She gets hot, then she gets rough!

[soobrdad02]

My n/a 4EAT '91 Leggy wagon is having issues after about 20 minutes of driving that lead to the idle drastically dropping, and unless I give it some gas, dying. Since it involves some driving(thus warming up) I was wondering if the issue was a bad coolant temperature sensor, and I wanted some advice before replacing it. Anyone?

[SubaruNation]

check to make sure your intake is all connected, i know that has happened to me.

maybe O2 sensor?

i dont see why a CTS would do this, unless you see that your coolant temp sensor on your dash either goes totally hot all the time, or it never moves at all.

[soobrdad02]

Checked for codes today and found nothing. I figured the O2 sensor would've set one off.

[Jessekrs123]

A CEL will trip if your o2 sensor is on it's way out. If there is no CEL, than my guess would be the coolant temp sensor. I had the same problem with my old wagon. $20 and 10 minutes later after replacing the coolant temp sensor, all was well.

[soobrdad02]

I swapped out the CTS to no avail this weekend, still warms up and starts running like poo. I looked at vacuum lines until I went cross-eyed yesterday and found nothing. I was wondering if, in fact it IS a vacuum leak, could I try my super-ultra-ghetto-leak-finder-idea? Here it is... I figured that if the there is a leak somewhere in the area of the intake, I may be able to find it by driving the car until it warms up enough to start with the rough idle, go back to the house, flood the general intake area with smoke(using a fog-machine) and use a timing light to see where the smoke(fog) is being pulled into the intake. Worth a try?

[subydaddy]

I've never had an issue like this but I'm thinking if it runs fine until it warms up than the main difference is going from open loop to closed loop... right?...maybe?

So I would think (I'm no expert though... not even close) that it would be a sensor sending the wrong signal to the ECU and therefor causing a rough running condition.

You'd think it'd throw a code though....

Do you have a manual that you could measure the resistance at the ECU of some of the sensor wires to make sure they're working properely and not corroded/exposed and touching the frame or such?

Anybody think it could be a problem with the Air Intake Control Valve or the Throttle Posistion Sensor?

Does the idle change (improve or worsen) when you go from Drive/Nuetral/Park Etc.?

How does it run while you're driving? Can you drive at cruising speed or accell at full throttle without problems?

[Legacy777]

I would check the IAC valve a little more thoroughly. There is a bimetallic strip in there that when warms up closes a little. It may be sticking and causing issues. Beyond that, I'd do the vacuum leak check you talked about. Have you put a vacuum gauge on the car to see what the vacuum is at?

[soobrdad02]

Just an update...I still haven't found the issue. I pissed away an entire can of carb cleaner searching for loose/unhooked/cracked/broken vacuum lines on/around the intake, pulled off the airbox/intake tube assembly to allow for a more thorough check of lines/connections, and still nothing. I have also realized that, at random times under heavy acceleration, my car can't get above 3,000-3,500 RPM's. not a good thing when trying to pass in traffic. Ideas?

[ericem]

What kind of vacuum are you making? Are you sure your bpv is fine, intake manifold gaskets? Possibly dead cylinder do a compression test, hook up a vacuum/boost gauge..

Also does your CEL work when you turn on ignition?

[soobrdad02]

As far as vacuum numbers, I haven't gotten any yet. No BPV, it's n/a. I replaced the intake manifold gaskets a little more than two months ago with a new set from Subaru.

[ericem]

Hmm almost sounds like your cat is clogged now. Than again I don't see where you said you have done a tune up such as spark plugs, wires, pcv valve, seafoam engine, fuel filter etc?

[soobrdad02]

Tune-up(plugs, wires, air&fuel filters) done last summer. Seafom'd her in January.

[ericem]

Recheck the plugs and wires out. Are you running ngk wires and plugs? Or atleast oem wires and ngk plugs? The spark plugs could possibly be loose too it happens. It that checks out fine maybe replace the MAF?

[soobrdad02]

Plugs and wires are okay. Took her for a drive and the damn thing still runs fine, warms up, and starts idling rough/shaking to the point of cutting itself off. I went crazy with the carb cleaner looking for leaks and found nothing. Tomorrow I'm testing every sensor that I can get my hands on with a multimeter and seeing if I can come up with anything. I'll also be pulling off the throttle-body and giving it the cleaning of it's life. This car!!! GRRR.....

[ericem]

Have you tried another MAF?

[Legacy777]

I'll second eric's suggestion about the MAF. I had an intermittant issue and tried all the normal tests. Replaced the MAF, no more problems.

[soobrdad02]

Installed a new Coolant Temp Sensor...no change. I've tried a used IACV...no change. I installed the used IACV this afternoon, took it for a test drive and she still warms up and idles like poo. I got her home, checked codes using the black connectors and found nothing. Tried pulling codes in D-check mode, nothing there either. I unhooked the battery and am leaving it unhooked overnight in the (bleak) hope that it'll reset the system and I'll re-connect it in the morning to find that she'll run like a brand new car. But I'm a realist and I know she'll still be running like crap. I've got spare MAF's lying around and figured that I'll swap 'em out for poops and giggles. I DO want to know though, when swapping out MAF's, do I just swap out the unit in the intake, or do I replace the unit that the wiring runs to as well? As for the IACV, is there a recommended technique/method/cleaner for cleaning it? This car's killing me...

[AWD_addict]

just swap out the unit in the intake

You could also get a can of MAF cleaner and try spraying it.

[Legacy777]

I DO want to know though, when swapping out MAF's, do I just swap out the unit in the intake, or do I replace the unit that the wiring runs to as well?

Not sure I follow you, the MAF bolts to the airbox, remove those four bolt, remove the MAF from the intake tubing and replace it with another one. The easiest thing to do is unbolt the clamp holding the MAF to the intake tube, then unbolt the top piece of the airbox from the lower piece, then remove the top part of the airbox with the MAF still attached. Then you get to the bolts much easier.