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Old 04-19-2004, 07:12 PM
Registered User
Join Date: Jan 2000
Location: San Francisco, CA, USA
Posts: 46
Question will a failed smog pump strand me?

hi, from reading about symptoms here in the forums, i think the smog pump on my 1995 E420 (116,000 miles) is in the beginning stages of failure. Every morning when I start the car, I got a slight squeaking/chirping sound that goes away after a minute or two. Last weekend, I got the squeak, and also a new noise that sounds like intermittent bearing/light grinding noises. These noises also went away after warm-up.

People here advise to just run it until it fails. My mechanic told me the same thing. What I'm wondering is: will I be stranded when the pump does fail one of these days? Also, will anything else be damaged as a result? The serpentine belt was recently replaced as part of routine maintenance.

If we determine that the noises are indeed coming from the smog/air pump, should we replace it as a preventive measure?



2011 C300

Gone but not forgotten:

1995 E420
1990 500SL
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Old 04-19-2004, 07:25 PM
Registered User
Join Date: Jul 2001
Location: SoCal
Posts: 1,303
With MB the case may be different - but I don't know why it should. With our Chevy 350, a check valve failed, roasting the interior of the pump, and causing parts to fail that locked it up. It was during a trip, and a couple of times I could loosen the belt, rotate the pump backward, and temporarily free it. Eventually that didn't work, and I had to snip the belt. That was a v-belt that only disabled the alternator, though, so not entirely disabling as in your case.

My understanding is that check-valve failure is THE typical cause of pump failure.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 04-20-2004, 10:56 AM
Registered User
Join Date: Apr 2000
Posts: 460
Sure, when that bearing seizes the pulley stops spinning, and your belt snaps. When your belt snaps you get stranded. I just faced this problem myself, and there are people out there that sell rebuilt pumps. I bought mine for $195 from a place in CA that sells them on ebay, but there are others. A new one costs $1,000! Alternatively, you can get a shorter serpantine belt and bypass the pump altogether. I would have done this, but smog checks are pretty strict where I live, so I decided to get a replacement. Oh, and its an easy DIY job.

'84 300D, 183k
'90 300CE, 173k
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Old 04-20-2004, 11:38 AM
Fimum Fit
Posts: n/a
A candidate for worst design mistake ever:

Starting in 1973 a number of FIAT models for the USA market only had the smog pump driven by a toothed belt (a sort of miniature timing belt) to avoid the cost of a tensioning idler pulley, but the drive came off the camshaft. The result was that when, inevitably, the smog pump seized up, the jerk against the camshaft pulley stripped the teeth off the timing belt at the crankshaft pulley, and since these were all interference motors -- oh what a mess, just to save a few thousand Lire (a couple of Bucks). Why didn't they just use a VW style shimable pulley?

Last edited by Fimum Fit; 04-20-2004 at 12:18 PM.
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