[HeartMan]

I've got the strangest problem that my independent Subbie mechanic cannot fix.
The car has about 116,000 miles. It had been running like a top. About 3 weeks ago as I was exiting the freeway, the car had a hard downshift. As I drove down the off ramp it stalled. It restarted easily with somewhat of a rough idle. At this point the Check Engine light came on. OK
1. The OBDC indicated a bad TPS.
2. Replacing the TPS cleared the code but made no difference.
3. This engine has a MAP sensor but no MAF sensor. The MAP sensor seems to be fine.
4. If I start the engine in neutral with my foot on the brake on a slight hill, the engine will idle all day long. But, the absolute instant I take my foot off the brake, the engine stalls.
5. If I can get the car moving by giving it gas, the engine keeps running but there is a lot of shift shock. Most times stopping at a red light, the engine stalls.
6. Out on the freeway the car will run very smoothly at 65 mph. If I slow down to about 40 and put my foot into it, the trans will not downshift. However if I put the lever into 3, the trans does downshift.
7. At this point the Check Engine light is off and the scanner shows no codes.
8. Using the "magic sequence" and the trans oil temp light, the trans reports no problems with the trans.
9. My mechanic has replaced the TPS twice and even tried a brand new VIN matched ECU. No change. He traced all the wiring between the engine and ECU and trans and the ECU and found no issues.
10. He has conferred with a friend who is the top mechanic at the Subaru Dealer where the car was purchased. They have run out of ideas.

Help!!!!

Thanks, HeartMan

 

[plain OM]

If I start the engine in neutral with my foot on the brake on a slight hill, the engine will idle all day long. But, the absolute instant I take my foot off the brake, the engine stalls.

This is an interesting symptom; I don't have any suggestions, but wonder if this somewhat unique situation might provide some leads.

Is this repeatable, every time?

What happens if you start the car on a level surface (can the slope issue be eliminated -- in other words, is it the brake that is the factor here, not the slope?

If it's the same on a level surface, I'm thinking that the brake has two links to the engine -- the brake light circuit is sensed by the ECM and ABS modules, among others, so there could be an electrical involvement. (Could try disconnecting the stop light switch and see if the symptom changes. If it's the same, then the electrical connection isn't a factor; if it changes, then it could be.)

Also, the brake system is connected to the engine through the manifold vacuum used for the power brake booster. A vacuum leak in the booster when the pedal is released could cause poor engine running and the other symptoms you described, but when the pedal is pressed, the leak (inside the booster) could be blocked off.

Just doing some brainstorming here -- I would imagine that the mechanics have looked at these "connections", but who knows . . . .

 

[HeartMan]

I "think" I can rule out the brakes

This is an interesting symptom; I don't have any suggestions, but wonder if this somewhat unique situation might provide some leads.

Is this repeatable, every time?

What happens if you start the car on a level surface (can the slope issue be eliminated -- in other words, is it the brake that is the factor here, not the slope?

Thanks for your thoughts.

Yes, this is a symptom that keeps making my head hurt.

It is repeatable every time.

To rule out anything having to do with the regular brake system, I have tried the same test except using the parking brake instead. It behaves exactly the same way! A wheel sensor problem keeps rattling around in my mind. Can anyone recommend the best way to check the wheel sensors?

 

[plain OM]

So, if I may paraphrase, you're saying:

"If I start the engine in neutral with the parking brake holding the car, (not pressing the foot brake pedal), the engine will idle all day long. But, the absolute instant I release the parking brake, the engine stalls, and it doesn't matter if the car is on a slope or on a level surface."

If this is correct, it makes no sense, or at least I can't make any sense of it. The parking brake and foot/hydraulic brakes are pretty well separate systems. Why would the engine stall when the parking brake is released, yet start and idle when it's applied?

Wheel sensors do not affect engine operation. They can affect ABS VDC, but their main effect is when the car is moving. When the car is not moving, there's no signal from the sensors. If a wheel sensor malfunctions, the ABS warning light will normally come on; but still, engine operation would not be affected.

I'm really wondering if there's either a misinterpretation of the apparent causes/effects, or there's another symptom that hasn't been identified/mentioned. We can only work with the information we're provided . . .

 

[HeartMan]

So, if I may paraphrase, you're saying:

"If I start the engine in neutral with the parking brake holding the car, (not pressing the foot brake pedal), the engine will idle all day long. But, the absolute instant I release the parking brake, the engine stalls, and it doesn't matter if the car is on a slope or on a level surface."

If this is correct, it makes no sense, or at least I can't make any sense of it. The parking brake and foot/hydraulic brakes are pretty well separate systems. Why would the engine stall when the parking brake is released, yet start and idle when it's applied?

Wheel sensors do not affect engine operation. They can affect ABS VDC, but their main effect is when the car is moving. When the car is not moving, there's no signal from the sensors. If a wheel sensor malfunctions, the ABS warning light will normally come on; but still, engine operation would not be affected.

I'm really wondering if there's either a misinterpretation of the apparent causes/effects, or there's another symptom that hasn't been identified/mentioned. We can only work with the information we're provided . . .

And thus you understand my confusion. Tomorrow I will post a video of this bizarre behavior.

Thanks

 

[SubaruDan1]

Sounds like it could be something with the transmission torque converter lock up function.

 

[HeartMan]

Sounds like it could be something with the transmission torque converter lock up function.

I would think that this could be a possibility. My question is how would one diagnose such an issue?

Thanks

 

[Pdsr]

Thanks for your thoughts.

Yes, this is a symptom that keeps making my head hurt.

It is repeatable every time.

To rule out anything having to do with the regular brake system, I have tried the same test except using the parking brake instead. It behaves exactly the same way! A wheel sensor problem keeps rattling around in my mind. Can anyone recommend the best way to check the wheel sensors?

Wheel speed sensors, generally, can be monitored through the OBD plug, but require a scanner that has access to the ABS system (ie not the usual consumer grade scanner you can get for $100 bucks or so). You might get a mechanic to go with you as you drive to monitor the output from the speed sensors. However, a malfunction in a wheel speed sensor will cause the ABS warning light to come on. In some vehicles, it can also cause rougher shifting, but not stalling. My bet would be some sort of vacuum related issue, but that's just a guess. Too many electronic pieces and parts, I miss points and carburetors.

 

[HeartMan]

Too many electronic pieces and parts, I miss points and carburetors.

I can certainly agree with your sentiment. If you had issues with points, you could smooth them off with sandpaper and gap then with matchbook cover!

Thanks

 

[Seabass]

Sounds like it could be something with the transmission torque converter lock up function.

+1. That explains both the stalling and shift shock. It could be a valve body solenoid problem as well. Did the dealership reset the tcu by any chance? Any diagnostic codes?

 

[HeartMan]

+1. That explains both the stalling and shift shock. It could be a valve body solenoid problem as well. Did the dealership reset the tcu by any chance? Any diagnostic codes?

I'm not sure if the TCU was reset. I assume that it was. I can check next week. As I mentioned in my original posting, the trans indicated no problems as checked with the Trans Oil Temp light in the dash.

Thanks

 

[1 Lucky Texan]

fuel trim data might help with a possible vacuum leak diagnosis.

I assume the transmission fluid is OK?

the problem seems to indicate the transmission more than anything.

Can you drive normally in 3rd gear? that is, will it up and downshift smoothly when driving in 3rd? It might not (anyone?) use the TC lock-up in 3rd so, it it drives smoothly in 3rd, that may help with diagnosis. not sure though.

 

[HeartMan]

The trans fluid was my first thought. It was full and clean. I will check out shifting up and down from 3 tomorrow. As I mentioned, last time I tried it would not downshift from Drive to 3 by stomping on the gas, but it would if I selected 3 with the lever.

Thanks

 

[Seabass]

Another thing to check is whether the torque converter lockup clutch is working at all. Again in 3rd gear (selected, not in D) cruise at around 1500rpm for a little bit, then go full throttle and see if the rpm jump up to the stall speed. I'm guessing the stall speed is around 2200 like the other 4eat's, but I'm not sure. If the torque converter clutch is working properly, it will disengage allowing the rpm to jump up to the stall speed. If it is not working, it will slowly climb as a manual would at full throttle in 3rd gear at 1500 rpm.

 

[plain OM]

4. If I start the engine in neutral with my foot on the brake on a slight hill, the engine will idle all day long. But, the absolute instant I take my foot off the brake, the engine stalls.

This particular symptom is with the car stationary and in neutral. Also, it's same using the parking brake rather than the foot brake, on a level surface as well as on a slope, and it's repeatable, or so I understand. (I presume it's the same in Park.)

If this is correct, then how does the transmission come into play? The fact that the transmission appears to be malfunctioning could be because the engine is malfunctioning to begin with, rather than the cause of the engine stalling.

A locked torque converter shouldn't cause the engine to stall if the transmission is in neutral. Of course, if the torque converter is locked and the transmission is in a drive gear, despite the gear lever being in neutral or park, the engine shouldn't turn at all as long as the brake is preventing the car from moving. Yet the engine will idle (without having to press on the accelerator to keep it going) "all day" as long as the brake (foot, or parking) is applied. (Very strange!)

I would want to be able to start the engine and not have it stall when the brake is released (transmission still in Park or Neutral) before going on to diagnosing a transmission problem.

What am I missing here?

 

[1 Lucky Texan]

This particular symptom is with the car stationary and in neutral. Also, it's same using the parking brake rather than the foot brake, on a level surface as well as on a slope, and it's repeatable, or so I understand. (I presume it's the same in Park.)

If this is correct, then how does the transmission come into play? The fact that the transmission appears to be malfunctioning could be because the engine is malfunctioning to begin with, rather than the cause of the engine stalling.

A locked torque converter shouldn't cause the engine to stall if the transmission is in neutral. Of course, if the torque converter is locked and the transmission is in a drive gear, despite the gear lever being in neutral or park, the engine shouldn't turn at all as long as the brake is preventing the car from moving. Yet the engine will idle (without having to press on the accelerator to keep it going) "all day" as long as the brake (foot, or parking) is applied. (Very strange!)

I would want to be able to start the engine and not have it stall when the brake is released (transmission still in Park or Neutral) before going on to diagnosing a transmission problem.

What am I missing here?

I don't think you're missing anything. I guess I just want to grab a symptom and go! lol!

I can't figure any electrical connection, not even a sagging dual-element bulb, but, maybe disconnecting or unplugging the brake lights to determine if that makes any difference with the the whole 'starts with the brake pedal depressed' issue - might shift the blame more towards vacuum or electrical.

I still think fuel trims might help with a possible vacuum diagnosis.

odd not having a CEL with such a serious problem.

 

[plain OM]

I still think fuel trims might help with a possible vacuum diagnosis.

Yes, that or the manifold pressure readings that some scanners can track. Be interesting to see a sudden change when the brake pedal is released.

HeartMan reported that when the parking brake was used to hold the car in place while the engine is being started the same stalling occurs, or at least that's how I understand it. The parking brake is not linked in any way to the engine or its control system.

I'm wondering if that was a mis-interpretation of the situation.

If the car starts and runs well as long as the brake pedal is pressed, and stalls whenever the brake pedal is released, there is a very high probability that the brake booster is the problem. The internal air valve is probably sticking open, and this will allow outside air to be drawn into the manifold when the pedal is released, but not when it's pressed down.

Still thinking about it, but really need confirmation of all the different possibilities, and perhaps that video.

 

[1 Lucky Texan]

how can the vacuum booster be an issue if the parking brake creates the same symptom?

and I think now, it can't be an electrical 'load' change because, releasing the handbrake will turn the DRLs ON, but releasing a brake pedal will turn brake lights OFF.


seems transmission 'related' still.

 

[plain OM]

how can the vacuum booster be an issue if the parking brake creates the same symptom?

1 Lucky Texan:

That's exactly what I was questioning way back. It doesn't make sense, as long as when the parking brake was being applied and then released, the brake pedal was not also being used to hold the car and was released at about the same time.

I guess the same sort of question could be asked about the transmission. It's not that I'm saying the tranny can't be involved; but like the parking brake question, I'm looking for an understanding of how it could be.

While still in neutral (whether the selector is in P or N), how would a transmission fault, such as torque converter lock up, cause the engine to stall when releasing the brake pedal or the parking brake?

Here's a worst case scenario for the transmission possibility. When the lever is in P or N, the transmission is actually in a drive gear, not in neutral. In addition, whenever the engine is running, the torque converter is locked up. (Can't get much worse.)

In this case, why would the engine run "all day" as long as the brake is pressed, but fail as soon as the brake is released?

The engine would be pushing against the torque converter, which itself is locked all the way through the transmission to the drive train, which cannot turn because the brakes are on. In this situation, the engine shouldn't be able to idle all day; in fact, it shouldn't idle at all.

Even if in this scenario the transmission internal gear train clutches and brakes were being forced to slip, the engine would be idling under a lot of strain. It should be struggling to continue running.

Moreover, when the brake is released, the engine loading would be lessened because the wheels could then turn. Consequently, the idling engine should start to run more normally, rather than stall.

I also appreciate that a locked up torque converter can cause shift shock, and possibly prevent shifting at appropriate points, but that's when the car is in gear and moving. The problem, however, is already apparent when the car is still parked and in neutral.



These points are significant, but could use some clarification:

4. If I start the engine in neutral with my foot on the brake on a slight hill, the engine will idle all day long. But, the absolute instant I take my foot off the brake, the engine stalls.
5. If I can get the car moving by giving it gas, the engine keeps running but there is a lot of shift shock. Most times stopping at a red light, the engine stalls.

HeartMan:

What happens if, after starting the engine and while still pressing the brake pedal (but not the gas pedal), the transmission is changed to D, or R? Does the engine stall, or does it continue to idle and again stall as soon as the brake is released?

When coming to a stop, does the engine stall if the gear selector is moved to neutral? Also, if the gear is moved to neutral before the car comes to a stop, is there any difference in what happens to the engine if the brake is used to bring the car to the stop, or if the car is just allowed to come to a stop on it's own (no brake) with the gear in neutral?

Will the engine start and run properly if neither the brake pedal nor the parking brake is being used? (E.g. transmission in Park, or car parked on a level surface where it won't roll.)

 

[1 Lucky Texan]

the way the facts/symptoms have been presented are a little odd, but, I wonder, if the stalling isn't happening just , off-idle , as if there were an IACV or weird vacuum problem?

these;

4. If I start the engine in neutral with my foot on the brake on a slight hill, the engine will idle all day long. But, the absolute instant I take my foot off the brake, the engine stalls.
5. If I can get the car moving by giving it gas, the engine keeps running but there is a lot of shift shock. Most times stopping at a red light, the engine stalls.

might both be related to IACV/vacuum or other idle problems, since, on a hill, one might try to press the accelerator slightly after, or concurrent with ,removing a foot from the brake pedal.

there could be some overlapping problem with the TPS and/or the transmission. very weird .

I think it may be necessary to triple-check any hoses/wiring disturbed by the mechanic at the first TPS change - maybe something at that juncture led to the stalling problem? Although I think it's a remote possibility, checking the timing chain and maybe a compression test should on the list of tests - a hard downshift exiting a freeway could over-rev an engine I suppose. How many miles on this car? did I miss that? Sometime older H6es need valve adjustment IIRC. maybe there's now a stuck or burned exhaust valve?


i dunno - just throwing out some brainstorming.