Runs a bit rough and hesitates when cold

When warm the car runs great but when cold it wants to hesitate and it revs higher then it should on cold and cool starts. Seems like a vacuum leak but all the hoses seem to be in good shape. I also thought about plugs or wires but I just did put in new plugs and wires unless I got a dud. I heard a bad coolant temp sensor can cause similar issues also. Anyone have a similar issue before?

I also got a new fuel filter and my injectors had been cleaned and flow tested. I thought about replacing my IAC also I have cleaned it with no help.

Vacuum leaks will cause a high idle because unmetered air is getting past the throttle plate. So unless the IACV is compensating for the leak at idle, I doubt its a vac leak.

 

I'd clean your mass air flow sensor, intake air temperature sensor, clean/test your throttle position sensor. If O2 sensors are old, those def can cause issues as well.

 

I don't know much about Subaru's though... Those things are cheap/free and have solved similar problems in the past for me.

Ive cleaned the MAF several times,TPS voltage was checked, o2 sensors are brand new Denso units. The idle is high for a few seconds then calms down which the IAC could be adjust for a vac leak or maybe my iac is just a bit slow when cold. hmmm

My car does not have IAT sensor. I was thinking maybe a CTS...

What is the CTS?

 

Have you tried running the car at night/look under the hood in the dark to see if your plug wires are shorting out?

 

Why would the coolant temp sensor cause an issue?

 

Essentially this sounds like something you will need an advanced OBD2 scanner to diagnose... Scan the numbers for anomalies etc.

I do have a pretty good OBDII scanner I will run a log again later. CTS is a coolant temp sensor and it will mess the AFR up if the ecu does not know the correct temp of the engine. I will have to check the engine at night tho.

as far as coolant temp. compare it to iat upon cold start. should be similar. then watch the coolant temp as the engine warms up, should be nice and even increase in temp, not jumping 10-20 degrees at a time.

 

as for vacuum leaks, check your air fuel trims, short term and long term, you want them near zero 25 is max and bad indicating an air leak and you will probably actualy hear a sucking noise. if they go neg that means not enough air and to much fuel. check fuel.

 

and check for 02 functionality. should very between .125v and .850 volts and be switching. watch the first o2 and not the second, the second monitors the cat functionality based on the first o2 but does not effect engine performance.

 

lets take it from there and see what info we get

Alright ill check it out again when I get some time. I have done a bit of tuning in the past so I know the basics.

i was having the same problem and once i changed the coolant sensor it resolved everything. The sensor is a pita to get to, also disconnect the battery and change the sensor while the car is cold and have the new sensor in hand when you are about to remove the old sensor or you will lose alot of antifreeze.

Damn wish I replaced the sensor when I had the engine out lol.

Sounds like the IACV needs replacing, along with a good flush of the throttle body coolant chamber.

Is it common for the EJ25D to idle high upon a cold start (~1200rpm), but maintain a [proper] lower idle as soon as you feather and let up the throttle? Or would that also be worth changing /cleaning filters, IAC, MAF, etc?

^Yes. That's normal.

 

to the OP: If your front o2 sensor has a heater circuit malfunction it will run like s#$t if you try to drive it when it's cold/right after startup but will be fine once warm. Check it out.

thanks for the suggestion ^ I think my car is having a problem with this on my leggy. here is a really good video on diagnosing a heater circuit malfunction [ame=http://www.youtube.com/watch?v=SEuGtggZAvo]1997 Jeep Blown O2 Heater Fuse Case Study - YouTube[/ame]

I went ahead and ordered a CTS since it is cheap only 21 dollars. If that does not work I might go ahead and get a IAC Valve. Sorry I took so long to reply I been pretty busy lately. I still havnt had time to hook up the lap top when its cold either.

I might of killed the CTS when I sand blasted the crossover pipe to paint it even though the cts was taped off the static caused from sand blasting could of affected it.

let me know how that works out. i am tired of my car doing the same. hopefully i will have a Bluetooth scanner tonight and be able to diagnose this problem

Ill let you know I just picked up the cts and will prob install it this weekend. Got a new pcv valve from subaru also.

Replaced the cts not sure if that fixed it yet but im guessing not. Broke the damn clip off the cts connector really pisses me off usually so careful about that. Anyone know where I could get a cts connector?

Drove the car to work this morning and it seems to be running very well now when cold. It does still rev to 2k when started but the engine just seems to sound better when revving also. For the 2k rev I think I might just need to flip around the rubber spacer behind the IAC. I need a few more drives with it to be a 100% sure but it had no issues this morning so I hope it continues to run good.

good to hear its running smoother. i am going to probably be replacing my upstream o2 to see if that fixes my issues. strangely i haven't had the high idle when its cold for the last week but still isnt revving smooth when cold.

good to hear its running smoother. i am going to probably be replacing my upstream o2 to see if that fixes my issues. strangely i haven't had the high idle when its cold for the last week but still isnt revving smooth when cold.

Thanks the car is definitely running better now I think it was getting some detonation before also during wot. Closed loop is much better now.