OK, a couple weeks ago when I would start my car it would go up to 1500 RPM's or so and then kill immediately. Start it again and it's fine. Now it idles rough at @ 700 RPM, I think it should be more around 900 RPM. Also when the air is on and the compressor is cycling, the car can barley idle, it will go down to 500 rpm's, even kill sometimes and then recover to 700 rpm's. Also the car will just decide to go on fast idle sometimes, car fully warmed up, it will just go to 1500 RPM's every time I put the clutch in. Any suggentions, I assuming it's a idle speed sensor or something. Any suggetions as to what's wrong.

Thanks!!

Car is a 1991 305 TBI 5 speed.

 

With the car running, unplug the IAC (Idle Air Control) and see if the fluxuations (sp?) stop. It is on the plug closest to the firewall on the passenger side of your TBI. Let me know what you get.

 

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Ok...

I have the same problem: 91 305 TBI Auto...

 

I have the same problem but my car will idle rough at 450 rpm's when warm and almost stall out before bouncing around with the AC on. Is this an IAC adjustment. Occasioanlly the car will idle around 650 rpm which is perfectly smooth, which I like.

Moderator: I posted this problem the other day and didn't get any response. i will take it from here since these guys have the same problem and you can delete my post to concentrate replies to this one.

 

I was going to see if a new set of plugs and a timing check fixed it, but I've discovered worse problems to fix (like my drivers-side outer tie rod end is connected to the wheel by nothing more than a sheared off bolt, and my sway bar isn't connected to the suspension at all...)...

 

hmm, my car does something similar. It always has idled around 600 RPM or so, ever since i bought it a month ago, although it seems pretty smooth to me. Sometimes, especially when going around corners, the engine will be turning around 1500-2000 RPM depending on how fast im going, then drop down to 500 and cough for a few seconds, then jump back up to the proper RPM. When it's low on gas (3-4 gallons), the engine wont recover and will stall out. It does it a whole lot when its running low on gas, although it does it when its full too. Two things that make it happen fairly often is going around corners, and pressing the gas hard. Sometimes it also starts and idles around 1000-1500 RPM and just never goes down until I start driving. Any ideas on whats wrong?

As an added note, it smokes a bit when its started, mostly the first time each day. sometimes just a little, sometimes a huge cloud. I'd imagine the valve seals are going perhaps? Could that also cause this problem? I really dont have the money to do the seals right now.. I'm sure the extremely dirty air filter isnt helping, i need to get some money and buy one. Also, When I bought the car, I immediately had the oil changed as, guessing from the window sticker and the blackness of the oil, the oil was about 8000 miles old. However, I think some of the real sludged up old oil has mixed in with the new oil, so I'm going to change the oil again real soon. I dont think it being low on gas is the sole problem, since it does it at any time, although keeping it full is certainly a good idea. Currently, my car has 210 miles on a full tank of gas, and it died about 5 times coming home from school. I wouldnt imagine that my car is getting less than 14 miles a gallon city, as it gets about 25 on the highway, with extremely unconservative driving.

 

Out of curiousity, how's your injectors?

Check the spray (take the air cleaner off and all that stuff -- the rod, etc. and start the car) - it should be nice and even, getting a little denser with higher throttle. Sudden drops in RPM, then behavior that is normal and then goes beserk might indicate leaky or clogged injectors.. If the injectors are fubared, get a Borg-Warner rebuild kit for (~$80 at Pep Boys).

Check your hoses.. it's OK to hear hissing until the ECM enters closed loop (read: your car warms up and everything is ok), but if you hear it after that, some of the hoses on and around your TB are leaking or not plugged in properly.

If not that, might be bad spark..

 

You could be too far advanced, and too low of an idle. Back off the timing a bit and increase idle to around 800.
I had a lot of code 43 43 43 43 43 43 43 43 43.
(I think it is called Petty-itis)
Also see if you're running too rich. (Easy to see) or too lean (Code 44)
That is how I fixed mine.
The most frustrating, touchy thing was that problem.
Grrr.
Now everything is ok. (Knock on wood.)

 

I've had similar problems a few years ago. The IAC, spark, sensors, and wiring are all areas that I checked. I even swapped ECM's. The problem got so bad that my car couldn't get out of its own way.

The problem turned out to be a bad fuel pump. More specifically a tear in the rubber hose that draws gas from the bottom of the tank into the pump.

The tear would expand when the pump was operating. This explained:

Mostly normal cold startup
Difficult warm startup
High idle
Sputtering and stalling under load or when cornering (especially when the gas tank was low)

Another reason why this problem is misdiagnosed is because the tech manuals don't specifically say to check for fuel pressure. And we all know that these engines don't even have a schroeder port for a guage connection, you have to fabricate one.

Anyways, my point is although you guys have all come up with areas that should not be overlooked......take the time and check your fuel pressure. It may save you a lot of unnecessary time and effort.

 

My car has idles smooth at 650 RPM all the time and it has since it was new. I think that would be normal.

 

I know that our cars should idle between 600 and 650. my question is, how do you adjust the idle. Is it by the IAC? Do you have to pull it out and adjust the little srpingy thingy so that a certain length of it sticks out?

 

First of all, you don't and can't set the idle. You set the minimun air and the computer will adjust the idle. The only "true" way to adjust the idle is to program the PROM. If you set the minimum air so that the car idles at 1000 (what a lot of people think is "setting the idle"), it will always be trying to achieve it's "desired idle RPM" and will result in poor idle/drivability.

The main two pieces of this puzzle is the CTS (which the ECM uses largely to decide desired idle RPM) and the IAC motor. If your IAC motor is old or clogged with carbon, or the IAC passages themselves are clogged, then the ECM will have a hard time achieving it's desired idle. If the CTS is bad or inaccurate, it won't know what the desired RPM is to begin with (or the correct RPM anyway).

For the most part, you are supposed to be idling at about 700 RPM. You set the minimun air so that the car idles at 450 (in drive BTW for autos) or so, and the computer will correct from there. It has to have some room to play.

And another big thing (very big) is people assuming the stock tach is accurate. 95% of the time, it's not. Especially for setting the minimum air screw. For that you'll need a dwell meter or scantool. The stock tach might be close, but it'll never be right. (But mine is!) If you say so.

 

It is starting to make sense now. Ok what the IAC is but don't know that the IAC motor is. I had the TBI unit off the other day to do the ultimate mods and it wasn't clogged or gummed up at all. So my fianl question is, what do I have to do/adjust to get my car to idle between 600 and 700 rpm when it is hot.

 

My car idles at 750 in D