Intermittent "rumbling" and loss of power 2007 Legacy Station Wagon

[millers15]

Our 2007 Legacy Station Wagon (owned since january 2012) has recently started to give us intermittent rumbling sound/loss of power for 2-4 seconds only intermittently. It is not loud. On a 15 minute drive it may occur 2-6 times. It feels like maybe transmission/axle/all wheel drive related. (Computer or sensor related ?). Our mechanic, who we have had take excellent care of 9 different extended family vehicles over the last 15 years, is somewhat baffled but has experienced this issue himself on test drives. He just did replace the transmission oil in an attempt to resolve the problem and also looking for bits of metal at that time, but not found no bits of metal and said that the result often indicates that there is probably on transmission issue. We recently experienced a failing transmission on a 1996 Montero and the issue with the Legacy does not feel similar. The local Subaru dealer is a train wreck and dishonest. Our mechanic says he is getting no error codes (don't know if it is possible to check for diagnostic error codes for a long period of time during a test drive). I read one post on another site that seemed to be describing a similar issue and that 5 trips to the Subaru dealer yielded no error codes or resolution but that when the issue worsened, they happened to be near a Firestone dealer who immediately diagnosed a bad sensor, and they replaced the sensor and resolved the issue.

 

Any ideas ?

 

Thanks-

Doug

[millers15]

Let me correct bad grammar and a lack of clarity on my above question (next time I need to proof read).

 

Middle of the post should read "he found no bits of metal when doing transmission service and he said that this usually indicates that there is NO transmission issue"

 

Thanks

Doug

[Fairtax4me]

Does this happen while turning, or when driving straight?

 

Intermittent sensor issues can be very hard to diagnose, and often will not set diagnostic codes. If this were a sensor issue I would expect it to be either O2 sensor or MAF sensor related. Though I'm not sure if your 07 has a MAF sensor.

 

This problem sounds like you are having an intermittent misfire. Often a few quick misfires will not trigger a code, so hooking up a basic code scanner will offer no help. A more sofisticated diagnostic scanner is needed to monitor possible misfires while the vehicle is driving.

 

At your vehicles age, if we assume you've driven an average yearly 15,000 miles, you are approaching 100,000 miles and possibly the end of the recommended lifespan for your spark plugs, as well as the O2 sensors.

[millers15]

It only happens when driving and not turning.  However the mechanic working on it for us noted that he felt there might be a turning issue.  It basically happens while accelerating and usually at low rpms.  It feels like a "hopping" of the rear wheels as if the transmission and engine are not communicating to each other.  The transmission has been checked and looks good.  Just did a tune up and changed the transmission fluid and filter.  There is only 65,000 miles on the vehicle so while we did the spark plugs I am not sure the O2 sensors are ready to be changed.

 

The mechanic also indicated that he couldn't get a reading on the diagnostic scanner.  Just read a blog that advised that their Subaru was doing something similar since it had 40,000 miles on it (now has over 130K) and finally someone at Firestone replaced the TPS4165 Trottle Posn Sensor and it seems to have helped.  Would like your thoughts???

[Fairtax4me]

Do you know what kind of spark plugs were installed?

 

TPS is possible. I'm thinking that 07 has an electronic throttle body. According to Subaru the TPS is a non-serviceable part on the electronic throttle body, which would mean replacing the entire throttle body if the TPS is bad.

[millers15]

Don't know which plugs were used, but I could find out. The issue was going on before the plugs were replaced (although the plug replacement was probably due, the plug replacement was an attempt to fix the issue)

 

Any idea how we would narrow the issue down to a bad TPS ? (more sophisticated scanner than what my mechanic possesses, as you mentioned in your earlier post ?). Also, what might be a good vendor to purchase a throttle body ?

 

I really appreciate your help !

[Fairtax4me]

Basically, yes, you need a pretty high end scanner to be able to watch the TPS signals while actuating the throttle control and look for irregularities in the signal.

 

Thnig is, the electronic throttle body uses TWO position sensors. Redundancy here improves the accuracy of the throttle position detection circuit, which also means that if one sensor is reading differently than the other the ECU will pick up on the difference and set a code because there is a failure.

It seems far fetched to me that there could be a problem with the TPS and not have a Check engine light with a code. But anything is possible with the right type of failure.

[millers15]

Thank you so much.

 

Will look into diagnosing with a high-end scanner as you suggest. Meanwhile, have I misunderstood, or is the best way to proceed at this point, or is there are other possible causes that would make sense to look into right now ?

 

Thank you again

 

-doug

[millers15]

Is this a correct method to diagnose if I have a bad TPS (found in another site) ? Also I want to confirm that you are saying that I have TWO TPS's. Thank you.

 

 

The throttle position sensor, TPS for short, is a sensor that is used in an internal combustion engine. These can be fuel injected engines or carbureted engines. The TPS is used to monitor the position of the throttle butterfly valve. 

When we accelerate, the butterfly opens a little or a lot to allow air into the intake manifold, depending on how much you are accelerating. There are a few signs that could be a signal that the throttle position sensor is failing. The first and foremost is a possibility that your check engine light could come on; this is a light on the dash that sometimes appears as a little engine or sometimes it simply says check engine. 

Next scenario is that the car idles rough, this is when the vehicle is in a stationary position and you get a rough feeling from the engine almost as if the vehicle wants to stall out. Finally you may find that the vehicle hesitates on acceleration. To make a proper diagnosis we first need to get our tools in order. 

The main and only item in the tool list is what is called an ohm volt meter. Once you have identified the TPS sensor, turn on the ignition, do not start the car, make sure the volt meter is on twelve volts DC. Locate the connector that goes from the ECM (engine control module) to the TPS. Once you find this disconnect it completely from the throttle position sensor. There are three prongs; the center prong is usually the negative connector. 

Once this is done, take the volt meter and connect the negative side (usually the black wire) to the ground terminal on the battery. When this is done take the positive lead, (usually the red wire) and attach it to one of the other prongs. The volt meter should read five volts, if you are on the reference wire for the TPS. If it reads anything below five volts then you are on the signal wire and need to go to the other prong in the connector. If it reads nothing on the volt meter then you are on the negative connector. Once you locate the reference wire, and it reads five volts then you know that you are getting proper reference voltage to the TPS. Therefore you have just eliminated a faulty power feed to ECM.

The next step is to put connector back on the Throttle Position Sensor. Take the positive lead for the volt meter and put it to the signal wire in the back of the connector, this is known as back probing. Take the negative lead from the volt meter and put it to the center wire which is negative on the connector, take the butterfly shaft and turn it gradually and as you turn it you should get a gradual count upwards to five volts. If the volts fluctuate backwards or forwards then you know you have a faulty throttle position sensor.

[Fairtax4me]

That method will work for an older style TPS. This style works a bit differently, though the method above can apply if its modified a bit. There are 4 wires for the TPS, 2 signal wires, a 5v reference, and a ground.

 

I'm not convinced the TPS is the problem. If it is a problem I think the ECU would set a code for it. The ECU monitors the TPS signals continuously and will set a code immediately if it detects a fault.

 

Also since the throttle body is probably very expensive to replace, and likely only available from a dealer, I think its better to rule out fuel or spark related misfires first.

 

I would try to find a shop that specializes in Subarus, and has the equipment to properly monitor the various sensors on the engine, to look at the car and try to get an accurate diagnosis.

[Bushwick]

The mechanic also indicated that he couldn't get a reading on the diagnostic scanner.  Just read a blog that advised that their Subaru was doing something similar since it had 40,000 miles on it (now has over 130K) and finally someone at Firestone replaced the TPS4165 Trottle Posn Sensor and it seems to have helped.  Would like your thoughts???

 

 

I've run into this with 2 completely different cars I've owned, and years apart (both Lincoln and Saab). What was happening is the code scanner is being inserted into the OBDII port at an angle by a careless parts store employee or mechanic. What this does is it backs a pin or 2 OUT (might not even look like much, but even a TINY amount will prevent connection) in the connector preventing the scanner from being able to connect. This first happened to my Lincoln. Had codes pulled, everything fine. about 2 years later had codes checked and no connection. Left it alone for a couple more years. Then got the Saab which had an O2 sensor issue. Codes were pulled fine, then one day nothing. No connection error. Reflecting back on the Lincoln, I knew no one else had scanned it in between and no CEL was on, so on a hunch I removed the 2 screws for the OBDII connector, and inspected the wire crimping from behind. About 3 wires were back out slightly and NOT lined up with the other crimps. I pushed them back in with small screwdriver, and was able to get a successful scan once more. So I inspected the Saab's connector. EXACT same thing! One problem I've found though is once they've backed out once, they'll do it again. In that situation, attach the scanner plug while leaving the OBDII connector hanging. Take a small screwdriver (do NOT touch more than one wire crimp at a time or risk shorting something out!) and push them back while it's attached. Then try and get the codes pulled.

 

 

EDIT: I absolutely will NOT let a parts store person insert a scanner into my OBDII ports anymore because of this. I'll insist they let ME do it. Had one guy get slightly offended as he was clueless and thought it was something "HE" could do. 

Edited November 4, 2013 by Bushwick

[millers15]

Dear Fairtax4me,

 

Thank you again. I appreciate your help very much. You are probably correct that the TPS is not the issue, and I will most likely either try to find an independent garage familiar with Subaru's as you suggested or take a longer drive to a Subaru dealer about an hour away, as the one extremely close has a terrible track record, including with their lack of honesty.

 

Meanwhile, will try to rule out the issue Bushwick had. 

 

Thank you Bushwick, and thank you Fairtax4me

 

-doug

[millers15]

Re-read the posts and I may have mislead Fairtax4me and/or Bushwick. I am not familiar with code scanners, but I was under the impression that my mechanic could in fact "establish a connection" with the scanner but that he was not getting any error codes. (I can double check with him tomorrow)

 

I am sorry if I was communicating something differently. Bushwick: are you saying that in those 2 instances that the scanner was not even "connecting" properly due to bent/misaligned pins ? (regardless of the presence or absence of error codes)

 

Thank you again

 

-doug

[Fairtax4me]

Do you have a link to the blog you mentioned above?

 

There are a lot of relatively simple things that can cause misfires, the two biggest are plug and wires. If you replaced the plugs hoping to fix this problem, did you have the wires replaced as well?

What else was replaced?

How long has this problem been going on?

[Bushwick]

Yes. It depends on which wires are backed out. Forget the number of pins, but it's either 16 or more. Scanner will get a "NO Connection" error like it's not even hooked up, or it'll connect but can't go any further. What was interesting is NO one had a clue what was wrong and I heard stuff like "xxxx might be faulty" "you might have a bad fuse" etc. So long as you insert the scanner slowly and evenly into the slot, you should be OK. But like I said, I had this happen years apart, and different stores in different states to be honest. A quick google search will show it's a common occurrence with most not figuring out a solution.

Edited November 4, 2013 by Bushwick

[millers15]

Fairtax4me,

 

Which blog ?

 

do you mean where the person mentioned that they had occasional bucking,etc from 40,000 miles to 139,000 (with subaru dealer failing 5 times to see an error code) before a bad episode of it sent them to a nearby Firestone dealer who correctly diagnosed it and replaced a sensor ? Or the blog where the person listed (an older method per your comment) the method to check a TPS to see if it was faulty ?

 

Thank you Fairtax4me

 

-doug

[millers15]

Fairtax4me,

 

My mechanic, who I have a lot of reason to trust completely, says he changed plugs and wires. This has been going on about 3 months. 

 

thanks again

-doug

[Fairtax4me]

The first one you listed there. About the car that went to the dealer 5 times.

[millers15]

Fairtax4me,

 

nothing else replaced. On a seemingly unrelated matter, the dealership checked brake linings as part of a Subaru recall. Obviously would seemingly have nothing to do with this, but my wife pointed out that this "rumbling" issue started around the time the dealership looked at the brake linings. If you are thinking I am cynical enough to believe that there are cases of mechanics/dealers accidentally or purposefully causing a new unrelated problem, you are correct, I have become that cynical.

 

thanks again

-doug

[millers15]

Bushwick

 

thank you again, and excellent information to be aware of

 

-doug

[millers15]

Fairtax4me

 

I will look for that link

 

thanks

doug

[millers15]

Fairtax4me

 

http://www.subaruoutback.org/forums/66-problems-maintenance/26509-help-someone-anyone-hesitation-returned.html

 

it is about the 8th post (by dpregs)

 

thanks again

-doug

[millers15]

Fairtax4me

 

more info/clarification. my wife reminds me that a Subaru dealer that we sometimes use out of state put 4 new snow/all wheel drive tires on in December. The ride is a little rougher than before, but Subaru dealer said that this is to be expected. There is not much "profile" to 2007 Legacy tires (distance between wheel and the road.) And we are happy to exchange a less than "super soft" ride for excellent traction

 

Our mechanic, in trying to figure out this issue, mentioned to my wife that if tires are not placed on the car "correctly" that their could be significant consequences and damage to the vehicle.

 

I do not know what he meant about "correct" tire installation (tires that belong on the back being put on the front ? who knows ?) or what he meant in terms of damage but I think he was saying that it was a possible source of this issue, unless we completely misunderstood him. Anyway, he made these comments a while back, before he changed the plugs, wires, and did transmission service.

 

My wife is confident that this issue started a couple of months after the new tires were put on (earlier than what I had said in a previous post, so NOT around the time the dealership looked at the brake linings due to a recall concern-the issue started before that.)

 

She says that the first time that it happened, she was accelerating after the vehicle was idling at a red light. It has happened many times while driving straight down the road at various steady speeds. 

 

Thanks again so much for all your time and effort

 

-doug

[Fairtax4me]

Thanks for the link Millers. What's described in the posters situation there is a sharp jerking and lurching type of problem. This is common with a failed TPS, and that dealers inability to diagnose it is based on the tendency of many technicians to have a heavy right foot.

The TPS sends a voltage back to the ECU that varies based on throttle position. Low voltage is low throttle, high voltage is heavy or wide open throttle. It varies from about 0.5 - 4.5v, and will sweep smoothly from low to high and back as the throttle is opened or closed.

The most common failure of a TPS is that it develops a "dead spot", basically a point in the resistive contact that has worn out or been damaged. This causes the TPS voltage signal to be jumpy and irregular, and confuses the ECU.

The ECU uses the TPS to determine fuel delivery, spark timing, and transmission shift points. If the TPS signal quickly jumps between high and low, the ECU may not detect that there is a circuit problem, but it will react to the changes in the signal. The ECU can make these changes in real time, usually only taking a few milliseconds to alter fuel and spark, which is where we get most of the problems when a TPS fails.

The ECU sees a smooth increase in TPS voltage up until the throttle opens far enough for the sensor to hit the "dead spot", now the ECU sees a jumpy signal and assumes you've opened and closed the throttle very quickly several times. It adds fuel, cuts fuel, adds fuel, cuts it, in rapid succession, which causes the jerky jerky feel. It could also decide to make the transmission downshift or upshift, throwing another mechanical motion into the mix. The result is you get a very rough ride until you press the throttle pedal down far enough to open the throttle past that "dead spot". Now its smooth again, but you're accelerating much faster than you might want to, and when you reach the speed limit, letting off the throttle moves the TPS over that dead spot again. Jerkiness ensues until you either let off the throttle entirely or give it the beans again.

And most of the time when a TPS gets a dead spot, its right in the range where you tend to keep your foot the most, right where you need it to sit in order to maintain speed, or accelerate very lightly.

 

The other type of failure is for the TPS to short to the 5v reference, so any touching the throttle sends a 5v signal to the ECU which it reads as WFO! The engine will redline itself even if your foot isn't on the pedal anymore. Very scary, but also very uncommon.

 

This is why I say if you're getting a more of a light stumble or hesitation, its probably not the TPS.

 

 

Tires I could probably go on about all day too. But in a nutshell, only tires of different sizes and tread depths will damage the vehicle. Subaru specifies no more than 1/4" difference in overall circumference between the tire with the most tread, and the tire with the least tread. Any more than that and you could cause long term damage to the AWD system in the transmission.

 

Tires will not generally cause an intermittent rumbling sensation, though they can if they have just the right amount of imbalance, but this is generally going to correlate with road speed, and not engine speed or throttle angle.

I would have to think that the rumble/stumble issue is just a coincidence, and not related to the tires or brake services.

[millers15]

Fairtax4me

 

Thanks so much. It may be a little while until I post again. I most likely will take it to Subaru dealer an hour away. Talk soon thanks again. Will post when I have an update

 

-doug