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My wife's '90 4EAT Wagon picked up a stumble last week, only under light or trailing throttle, and has died a couple of times rolling up to a stop. She's good about tracking fuel milage, and it's still getting 21+mpg around town. I took it to work and noticed the performance was off, and it felt rough, so today I threw a bunch of parts at it:

Plugs
Plug wires
air filter
cleaned the MAF
Cleaned the throttle body
cleaned the IAC
ran a spray can of Sea Foam thru it while running at 2-3k

Checked the knock sensor and yes, it's cracked...

drove up to the corner parts place (Schmuck's) to get one, of course they said 3-5 days... anyway, it is running much better, smoother, performance is back, but heading home it chugged a couple of times again. Applied a little more throttle and it smoothed back out. Idle sounds normal again, almost dead smooth.

Car Quest has a Knock sensor waiting for me in the morning, so I'll see what that does for it. I'm wondering if it's occasionally picking up a knock signal and confusing the ECM due to rapid change??

One other possibility is an O2 sensor, but I'd like to avoid the $180 tab on that one...

Anyone have thoughts or input on my line of thinking?

any cel or codes?

doesn't sound like the knock to be honest. That will cause a serious loss of power. When mine was bad it would only act up when it rained. and it would put the engine in "limp" mode which is very noticeable and never stalled the engine.

As for the O2 I doubt its that if you are getting 21mpg with the car. Sounds more like the TPS to me.

look here to diagnose that http://www.surrealmirage.com/subaru/engine.html#tps

No CEL, haven't checked for codes since the CEL hasn't come on.

I had a knock sensor crack on my daughters '92 Leg on the way to Texas, and it never went into "limp" mode, if anything it let the timing avdance further as I was getting pinging under hard load conditions, which is what my wife's is doing now..

Daughter's Legacy got 32 MPG average Tacoma to San Antonio with a bad knock sensor from the start of the trip and an injector failing on the last 800 miles!!! Gotta love that kind of failure...

The more I think about it the wife's car is acting almost exactly like the daughters did, except for the stalling...

I had thought TPS also, and if the knock sensor doesn't fix it I'll check the TPS readings... It is most noticable when it's been raining, so it's a possibility.

Latest update: the knock sensor took care of the pinging coming off idle, but still had the stumble at idle. while I had it sitting idling I realized the cooling fans weren't cycling, so I changed the water temp sensor, that that seems to have helped some,and the fans are workjing again, and now it isn't stumbling as much, but when it does it dies most times.

We drove it up for lunch, ran fine most of the way there, but as we were waiting to pull into the parking lot it shuddered, and when we left it died twice, once in the parking lot,a dn once at a traffic light.

Overall the car is running better than it has since we've had it, it's smooth, good power, and idles nicely until it stumbles.

I'll go out and check the TPS after my neighbor gets home, as I don't ahve a good digital voltmeter.

Would the cam or crank position sensor cause this, I didn't think it would as it should have a problem at speed if they were getting flakey, right?

Have you reset the ECU after all this work you've done?

Got an extra MAF to try? My car would stumble/die, and didn't throw a code for it.
EDIT: Should have stated didn't throw a CEL. I did have codes when I checked.

Sounds like you should have access to lots of the JECS/AUTECS MAFs?

I'm resetting the ECU again overnight tonight, it was reset after the majority of the work, only thing swapped after last reset was the temp sensor.

I could swap parts from my White wagon to figure it out, but would like to keep it running as that's what the wife is driving while I figure hers out... Daughters '92 is in Texas now.

wtdash wrote:Got an extra MAF to try? My car would stumble/die, and didn't throw a code for it.

Sounds like you should have access to lots of the JECS/AUTECS MAFs?

Ding, ding, ding - We have a winner!!!!! Swapped MAF/upper aircleaner housing with my other Leg, went out for a 50 mile drive around town and no stumble!!

I put the "bad" MAF back on the white wagon and fired it up, and it sat at idle for 20 minutes without stumbling or dieing, but it was running rougher than normal. I've put 30K miles on the White wagon without anything other than oil changes... probably time to tune it up too. Also found out it has a broken pass side motor mount...

BTW, even with all the idling I put on the wife's car while trying to figure it out and the bad MAF it got 19.8 mpg on this tank!!!

Verified today that the MAF IS the problem, my white wagon is now stubling and acting up... Off to pull a part tomorrow to see if I can find a good used one.

tahiti350 wrote:Verified today that the MAF IS the problem, my white wagon is now stubling and acting up... Off to pull a part tomorrow to see if I can find a good used one.

Good to hear...seems there's a rash of bad MAFs lately.

Yes when MAFs go bad you definately know something is wrong, although I've never had a bad MAF with no code yet... I wonder why the code wouldnt come up?

Most likely because our ECU's are not smart enough. They just typically provide a code if the output from the sensor is outside a predetermined range.

It does seem strange, but I never got a code on either car when it stumbled

When I had my MAF issues, out of all the time I had the issues I think only once or twice did I get a CEL.....and that was over a year's time of having intermittant issues.

Come to think of it, wite had told me of a couple of CEL's over the past few months, and about 6 months ago had a hard/no start issue that went away after she had it towed home.

Maybe it was the MAF then?

Could have been...

Yea, the MAF can cause alot of strange problems, it seems like the list never ends of problems that have shown up just because of a bad/dirty MAF

Picked up a used MAF at Garraway's for $30, swapped it in out in the parking lot,adn it ran really rough, stalled, etc, but got me home. Reset the ECU until I had to head in to work, drove it easy for a while,adn it just kept running better. After putting about 50 miles on it it's running better than I remember since I've owned it.

Wife's wagon is running ggreat, too. Funny though, I put about $200 into hers doing the tune-up and misc stuff, and wonder if a $30 junkyard part would have taken care of it... Oh well, it was probably due for a little attention (especially the Knock nad water temp sensors).

Next on the list: fix the broken motor mount I found on mine, and get the winter tires mounted up on both of them.