86 944na: what's normal for the MPG gauge..?
 

Must be a vacuum leak, the gauge slowly fluctuates between 29-31 at idle when engine is hot. What's normal at idle? I would expect it to be rock steady. I read a previous post that said his was 50!

When cold, the idle RPM is goes low, almost dies, about 300, then surges to 2000, then back down again to 300.
I sprayed all the vacuum lines with carb cleaner, but unable to locate any leaking hoses/connectors. I do hear a faint hiss from somewhere in the top center intake area when the engine surges, then hissing stops when rpms lower... is this normal?
After warmup, RPM is about 1500.

Sorry for the ramble... just replaced cap,rotor, wires, plugs, fuel filter, air filter which improved this condition but didn't fix it. Exhaust is tight, no leaks, car is stock. I'm thinking next idle control valve and o2 sensor. Is it true the o2 sensor only is activated when hot?

Any ideas? Yes, I already did a search for previous posts. Many many thanks

BOB
86 944na

with the fluctuations and stumble you describe, i'd say the "idle control valve" is the next logical place to check.

The idle control valve is just a little electric motor that open and closes depending on the electrical signal sent it from the DME. It is most likely the signal that is at fault. My computer always sends a closed signal to mine and it stays closed so mine is low rich idle at start up and ok when warmed up. Yours sounds like a open and closed signal at alternating times. I have been after this problem of the wrong signal to the idle control valve for a long time and can't find the solution. I have checked several idle control valves , several loaned to me and all are ok. I'm not saying none go bad but if an electric motor stops working, it normally stops.

[This message has been edited by T86951 (edited 09-21-2001).]

The MPG gauge is not directly connected to anything like a vacumme line. It comes from your computer and shows (in general) the relative time your fuel injectors are open. I've had a scope hooked to it and it's is a square wave the varies between 2ms to 45ms or so, depending on the throttle. (idle 2 ms, wide open 40ms or more)
If its jumping around at a steady RPM, then your injectors are doing the same and something is messing up the computers output. The mpg reading is a end result of all the computers work in figuring out how much fuel is needed, and how long the injectors are open.
Mine would normally hang to the left side , about 50 or or so at idle, and to the FAR left when de-celerating but the care still moving above 5mph. 50 at idle sounds normal.
It could be your idle control valve, but consider checking the O2 sensor, or swap it out. You can wire in a BOSH 02 sensor from a ford mustang for $30 or so. The 02 sensor is heated with 12 volts, and after about 3 min running from a cold start it is hot enought from the exhaust and the heater it begins to work. (regardless of engine speed).
You can test it, if you have a high impedence volt meter, slap it on at the connector at the firewall (the green wire, to ground) At first it should read about .45 volts steady until it warms up from a cold start, then SHOULD varie from .2 to .9 volts if it is working correctly. I pulled mine out, and the tip was gone..it would stay at .45 volts forever..but I didn't have the exact problems your having, mostly just bad gas mileage.

thanks for the insights. I had thought the mpg gauge was just a vacuum gauge.

I'll start with checking the ICV, then on to checking the o2 sensor.
94k on car
I'll post updates
thx
BOB
86-944

[This message has been edited by 41V (edited 09-29-2001).]

[This message has been edited by 41V (edited 09-29-2001).]