I had another thread opened, but since i'm technically troubleshooting a different problem, I'm opening another one.

My car's idle drops down too low when I let off my clutch pedal while decellerating. The whole car shakes and then it goes back up. I have new front and rear motor mounts. The idle just drops too low. The car shakes when I turn on the A/C pretty badly. It only does this after the car warms up. I am not throwing a CEL.

It's almost like the computer tries to drop the RPMs too low, then says wait, lets go back higher, and then eh.... that's good enough. It should be around 1,000 for A/C on, but it is around 700-800.

Here is a video. It is poor quality, but you can see my RPM drop way down, then come back up. It still feels too low when it comes back up.



Webtech says Terminal 2 and 3 on the IAC should be 10-12.5 OHM. I am reading 12.8 ohms. .3 higher then specifications.

What do you think?

 

I'd have to agree with it being the IAC. I go ahead and clean it out just to elminate that being the problem, if it doesn't fix it. Another thing would be the TPS but if i recall you said the one you have is fairley new and not a dae sung. Does the OBD diag you have read IAC count?

 

I've been having the same issue with my Elantra for awhile now. The idle rpm's seem to be off about 100-200rpm's. I cleaned out the IAC, MAF, but only have a code for a low voltage on the primary oxygen sensor which I still need to replace.

 

^^ well, maybe you can shed some more light on the situation?




No, there's no IAC reading. Hi-scan may have that, but OBD-II does not include that in it's standardized set of readouts..

btw.. I've cleaned it many times. I'm now thinking about replacing it. I checked with autozone and they wanted $280 for a new one because "emissions parts are expensive"... even though this has nothing to do with emissions, it is an idle speed regulator.

I found a junk yard locally who wants $35, a man on e-bay who wants $20 and calls it a "water pump" but the picture is most definately a IAC, and Ericy321 from this site who wants $15 shipped for his. I'm taking Ericy up if his ohm values read out properly and the IAC moves smoothly.

Most things on cars are either work or don't work things. If this fixes the problem, it will be the first time I've caught a computer controlled object out of tollerance.

 

My car used to do the same stuff...
I said it before, but check the wiring on your TPS and MAF. there is no stress break on the wires harness, and over time (8 years now?) it will start to corrode if there is a bare spot

 

^^ man... stop with that. I've told you they are working perfectly 3-5 times now. I can see the values on my obd-ii. the problem is the car won't idle up when a/c is turned on, does not compensate for engine load, and the idle dips when gas is released.

 

Bad ground? voltage stabalizer? Maybe not the problem, but I'm sure it would help.

 

Have a voltage stablizer. Checked, cleaned, verified grounding, added 2 extra grounds.

 

The most common causes of idle issues are improper AFR, timing or idle air valve issues.

Easiest first. Vacuum leaks on a MAF-controlled car will cause lean operation, which will DEFINITELY impact the engine's ability to idle, especially with a load like the A/C compressor on it. Have your system pressure tested. If you live in/near Denver, CO, I can do this for you.

The ECU "knows" if it should go into idle air control mode and modulate the valve if it sees certain values out of the TPS. That's the 2nd easiest place to start. Make sure you include the car's wiring harness when you test sensor resistances! In other words, do it from the back of the ECU plug.

Don't sweat the slightly-out-of-spec resistance on the IAC valve. Only sweat it if your resistance was WAY out, like 20 ohms.

You can test your IAC's operation by hooking it to the battery VERY quickly. Just touch it quickly to the terminal. It should tick. Test both open and close circuits. The wires are switched high, open ground and closed ground.

Another easy way to make sure it's not the valve is to unplug the electrical connector and manually open the valve using the battery trick with the motor running. The engine should race and bounce off the fuel cut limit at around 2000 RPM, give or take. This will rule out a dirty valve.

 

Hey, I just got a wild hair up my butt and pulled the vacuum line off the fuel pressure regulator. It seemed to run better with the a/c on!??!!??! WTF?!

I did blow up my intake manifold with nitrous once to the point where each and every silicone coupler joint was blown apart, my MAF was destroyed and my filter was blown up like a baloon. Maybe the shockwave through the vacuum lines damaged my FPR?

What should happen if you remove that tiny little vacuum line from the Fuel Pressure Regulator?

My MAF is new and good. The IAC has the ability to control the speed, but not very well I think... The car just won't idle up.