93 Legacy backfiring at start-up

[88coupe]

Recently bought a 93 Legacy wagon off Craigslist for my father to commute to work out of town. It runs good and is used to commute 250mi's a week (highway drive 125mi there & 125mi back). I check it out every weekend when he gets back.

 

THE MAIN issue is it backfires thru intake at startup & the idle is high, only when engine is cold! After it warms up it runs better & idles fine. It had the Bosch 4 prong platinum spark plugs, runs better with copper NGK's, but still backfiring. I cleaned the PVC vavle, MAF sensor, IACV, & throttle body also. Had the exhaust checked out & they replaced 1 of the cats but not sure if they really checked EVERYTHING out. Previous owner had replaced the plugs/wires & air filter. I've researched this & came across a few possibilities but the car runs fine once warmed up.

 

coolant temp sensor could be failing-dumping too much fuel at warm up. Timing belt could have jumped a tooth. Exhaust leak near the head or at the Y-pipe allowing air in & then closing when warmed up. Low or uneven compression. These are some of the problems that I read can cause a backfire but the car runs really nice once warmed up.

 

Another issue is the speedo doesn't work. Check engine light is on for Vehicle Speed Sensor. And the POWER light for the trans blinks when you turn the key on. I figure that the tranny Control Unit senses the VSS & is why power light blinks. So does this mean the Tranny is in a "SAFE MODE" not performing at its best because the computer sensed a problem? It feels like it could have more acceleration & performance is what im getting at.

 

Any helpful suggestions, comments, questions or information is highly appreciated! THANKS.

[1 Lucky Texan]

check the timing.

 

you might pull the plugs after it sits overnight and see if one is wet - maybe an injector is leaking?

[johnceggleston]

two possibilities for the ''back firing'' and high idle,

probably a vac leak, spray some car cleaner or the like around the engine bay while idling.

if the engine revs you have located the leak.

 

or check your timing,

back fires are usually a timing issue.

i'm not an expert, but this is where i would start.

 

the speed seanor could be the speedo cable.

the sensor is part of the speedo head, but the cable is a weak point in a 20 y.o. car.

and not uncommon.

 

i'm assuming this is an AWD car.

the flashing power light at start up indicates there is /was an electrical problem in the auto trans

during the last drive cycle.

(if you start the car and do not drive it, the power light will, should, may, not flash on the next start.)

the most common cause is the duty C solenoid on the transfer clutch in the rear extension housing of the trans.

this controls how much power is directed to the rear wheels,

and it's failure will cause binding in slow tight turns.

give it a test drive in slow tight turns.

 

duty C test,

there is a fuse holder under the hood, passenger side, at the fire wall near the wiper motor.

put in ANY fuse and the car will be come FWD and there will be a FWD light on the dash.

if the duty C is bad, the light will not come on.

the duty C is working the light will come on and any binding should be gone.

 

do a search for ''how to read trans codes'' and you can learn exactly what is causing the power light to flash.

 

the temp sensor can cause starting issues, but i have never heard of it causing back fires.

it sounds like once it warms up and reaches ''closed loop'' the computer compensates for what ever is causing the issue.

Edited March 18, 2014 by johnceggleston

[88coupe]

But wouldn't the timing cause backfiring/misfire even when the engine is warm? I was thinkning maybe it was something that closed up when it got warm like a exhaust leak or ...

[heartless]

But wouldn't the timing cause backfiring/misfire even when the engine is warm? I was thinkning maybe it was something that closed up when it got warm like a exhaust leak or ...

 

not necessarily - some wont run at all, others will run fine once warmed up - depends on which way the timing is off and how far. the computer will compensate for small differences if it is in the right direction.

 

pull the outer timing covers off and verify the timing is correct and go from there.

[88coupe]

 

 pull the outer timing covers off and verify the timing is correct and go from there.

 

Check the ignition timing with a light gun & the timing of the

cam/crankshafts & belt? Or is that how you check the belt & such is with the light gun? Start there then check the coolant temp

sensor and so on? Do you not think it could be the coolant sensor?

 

 

 

 

the flashing power light at start up indicates there is /was an electrical problem in the auto trans

during the last drive cycle.

(if you start the car and do not drive it, the power light will, should, may, not flash on the next start.)

the most common cause is the duty C solenoid on the transfer clutch in the rear extension housing of the trans.

this controls how much power is directed to the rear wheels,

and it's failure will cause binding in slow tight turns.

 

Well the CHECK ENGINE light was triggered by the Vehicle Speed Sensor and I had read that the tranny has a control unit also (VCM) and the POWER light is its own equivalent to the check engine light, so I assumed that they both picked up the VSS code. Does the tranny go into a safe mode when a trouble code is detected?

[johnceggleston]

there are some problems that will throw both an engine and trans code like MAF or TPS.

the speed sensor is not one of them.

 

safe mode for the auto trans is actually called ''limp mode''.

the trans defaults to 3rd gear and reverse mechanically and there are no electronics.

but you can limp home.

[heartless]

Check the ignition timing with a light gun & the timing of the

cam/crankshafts & belt? Or is that how you check the belt & such is with the light gun? Start there then check the coolant temp

sensor and so on? Do you not think it could be the coolant sensor?

 

no timing light involved.

 

remove the 2 outer covers - 3 bolts each - this will expose the cam pulleys.

using a socket on the main crank bolt, rotate the engine by hand until you can see the hash mark (not the arrow) on the outside edge of one of the cam pulleys pointing straight up - there is a notch in the rear timing cover that it should be lined up with.

once you have one pulley lined up in the correct position - check the other pulley to see where the same marking is - it too "should" be pointing straight up - if it is not, the cam timing is off and this is the source of your backfiring problems.

 

there are some really good pictures of the hash mark you are looking for in this thread: http://www.ultimatesubaru.org/forum/topic/117660-timing-and-belt/

[88coupe]

UPDATE*** So I checked the timing & to not much surprise it was off, but what was surprising was how off time it was. It looked to be at least 2 - 2.5 teeth off (maybe 3)!!! WHAT THE ****? I was under the impression that being a 1/2 tooth off could cause it to not start or at least barley start and run like crap. And being a whole tooth or more would warrant the engine nonoperational, yet it was more than that and it started up every time & would run & drive all day but would backfire & idle high on initial cold start. So anywho it was the timing so changed the belt and water pump, checked the idlers and sprockets put it back together & it fired up right away no backfiring what so ever & MUCH better performance. There were a few things I found that I want to know more about tho, such as the crank shaft position sensor had a slight dent in it. I searched and found a few pictures of others that I would consider "smashed" at the end & how the owner couldnt even get it out of the hole without ripping the thing apart. But not to much explanation was given besides assumptions of belt jumping teeth & hitting the sensor, that is quite a ways for the belt to jump to touch the sensor. Any thoughts on what could cause a dented/dinged/smashed Crank Shaft Position sensor? Also the idle is still what I would consider "above average" on start up, but lower than before the timing was done. Where do the most common vacuum leaks occur on this generation of Legacy? What else should I check out? Thanks in advance for helpful info & Thanks to the few who replied already, Much appreciation.

[heartless]

depending on ambient temperature, the idle will jump up fairly high at first start - 1500 rpm or so.

My 95 will jump to 2000 for a second or so, then settle to around 1500 and gradually drop down as it warms up.

 

normal warm idle should be around 700 rpm or so.