Sometimes runs way too rich - Computer or oxygen sensor?

[Vanguard]

Vehicle info: '92 Subaru Legacy L wagon 4WD non-turbo

 

Sometimes when I start my car, the exhaust is far too rich and detonates

back in the catalytic converter. When it happens seems sporadic. The

weather could be cold or hot, humid or dry, and the car may have been

sitting for several days or only for 4 to 8 hours since the last time it

ran. I can compensate by shifting manually to keep the RPMs way up. When

the problem arises, I have to keep the RPM up to around 2500 when at a

stoplight (so it doesn't peter out). When I take off, I'll put it in first

gear and shift manually to keep the RPMs up high; otherwise, it will

hesitate on acceleration and I hear the detonation in the exhaust system.

Even after the temperature guage shows normal level, it is only after it has

been at normal for about 15 minutes before the over-rich problem fades away.

It only runs overly rich occasionally.

 

The other problem is that the "Check Engine" light comes on sometimes. I've

had it in the shop 4 times to have them read the codes. The first 3 times,

they got no error code. On the 4th time, they got no error code but when I

started my car to leave then the "check engine" light came on and they

hooked up while the car was running. I don't remember what was the code but

it didn't make sense at the time and the mechanic said it was wrong. When

the "check engine" light comes on is *not* when I have the first-startup

overly rich mixture problem. They seem to be independent symptoms but maybe

not. I suppose the "Check Engine" light might not be coming on but the

computer is still screwing up the mixture when the car is started.

 

So my choices (that I can figure out) is to replace the engine computer

and/or the oxygen sensor. I replaced my exhaust system about 4 to 5 years

ago but they reused the original oxygen sensor (they were going to replace

it but couldn't get one at that time). As I recall, this sensor check how

much oxygen is in the exhaust and recirculates a portion of the exhaust if

the oxygen is too low. Well, if it is running overly rich then wouldn't the

oxygen level would be low?

 

Any opinions if it is more likely the computer or the oxygen sensor screwing

up the mixture (so it is sometimes too rich on startup)? Might it be

something else? Besides the 4 times to take it into the shop to see if they

can nail the cause of the "check engine" light coming on (but never finding

anything), I've even left my car at their shop over the weekend to let them

start it cold to see if the over-rich problem crops up, but it never failed

when they had my car.

[1 Lucky Texan]

Vehicle info: '92 Subaru Legacy L wagon 4WD non-turbo

 

Sometimes when I start my car, the exhaust is far too rich and detonates

back in the catalytic converter. When it happens seems sporadic. The

weather could be cold or hot, humid or dry, and the car may have been

sitting for several days or only for 4 to 8 hours since the last time it

ran. I can compensate by shifting manually to keep the RPMs way up. When

the problem arises, I have to keep the RPM up to around 2500 when at a

stoplight (so it doesn't peter out). When I take off, I'll put it in first

gear and shift manually to keep the RPMs up high; otherwise, it will

hesitate on acceleration and I hear the detonation in the exhaust system.

Even after the temperature guage shows normal level, it is only after it has

been at normal for about 15 minutes before the over-rich problem fades away.

It only runs overly rich occasionally.

 

The other problem is that the "Check Engine" light comes on sometimes. I've

had it in the shop 4 times to have them read the codes. The first 3 times,

they got no error code. On the 4th time, they got no error code but when I

started my car to leave then the "check engine" light came on and they

hooked up while the car was running. I don't remember what was the code but

it didn't make sense at the time and the mechanic said it was wrong. When

the "check engine" light comes on is *not* when I have the first-startup

overly rich mixture problem. They seem to be independent symptoms but maybe

not. I suppose the "Check Engine" light might not be coming on but the

computer is still screwing up the mixture when the car is started.

 

So my choices (that I can figure out) is to replace the engine computer

and/or the oxygen sensor. I replaced my exhaust system about 4 to 5 years

ago but they reused the original oxygen sensor (they were going to replace

it but couldn't get one at that time). As I recall, this sensor check how

much oxygen is in the exhaust and recirculates a portion of the exhaust if

the oxygen is too low. Well, if it is running overly rich then wouldn't the

oxygen level would be low?

 

Any opinions if it is more likely the computer or the oxygen sensor screwing

up the mixture (so it is sometimes too rich on startup)? Might it be

something else? Besides the 4 times to take it into the shop to see if they

can nail the cause of the "check engine" light coming on (but never finding

anything), I've even left my car at their shop over the weekend to let them

start it cold to see if the over-rich problem crops up, but it never failed

when they had my car.

 

I'd expect a vehicle this old may have more than one problem - but, if its equipped with an Engine Temp Sensor (seperate from the coolant temp sensor) it may be bad, basically keeping the car in choke condition. Though I'd expect it would not go away as you indicate. O2 sensors, IAC, maybe some other system could possibly be involved.

 

maybe somone else will chime in soon - I'm guessing too much!

 

Carl

[gbrand]

If this machanic can't read your codes, find one who can or do it yourself-it is very easy. Legacy777(Josh) has an excellent write up on how to do it. http://www.surrealmirage.com/subaru/engine.html

 

It could be many things, a coolant temp sensor is certainly possible. i would also look at the fuel pressure regulator, plugs, vacuum leaks, etc. Read the codes and do a visual inpection.

 

Let us know how you do.

[Vanguard]

I don't recall seeing an *engine" temperature sensor, just the coolant temperature sensor. I'll have to check the diagrams again. I had not thought about the knock sensor. I haven't yet tracked down where it is.

 

Since the car shop and dealer couldn't find the source of the problem after taking in 5 times, I suspect that I'll end up piecemealing the sucker by replacing parts one-at-a-time until the problem goes away: oxygen sensor ($85), knock sensor ($66), purge valve ($86), PCV ($8), and the computer last ($???).

 

-----

 

Regarding the engine computer, I figured it would cost $400. But a search at car-part.com turned up the following:

 

1- Ignition Cont, (center of firewall), w/o turbo; AT

2- Ignition Cont, (center of firewall), w/turbo

3- Ignition Cont, (center of firewall), w/o turbo; MT

4- Elec Cont Unit (ECU), (behind left side of dash), w/turbo, thru 9/91

5- Elec Cont Unit (ECU), (behind left side of dash), w/turbo, from 10/91

6- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; LHD

7- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; RHD; thru 8/91

8- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; RHD; from 9/91

 

With a price of $30 (salvage price) if I selected the 8th item. However, I'm not sure which to pick. LHD? RHD? It is non-turbo and referred to in the diagrams as the ECU (don't know what they mean by "Ignition Cont[roller]"). I was a bit surprised the ECU would only cost $30 even for a used one.

[1 Lucky Texan]

I don't recall seeing an *engine" temperature sensor, just the coolant temperature sensor. I'll have to check the diagrams again. I had not thought about the knock sensor. I haven't yet tracked down where it is.

 

Since the car shop and dealer couldn't find the source of the problem after taking in 5 times, I suspect that I'll end up piecemealing the sucker by replacing parts one-at-a-time until the problem goes away: oxygen sensor ($85), knock sensor ($66), purge valve ($86), PCV ($8), and the computer last ($???).

 

Does this car have a MAF? Ever run a K&N type oiled filter? They can sometimes destroy MAF sensors.

 

I dunno but Legacy777 could definitely put you on the right track I bet.

 

Carl

[Vanguard]

If this machanic can't read your codes, find one who can or do it yourself-it is very easy. Legacy777(Josh) has an excellent write up on how to do it. http://www.surrealmirage.com/subaru/engine.html

 

It could be many things, a coolant temp sensor is certainly possible. i would also look at the fuel pressure regulator, plugs, vacuum leaks, etc. Read the codes and do a visual inpection.

 

Let us know how you do.

 

Thanks for the link. Looks like the same article that I read awhile ago when I decided to check for the codes (when the check engine light came on). I don't recall the numeric code that I got back but, I think, it was for the oxygen sensor. I'll have to run through the procedure again to see what code it reports.

 

The dealer and car shop tried retrieving the code(s), they were using their equipment which presumably would know how to read the basic and advanced codes (I've heard that the above procedure only gets basic codes and there are advanced codes, too). When I managed to get a code reported using the above procedure, it only reported a code once out of 6 attempts on different days and after clearing out the ECU and waiting for the error to reappear. So it is like the ECU is setting off the engine check light but won't report any codes for it. Maybe the ECU is defective and why no one can get codes reported from it.

[Vanguard]

Does this car have a MAF? Ever run a K&N type oiled filter? They can sometimes destroy MAF sensors.

 

I dunno but Legacy777 could definitely put you on the right track I bet.

 

Carl

 

According to http://www.cobbtuning.com/tech/airflow/ for the MAF sensor, "Subaru uses a cylinder shaped sensor mounted securely to the stock air cleaner box behind the passenger side headlight assembly". I don't remember seeing anything like this. I've replaced the air filter several time but then I wasn't looking for something attached somewhere to the side or underneath of the air cleaner box.

Also, a few times the dealer or oil change shop would replace the air filter so I don't know what they put in, but presumably it would match the type that is specified for my car. When I have replaced the air filter, it was just a folded dry paper type, like a Fram brand air filter. Basically I hit the car parts shop, use their lookup list to find which part number for the brand they carry, and use that. I don't remember ever seeing an oil-impregnated air filter in any car that I've had.