1991 Mustang Rough Idle

Discussion in '2.3L (N/A & Turbo) Tech' started by tokillamurderer, Oct 4, 2007.

  1. I have a 1991 4 cylinder EFI Mustang. When I first start it up, it idles rough, and if you don't hit the gas a little, it will almost stall out, or sometimes it will stall out. (This car is not currently on the road, so I haven't been able to drive it and see how it reacts; however, when it was last driven, there was no problem at all while it was driving, just when idling or stopped.) Recently though, after about 15 or 20 minutes of running the car, the problem seems to go away. However I don't know if it really goes away, or if a vacuum cable moved and blocked a vacuum leak, or what? Also, it doesn't really completely go away... even after the rough idle dissipates after that time period, the RPMs change every once in a while while it is idling. There is no problem at all above idling speed. I am not sure this is a vacuum problem though, because the RPMs don't pulsate or sway up and down, but rather they just slow down until the engine almost cuts, and then they go back up a bit and start over, or it dies, one of the two.

    Also, I disconnected the intake tube from the throttle body and inside the throttle body tube, before the valve, on the bottom of the tube was what appeared to be greenish sludge. It looked like it was dried on there though. What could this be, and what would have caused it?

    I'm kind of new to the vacuum system... I don't know how to find the vacuum lines. Which ones are they and where would they go to and come from so I can trace them to see if it's a vacuum leak?

    Also, I was wondering where to get a code reader that will read codes in 1991 year Mustangs? The ones that I found online at sears.com say they're only compatible with 1996 and up cars.

    Thanks for any help you guys can give me!
  2. To answer your question about the code reader first: You do not need a dedicated tester. All you need is a 12V light bulb, a few pieces of wire, and a Chilton or Haynes manual. Once the computer recognizes it is supposed to output codes, the light will begin to flash out the codes. Just count the flashes (instructions are in the repair manual, the usually use a voltmeter but a light bulb is easier to use).

    To answer your question about vacuum leaks: Your vacuum lines all come off the intake at one place or another. You should have a vacuum routing diagram either affixed to your hood underside or your ignition coil/starter solenoid cover. All vacuum lines are either rubber or hard plastic. Very easy to identify apart from the wiring harnesses. First do a visual inspection of all hoses and connections. Any cracking rubber or damaged lines should be replaced right away. A trick to finding vacuum leaks is to allow the engine to warm up and go to curb (hot) idle. Carefully spray carburetor cleaner around all vacuum lines and vacuum connections (use it sparingly as it is flammable and you are working around a hot engine). Spray about 3" of hose or connections at a time and wait about 2 or 3 seconds. If the engine idle comes up or evens out suddenly, it means carburetor cleaner has been sucked through a vacuum leak and into the engine. Remove the fitting or line for further inspection.

    To answer your question about your throttle body: Deposits build up over time. Green? Not sure what that is, but if it is dried on, it could have been something else spilled there at some time. Usually oil and dirt accumulate in the throttle body and intake due to EGR and PCV system operation. This is easily combatted by spraying carburetor cleaner into it with the engine off or idling. If the engine is idling while you are spraying it, it may be necessary to occasionally rev the engine a little to clean it out. If the engine is off, it may take a little while to start. Don't use excessive amounts either way as it can damage your engine.

    To answer your low idle question: There are a multitude of things that can cause low idle, but your situation is classic for a bad or gunky idle air controller. This would be on the throttle body with 2 (not 3, that's the throttle position sensor) wires connected to it. It can be removed easily by unplugging the electrical connector and then the 2 holding bolts. Be careful not to tear the gasket or you will have to run to the auto parts store to buy another one. Clean it as best you can with carburetor cleaner or some type of non-corrosive solvent. Even PB Blaster works really well. Reinstall it and see if that fixed it. If it is still idling low and rough, pull back into your driveway or into a nearby parking lot and, with the engine at idle, unplug the electrical connector from it. No change whatsoever in idle would indicate it is totally dead. If it stalls immediatly, then it is still working. Something else that causes low idle a lot is an improperly sealing EGR valve. This is in close proximity to your idle air controller and has a steel pipe coming up to it from your exhaust manifold and a vacuum line going to the valve along with an electrical connector. Remove the valve (2 bolts and one large nut on the tube) and clean the part that the exhuast gas flows through (will be all black and sooty) well with PB Blaster or some mild solvent. Look around where the valve needle drops down to block the passage and ensure it is sealing.

    Hope this helps.

  3. This seems to have helped so far... I accidentally tried to start the car with the electrical connector on the IAC disconnected, and the car wouldn't run, so that means the unit is still working. I plugged it back in and the car started right up and idled fine. After a while, the idle dropped about 2-300 RPM, but I think that was just the car warming up. So it seems to be fine so far. All I did to achieve this was remove the IAC and put it back in, as it didn't seem to be blocked, and I didn't see a way to clean it with any spray. When I took it out and shook it a little bit, it must have freed up the internal parts or something, because when I put it back in, the car seemed to run fine. Also, there wasn't a gasket on the unit when I removed it, so I didn't put one on when I put it back in. What does that mean?

    Also, on another note, I don't know if you'd have any ideas for this, but I also own a 1996 Ford Explorer XLT (my daily driver) and the check engine light is on. I checked the codes with a code reader, and it came up with EGR Flow Insufficient, and EGR Flow Excessive. I was told that I should replace my EGR valve, so I bought one at Auto Zone. I went home, and tried to remove my EGR valve, but after removing the vacuum line and the two holding bolts, I discovered that there was a threaded nut on the EGR pipe that connects to the valve. I could not budge this nut, as it is completely rusted in place. I bought some PB Blaster and have been spraying it on there religiously for two days now, and tapping it with a hammer to try and loosen it. But I still have not had any luck removing it. My car needs to be inspected by the end of October, which is three days from now, but I can't get this valve off! Does anyone have any ideas? Also, I don't have the right size wrench for this nut, so I've been using a pair of long handled adjustable pliers... how can I determine what size wrench I need for this job? I can't get in there well enough to measure the size of the nut, so I can't determine what size wrench to buy. Thanks for any help, and thanks to ethangsmith for all your help!!