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Discussion Starter · #1 ·
The car shuts off if I'm too slow backing out of the driveway, it does the same thing moving forward at low speed. The rpms drop to like 200 and it shakes a lot right before it dies. My brother and I have tried everything we can think of.

I included the conversation below because this AI mechanic asked me some good questions you guys might want the answers to.
517469
 

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Discussion Starter · #2 ·
When the car is stopped and I press the throttle the revs drop before they go up. I'm more familiar with dirt bikes, so to me this feels like twisting the throttle with a dirty carburetor. We thought maybe the gas/air mixture was off somehow? The car drives fine once it gets above 5 mph.
 

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2020 Onyx
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You could get a code reader to gather more info, but there is a sensor or something to tell the engine that the transmission is engaged and it is supposed to boost idle so that the engine doesn't die - similar to the way power steering and A/C have a way of telling the engine to expect more load and maintain idle.
 

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Discussion Starter · #4 ·
You could get a code reader to gather more info, but there is a sensor or something to tell the engine that the transmission is engaged and it is supposed to boost idle so that the engine doesn't die - similar to the way power steering and A/C have a way of telling the engine to expect more load and maintain idle.
Ok, that makes sense, it definitely idles higher when the transmission isn't engaged. Do you know what that sensor is called or where it is?
 

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Discussion Starter · #5 ·
This website mentions a transmission position sensor, but it doesn't say anything about it having an effect on idle. It just says the sensor prevents the car from starting in gear. Is this the sensor you're talking about @SilverOnyx ?
 

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I'm not sure if the ECU uses the same sensor to disable engine start as it does to tell it to increase idle for transmission engagement. The details are unknown to me - there are experienced real mechanics in this forum but I'm not one of them, thus my vague description. A code reader could tell you more, either borrow one from a parts store or you can buy an inexpensive one.

I'm hoping it's electronic and not a mechanical issue. There are other threads on idle problems but an idle problems and various things can cause poor idle when put in gear.

Oddly enough aftermarket axles seem to be a common problem but I can't understand.

 
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This kind of problem may be the xMission NOT releasing from the engine when it is supposed to.

Perhaps now is a GREAT time to have your xMission fluid changed. You may find that the torque-converter behaves better (releases from the engine) better.

If the problem is not xMission-related, here are some other possibilities:
  • Vacuum leak in the intake plenum (between MAF sensor and throttlebody)
  • Idle-compensation
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The Idle-compensation is supposed to bump the idle-speed a little under specific conditions. One of these conditions is when xMission is put into Drive. (but the load of the xMission kinda 'hides' the bump)

A quick check of the Idle-compensation would be the following:
(Engine idling in neutral (AC OFF))
  1. Turn steering-wheel a little.... the idle should jump up about 50 RPM
  2. Turn on AC... the idle should jump up about 50 RPM
 

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Discussion Starter · #8 ·
@SilverOnyx
Thank you for your vague description, anything helps. My brother has a code reader, I'll see if we can glean anything from it.
I don't think the car has aftermarket axles, but I did buy it used. Maybe I'll check that.

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@brucep
Those are good ideas. I actually just had the transmission fluid flushed and replaced. The problem existed both before and afterward, so I think we can check that box.

I'll check for a vacuum leak in the intake plenum and I'll do your quick check of the idle-compensation.

If the idle-compensation is bad, how do I fix that? Is it a part that I buy or do I have to reprogram something?
 

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If the idle-compensation is bad, how do I fix that? Is it a part that I buy or do I have to reprogram something?
FIxing a car (or anything else) is not just about replacing stuff. It is FARRR better to diagnose a problem rather than just replace parts hoping it helps.

If you do get ahold of a descent code-reader... You could monitor the DESIRED idle rpm -vs- the ACTUAL. Also, the readings from the MAF sensor would be useful. Perhaps you could determine if the idle-compensation is at its limit trying to keep the engine running,

Lets hope this information will help lead to a resolution.
 

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Start with the easy things first.

The cause to your problem is more than likely the “misfire on cylinder two” that you reported to the “online tech”. If a Subaru engine is misfiring the idle will be very rough and will cause low idle speed.

Please remove the spark plug wires and check for oil on the spark plug boots. This is a common problem on the EJ engines and causes misfires. If there is oil on the spark plug boots the solution is to replace the valve cover gaskets and spark plug tube seals to prevent further oil leaks. You may also need to replace the spark plug wires and spark plugs.

Seagrass
 
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What happens if you turn off the air conditioner? Does the car drive OK? The idle is supposed to increase when AC is activated. I'm wondering if yours isn't.
 

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I would start with the obvious. If you are getting oil in the spark plug boots, this can cause a misfire. You need to change the spark plug tube seals. After that if it still idles low... do you know if the throttle body has ever been removed and thoroughly cleaned from both sides? A dirty throttle body can cause a poor idle.

@seagrass

When we replaced the spark plugs and wires there was a lot of oil on the spark plug boots, so maybe that's our problem.
 

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Another thought, you may need to do the “idle relearn” procedure.

This can be required after removing the battery connections for a period of time..

Seagrass
 
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