Mike Frye

I'll try to keep this brief because I have a habit of writing really long posts.


Symptoms: Oscillating idle when cold (sitting for 5+ hours), settles down after 45 seconds or so. Not a problem when warm. When warm, it starts and idles smoothly until I hit the gas once, then when it drop down to idle again, there's a miss.

Tests: Adam and I went through the test plan of the '85 LH and EZF, and when we got to the MAF test we got some funky readings. I need a little help with it from here though.

Results: One of the tests says to meter for voltage across pins 2(+) and 4(-) of the MAF plug and I should see battery voltage (with pins 87 and 30 of relay XXV jumpered). No dice. Further tests showed that pin 2 had positive voltage (12 volts from 2 to engine block ground), but I got no continuity between 4 and engine ground.


The ground point for this is MP 9 (I'm pretty sure) which is the ground at the passenger end of the ground bar at the rear of the valley (by the fuel return lines). There are about 4-5 wires that are all grounded here including (if we're reading the diagram correctly) the ISV, the MAF and the LH and possibly the injectors. When I took that off of the ground, the fuel pump started running(!) and when I metered across between those wires in the wire ring and ground, I got 9 volts (really). When I cleaned up the ground connection and wire ring and put them back together, the fuel pump stops.


So: With the key off and relay XX jumped out, I'm getting voltage backfeeding through that set of wires in that ground bundle.

I think this means I've got a grounded wire somewhere or else a relay is triggered that shouldn't be.

I appreciate any help or suggestions you can offer.

Edit: 3/16> Changed relay XX to XXV, that's the one I jumpered.

Lizard928

Peel back the MAF and ISV boot and look for frayed and touching wires.

Adam Geist

To all,

The other oddity we observed yesterday is when jumpering the B&C terminals in the diagnostic port, the idle speed increased. According to the test plan, the idle speed should decrease and that is when the base-line idle speed is set. We performed the same on my green '86.5 and the idle speed decreased (as it should).

According to the wiring diagram, the ISV is also grounded through MP 9, if this helps us figure out the issue.
Adam

Lizard928

did you unplug the ISV and do the readouts for the pin outs?

Adam Geist

To all,

I'm sure Mike will chime in on this. When Mike disconnected the ISV connector, the idle speed increased significantly. Mike shut the engine down immediately.

What should the voltage readings be at the ISV connector when it's disconnected and the B&C terminal grounded? From what I understand, the center pin is 12v positive and the outer pins are the grounds.

Adam

Mike Frye

Thanks, will do. I was thinking this was unlikely since we were moving things around quite a bit and the results weren't changing (consistent readings instead of swinging around). But I'll check that.
Yes, no change with the ISV plugged in or unplugged.
The test was: Unplug the ISV, connect the MAF and Temp sensor in the airbox and start it. It started at 2k rpms and stayed there without attempting to come down to idle. I assume this means the last position of the ISV was pretty much wide open, so it just stayed idled at the top of the possibly range.

Adam, if that's true then we tested your spare wrong. We had the alligator clip on the center pin and applied 12v+ to each of the outer pins to get it to open and then close. Granted, we had to jiggle it to get it to work after each cycle, but it did work. I'm pretty sure it should work with either polarity, it just means that the one we thought was open is close and vice versa. However, you ask a good question, with B&C jumpered, we should meter the ISV plug in your green car and then compare it to mine.

Thanks again for your help Adam. You should have come back down to my place for some Guinness Saturday. The craic was good!

WallyP

I think that the ISV should close with no power applied. Sounds as if yours is sticking.

aggravation

Yes a good ISV will close with no power leaving a small opening...maybe 10 to 15% of the total window. I've tested a few good used ones and one new one they all behaved the same way.

Mike Frye

Thanks Wally and Craig,
Interesting. I have my old one and Adam has a couple of spares, Adam wanted to change it out to test it but I was stubborn because it was brand new when I put it in last fall.

I'll try swapping it out, because if it's sticking when brand new, lubing it up will probably only help for a while.

So that would explain the weird issue when I put the test jumper in where it idles up higher instead of lower, but what about the ground issue with the MAF plug. We were thinking that the power that is backfeeding into that ground is causing the ISV to go out of whack.

Maybe if I just try removing relays for all the things that are grounded through MP9 until the voltage disappears?

John Speake

Pin 4 goes to MP VIII (8) but OI think you have found the correct point. There is another feed of ground to the MAF to pin 3 via the LH ECU, but that is supposed to be a dedicated analog ground, not a pwoer ground. You need to take that bundle of ground wires apart at the ground terminal. Sometimes there is corrosion within the bared wired at that terminal.

First, check that the wire is well connected at the MAF connector. then imvestigate that terminal.

Don't get sidetracked by fuel pump running when ground is lifted off the block. In fact, don't do that, you are running sevweral amps through something else..... disconnect battery before lifting any ground point.

Have you set the idle mixture to 1% with O2 sensor disconnected, as described in the WSM ?

Mike Frye

Thanks for chiming in John,
Thanks. It looks like it's soldered together so I was hesitant to start breaking it apart and making new splices thinking it's more likely some terminal device or relay than the actual wiring.
OK

I didn't know I could damage anything by lifting ground points while the battery was connected. I hope I didn't cook anything. Thanks for the tip.
I'm working from the LH/EZF test plan and the only way I can set the CO is using Ken's Blink'r, but it it did set up properly and blink as described in his documentation. I thought the test plan for '85 would have the same info as the WSM, but I don't remember anything in the test plan saying I should disco the O2 sensor while adjusting anything.


It just seems really weird that something I didn't touch should go bad instead of something I worked on. It idled fine before the top-end refresh and it didn't afterward. Common sense says it should be something I did, or related to something I did (or didn't do properly).

Thanks again, I'll print out the part of the WSM that deals with this and see if there's any difference.

John Speake

CO adjustment is on Page 30 of the test plan.....

Mike Frye

Thanks, I'll look it up. I don't remember anything about the CO test requiring the O2 sensor to be disconnected.

Seems like I may have a whole raft of possibly unrelated issues going on. Weird.

Adam Geist

I'm trying to figure out the impact on the idle speed when the B and C terminals in the diagnostic plug are jumped. If I'm reading the wiring diagrams correctly, terminal C is the ground (MP IX) and terminal B is tied into terminal 3 (full load contact) of the TPS.

Am I correct in assuming that grounding terminal 3 on the TPS via the B&C jumper provides the same input to the LH & EZF as if the engine is under load?

Does this retard ignition timing and change fuel enrichment which results in a slightly lower idle speed?

Thanks in advance,
Adam

Cosmo Kramer

From what I remember when I had mine out on my 85, the ISV is a 3 wire and not spring loaded and stayed in position after the power was disconnected. I think the 87+ with the 2 wire connector is spring loaded and snaps back into the closed position when power is removed.