Need Help Diagnosing Steering Problem

[Burbman]

Nagging little problem is getting worse and needs to be addressed, but not sure where to start.

 

Vehicle is a 2001 Suburban 2500LT with 4WD, 8.1L, Pro-Comp 265/75-16E tires on 16x8 American Racing rims.

 

The symptom is steering wheel shimmey. At about 40-45 mph, the steering wheel will start an oscillation of about 1/2" to either side of center, for about 1" of total movement. Driving straight ahead, the wheel will start this oscillation when you hit 40-45 mph and continue up until 60 mph or so. No vibration or roughness like an out of balance tire. No pulling at all to indicate an alignment issue. If you let go of the wheel, the truck goes straight as an arrow, you just sit there and watch the wheel oscillate.

 

Now, interstingly enough, when you enter a turn and move the wheel off center, the oscillation stops. However, if you are in an "S" type turn, the wheel is calm in turn #1, but as you pass back through center to turn the wheel the other way, you can feel the oscillation return then go away again in turn #2.

 

The oscillation stops over 60 mph, but is replaced by a general sense that there's too much play in the steering, expecially in crosswinds. No vibration or tire noise though.

 

I spent some time reading the Helm service manual, but the problem I have is that every trouble tree starts with "connect scan tool"... In my own mind I'm thinking the problem is either mechanical ie, a worn suspension component in the front end, or electronic, ie EVO sensor.

 

This kind of problem can be tricky to diagnose, and I'm hesitant to just bring it in to my dealer because they have a tendency to do a quick diagnosis and change a part. I'd like to be able to point them in the right direction, or do some additional checking to narrow it down so I don't wind up paying for a parts chase.

 

The wrinkle is that according to Helm, the EVO and power steering stuff is connected into the ride control module (RCM), which is not accessible with generic garage scan tools. Only the GM Tech-II scan tool can access the RCM, this I know first-hand because of a previous problem with the Autoride system. [Message center said "service ride control" and my buddy's Snap-On scanner could not access the RCM to see what codes were stored. Took the dealer about 10 mins to connect the Tech-II and declare the LR ride sensor inop, which fixed the problem.]

 

In addition to EVO sensor, could also be PS related or the steering box itself. Helm has a several page test procedure for the PS system that includes measuring line pressure and flow rates at specific points, data from which pinpoints specific issues with the steering box/booster system.

 

Already did the obvious, checked tire pressure, rotated tires, checked for bent wheel, etc., made no difference. Anybody have an idea of where to start looking for this problem??

[GM-Tech]

Definitely sounds like a tire/rim problem. Out of round, slipped belt in a tire, etc. This being a steering system problem seems highly unlikely.

 

The tires need to be checked on a road force type balancer that can do a thorough analysis.

[Burbman]

Thanks for the quick response. Here's where I get off-track, though: if it is a tire/wheel problem, why doesn't the problem get worse as you go faster? Once you're at 70 mph, the steering wheel is smooth again, no oscillation, no vibration. Maybe I'll rotate again and see what happens.

[GM-Tech]

why doesn't the problem get worse as you go faster?  Once you're at 70 mph, the steering wheel is smooth again, no oscillation, no vibration.

 

It's not unusual for a tire vibration to be speed specific. To me, your description of the wheel going back and forth is a tire problem. Not necessarily a balance problem but a belt or lump in the tire.

 

Take both front tires and put 'em in the back. Hopefully, you don't have more than one that's a problem.

[Burbman]

Got it! Will rotate again at the next oil change and see where that leaves me. However, that makes me think about the tire...assuming that the tire is the issue and putting it on the back solves the problem, is it safe to leave the tire there? Come spring, I'll be towing the 9K lbs travel trailer again and loading the rear axle to the max...I don't want to shred a tire somewhere on the interstate. I was hoping to get another year out of the tires, but will replace early if I have to..

 

Thanks,

 

Don

[GM-Tech]

If it's a slipped belt, it's not safe. Dunno what's wrong so can't say more than that.

[Burbman]

Understood and appreciate the info. Will report back after rotation with an update.

[alvocado]

Thanks for the quick response.  Here's where I get off-track, though: if it is a tire/wheel problem, why doesn't the problem get worse as you go faster?  Once you're at 70 mph, the steering wheel is smooth again, no oscillation, no vibration.  Maybe I'll rotate again and see what happens.

 

<{POST_SNAPBACK}>

 

 

 

Mechanical systems have natural modes of vibration at different frequencies. You can compensate through the use of dampeners (like the rubber mounts on your exhaust systems) or through changes in the weight of the system (wheel weights on your wheels.) Tire balancing is performed at a specific rotational speed, generally set at one that you commonly drive and typically covers the driveable range. However, if you have thrown a tire weight or if it needs rebalanced, the tire/wheel system may have a natural vibration point at the lower speed where you are experiencing the issue. Once you pass through that speed, the vibration would disappear, hence the reason you don't have the issue at 70 mph.

[Burbman]

UPDATE:

 

Well, you guys were right, both front tires were out of balance. I just had them balanced when they were rotated not that long ago, but neither wheel had any weights! Not sure what happened to the weights, but balanced the wheels and shimmey is gone. If I remember, L took 1.25 oz, R was 1.0 oz. Not a lot, but enough to matter.

 

Never had a tire balance issue produce a steady shimmey in the wheel like that but I guess there's a first time for everything.

 

Now to figure out why the weights are flying off like that....