperture sized to pass the amount of. If you disconnect the power supply to the ISV the engine should hold a stable idle with the engine warm and the a/c switched off. If in this condition the engine is racing then for sure you know you have an air leak and then a question of finding and isolating it. .False air has to come from inboard of the throttle plate. If you have a viable signal from the throttle closed switch then logic suggests it is not caused by a partially opened butterfly. After that the other sources of air entry are likely somewhere in the tank vent system [split hose or opened valve when it should be closed] or possibly in the servo venturi system- both can be mostly isolated for testing purposes.

It certainly does not seem to take much to upset the idle system.

928NOOBIE

Bureau curious...have you done a vacuum test at (I think it's the EZK...I get those mixed up)...curious what you are reading at idle..I think minimum is around 17 inches with a high around 22. I'm sitting at 19 right now...sort of in the middle but I have noticed the higher the vacuum reading the better our vehicles run....

docmirror

The ISV is controlled by the LH developed from a group of signals including the RPM sensor, temp sensor, temp II sensor, O2 sensor, MAF air flow. There are three wires to the ISV. The center wire is tied directly to 12V. Check that for 12V when the car is running. The other two wires are going to be the slew voltages. First measure the resistance from the center pin of the ISV to the outer pins. I think it will be about 12-15 ohms each side. If they are approx equal, then the slew voltages will cause the ISV to either open or close depending on the driver voltage from the LH. Those two voltages are developed at pins 10 and 23 of the LH. Since it's very hard to test those wires, I would suggest getting a voltmeter across the outer two pins which will give a differential reading of the slew voltages(after checking for solid 12V at the center pin to ground). Once you have the voltmeter on those two pins, you can get some real world results from changes in the voltage. Start the car from cold, and make a note of voltage. Then as it warms, the voltage should go down one way as less air is needed to keep running. Turn on lights, blower, wipers, and AC to load the engine and note the voltage change. Then turn everything off and note the voltage change again.

Report the results of the voltage readings. It can be done with an oscilloscope, but you will need a 2 channel unit, and they must be able to run a differential voltage, or you can use the ground lead of one probe on one outside wire, and the tip of the probe on the other outside wire, but make sure you float the input of the scope on AC(capacitive block) so that you are not grounding the ISV pin.

bureau13

Fred: I'm pretty sure I don't have a false air situation, based on smoke testing, etc.

928Noobie: I haven't checked vacuum in it's current state. When I had the extremely high idle I did, and it was roughly correct. I'll check it again.

Doc: Good detailed test info there. So the center pin is +12V? I see a problem with my previous testing already, in that case. I thought center pin was ground, and one or the other outside pins would be given +12V as needed. No wonder my 9V battery test failed...

docmirror

The ISV center pin is supplied from pin 87 of relay XX(fuel pump), via CE panel pin W13, to the injector tie point for cyl 5-8, then to the ISV on red with green stripe. Check for sold 12V(13.6VDC) on that center pin, and it does not vary. The differential voltage drives the shutter either open or closed. Also, check the resistance of the two coils. They should be very close to the same.

bureau13

Will do. Thanks!

FredR

When one does not know what the cause of the problem is and one is sure one knows what it is not, the probability of that being the cause skyrockets -

bureau13

You're not wrong!

Brian'sIris

Docmirror has some good info there. I would check that stuff for sure, and I will if I ever have the same problem again. FWIW, mine tested fine with a small 9V battery.

Jeff confirmed that the ISV is the source of excess air by pinching the outlet hose, so whether the new ISV is again the problem remains to be seen. The LH could be the cause at this point, and swapping it with another would be a good check if you had such. The fact that jumpering the diagnostic leads has no effect on idle makes me suspect the LH. But you should also check for harness continuity from the LH end to the ISV, perhaps there's a broken wire ? This in addition to checking for 12V at the center pin.

As I noted in Jeff's previous thread, the ISV is (should be) free to move by itself with the electrical connector disconnected. I tested this myself, the thing moved and idle changed after revving the engine. There is no return spring that closes or opens it in case of a fault.

Good luck!

bureau13

Just to clarify something, pinching the hose and then replacing that ISV fixed my roaring 2500+ idle. Now it's back to it's old tricks of 1000-1200 idle. I think in both cases the ISV isn't moving (or at least, it isn't being moved properly by the LH...but I'll verify that) but your comment about it moving on it's own as a result of reving the engine is interesting. When I first started the car after replacing the ISV, it seemed to be idling about 800 or so...pretty normal for a cold start I believe. It went up after, but it didn't seem to be so much as it warmed up as after I started driving. Now, it may have simply been warming up, or it may be I revved the engine and it pushed open the (presumably uncontrolled) valve a little bit. It's possible.

928NOOBIE

Just checked the resistance from middle to outer pins on the ICV I removed several years ago and 21 ohms on one side and 20 on the other....any ideas what causes the resistance to go up so quickly? I think I only had that unit in the car a few years....maybe it was just defective?

928NOOBIE

Just looked on eBay there are LOADS of ICV's that will fit the 85-86...cheap cheap cheap! too bad mine's such a bear to replace I'd think about doing it...I have a Bosch one in there right now....I hope I get more mileage out of it than the last one...