Solid Snake

Thanks, I'm going to print that out and go hose hunting after work.

Solid Snake

I checked the hose diagram against the car and it's dead-on.

I checked the vacuum after the thermo valve. There was none while the engine was cold, as there shouldn't be. After the engine warmed up, there was vacuum. I triple checked this line, and it was correct. I took a long screwdriver and forced the throttle plate shut and the vacuum stopped. I realized that the cruise control cable was preventing the throttle from shutting all the way. I unscrewed the CC bracket from the throttle body to get it out of the way and surely enough, no EGR at idle. Great!

So what I have right now is a good fuel mixture, a smooth 800 rpm idle, and a rock-solid vacuum of 15' hg.

When I feather the gas and EGR kicks in, the engine wants to die. It's as if there's a dead spot in the throttle from 5% to around 15%. It seems like there's less O2 available to the engine within this range than at 0%. If this is correct, wouldn't the idle be significantly leaner than part-throttle operation? Is this dead spot caused by too rich a mixture?

Dennis Wilson

Earlier you stated that "I measured the vacuum off the brake booster line. My distributor has only one vacuum connection and that line is shared by the evap control valve." . Have you fixed this?

Dennis

special tool

Dude....
why are you testing all this tming, fuel etc?
Was it not running fine before you "fixed" it?

You screwed something up!

I can tell just how you posted dude, it probably wasn't a good idea for you to be digging around in there.
Take off the obstinance gloves and take it apart and figure out what you f-ed up.

Solid Snake

No, it's still shared. None of the posted diagrams show were the evap valve is supposed to go. It's supposed to get ported vacuum so I T'd it into the distributor line.

Yes, it was running before I "fixed" it, but most hoses were either dry rotting or had been chewed apart by rodents. Also, the two main rubber fuel lines were cracked, the evap lines would turn into dust right in your hands. Most of the throttle body/CIS housing studs were broken because the rubber had turned into plastic and broke off. It was 1 stud away from floating in place by the fuel lines. Sure, it technically ran, but how far would you care to drive a car like this? Remember, this thing is almost 30 years old. You think taking care of this stuff wasn't a good idea?

This is my project car. I am new to 928s, but I am not new to working on cars. I followed every single vacuum line last night and they're all in their proper places. I have a smooth idle now, so if I get rid of that part throttle dead spot and improve throttle response a bit, I'll be all set. There's no reason to tear the car apart again.

Dennis Wilson

It is very possible that a PO adjusted the A/F mixture to compensate for a major vacuum leak which you have now fixed. If that is the case you will be running too rich. Try reconfiguring the vacuum advance line and setting your timing. If that doesn't cure the lag, then go for adjusting the A/F mixture. Definitely take the evap valve off the vacuum advance line. If the valve is faulty it will mess up the advance and cause a sluggish engine.

Dennis

Lizard928

afaik, the vacuum advance to the distributor is not supposed to be MAP, it should only get vacuum under WOT, or sudden bursts of throttle.

Solid Snake

And stick it where?

Dennis Wilson

Solid Snake Wrote "And stick it where?"

If it isn't one of the components diagrammed by Dave on page 3 of this thread, I would just plug it and see how the advance works by itself. The evap valve that you are referring to is probably also named an EEC valve/switch.

Dennis

Solid Snake

After reading the manual a bit I realized that the '79 doesn't use vacuum advance, it uses vacuum retard! 1980 onwards uses vacuum advance. So the line at the distributor gets vacuum at idle and now I see why. Only problem now is that I can't seem to advance it enough to get power out of it.

Solid Snake

I just found the problem tonight. It wasn't vacuum or fuel, it was ignition. The distributor gear looks like it skipped a few teeth somewhere during the rebuild process. I remember having to pull it out a bit in order to get to an intake runner bolt. I pulled it out and repositioned it so I could get enough advance. I don't have a timing light handy so I used the method one of you guys mentioned. I set it conservatively as I didn't want to risk detonation. I noticed when I did this, the idle jumped up to 2K rpm. It was running lean and very retarded, no wonder there was no power. I put in the idle screw until it was at 700 rpm and measured 19'' hg at the manifold. I stabbed the throttle a few times and the engine responded instantly. I took it out on the street, dropped the hammer, and it threw me back in my seat. My only concern is some part throttle hesitation and bucking, to the point where I may unintentionally lay rubber in 1st gear around corners. I may need to lean out the mixture a bit. BTW, the EGR valve is connected and operational. Now the only mystery is where to connect the EEC. I thought the distributor hose was the way you thought it was, ported vacuum, atmospheric at idle and increasing with airflow, but this is not the case.

Thanks to everyone for your replies. After I get a timing light and fix the hesitation, I'll have myself a perfect ride.

Mrmerlin

Hey great you figured things out now if you would have followed the #4 thread the next 3 pages would never have happened, like i said it is probably something simple, and since you did a top end rebuild it stands to reason that you moved something out of place. Goodluck

By the way to check the static timing of the distributor line up the timing marks and then remove the dist cap if you look carefully there should be a hash mark on the rim of the distributor and the rotor should be pointing at it this is TDC for #1

special tool

There you go, good job.

SharkSkin

Glad you're making progress. The last diagram that I posted clearly shows that the distributor should see vacuum at idle, dropping toward atmospheric as you open the throttle.

The only right way to set mixture is with a CO meter. Here is a method that will get you in the ballpark though:

1) Remove spark plugs and disconnect green wire from the ignition box end(to prevent hydro-lock and fire)
2) Jumper the fuel pump so it's on.
3) Turn the mixture screw full lean, until you no longer hear the injectors squealing.
4) Turn the mixture screw toward rich until the injectors just begin to squeal.
5) Slowly turn the mixture screw back toward lean until the squealing just stops.
6) Remove fuel pump jumper
7) Crank engine several revs to make sure any pooled fuel is blown out.
8) Reinstall plugs and green wire.

You want to do the above steps quickly so you don't get too much raw fuel in the cylinders. This procedure will get you close enough that you can set the timing to spec, then beg/borrow/rent a CO meter to dial in the mixture properly. Also, unless you know when the fuel filter was changed, change it. It takes a different size crush washer than the other banjo fittings that you've been dealing with. If you don't know already, the filter is in the front of the RR wheelwell(below the accumulator):

Solid Snake

Thanks for that procedure. I can probably borrow a CO meter. I'm also thinking about drilling the downpipe somewhere before the cat to install an O2 sensor so I can keep an eye on things and adjust the mixture, should it drift. Good idea?