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Discussion Starter #1
I'm having a strange acceleration pull on my 15 STI. Bought it new in 2014 and it has never been in any accidents.

Recently in Nov 2019 the power steering rack was replaced under ext warranty because the original one was leaky. Right after this I bought Michelin PS4s from Costco and noticed that the car consistently pulled to the right during any highway speed acceleration. I got the alignment checked at the dealer that did the rack replacement and they said everything still looks good.

2 weeks later the car still is pulling to the right at highway speeds to I decide to pay for another alignment at a different Subaru dealership. The car still pulls to the right at highway speed acceleration after the alignment. I decide to leave the car with the dealer to check the power steering rack and anything else that could cause this issue. They report back that they could not find anything wrong with the car (they manage to scratch the steering wheel during this time but kindly offer to replace it).

I then took the car back to the original dealer that did the power steering work and the technician and service advisor rode along with me. They also agree the car is pulling hard to the right "enough to change lanes". The technician swaps the front two tires and adjusts the alignment. The car now pulls slightly to the left while accelerating but is driving much better. The service advisor recommends that this is a tire pull and that I contact Costco and get 4 new tires.

I then got my steering wheel replaced and toe re-set by the secondary dealer trying to work on the car.

The Costco saga was long and arduous but in short they replaced all 4 tires with a new set of Michelin PSS. I had been running these same tires previously for 20k miles so I figured it would be a good idea to go back to them. I drove the car around right after the install and the car STILL pulled to the right.

At this point I am baffled by what could be causing this issue. The car drove straight before the steering rack was replaced... but its interesting to note that the first set of PSS tires I replaced the OEM tires with in 2017 also had a strong pull that switched directions when front tires were swapped. The tire shop just replaced all 4 tires and the car drove straight for 20k miles after that. I would be more than shocked if I have had 3 sets of high end Michelin tires that were defective. I honestly am not sure where to go from here. The car is still under extended warranty for a few months, but the dealers can't seem to find anything actually wrong with the car. I'm not sure if replacing the rack bushings would help...but it makes me nervous to start throwing money and parts at this problem to potentially see no results. I guess I also learned its better to do repairs and maintenance one thing at a time, I could have probably avoided the steering rack vs. tires blame game. Regardless, I am pretty close to giving up and selling the car at this point.. but if anyone has any advice it would be much appreciated.
 

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Rough guess, it’s a directional tyre and you’re running a pair of lefts OR rights on the front wheels.
Which would explain why it pulls to the opposite side when the tyres are swapped over.
 

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Rough guess, it’s a directional tyre and you’re running a pair of lefts OR rights on the front wheels.
Which would explain why it pulls to the opposite side when the tyres are swapped over.
MPSS are not rotational, but they do have a label to show clearly which side of the tire goes outside.

To the OP, i wonder if the steering rack was centered correctly at the time of replacement. Do you feel that you have the same amount of travel or turns when turning the steering wheel to full lock on either side? Ive read that alot of guys with GR/GV chassis sti's upgrading to 2015 racks and having to center it prior to installation. Based on what i have read, the rack comes maxed out to one side and the tie rods have to be manually adjusted to be somewhat even prior to installing.

Also you said the alignment was good, do you have the print outs to show? Having excessive camber or caster will cause a pulling issue. Oh and one other quick thing, have you verified that all of your tire pressures are ok? This is something that is so easily overlooked its almost embarrassing lol.
 

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On my 18 the pull showed up after the first tire rotation. The dealer did a 4 wheel alignment under warranty and found the rear toe was out enough to cause uneven wear of both rear tires that were now on the front. They replaced them with new shaved to depth tires and cured the pull. I've since put PS AS3+ on it, rotated twice and the pull has not come back.
 

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Discussion Starter #5
MPSS are not rotational, but they do have a label to show clearly which side of the tire goes outside.

To the OP, i wonder if the steering rack was centered correctly at the time of replacement. Do you feel that you have the same amount of travel or turns when turning the steering wheel to full lock on either side? Ive read that alot of guys with GR/GV chassis sti's upgrading to 2015 racks and having to center it prior to installation. Based on what i have read, the rack comes maxed out to one side and the tie rods have to be manually adjusted to be somewhat even prior to installing.

Also you said the alignment was good, do you have the print outs to show? Having excessive camber or caster will cause a pulling issue. Oh and one other quick thing, have you verified that all of your tire pressures are ok? This is something that is so easily overlooked its almost embarrassing lol.
The steering does feel to have the same number of turns from left lock to center and then to right lock. There might be a small discrepancy there but nothing immediately noticeable.

As for tire pressures, I would gladly take the embarrassment and eat my words if this would solve this issue, but I thought about this and had Costco verify pressures with spec for the PS4s a week or so after they were installed. The car still pulled to the right. I checked the PSS pressures myself this morning and they are even around 32/33.

I also attached a picture of the latest full alignment (3rd alignment). The right rear camber has always been slightly in like that, or at least as long as I can remember. I’ve been told this is not adjustable and that it wouldn’t be causing this aggressive of a pull.
20649DE2-095F-4B60-B107-9FE83A07A624.jpeg
 

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Ok just making sure lol. Yes camber is not adjustable on our cars but it still should not be in the red. I would recommend you go about trying to replace RR suspension components in order to get the camber into spec. Its possible maybe the trailing arm or knuckle is tweek'd. Of course another another alignment would be required after repairs to verify that the camber is back into spec.

I also think that the rear toe should come in a little more in order to get the rear total toe measurement closer to center.
 

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On my '17 (which has been modded so the alignment is as near perfect as it can be) as little as 0.5 PSI difference (which causes the diameter to slightly change) between the 2 fronts can cause a slight acceleration pull.

When I got my winter tires a while back there was one that was visibly different in diameter (I didn't notice
until after I drove it).
It pulled so bad under accel that I almost had an accident.
Discount gave me a whole new set which fixed the problem.

Because of the AWD, wide tires and limited slip front diff STI's react to even slight differences
of tire diameter between the front left and right tires.

To check this inflate the front tire on side it is pulling towards more than the other....maybe even as much as 5PSI.
 

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Discussion Starter #8
Ok just making sure lol. Yes camber is not adjustable on our cars but it still should not be in the red. I would recommend you go about trying to replace RR suspension components in order to get the camber into spec. Its possible maybe the trailing arm or knuckle is tweek'd. Of course another another alignment would be required after repairs to verify that the camber is back into spec.

I also think that the rear toe should come in a little more in order to get the rear total toe measurement closer to center.
Yeah, I agree that it should be in spec. It is just puzzling to me that the car drove straight for years before this with the same exact camber set up.

On my '17 (which has been modded so the alignment is as near perfect as it can be) as little as 0.5 PSI difference (which causes the diameter to slightly change) between the 2 fronts can cause a slight acceleration pull.

When I got my winter tires a while back there was one that was visibly different in diameter (I didn't notice
until after I drove it).
It pulled so bad under accel that I almost had an accident.
Discount gave me a whole new set which fixed the problem.

Because of the AWD, wide tires and limited slip front diff STI's react to even slight differences
of tire diameter between the front left and right tires.

To check this inflate the front tire on side it is pulling towards more than the other....maybe even as much as 5PSI.
I'll need to check this theory as well, but when I checked the tire pressures this morning they looked uniform. Granted, the resolution on my gauge isn't the best, so I'll probably verify with another.

For now it looks like the car is going to go back to the dealer to verify the current alignment. If it's still "in-spec" but driving poorly I'm going to take it to a performance shop nearby and let them have a go at it. I appreciate the feedback y'all.
 

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Not an expert here, but my 2 cents: That is a LOT of rear negative for a stocker. Not even sure I have that much on my lowered '06 in the rear. That plus the fact that it's uneven makes me wonder if it is a contributor.
My car has an aggressive autocross alignment, IIRC it's about -3.8 front, so even with rotation the street tires wear and I have to make totally sure that the toe is about zero otherwise it will chew them up. As a result when I rotate the race tires back off (I don't autocross with the street tires) I have found that some combinations of the worn tires will result in a strong pull to one side on acceleration/ opposite on deceleration, to the point that with the worst combo that going up a steep grade and then having the car decelerate on going over the top of the hill will result in a move toward the next lane. Usually swapping one side or the other f to r will minimize that.
My experience has been that it's rare for modern tires to have defects that affect pulling like this, in fact I balance my tires WAY less now than I did even 20-25 years ago so the quality seems to be improving.
 

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Discussion Starter #10
Update:
I took the car to a well regarded performance shop in the area. Interestingly enough, they found that the car was actually out of alignment and corrected it, meaning Subaru’s alignment rack was probably not properly calibrated. They also verified that the tire pressures were equal side to side and that there were no obvious mechanical suspension issues.
43332141-646E-4573-AC15-69F6FAFE46BD.jpeg


I expected this to solve my problem... but it did not. The car still pulled hard to the right at higher speeds. The shop then checked the balance for the front wheels as well as for any wheel damage, and found them to be correctly balanced and un-damaged.

As a last ditch effort, they switched the FR and FL wheels and tires to see how that would affect the pull. Sure enough, the car now drives straight-ish with a slight pull to the left at highway speeds.

To me, I guess this means that there is either a tire pressure issue I’m missing or I got another set of defective tires. I find it extremely hard to believe that I would have 2 sets of different Michelin tires that pull, but at this point I really don’t know what to believe. I’m also almost willing to just leave the tires as is and not go through another fight with the tire shop to get them replaced. I guess I’ll see if the pull worsens over a few weeks and decide from there.
 

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You can't tell right away if the alignment fixed your issue because your tires will still have some improper wear in them from the previous incorrect alignment. I'd say wait a couple weeks and see if it all evens out.
 

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Discussion Starter #12
You can't tell right away if the alignment fixed your issue because your tires will still have some improper wear in them from the previous incorrect alignment. I'd say wait a couple weeks and see if it all evens out.
That thought crossed my mind.. but I also put less than 50 miles on those tires before the alignment got corrected. I figured that was too few miles to cause drastic tire wear but I’ll believe anything at this point. As of now the car still will pull to the left during highway acceleration but it’s much more livable than before.
 
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