Working on an '85 US auto.

First symptom was a giant sucking sound when warm - the hose from the ISV to the top of the manifold is being sucked flat at idle, as if there is no airflow through the ISV. Car will idle with idle screw turned fairly far open and runs strong off-idle.

Capped the idle stabilizer in/out hoses and with the ISV free to see, can see that when the car is idling the ISV is fully closed and is being told to do so - unit is pulsing itself fully closed. So, there's the problem. ISV is brand-new - old one was hanging up (verified on bench).

I've confirmed that the idle switch is contacting at both LH and EZF, WOT switch not active at idle, confirmed wiring to ISV with no cross-short or short-to-ground. Jumping the test connector at the front of the motor does not change the ISV response - stays fully closed.

Anyone know if this is a possible failure mode of the LH or if there's something else I'm missing?

Thanks very much - really stumped here.

 

I suggest you test the new ISV to make sure it's capable of operating correctly.

The '85/'86 ISV has a 3 pin connector. The middle pin is the ground and the outer pins receive a signal from the LH to open and close the ISV. Get a hold of the wiring diagram and check the 3 pin connector to ground and back to the LH connector under the dash for continuity.

 

On the ISV/IAC/ICV connector, the center terminal is 12V+, the outer two are grounds to open or close. (LH/EZF pinouts)

The 12V+ is shared by the injectors, O2 sensor heater, and fuel pump, through the fuel pump relay, so normally, the engine has to be running to have power at the center terminal.

 

If the engine idle speed is above spec, the ISV would drive closed trying to bring it down to spec.

 

Yes, this is a possible failure mode of the LH. When I bought my '85 in 2006, I did an intake refresh, timing belt, plugs, wires, caps, rotors, MAF, and it still had some weird running issues. Wasn't going to spend the money for a LH unless I could verify that it had failed. By chance I was browsing here one day, and someone described this specific symptom: attaching a jumper wire at the test plug at the front of the engine produced no noticeable change in the idle speed. No burp, flare, drop, etc. Nothing. It did not respond at all. Someone from 928 International quickly responded and said that was a symptom of LH failure on the '85-'86. Mine was behaving the same way. To take things a step further, I was lucky enough to have a friend nearby with a 86.5 (which uses the same LH), and he plugged mine into his car. Sure enough, his car ran just as poorly as mine. Mystery solved.

I would reinstall the ISV and then attach the jumper to the diagnostic plug to see if there is any change whatsoever in the idle speed. If there's no change, I'd put my money on a defective LH. Just curious, where are you located?

 

Thanks, all.

Adam - have previously confirmed new ISV operation. Opens and closes correctly with voltage applied; compared to another currently operating unit I sourced today; operation is the same.

Ken and Adam - I was with Adam on this one; the car shows continuity to ground from the center connector, voltage at end pin when running. Either this is the only time in my life I'm going to disagree with Ken or we may be onto my problem.

Wally - Independent tach shows me approx. 550 rpm with the ISV lines plugged and the bypass screw opened slightly. Though I will take the polite inference of your comment and check again for vacuum leaks that may be driving my idle.

Further info, if it helps: only happens warm. ISV is open and functioning during cold starts. If I get on the gas at all, ISV opens proportionally with RPM up to about 4K. ISV opens slightly with A/C request, opens partially when I disconnect Temp II. Temp II wiring and resistance check out.

'Preciate you letting me pick your brains.

 

JHowell - we overlapped posting. Thank you. That is as I feared; I spoke with a PCNA rep who is a longtime tech who said the same.

I'm in Minnesota. I'll try to source an LH locally; could be tough.

 

I stand corrected - Ken is right that the middle pin is 12V+.

I use a Power Probe to test small motors and should clearly label the test leads.

Sorry for the confusion.

 

Many of the inputs and outputs of the LH/EZK have voltage on them until they are used for grounding.

For example, the WOT and idle inputs measure +5V until the throttle switch grounds them. (Large resistors limit the amount of current.)



Note: I only barely understand this e-lec-tronical stuff.

 

I've got a spare set of LH/EZF. You're welcome to borrow them for a few weeks for the cost of shipping both ways. I just need it back in time for SITM (dyno day the following week and I have some tests I'll need them for.

PM with address if you need it.

 

LH should be repairable if you can borrow a spare while I repair it for you.

 

The old stuck ISV/IAC/ICV might have overheated the 'open' power transistor.



This pic is a work in progress, don't treat as gospel!

 

Nice!

Thank you all...it'll be a day before I can get back to 'er.

I have sourced a good LH locally for testing - charitable owner - and will be trying it shortly after I review everything *I* could have done.

Will report back.