There is a ground in the battery tray area on the back side of the firewall, is this the one you are thinking of?

As for the 5V to the AFM, as your car is an 87 you should only have 5V at the AFM. Early cars ran a higher voltage (I forget what, 12, 8, 7, something over 5 for sure), and swapping the 924S DME into your car could damage your AFM as a result. Early DMEs should be kept with early AFMs only.

The spark plug wires are an issue but they should be causing a stumble or low idle, not an occasional high idle. Change them, but I don't think they are causing this issue.

You say you checked for vacuum leaks, but you don't say _how_. The best bet is to actually pressurize the intake (not much, 2-5psi is enough) and listen for hissing anywhere. All the lines can be good but fittings or other items can be leaking. Some people on here have cobbled together smoke testing devices but if you have a good ear and some soapy water in a spray bottle you can do it without smoke. Plug the AOS hose and use half an oil filter with a tire air valve in it as this should fit the intake boot perfectly. Pressurize the intake (you may need to rotate the engine a bit to get all the intake valves closed) and find the leaks. This issue screams of a vacuum leak.

 

I agree that the spark plug wires shouldn't be it.

Strange that the Shop Manual only states 8V for the AFM and doesn't differentiate, but good catch. I was able to confirm that the ISV is working with the key on today, so that's looking good. Slowly ruling out the DME as any issue with that one.

I checked the vacuum stuff mostly with an ear and a stethoscope and for the small lines pulling a vacuum by sucking on the hose and holding it for a little bit to see if it bled off. It doesn't taste the best, but seems to be a good check.

Don't have an easy way to pressurize it at home, though I DO have a good air compressor.

I've seen people say their AOS was bad, but frankly I'm not certain how you know that.

As for the ground, I know there is one by the battery. The shop manual scematics also lists 7 ground under the hood. Ground VI is the battery, ground V is by the right headlight, ground IV is by the left headlight, Ground III is called 'inside firewall ground', ground II is called 'inside firewall electronic ground', ground VIII is enginer blcok to DME ground and Ground IX is the cluth bellhousing to battery and DME.

I know where all of them are except ground III and II, both of which are shown just in front of the driver. I'm assuming ground II is under/at the fuse/relay panel, but I don't see it unless it's under it, nor do I see ground III at all which is listed as in front of it.

 

It does, but your copy probably doesn't have that page updated (the send out individual pages throughout the different production models of the cars the manuals cover)

When your intake, J-Boot, and air box are full of oil, that's your first hint to start looking in that direction.



If they are what I think they are, if you look straight up under where your feet are when you drive, you should see a couple bundles of wires going to self-tapping screws in the chassis. Not sure if these are the ones that are indicated by those numbers, but by the descriptions it would make sense (one is a bundle from the instrument cluster and some other things, and I believe the other has grounds for the radio, etc).

 

DarylJ - Thanks. I don't have the oil problem, which is good (just replaced those seals when I took the intake off, and no oil or milkshake).

I was beginning to think those two grounds must be under the dash somewhere, just haven't had time to look. The schematic makes it a little hard to tell since it shows them in the engine compartment. Sounds like they look just like the ones behind the headlights.

 

The 924S uses a LATE DME and Late AFM.

 

Well, it hasn't repeated it's wild idle since last weekend. I'm starting to think that maybe that was related to the high humidity last weekend and the faulty plug wires shorting to the block and maybe causing a floating ground.

Put new plug wires on tonight and threw in a new set of plugs just for fun while I was in there (and to test out my new torque wrench from chistmas. Gotta love father-in-laws who are gear heads and know their way around a Northern Tool).

All seems to be better now, other than when the car gets warm it seems to idle up just a hair to about 950 to 1000. I'm starting to think I shouldn't worry about it until it does something drastic. The car runs fine otherwise.

The only things left under the hood I haven't touched are the AFM, the ref and speed sensor (though my father tells me he had one of them replaced last year before I got the car, but can't remember which), the O2 sensor (which I have unplugged at the moment), the oil pan gasket, the alternator and then inside the DME itself.

Just about everything else has been resealed (that includes resealed AOS, throttle body, front seals, intake manifold and fuel injectors) or replaced (new intake hoses (including vacuum lines), throttle switch, DME temp sensor, fuel pump, dampener and pressure regulator, rotor and rotor cap, spark plugs and spark plug wires, and engine mounts).

Whew.

 

So today it's pretty cold in South Carolina. I went out after work to start the car which has been sitting in 30ish degree weather most of the day and start the car and it DOES NOT go to high idle like I expect it too (I'm not certain it ever has, but it's rarely cold enough here to find out frankly). After warm, it runs fine (other than my normal slight variation in idle I'm still hunting down).

As you might recall I've done most of the typical parts. The only intake parts left to replace with new are the AFM and the fuel injectors (though those were resealed when I had them out for the ISV replacement late last month). I also haven't replaced the O2 sensor, but it should be ignored when the car is cold from what I understand.

I've checked the AFM and it appeared to be okay, but like the DME block sensor, it's hard to tell if it's completely right since it works over a range of temps.

So what's the likelyhood that it's the AFM? Or that I got a bad DME block temp sender? Anybody know?