$20 for a Mighty-Vac from Sears and you would have figured this out in less then 10 seconds.

If you really want to see what kind of vaccum each side is seeing it needs to be under a load. Not in the garage under idle.
T in a vacuum gauge to each side and go for a drive. It will then be clear at what point each side of the diaphram is seeing vacuum.

The highest vacuum is under decel, especially if your vacuum limiter is broken.

 

Let us know how it goes, Tom. And as Erik says, you really want to pick up a MityVac ASAP. It's also good for troubleshooting HVAC issues, getting the spare deflated enough to fit back in the spare well, etc. etc.

Don't go for the one with all the brake bleeder crap... just get a basic model. It will do everything you need. If you had EGR the extra bits that make the MityVac capable of producing pressure would be useful as well.

Erik, the road test may be a bit overkill here. The pot failed a basic test, replacing that may be the end of the story. Usually with these vac advance systems, once you have the right ports hooked to the right vac sources and the pot is moving the advance plate freely you're done. ~99% of the time anyway. Vac readings at idle and just off idle only serve to identify the correct vac ports on the TB in this case(at least, that's all I intended).

 

Well....it didn't go very well.

The "new" (actually used) vacuum advance arrived, and I installed it. Although it "felt" different than than old one before installation (i.e., amount of suction required to move the actuator seemed less), the timing advance seemed to behave about the same as the old one. Maybe just a little more advance at the higher RPM. And I still just don't see the retard vacuum line making a difference when connected. And when driving it, there was still some sputtering or hesitating at high RPM and full throttle, and what seemed like a lack of power. Maybe a little less than before. It did seem to have more pick-up at lower RPM. And, it would achieve about 5000RPM at full throttle whereas before it wasn't much over 4K. (5K still seems low at full throttle.) So...bottom line is that this new vacuum advance seemed to help some but didn't completely solve the problem.

And....some other bad things. Now the tach in the dash stopped working. I have no idea why. And, of course, this made it hard to see at exactly what engine speed the hesitating was happening. And worse...the car started stalling frequently while out driving. It wouldn't stall while idling or reving while parked in the garage. But while moving (coasting or accelerating) it just died quite a few times. Thankfully it would always start back up readily. But it started doing this so frequently that I considered myself lucky to make it back to the garage without too much trouble.

And on top of that....I think there is an electrical short somewhere, because the battery had to be charged before I could get it started when I first started working on it tonight. It is a new battery, so it should not go dead sitting for just a couple weeks. And when I disconnect the battery, I measure a short between positive and negative cables going to the battery...with all the fuses out.

So, darn it. Not sure what all is wrong. But I have more work to do.

 

Tom,

The tachometer passes thru G7 on the CE panel and terminal 2 of the middle pod plug inside the pod.

You can easily remove the CE panel itself to set it on a table and more conveniently clean the contacts.

Also, the ignition switches sometimes go bad and cause a host of problems and inconsistencies, but you probably know that already. I've only had to change-out one so-far, but it seems to be mentioned frequently as a possible suspect. You might want to change yours preemptively, or at least consider it. Remember, I'm somewhat new at this. Wally and Alan and others have given a lot of help and left a lot of good historical posts, too.

 

OK, before you spend another dime on anything, get yourself a MityVac and check that advance pot against the tech data that I posted above. Used parts are generally not tested, and they tend to come from cars that ended up being junked for one reason or another(often because there are too many things wrong with them to be worth reviving). Keep in mind that the used part may be just as old as the part you are replacing. If you bought it where I think you did, they will be happy to swap it with a different part if it doesn't check out. But check it out, diagnosis by swapping used parts with used parts can drive you insane. Also check the vac hoses for correct behavior. Maybe now it's time to get some vac tees and do the road test mentioned by Eric above, though I think there are other things you can do to diagnose the problem.

In any case, you're going to have to be methodical about this unless you want to keep throwing parts at it. It's a lot cheaper and easier to diagnose the problem first, usually.

Inspect the green wire closely for any signs of aging. With the car running, jiggle the ignition key to see if the engine or any of your accessories seem to falter when the key is jiggled. check the resistors near the coil to make sure they are in good shape. Connect a small light bulb from coil + to ground, and put it where you can see it while driving. If it stays lit strong while the car is faltering, then it's not the ignition switch giving you trouble.

Something just occurred to me. Make sure you have the correct fuel pump relay. It picks up the tach signal to prevent the fuel pump from running if the engine is not running. An incorrect(or faulty) relay here can ground the tach signal, preventing the fuel pump and the tach from operating normally. A quick way to check this is to put a jumper across the contacts in the relay socket, the ones that are at right angles, and take it for a drive. An example(not the correct socket for what you're doing, just trying to illustrate the concept) is the jumper in the fourth relay socket from the right, bottom row in the pic below.



I don't suppose you have an oscilloscope, or a DMM with a frequency counter, do you?

 

Yup, hunting for a mitivac. Found a couple on ebay, but need to go see if there is one readily available at a nearby store so I don't have to wait for shipping.

After two nights of sulking about the new unexpected problems and poking around, I think I have a plan of attack. I think I need to focus first on the unexpected electrical problems (born out by the battery going dead) before I start running it again. And, like you said, I want to do that very methodically. I've already found at least two shorts (intermitent short in the headlight switch, and a partial short in one of the door lights) after a quick and dirty check of things. I actually did a methodical check for shorts many months ago before even attempting to run it, because I knew there was some bad wiring. And I replaced a lot, and then it all seemed good at the time. But clearly some problems have developed since then (maybe just from the stress of actually getting it running again and driving some), because there are shorts now where there weren't any before.

Anyway...that's what I'm doing now: going through every fuse circuit one at a time to see if there is current being drawn when there should be none....and addressing the problems I find. I don't know if any of these things will be related to the new problems of stalling and no tach, but I want to eliminate shorts from the equations first. Then I'll see if it is still stalling, and work that next. Then get back on the timing advance/vacuum testing. And tach.

Regarding the fuel pump relay...I actually put a new one in months ago because I discovered very early that the original one was bad. Verified by jumping the contacts just as you describe. Doesn't mean this new one didn't go bad too, though, of course. So it is certainly something to try again (jumping the contacts) if the stalling problem is still occuring.

Also...I decided to send my DME in for a bench test. That may or may not have anything to do with the problems. But...I had great success doing that with a 944 DME recently (and greatly regreted not doing it earlier since I spent more than a year chasing a poor idle problem that turned out be the DME). Either knowing it is good, or discovering it is bad so I can repair/replace, is worth the $50 for the test.

That's an awesome suggestion about putting in a light bulb to test if ignition is good or bad when/if it stalls again. I did take a good look at the "green" wire, and verified good continuity from the distributor to the connector going into the ignition controller. And I also did jiggle the key when I was struggling with the repeated stalling Friday night to see if that made things worse or better....and it had no affect.

Anyway....that's where I am at. Will be chasing shorts for a while till I'm sure I've got 'em all fixed (again), and waiting for my DME to get back.

Thanks!

 

If you have a number of electrical problems you are chasing at once, I recommend you disconnect the battery and pull the CE panel. Whether you decide to clean all the connections or not(of course I recommend that you do), poke around and look for melted wires back there. One day when I was looking through a stack of CE panels at the breaker's I found that just about all of them had some wires melted together on the back. It takes maybe 10 minutes to get the panel out on an early car, and 10 minutes to put it back. Make that 15 minutes on later cars with the blade fuses and the posts on top for the power feed, because Porsche didn't think the early setup was enough of a hassle. If nothing else it will give you peace of mind if you find no issues. Besides, if you have the DME out, what else are you going to do to keep busy?

BTW, when you make a repair I hope you are using solder/heat shrink. Crimp connectors are the bane of my existence, and I cringe whenever I see them. IMHO they do not belong on critical circuits. I found some recently when I had my radio out during a huge project involving a month of downtime, but I did my best to ignore them and stuffed them back in the console to be addressed another day. Who needs a stereo when you have BWAAAH?

A note about the MityVac... the silverline all-metal unit(which can be rebuilt if needed) with all the bells and whistles runs about $90. If it's something that you will seldom use, the plastic unit will probably do just fine. If you're sending the DME out it sounds like you will have time to wait for shipping.

 

Virtually every modern aircraft is completely wired using crimp connections...

 

Thanks Wally, as if I didn't already have enough reasons to not want to fly.

Kidding aside, I am sure that the tools used to crimp connectors, and even the connectors themselves, are of a different quality altogether than the crap that is usually sold in auto parts stores. I am referring to the latter style.

 

Get one of these with the correct die set:

http://www.wihatools.com/400seri/436_crimps.htm

 

Hacker, those are nice units and are along the lines I had in mind when I mentioned higher-quality. Thing is, it's a bit pricey considering how often I would need it, especially when you consider the many wire sizes and terminal types I end up dealing with over the course of a year. It's somewhere near the bottom of my wishlist though...

 

There are many sources for these, I picked mine up for $30. Friend of mine snagged one for $60 with four different die sets. The die in mine fits 99% of every wire I'll ever crimp on my 928. It also came with a spark plug wire die.

To seperate one of these I had to lock the crimped end in a vice and pull very, very hard. Solder isn't that strong.

If you deal with a lot of wire connections, this should be at the very top of your list

 

Haven't followed this thread all the way through, but couldn't a drop off in power and erratic vacuum at 3K+ rpm be caused by a plugged cat?

Dennis

 

which could be diagnosed with a vacuum gauge......

 

Solder is surprisingly strong, especially if you crimp then solder as I do. It also has the advantage that it will not allow fluids to wick up into the wire as a simple crimp will. I'd put one of my soldered terminals up against one of your crimped terminals in a pull test any day!

Anyhow, the 928 isn't the only thing I work on. I find myself attaching terminals for anything from 30 gauge to 4 gauge wire over the course of a year. Most are pretty common terminals, but I could see needing at least 4 distinct types of dies and I would STILL encounter odd stuff occasionally that I wouldn't have the die for. Like I said, one of these days...