D-Jet Vacuum question - PeachParts Mercedes-Benz Forum

Graham · #1 09-10-2009, 07:56 PM

My '72 SL only shows about 16" HG vacuum.

In looking for leaks, I disconnected the Air Valve and installed a manual needle valve. This allowed me to set the leakage through the AAV inlet to the idle valve.

If I closed the needle valve altogether, the engine would stall. But, the vacuum still stayed around 15-16". I had to open the idle screw just to keep the engine running, but even then vacuum hardly changed.

What would reason be that vacuum would stay low?

Could that happen if leakage was on say just one or two of the inlet manifold donut seals?

GGR · #2 09-10-2009, 08:26 PM

If your pressure sensor is on its way out, say with a broken diaphragm, it will let air in the plenum. It will also send the wrong info to the brain and the engine will run rich at low and medium range.

Tomguy · #3 09-10-2009, 10:06 PM

To be honest, I know my 4.5 has no vac leaks, and it can pull over 18" of vac, but for some reason runs best at around 15-16" @ idle. I'd check for injector seals and vacuum door locks because they are the main suspect. Your ATF modulator may have a torn dia as well.

Graham · #4 09-10-2009, 10:18 PM

Quote:

Originally Posted by Tomguy

To be honest, I know my 4.5 has no vac leaks, and it can pull over 18" of vac, but for some reason runs best at around 15-16" @ idle. I'd check for injector seals and vacuum door locks because they are the main suspect. Your ATF modulator may have a torn dia as well.

I have checked the MPS with mity-vac and it holds vacuum indefinitely.

There are not too many vacuum connections on my car - One that goes to aux users, one for brake booster, one to MPS and the one off the throttle to the distributor. I should try plugging the one to the aux users.

I suspect I may have some leakage on the inlet manifold seals. But, no easy way to be sure that I know of? Smoke Machine?

I did mess with the timing and got the vac up to about 16.5-17" with almost max advance (~27deg w/o vac). I seem to recall it used to be more like 18".

Tomguy · #5 09-11-2009, 07:38 AM

You're not mentioning the ATF modulator. It takes manifold vac off in a hard metal line down to the transmission. If the diaphragm in that is ruptured, you'll be losing vac. You may need to plug it to measure correctly. (Don't drive with it off unless you want very hard shifts!)

Graham · #6 09-11-2009, 09:54 AM

Quote:

Originally Posted by Tomguy

You're not mentioning the ATF modulator. It takes manifold vac off in a hard metal line down to the transmission. If the diaphragm in that is ruptured, you'll be losing vac. You may need to plug it to measure correctly. (Don't drive with it off unless you want very hard shifts!)

I have not actually seen that line at the engine end, but do recall seeing a line that runs under the car to the back of the transmission. The transmission was out last year for a front seal leak, so the line may have been disturbed. Now, just how do I plug that line?

By the way, transmission shifts smoothly, but I guess there could be a small leak.

230/8 · #7 09-11-2009, 10:59 AM

Quote:

Originally Posted by Graham

My '72 SL only shows about 16" HG vacuum.

What would reason be that vacuum would stay low?

Common diagnosis areas for this, if you do not suspect a genuine leak, are late ignition timing and/or late valve timing. Both will reduce manifold vacuum, particularly at idle speeds. Does the engine maintain a good, i.e. higher, vacuum above idle? If so, you have a leak, if it still is low relative to new engine standards then you should suspect late valve/ignition timing.

Graham · #8 09-11-2009, 11:48 AM

Quote:

Originally Posted by 230/8

Common diagnosis areas for this, if you do not suspect a genuine leak, are late ignition timing and/or late valve timing. Both will reduce manifold vacuum, particularly at idle speeds. Does the engine maintain a good, i.e. higher, vacuum above idle? If so, you have a leak, if it still is low relative to new engine standards then you should suspect late valve/ignition timing.

Thanks for that, it is what I planned on working on this morning.

I planned on setting timing to max advance (About 30degBTDC at 3000rpm ) as per the Standard specs (not US) which is where I had it before. The recheck vacuum at idle.

Does that make sense?

I have separate question re adjusting the MPS - see new post.

werminghausen · #9 06-25-2011, 11:47 AM

Graham:
I went through this exercise. I was spraying carb cleaner while the engine is idling. Spay to various areas at the intake with the aircleaner removed...
If idles changes by spraying cleaner then there is an idication that engine is sucking air through a leak. I had my intake removed and changed all rubber.
8 big donuts for the intake halves and the injector seals.
I can easily pull 20 inch now with advanced timing and the vacuum line at distributor pulled, but run at around 18/19 (without vacuum line to distr.) which is about 16 inch with the vacuum line....which ends up at 3 degrees after TDC at idle = 7 degrees before TDC without vacuum line at distr.

Before the intake renovation the max I could reach was 14 inch at idle.
Martin

Graham · #10 06-25-2011, 01:21 PM

Quote:

Originally Posted by werminghausen

Graham:
I went through this exercise. I was spraying carb cleaner while the engine is idling. Spay to various areas at the intake with the aircleaner removed...
If idles changes by spraying cleaner then there is an idication that engine is sucking air through a leak. I had my intake removed and changed all rubber.
8 big donuts for the intake halves and the injector seals.
I can easily pull 20 inch now with advanced timing and the vacuum line at distributor pulled, but run at around 18/19 (without vacuum line to distr.) which is about 16 inch with the vacuum line....which ends up at 3 degrees after TDC at idle = 7 degrees before TDC without vacuum line at distr.

Before the intake renovation the max I could reach was 14 inch at idle.
Martin

Those posts were quite a while ago.

Once I set up timing properly and made sure AAV was closing, I had no problem getting good vacuum, but like Tomguy found that at about 16" car runs well. It does depend on where you measure it. Memory is bad, but that might have been on line at back of engine, because that's the easier place to attach my gauge.

I haven't ever changed the donuts. Car runs really well, so don't plan on messing with it!

werminghausen · #11 06-26-2011, 02:40 PM

I measure vacuum at the vacuum line to the central locking reservoir...before the check valve.
The vacuum line to the transmission give signal to the modulator, a vacuum device in the transmission that controls the 'hardness' of shifts.
If you have no hard shifts when you drive slowly then this line should be good.