A hesitation between 1500-2000 rpm?

So my 2008 Tribeca, 3.5L H6, has developed some kind of a hesitation when accelerating at low RPM (usually in 1st or 2nd gear, I assume). The
problem is most pronounced at starting at 1500 RPM, and it may last all of the way upto 2000 RPM, before everything normalizes again. What could be causing this?
    Yousuf Khan
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On Thu, 3 Aug 2017 09:19:09 -0400, Yousuf Khan

How many miles on the plugs??
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On Thursday, August 3, 2017 at 3:18:21 AM UTC-10, Yousuf Khan wrote:

First check your spark plugs and wires. If they haven't been changed yet, change them.
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On 03/08/2017 2:08 PM, dsi1 wrote:

They had been changed maybe 30,000 km back, relatively recently. Previous plugs were working fine until about 130,000 km (a little over the limit, I know, should've been changed at 100,000 km, but they had no problems). There's no Check Engine Light that comes on to say there's anything wrong with the plugs yet.
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On Thu, 3 Aug 2017 14:31:36 -0400, Yousuf Khan

Did you replace them with the specified OEM platinum or dual plat plugs, or some other plug? You can get a fair amount of sporadic misfire before the CEL comes on - and a misfire from ANY cause will turn on the CEL. It doesn'tKNOW what caused the misfire.
90+% it's plufs or wires or coil - and bad plugs WILL kill wires and coil ---
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On 03/08/2017 2:42 PM, snipped-for-privacy@snyder.on.ca wrote:

I don't remember what type of plugs I used. But I didn't use OEM's, just whatever was available at Canadian Tire.
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On Fri, 4 Aug 2017 10:39:27 -0400, Yousuf Khan

Then the FIRST thing to do is buy the right plugs and install them - I had the same issue on my PT Cruiser when I "upgraded" to champion irridiuma. Use OEM plugs ONLY on today's cars if you wannt assurance you will not have issues. On engines with waste fire ignition using the "double platinum" option if it is available is often worth the extra expense as one plug per coil fires in reverse polarity
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On 8/3/2017 9:19 AM, Yousuf Khan wrote:

Could you be interpreting vibration as hesitation? That opens up a larger range of possible problems. That is such a narrow and specific range that I don't see it as plugs or similar -- normally bum plugs or wires will give worse symptoms the higher the RPM but having it stop abruptly as you accelerate doesn't seem right.
One test is to try going through the range going up and then going down (RPM that is). If it is vibration then you will feel it either way but if it is hesitation then it should be evident only while accelerating. Far easier with a stick than an automatic but it may be worth a try. What are the symptoms when trying to accelerate up a grade? Worse? Better? How about going down a grade?
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On 03/08/2017 4:00 PM, John McGaw wrote:

Actually, my first guess was that it was some kind of a vibration, but then I noticed that it only happens while accelerating up the range. Doesn't happen while decelerating. I have tried going up a grade, and the symptoms are more or less the same. Going down a grade, you're normally braking not accelerating, so the problem doesn't occur.
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Yousuf Khan wrote:

Was the vehicle worked on and afterwhich the symptom appeared? Did the battery get disconnected and not get reconnected for quite a long time later?
In the past, and after a couple repairs that had the techs disconnect power from the battery, I had to disconnect the battery for a while, like 30 minutes. Typically I'd remove the ground cable from the battery, come back sometime later, like an hour, or more, and reconnect the battery cable. Then I had to drive the car a while along with repeated engine starts. The computer re-learns how to adjust the idle mixture for smooth running. I could smell the fuel from the tailpipe so the idle mixture was way too rich. Had to do an "idle re-learn" by the computer.
http://www.subaruforester.org/vbulletin/f113/how-idle-re-learn-after-battery-disconnect-154161/#post1640665 http://www.aa1car.com/library/battery_disconnect_problems.htm https://itstillruns.com/disconnect-car-battery-reset-computer-4963429.html
I don't remember the procedure that I was told to use to get the ECM to re-learn its idle mixture settings. Just remember having to go through a procedure.
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On 03/08/2017 4:27 PM, VanguardLH wrote:

No, the battery didn't get unplugged at any time. However, now that you mention it, the symptoms started appearing after my last engine oil change, and as well there was a transmission/differential oil change that occurred around the same time. I was actually thinking that there was some kind of a connection to one of these oil changes, but eventually I discarded that idea.

Yeah, I used to actually use this method on my old Outback to have it relearn its engine mapping, but so far I haven't done that to my Tribeca. Is it possible that this is a fuel pump issue? And will a fuel pump issue be corrected by resetting the ECU?
    Yousuf Khan
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On Fri, 4 Aug 2017 10:57:01 -0400, Yousuf Khan

Not likely, and no. Could be a transmission issue - who changed the trans fluid? It takes a SPECIAL trans fluid. Using the wrong fluid could very well cause a problem that COULD manifest itself as a "stutter" on accelleration in certain gears.
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On 8/4/2017 12:54 PM, snipped-for-privacy@snyder.on.ca wrote:

I went to Mr. Lube, and they said they looked up the specific differential fluid and looked to see that they had that type in stock, and they did. I've had them change my diff fluid before, and no problems developed before.
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On Wed, 9 Aug 2017 23:58:03 -0400, Yousuf Khan

What about the trans fluid? And Mr Lube can screw up something as simple as greasing a door hinge - v- - -
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On 10/08/2017 8:39 AM, snipped-for-privacy@snyder.on.ca wrote:

They changed the trans fluid too. I think the front diff and trans are part of the same assembly, so one gets done when the other gets done.
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On 2017-08-10 2:08 PM, Yousuf Khan wrote:

There was a report on the Audi forums some time back where the owner of one of the more expensive Audis took his in for an oil change and discovered after the fact that the techs had cut a huge hole in the underbody shielding to get at the oil drain plug. One would have thought it would have been faster to remove the screws holding the shield in place, but that apparently did not occur to the Mr. Lube staff.
I wouldn't take any decent vehicle to Mr. Lube. Yeah, it may save a few minutes of your time, but at what cost if they screw things up?
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On Thu, 10 Aug 2017 18:29:49 -0400, Darryl Johnson

I can't remember how many we ran into when the Tercel first came out - thelub shops drained transmissions and overfilled crankcases. I can remember at least 3, plus at least 2 done by DIY owners. Cost a few transaxle assemblies.
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On 10/08/2017 6:29 PM, Darryl Johnson wrote:

Okay, let's get off of the kicking the staff at Mr. Lube here, it's getting off-topic. I've been going to their place for years, it's not like as if they've never seen Subarus before; and it's just an oil change, not rocket science. I hesitated to even mention that there was an oil change that happened prior to it, as I knew that this blame-game would start, and take away from the actual topic here. The problem might have already been developing, and the oil change was just a coincidence.
Let's just stick to discussing theories about why this hesitation might be happening. Shall we?
    Yousuf Khan
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On Fri, 11 Aug 2017 10:17:11 -0400, Yousuf Khan

Well - let's start with EXACTLY what is happening - and exactly what is occurring when you feel the "hesitation". Does the tach flutter? Is it a "miss" or a "surge". Does it happen if you "lock" the transmission into a fixed gear (you can lock the Soob into second?)
We need to determine if it is an engine missfire, or a rich or lean surge, or the transmission shifting, or the torque converter locking and unlocking. Does it happen at all operating temps, or only after warmed up? If it doesn't do it cold, it COULD be the torque converter locking/unlocking. If it doesn't do it when locked in second, it COULD be the transmission hunting.
If it is an engine "miss" it COULD be a spark plug issue - particularly if you have "3rd party" plugs installed. It could be a variable valve timing issue if the wrong engine oil has been used or the oil has not been changed often enough. It COULD be a lean surge from dirty injectors - it COULD be a rich surge from a dripping ir squirtinf (as compared to misting or spraying) injector.
If it was mine I'd make sure it has good plugs and wires, that the oil is fresh synthetic of the recommended grade (and if any question about oil change intervals longer than 5000KM I'd run a good crankcase cleaner - like MMO or Sea Foam before the change). I'd also run a good fuel coditioner, like Syncron ot Sea Foam in the fuel system and fill up with a tier one ethanol free fuel like Shell premium. That's the easy stuff.
Also make sure the VSS (vehicle speed sensor) is working 100%.
If that doesn't fix it, I'd be double and triple checking the trans fluid - making sure it had been properly flushed (including dropping the pan and changing the filter) with EXACTLY the recommended fluid. If this was not done ONE HUNDRED PERCENT to spec, I would do it again - and do it right - Possibly adding a can of Sea Foam Trans Tune or BG ATC Plus (both high quality "professional grade" ATF additives with a long record of success)
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On 8/11/2017 12:32 PM, snipped-for-privacy@snyder.on.ca wrote:

The hesitation is almost like you're lugging the engine, like in a manual transmission. But of course, this is an auto, so how can you lug an engine if you're on a straight level ground with just normal amount of cargo and passenger weights inside? It's just a light lugging sensation though, at these RPM's.
Now, interesting development just happened earlier today, seems to have fixed the problem. I'd been having a problem with the warning light that came on the dash for the past few months, the "Rear Differential Temperature" light had come on. I was told that 99% of the time that this light doesn't actually mean that there is anything actually wrong with the rear diff, it's usually some corrosion on the wire leading to the sensor in the diff. That's what it turned out to be this time too. I hadn't mentioned this issue, as I was scheduled to have it looked at anyway, and it didn't seem related to the engine hesitation issue, it seemed like a separate issue. Now I'm thinking maybe when the Rear Diff Temp light came on, maybe the engine management system was trying to reduce the power going to the rear diff?
    Yousuf Khan
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