85 300TD vacuum problem - PeachParts Mercedes-Benz Forum

Jatimo · #1 03-19-2012, 12:28 AM

Hello folks.

A couple days ago I bought my first MB, and 85 300TDT. As you might expect I'm working out some vacuum system issues. Looks like I'm going to have to do an item by item inspection, as pretty much every vacuum subsystem is losing pressure when isolated and tested.

Is there a good reference on what the nominal readings should be at various points of the system?

Also, should my vacuum gauge be giving a steady needle reading just after the VCV? Its oscillating so quickly that it creates a nearly solid arc (as seen by the human eye) with about a 5 inch Hg spread. I ran out of daylight today before I got to check the the pressure directly from the pump.

Could this be a sign of a worn rubber diaphragm in the vacuum pump or fault in the VCV?

The shutoff valve was previously disconnected completely (as well as the transmission tube after the transducer), and oddly there were no free hoses or connector points. I added a T-type connector to the appropriate line after locating it with the MityVac and hooked in the shutoff valve and the engine stopped as expected when key was turned off. but this had the unfortunate side effect of causing the engine to not go above 1.3krpm or so, no matter how much throttle is applied.

Faulty shut off valve, partially engaging, or perhaps I installed the fitting wrong? faulty engagement at the ignition assembly, causing partial engagement of shut off valve, and starving engine of fuel for higher rpm?

possibly related, the vehicle might also have low power at low rpms, but its hard to gauge at this point because its my first diesel, and the accelerator has a lot of play in it until i install the new bushing against the firewall.

Thanks in advance for your suggestions, and for your past postings, which gave me the confidence to buy this vehicle in the first place.

Jatimo · #2 03-19-2012, 12:30 AM

to clarify the low power at low rpms comment, i get a noticeable surge in power at 2krpm and have to back off the accelerator considerably to get steady acceleration.

Jatimo · #3 03-19-2012, 12:34 AM

By the way, tomorrow when there is sunlight again I will take vacuum readings of all the pertinent points and post them. I didn't write them down, and not sure if I'm remembering correctly. I think I was getting between 2.5 and 7.5 inches right after the VCV.

TnBob · #4 03-19-2012, 12:55 PM

Before the booster you will be reading the full vacuum pump output with both of the smaller lines off and the 2nd hole closed. Certainly should be 22"+.

Jatimo · #5 03-20-2012, 12:27 PM

Thanks very much TnBob!

I guess I need to take the brake booster cable off and test it there. My brakes are not getting boosted right now, and with the very low vacuum readings I'm getting elsewhere in the system, I suspect the pump could be faulty.

Jatimo · #6 03-20-2012, 01:46 PM

Oh btw, is this reading for a particular rpm or idle?

Jatimo · #7 03-20-2012, 02:46 PM

Thanks to the community again for all the wonderful info. I feel I am well on my way to having my problem isolated, and then I can go about getting all the other little pesky things taken care off like the door locks and such.

I plugged the two small lines coming off the brake booster and and hooked up the MightyVac to the end of the cable that bolts onto the brake booster.

My engine idles at 750rpm. Here are my pressure readings at various rpm
750 = 3"
1000 = 6"
2000 = 12"
3000 = 18"

After this i reconnected the brake booster line and tapped one of the small lines (still idling) and had 3". I connected just the engine shut off hose and turn the key off, but engine kept running. Tested the engine shutoff valve manually with hand pump. It shut the engine off once 7.5" was reached. Connected the two lines from the ignition side shutoff valve with a T connector and used Mighty Vac on third connection. As expected, holds no vacuum when key is in run mod, and held vacuum fine when in shut off position. Tested shut off valve with engine off to see if it held vacuum and it seems to slowly bleed at a rate of about .25 - .5 inches per second (starts off faster under higher pressure and slows down considerably once under 7.5).

With just the shutoff and brake booster in circuit and with the MightyVac T'd in to check, I noticed the needle was now oscillating violently between 1 and 3. I guess the one thing i didn't think to do was to remove the brake booster from the circuit and test that way, but it was holding vac just fine when isolated -- no oscillation.

So firstly, do these readings seem correct? TnBob had said 22" at least, and I'd read 23-25 is optimal elsewhere on the forum but it didn't say at what rpm range this should read. Is that at 5krpm or something or should I be getting that at idle? I guess that would tell me if the vacuum pump needs rebuilding/replacing. If that sounds about right on the readings, then perhaps the shutoff valve is losing the vacuum? In that case, perhaps its the gasket or does that rather isolate the valve from the fuel injector housing so that fuel doesn't leak, rather than vacuum?

Thanks again

TnBob · #8 03-20-2012, 02:54 PM

With a max of 18" being measured at the source it would appear you have a pump issue.

Having said that please redo the test. Leave the booster hooked up and connect your vacuum gauge to one of the two small outlet hose ports in the main vacuum line.
Pull the other line and plug that hole in the main line.

Remember when your working on vacuum connections they are plastic, 30 year old baked plastic fittings that are inclined to break.

TnBob · #9 03-20-2012, 07:32 PM

I went out and measured mine with the 2nd line still in place.

22" across the full rpm range. Really shows that either you have a pump issue or that the single line to your brake booster has a hole in it.

Jatimo · #10 03-20-2012, 08:06 PM

Thanks so much sir! That's exactly what I needed to know!

Jatimo · #11 03-21-2012, 01:00 AM

anybody know a good method for diagnosis in determining whether the main (external) check valve is bad as opposed to the pump itself?

TnBob · #12 03-21-2012, 01:46 PM

IF ... you dont have decent vacuum from the source ... all else is for naught.

If you continue to read the 3" at the booster line with no leaks you need to address that first.

To check a vacuum check valve you need to suck on it.

Jatimo · #13 03-21-2012, 04:59 PM

thanks.

removed the brake booster cable and tested it at the metal pipe directly from the vacuum and got 20" steady, so perhaps a bit anemic, but something at least. when i replaced the brake booster cable it now holds 20" as well. attached brake booster, still 20". frustrating because those same points tested yesterday with fluctuating numbers according to rpms.

I had done an italian tune up previous to this, and the engine was warmed up (mine stays at 83C pretty much). Other than that all I've done was to take the brake booster cable off and re-attach it. I'm thinking it may have been a rubber O-ring improperly sealing, and expanding when warmed up and then creating proper seal. if that's the case maybe a rebuild of the pump will be in order. I'll check it again with the engine cold.

sleepstar · #14 03-21-2012, 05:26 PM

did you examine the check valve that sits on top of the pump itself? really easy to remove. the pieces inside the valve disintegrate and fall into the pump inlet. i just replaced mine a few weeks ago because it was completely empty inside. apparently the parts can rattle around in the inlet causing erratic behavior (it might pull 20" today and 10" tomorrow)

Jatimo · #15 03-21-2012, 05:34 PM

thanks for the tip. i'll inspect and test the check valve next.