I've been trying to fix an idle problem on my 94FD. The idle fluctuates between 1100-1700rpm with no load, but when adding an electrical load, it jumps to almost 2500rpm and stays constant. Turning on the A/C drops the idle down to about 2100rpm and is constant. If I try to hold constant throttle at say 3000rpm, it will fluctuate up and down as well

The odd part is, when I completely disconnect the IAC connector, nothing is different! It still bumps the idle with the electrical load and it does not flash the check engine light. I measured the resistance from the IAC connector to the PCM and it seems fine at about .8 Ohms.

I've checked for vacuum leaks, checked that the IAC (and AWS) works properly when removed from the car (resistance is in spec per the shop manual and will click when 12V is applied). I've messed with all of the manual idle adjustments with no effect. I checked the TPS, it is within range on all 4 measurements. I checked the MAP sensor, it seems fine per the shop manual tests. When I pull the vacuum hose to the MAP, it drops the idle to about 1000rpm and stays relatively constant, though rough. I even tried a different PCM that I had sitting around.

Perhaps related, I've noticed that when I ground the TEN and GND terminals on the Data Link Connector, the fan will not come on when I turn on ignition. Also just yesterday (idle problem has been around for a while) I didn't get any boost at low rpms. In a 2nd gear WOT, I get 2psi until 4000rpm, then full boost all of a sudden. I used to get full boost even with the same idle problem.

So how can the idle get bumped with the electrical load and A/C when the IAC is disconnected? Can the PCM bump the fuel injection to raise the idle? If my idle problem was caused by an intake leak, how can it be so smooth when the idle is bumped to 2500?

Please help, I'm starting to think there's a ghost in my car!

 

Still looking for some more ideas.

 

It sounds like you may have several simultaneous problems, but two specific thoughts occur to me:

• Have you verified/adjusted the mechanical stop screws for the throttle-plates?
• Could you have one or more faulty solenoids and/or miss-routed vacuum lines?

Attached is a listing of items affecting the FD's idle RPM that I've collected over the last year or so. It's not necessarily all-inclusive, but maybe it will give you some other ideas what else to check.

PS - My own investigation indicates that testing the correct operation of the IAC is difficult because it's a duty-controlled solenoid; no constant DC voltage. MY own IAC has developed a sticking problem causing the idle to 'hang' at about 1,100 RPM unless the A/C is on.

 

Thanks for the file, I have already checked most of that stuff, but there's a few others I should check out. I think the boost problem may not be related, since it started after I re-installed the UIM, I probably pinched some hose to the precontrol or something.

I have checked the throttle stops, they are all the way out, the throttle doesn't even touch them. I also doubt it's a solonoid, vac line problem, since it started during an autocross run. It was fine before the run, but was hunting after I finished. It's also not intermittant like solonoid problems tend to be. Could be a loose hose I guess, but I've checked those pretty well, especially since I had the manifold off.

But what I still don't understand is why does the idle still bump when the IAC is unplugged? How is this even possible?

 

It does seriously sound like a vacuum leak. A vacuum leak after the throttle body will give you a high idle, and the car will run weird. Might be the lower intake manifold gasket - many FD's had a paper gasket that splits and warps over time. The newer part is a metal sandwich, and lasts virtually forever.

Vacuum leaks are tricky to find. I typically use a can of starting fluid - spray slowly and carefully around the intake manifold, the car should change its idle when it sucks the fluid in. Be careful - the stuff is flammable, but it doesn't leave a residue and evaporates quickly, so it's much safer than other chemicals for this purpose.

You could also have a really stuck throttle cable or something.

Dale

 

have you adjusted the tps and reset the bleed screw under the manifold? do you have a stock ecu?

 

So how do you spray starting fluid (you're talking about the Barbeque stuff right?) around the LIM gasket? My car is a Feb 94 build, would it still have the paper gasket?

 

No, no, not the barbeque stuff! Boy that would be interesting! Auto parts stores sell cans of starting fluid - you typically use it on an old carbed car to help start the car. It's a spray can, like a can of WD-40.

Feb 94 I don't think would have the paper gasket, but I'm not 100% sure.

Anyhow, spray the fluid in slow bursts around the areas where the leak could potentially be. Take your time and go slow. You can also remove intercooler piping and the like to help access - the car will run fine as long as nothing past the throttle plates is disconnected.

Dale