jkowtko

I have been working on getting rid of a shudder problem in my power steering.

The car has 100k miles on it now. Around 50k it had a "PS fluid flush" from the local indy, and over time the steering developed a strong shudder, mainly when the engine was cold, backing out of the driveway and turning the wheel. So I don't know if they put in the wrong fluid, or if the fluid just got old over time. The car also developed a problem with tensioner pulley chatter and I had that replaced around 90k.

Over the past several months I flushed the PS fluid a few times using Pentosin CHF 11S, which resulted in an immediate and dramatic improvement in the steering. It's down to the point where there is a very small noticable amount of shudder when turning the wheel slowly under load. I would like to figure out what is left to do short of replacing rack and/or pump ... hopefully you guys can help.

Here are the symptoms:

• happens warm or cold. slightly more noticable when cold
• happens only when the steering is under load, and usually when I turn the wheel slowly so that the power assist just starts to engage.
• generally noticable when engine RPMs are low (~ 600)
• the shudder is accompanied by a momentary drop in engine RPM (~50)
• steering at speed is clean and feels fairly light and fluid (not sure if it's a bit too fluid though)
• suspension should not be an issue ... I've renewed the front suspension with struts, strut mounts, control arms, sway bar links. Tie rod ends still seem solid.

So, the candidates for what is causing this seems to be:

1. Still some old or bad fluid in there. Buy another quart or two of Pentosin and keep flushing.
2. Pump is getting weak and/or parts within it need to be cleaned.
3. Rack is getting worn (torsion bar or seals in valve assembly?) and may need replacing.
4. Something else causing unnecessary load on the engine that allow it to drop in RPM under steering pump load.
5. Nothing's wrong -- it's a Mercedes!

Any suggestions?


Thanks. John

Glyn M Ruck

I'll bet this steering system had the wrong fluid in it! If you hold your foot slightly on the throttle to maintain 600/700 RPM do you still feel a shudder when turning the wheel? You have to have stable idle & good engine mountings for zero shudder.

If you still have slight shudder at stable RPM the likely causes are:

Valving in the rack & pump circuit playing up a bit. (as long as pump & rack are both quiet)
Stiction (stick/slip motion) of the rack seals possibly due to wrong fluid causing too much seal swell.

The fact that a fluid change has made a big difference is encouraging. You can do another fluid change but I would be inclined to drive it. If it is a seal stiction problem it will likely return to normal with more use. Say 5000 miles.

All this is predicated on the M271 engine maintaining a smooth idle & holding RPM under load. Some M271's lose their perfect idle as the miles pile on mainly IMHO due to throttle body actuator wear or fouling or chattering. Sometimes a TB clean including the actuator & electrics will resolve this.

jkowtko

I tried several things this morning. Initially at high idle (~1200) the steering at a standstill was smooth. And at low idle if I gas it up to ~900, smooth. Below 900 the steering load will pull the rpms down a bit and I start to feel the shudder. The shudder gets more pronounced as the RPMs are lower. After the car is warm, at low idle in gear I'll always feel something. If I go into neutral or park, almost nothing. Again gas it up to ~900, nothing. So I would say 850-900 is the threshold.

Pump and rack both seem quiet to me. No noticable squealing or hissing. I haven't gone under the hood and listened carefully though, to see if the shudder comes from the pump or the rack. I will probably do that next.

Now that I've changed the oil (5w-40 Formula M) and trans fluid (Shell ATF 134) the car runs incredibly smooth at speed, and when the engine is warmed up I can't hear or feel anything when stopped at a light.

But once in a while (maybe <10% of the time? ... and this was more frequent in the past) when waiting at a light, I get those momentary dips in rpm with a slight amount of engine vibration. Picks right back up. Is this indicative of a throttle body which needs cleaning?

Glyn M Ruck

The moment that the engine is placed under load (from steering pump, alternator, HVAC, transmission in Drive etc.) the ECU should bump the idle to compensate & maintain stable idle & RPM. If the TB does not react accurately it can't do this so I would give it a good clean.

jkowtko

Okay, a throttle body cleaning is next. I found the DIY ... should I take it off the car to clean it front and back? (does the back get as dirty as the front?)

Glyn M Ruck

You need to take it off the car to clean. You need to take off the cover & clean the contacts & tracks as well as the bore.

jkowtko

Okay thanks, this is good info -- unfortunately bringing up some more questions:

* What should I use as cleaners for the tracks and contacts? (I have electrical contact cleaner)

* Should I lube the gears? (I have a variety of lubricants including silicone grease and silicone spray)

* Are there specific torque settings I should use on the mounting bolts here, or just nice and snug (5-10 ft-lbs)?

Thanks. John

Glyn M Ruck

Clean with contact or switch cleaner

Wipe a small amount of silicone onto the gears with a cotton bud.

Tighten snug.

sammydragon3

i too had wondered why the rpm drops very slightly then back up when u then the wheel

my brother has a brand new honda civic and it does the exact same thing with 10k miles on it

jkowtko

Okay, throttle body actuator is clean.

Pulling it off the car was easy ... getting all that grime out from the bore and butterfly valve was a pain. A toothbrush got most of it, but I finished it off with a stiffer auto cleaning bristle brush that I bought a while back and until now had no use for. I used maybe half a can of throttle body cleaner on this.

I opened up the back of the actuator -- the electrical contacts looked pretty good as I wasn't able to get much of anything off of them using the contact cleaner. Rolled a bit of silicon grease over the sprocket teeth, put it all together, and back in the car. Reset the transmission/throttle memory.

I've only driven it for one evening and one morning, but the idle seems to be a bit higher now and I haven't noticed the hesitation coming off of a slow-to-stop. So I think cleaning the throttle body made a difference.

However, the power steering shudder is still there. Idle in reverse has gone up maybe 100 rpm, but not enough to remove the shudder. That combined with the "clunk" -- it may be time to start thinking about getting a new steering rack ....

Glyn M Ruck

Oh dear! I would drive it for a bit with the new fluid. Sounds like the TB needed a clean anyway.

jkowtko

... or a new pump? If PS pumps are old or dirty can they put excessive drag on the serpentine belt?

What else besides the throttle body is allowing the engine to dip in RPMs when I turn the wheel? I've had no CELs for years, but is it time to consider replacing spark plugs (going on 40k), alternator bushings, battery, etc?

Yes, for now it isn't that annoying, but still a noticable difference compared to our '96 Accord with 230k miles, whose power steering is effortless and seems to have no effect on engine RPM.

Glyn M Ruck

It certainly seems to be the PS system ~ pump or rack ~ they are all that can provide load to the engine when turning. Plugs etc shoud be fine at 40K. Are engine mountings in good condition?

How many miles have you done since you changed the PS fluid?

jkowtko

I replaced the rear transmission mount a month ago, haven't checked the front mounts other than to note there is no visible sign of fluid leakage and the engine doesn't seem to vibrate at all when running.

My most recent PS fluid change was two weeks ago, so maybe 200 miles? The steering feels light when the car is at speed, which tells me the clean fluid is doing it's job ... it's only down in the idling range and in gear when I notice the steering pull down the engine RPMs. It's not a huge drop, maybe 50 or so RPM. But around 800 RPM and below I can feel the shudder. It just feels like something in there is causing drag. If PS should not shudder or pulse above 700 RPM then there is definitely something going on with the PS system.

What I fear is that the indy who changed the PS fluid a few years back put in the wrong fluid, and as you mentioned before, maybe this affected the rubber seals in the pump and rack. If you think continual flushing with good, fresh fluid will allow the rubber to regain it's normal characteristics over time, and there is nothing else in the pump or rack that the wrong fluid could have affected, then I'll just keep things as-is, maybe change the fluid again in a few months.

Is there anything else that you suggest I try to test?

I guess I could also start visiting dealerships and test driving other 2005 w203s to see if they exhibit the same behavior ...

bozapp

I. recomend that you change the power steering pump it has low pressure. damaged internally from wrond type of fluid. You need to take off the pressure and return line and blow all the old fluid out of rack n pinion and cooler if applicable. Then replace with correct fluid type and the system heeds to be bled. Take the weight off of the front wheels without starting turn the wheel from lock to lock several times then top off the fluid again,then start the engine very briefly one second then shut it off, do that four or five times while turning the wheel back and forth without going all the way to the end. Then top fluid off again after that then start the vehical there should be no noise continue cycling the steering wheel should feel easy then put the weight down on front wheels turn back n forth top off fluid test drive check for leaks. Done

Glyn M Ruck

Yes I agree - You either continue running for a while or I would start with a pump replacement if there are no leaks on the rack. Check by ensuring no fluid in the boots.

jkowtko

Okay, I'll keep driving the car for now and start researching pumps. Questions -

1) does the small amount of old fluid in the system still pose a problem? I've flushed it 3 times now with Pentosin CHF 11s including two drains from the bottom hose. I would assume by this point I have less than 5% of the old fluid in the system.

2) what in the pump is causing low pressure?

Looking at the rebuild DIYs, the internals of the pump appear to have a bunch of rubber seals and then the impeller fins. Are some of the internal seals leaking allowing fluid to seep back around the high pressure chambers? Are the impeller fins bent or damaged? I believe there is also an overflow relief valve ... is that weak and allowing pressure to escape? Can any of this can be dealt with by a DIY pump rebuild?

I don't mind doing a rebuild myself if a parts kit is available ... I'm a Mechanical Engineer and cars are a hobby to me ... not to mention saving a few hundred bucks

Thanks. John

bozapp

I would get a brand new pump for the simple fact that the body of old pump maybe out of tolorance and you don't get those parts with a rebuild kit.

jkowtko

Update ... the small amount of shudder I was feeling seems to be slowly but continually going down. So maybe it was the wrong fluid swelling the rubber seals -- as Glyn suggested -- and in the time use of the correct fluid will allow the seals to cure themselves.

My attention has become more focused on the momentary dips in idle RPM, which I have been noticing a lot more. Still possible that one of the components such as PS pump or alternator are putting excess load on the belt ... or something else? I will follow up with this issue on another thread.

Glyn M Ruck

Pleased it's improving.

Is there any way you can borrow a new throttle body to try. Sadly it often fixes the problem if everything else is normal. That said the 4 bangers battle to stabalise RPM more than the bigger engines for obvious reasons.

jkowtko

What I might do is try driving some used 2005 C230s (there are plenty for sale around here) to make a comparison with the feel of my car.

I don't know that I can "borrow" one ... unless I borrow it from a car that I've taken home to test drive for the day

These aren't cheap parts ... just under $500 and up. If it turns out to be the problem I have no problem replacing it .. just need to figure that out.

Fyi, there are used ones on eBay for $50 - 250 ... do I dare buy one? What exactly in the actuator would be causing the problem? Just the motor's ability to respond quickly enough to electrical signals?

Another question -- there is an "air flap" unit on the backside of the compressor, that looks identical to the throttle body but has a different part number. How are these two parts different? And does the air flap ever cause issues?

Thanks. John

lsdlsd

Regarding the TB if you've already cleaned it and notice no or just a slight improvement on rpm stability when idle with gear engaged I would suggest not wasting too much time with another TB. I've done this kind of test and the result is the same (with a borrowed one).
Correct me if I'm wrong but I would doubt more on the components that add load to the belt, more likely it is the steering pump or even the AC compressor with degraded pulley that will add the load. Is there anyway we can check without having to replace parts or buy new ones to find out which of the parts is giving excessive load to the belt causing rpm drop ?


Also regarding the steering pump what is the proper fluid code suggested by MB ?


Thanks, Lsd

jkowtko

The MB spec for our cars for PS fluid should be 345.0. There are only two fluids listed under that category now (Pentosin CHF11S, and MB brand)

Yes I pulled the cover off the TB and cleaned the contacts and greased the plastic cogs with some silicone grease.

I would agree with you that it seems a bit more like excess load from one of the accessories.

I did replace the idler pulley recently, and while I had the belt off I spun the water pump, alternator and kompressor pulleys. All seemed pretty smooth, although with the kompressor pulley I could feel a bit of resistance as it had to move the internal mechanism. So, if it's possible that the compressor is just wearing and resisting more, that could be it. If there is anything electrical in the alternator that could be causing excessive back-pressure, I guess that's also a possibility.

PLGDEES

The answer is point 5 !!