98 Impreza Outback stalls ... badly

[subarindio]

Hi there fellow Subie owners:

 

I have a 98 Impreza Outback, 5sp with 123k that I recently bought. The lady who sold it to me told me that the car would stall while driving and that the Subaru dealer told her that it had a CEL for Idle Air Control.

 

The car would sometimes start fine and run fine and suddenly stall, either on the freeway, or idling. Sometimes it will start and idle really rough. After it stalls it may start right on or may not start. If it starts, it runs like it has multiple misfires or huge vaccum leaks, which it doesnt. This is what I have done so far:

 

New spark plugs and spark plug wires NGK

Checked primary and secondary resistance for ignition coil, within specs

Replaced Idle Air Control valve with a used, but working one, plus I gave it a really good cleaning and used a new gasket

CKP and Camshaft sensor are within specs

TPS seems to be working fine (Checked with OBD scanner)

I replaced the Neutral Position Switch, but I noticed that both, the new and old, are open when in neutral and closed when in gear. I though it was supposed to be opposite.

When car is about to stall, if I unplug the MAF it will die right away. Also, OBD tells me that MAG output is about 250 grams per second, but the Hanes manual say it should put 400 to 700. Don't know if this may be bad, since when the car runs, it runs really good.

Fuel pressure was within specs, fuel filter is new

 

Sorry for the long list, but I am running out of ideas and I am wondering if you guys may have done something differently. Any help will be greatly appreciated, as a matter of fact, the car barely made it to the library where I am typing this. I just hope it starts right back so I can get back home:D

 

p.s. Yes, the car has gas in the tank

[jarl]

Bump for an interesting question... From your description there may be a problem with the MAF, but let's see what the resident Gurus have to say

 

BTW: look for the FSM for your car. For the '95 it says the signal for the MAF at the ECU connector should be 0-0.3V with the engine off, and 0.8-1.2V with the engine running.

Edited April 27, 2012 by jarl

[1 Lucky Texan]

OK, just some wild rambling ideas;

 

since you bought it used, triple check all the vapor/vacuum line routing in case someone misrouted something. Also, wonder if it had timing belt service at 109K? It seems to intermittently run well so, I doubt it has slipped a tooth.

 

Have you put a vacuum gauge on it? Sometimes that can help with several diagnostic hints.

 

You might clean and recoonect as many ground connections as you can find. Also, confrim that the wiring from the Engine Temp Sensor is good. Not the Coolant Gauge Sender.

 

Any hints that voltage falls off when the poor running condition shows itself?

 

It's my understanding that, with the mAF unplugged, the system should be in 'limp home' mode. I might be tempted to try troubleshooting why it won't even idle in that condition.

 

No other DTCs stored?

[Fairtax4me]

I lost a post!

I typed up a reply to this last night and I guess I never hit submit.

 

Anyway, assuming all the hoses are in the correct places, there are no vacuum leaks or anything like that, a bad MAF sounds likely. You made sure the clamps on the intake tube are tight? Common thing and easy to overlook, and will cause all kinds of trouble.

 

The IAC code could be due to a bad neutral switch. Could also just be a symptom of the running problem.

 

You can get a copy of the FSM for your car here: http://bustedfingermotorsports.com/forums/viewforum.php?f=18

[subarindio]

Hi guys:

 

Thank you for the helpful responses.

 

Yes, I inspected all vacuum hoses as it seemed like a vacuum problem at the beginning. I replaced some even though they were still good. When it was running good, I connected a vacuum gauge and the readings were excellent. No obvious leaks were present.

 

I have mainteinance records and the timing belt was done about 5k miles ago, so it should be fairly good.

 

When the car stalls, it may either start stumbling if it's, say parked, or just die while I'm driving on the freeway. The RPM may drop just before it dies, but I don't see any indication of voltage dropping. I would assume this would be a charging system light? No, none of that.

 

So far I haven't had any CEL code even when it's about to stall and running like crap. I'd think that if it was a misfire I should at least get a CEL code, but no, so far is clean. Since I have swapped the IAC and NPS I think these may not be the issues.

 

Yes, when the car is about to die, if I unplug the MAF it would just die, but I eventually it would stall anyway. An interesting thing is that after a stall, I may re-start the car and it will start and run fine, like nothing happened. Other times, it will take a few tries before it starts and moves.

 

The connectors to the ECT sensor look good. I have also monitor the engine temperature with an OBD scanner and it seems to be working properly as temperature increases accordingly until the fans come on and cools down.

 

Jarl, thank you for the info on the voltage. I will check the reference voltage of the MAF as soon as the download for the service manual is completed.

 

Thank you for the link by the way.

 

May there be something that it's obvious, but I've been missing?

[1 Lucky Texan]

is there any indication the car has run rich in the past? maybe the exhaust is clogged/bad cat.

 

eh, you'd think you get a 420 code or something.

 

I know you checked the coil, but, if you can get the 'Sometimes it will start and idle really rough.' condition to maintain for a few minutes, you might try pulling plug wires one at a time to locate a bad cylinder (sticking valve?) or a general HV failure (coil intermittent???)

 

dunno, crazy

Edited April 27, 2012 by 1 Lucky Texan

[Fairtax4me]

Yes, when the car is about to die, if I unplug the MAF it would just die

This is normal when the ECU loses the signal from the MAF.

You can restart the engine after unplugging the MAF and see how it runs. Should be kinda rough and stumbly, but it will run, and should continue to do so without stalling.

 

CELs for misfires don't set as easily as you might think. I've driven for several days with a slight miss before I've had a code finally set.

[subarindio]

Hello there:

 

I'm determined to try everything in order to get this Subie running. I tested the output of the pressure switch. According to the Hanes manual, it should output 3V idling and drop to about 2V when accelerating. Mine would actually go up. So I swapped that for a good one and also put in a different pressure source switch solenoid, but the voltage output was the same, so I guess the Hanes manual doesn't help much.

 

I have even swapped another ECU from the same year, but the car started pretty rough and it remained like that, so I guess it didn't make any difference.

 

I haven't tried to pull the plugs when the car is running rough because last time I tried that on a Legacy and got shocked .... pretty good. Those coils are powerful. I may try if I get a good set of insulated pliers.

 

I did pull the connectors for the injectors, but I noticed the engine bogging down a bit, plus that threw a CEL code right away.

 

I though about a catalytic or exhaust being clogged, especially because they would get super hot when the car was running rough. However, the one day that the car decided to run well, I took it to the CA emission test and it passed with flying colors. I'd think that a bad cat or oxygen sensor would cause the car to fail the emissions test.

 

I just plugged a used MAF from a 91 Legacy, they have the same part number. I looked at the output readings with my OBD scanner and I'm still getting 2.6 grams per second, pretty much like the old one, so again, is the Hanes manual wrong? The FSM that I downloaded doesn't have output specs for the MAF.

 

I will drive the car for a long trip and see what it does. I'm hoping that one of the multiple fixes that I've done will do the trick. We'll see.

 

Thank you for all the tips, any other ideas out there?

[1 Lucky Texan]

you seem pretty good with auto stuff, but I didn't notice if the car had O2 sensors changed, originals at that mileage are probably lazy or bad.

 

you did mention Engine Temp Sensor right?, not gauge sender. The ECT can be bad without throwing a code.

 

Do you have a code reader? any pending cat system codes?

[subarindio]

I also suspected the O2S and I noticed that the downstream was lazy, so I frankenstained one from a Nissan, They are the same, only the connector changes. I plugged it and then I monitored it with the OBD and it worked good after that. That also helped me pass the emissions test I believe.

 

Yes, I monitored the temperature changes in the ECT with the scanner. It seems to work properly. I also checked the connectors since I'm wary of these ECT sensors. One time, a loose connection caused a no-start on my old saab. Connector and performance seem to be OK.

 

Still no pending codes of any sort. Whatever is causing the engine to stall is very subtle or acting under the radar.

[1 Lucky Texan]

not that I'd be any good at fully interpreting them, but a couple of ideas occur to me, one would be freezeframe data - or otherwise retrieving short-term and long-term fuel trims. I believe they should both be quite low unless there's a problem.

 

Also, maybe monitor the vacuum with a gauge. There's some old school diagnostic approaches with that thing.

 

kinda wondering if a vacuum leak could fool the system into an incorrect A/F ratio.

 

I had a nissan once that would intermittently stall - it was a bad 'motor control relay' but it was a 'hard' failure, no rough idling. Just, turned the car off occasionally.

Edited May 1, 2012 by 1 Lucky Texan

[subarindio]

Well, it's been 60 miles and a few trips since I swaped the MAF sensor and pressure switch and the Impreza seems to be behaving. I'm not going to get all excited yet until a few more hundred miles, but so far it seems like the Impreza liked the implant from the 91 Legacy and hasn't stalled on me. I hope this will take care of this nightmare and I'm ready for taking on other little issues .... like jittering transmission.

 

Thank you all of you for your advise, and so far it seems like some of you hit it on the nail by pointing to the MAF.