failing vacuum pump? driving problems 82 300D - PeachParts Mercedes-Benz Forum

NH82W123 · #1 10-17-2011, 02:03 PM

Yesterday while driving the trans shifts got progressively firmer until it felt like no vac was getting to the trans at all. Made it home just as the brake pedal firmed up as if there was no power assist, then the engine wouldn't shut off with the key. I checked that all under-hood vac connections were tight (the lock system has been disabled and blocked for some time). This morning I drove carefully in the direction of my indy's shop and after a few miles everything returned to normal. Problems returned just as I pulled into the shop.

Time for a new vacuum pump, or is there possibly a check valve that has gone bad? (I haven't heard back yet on the diagno$i$ and am just curious of the experts' thoughts here.)

Thanks, all.

Charlie

kerry · #2 10-17-2011, 02:08 PM

Yes, could be a bad check valve. Go out and start the car again and see if hte problem has gone away.

NH82W123 · #3 10-17-2011, 02:11 PM

Kerry, the car is still at the shop (a half hour away), awaiting my indy's diagnosis. I just wanted to make sure I understood all of the likely causes before our discussion of his recommedation. I completely trust the guy, I just like to have all the facts... Thanks.

Biodiesel300TD · #4 10-17-2011, 03:41 PM

Sounds like vac pump issue for sure.

NH82W123 · #5 10-18-2011, 10:33 AM

Still waiting on a call from my indy. Hoping it's just the check valve and not the pump itself. Not that I can really complain with nearly 200K miles on the original pump.

funola · #6 10-18-2011, 11:04 AM

Make sure your mechanic measured the vacuum pump suction before condemming it. I just went through testing mine and it's easy. Disconnect the manifold plastic vac line at the vac pump steel line. Use a 1/2" heater hose with a clamp on the vac pump steel line fitting and adapt it to the vac gauge (I used a Mityvac) onto the 1/2" hose. Start the engine, you should see 19 to 20" HG or higher. Shut the engine off, vacuum should remain at whatever the reading was while engine was running. If the vac reading drops, the check valve is NG.

NH82W123 · #7 10-18-2011, 11:15 AM

Great method, funola. Thanks. I'll be sure this gets done before we look to the pump itself. Wish I had done this myself before I took the car in yesterday!

funola · #8 10-18-2011, 11:24 AM

Note that the check valve is at the other end of that steel line where I did the testing. Make sure the fitting at the check valve (on the vac pump) is tight. Mine was a bit loose.

tangofox007 · #9 10-18-2011, 01:01 PM

The intermittent nature of the problem is classic failed check valve behavior. The valve retainer plate breaks, dumps the parts into the pump inlet, where they rattle around and sometimes block air flow. Diagnosis is a five-minute evolution. (Remove valve and inspect.) It's also an easy way for a shop to turn a small job into a bigger one.

NH82W123 · #10 10-18-2011, 03:58 PM

Indy reports that the vac pump and check valve test OK, but the vacuum control valve on the IP for the transmission is leaking badly. When he blocked off the VCV, the brakes and key shut-off returned to normal. He's trying to locate a new VCV now.

Failing--rather than failed--vac pump or check valve are not ruled out, but I figure I may as well get a new VCV on there anyway.

tangofox007 · #11 10-18-2011, 04:57 PM

Quote:

Originally Posted by NH82W123

Indy reports that the vac pump and check valve test OK, but the vacuum control valve on the IP for the transmission is leaking badly.

Considering that the VCV is designed to leak, quantifying "badly" would be a good idea.

NH82W123 · #12 10-19-2011, 09:08 AM

kerry -- just heard from my mechanic that he can't locate a replacement from any of his usual sources, so sounds like you're absolutely right. He's going to make a few more calls.

tangofox -- that's a very good point. All I know at this stage, from what my indy reports, is that blocking off the VCV makes the brakes and key-shut-off return to normal.

tangofox007 · #13 10-19-2011, 09:25 AM

Quote:

Originally Posted by NH82W123

All I know at this stage, from what my indy reports, is that blocking off the VCV makes the brakes and key-shut-off return to normal.

It is quite possible that eliminating a "normal" leak from the VCV could adequately compensate for an abnormal leak elsewhere. It could be that blocking the door locks, the climate control or the EGR would have the same affect. Confirming the integrity of other vacuum circuits would be a good idea. And it's entirely possible that your mechanic has done just that.

kerry · #14 10-18-2011, 04:19 PM

No new VCV's are available. ( I think) You'll probably have to find a good used one.

kerry · #15 10-19-2011, 11:51 AM

Something seems odd. I failing VCV should not cause your brakes to fail since there should be an orfice in the system which limits the amount of vacuum going to the VCV. Did your mechanic pull a vacuum on the brake booster to see if it held? It could be that your brake booster is failing and most of the vacuum is being consumed by that failure and any additional loss via the VCV is just making the problem evident. I've had failures in the door lock system, shut off diaphragm and climate control systems but none of those caused the brakes to get hard. No engineer in his or her right mind would have connected such a crucial system as the brakes in such a sway that a failed pod in any of those systems would cause you to lose power brakes. It would be insane. Something else is wrong with your system than just a leaky VCV.