Curt

Thought I would throw this out one more time before I begin operation this Saturday.

My timing issue still stumps me after many hours of investigation.

The problem: With the vacuum retard and vacuum advance hooked up to the distributor the car will idle perfect but as soon as you crack the throttle the car will die. and I mean as you just crack the throttle is stumbles and dies.

The way I have been driving it is having a vacuum pump holding 5-10psi on the retard side and just keeping the vacuum advance disconnected. With this setup the car runs great. Absolutely no problem with running in any temp range or time duration.

The problem starts when I hook up the vacuum advance to the distributor. The car wont rev.

When both vacuums are hooked up the car idles perfect. I can disconnect the vacuum advance line and it will still idle perfect. If I disconnect both vacuum lines the car wont idle because the retard line is disconnected. As long as I have 5-10psi vacuum on the retard side and no vacuum to the advance everything is perfect.

I have looked at everything 5 times already. I have removed the entire intake and replaced every vacuum line. Most were leaking a bit, now they are not. I have checked timing marks with cam gears to crank. Those are within a 1/2 tooth or less. I have checked all fuel pressures and everything is within spec. I have run through the electrical diagnoses and all checks out. I have even swapped distributors with different vacuum advances and both act the same way. It was a used distributor but what are the odds of them having the same problem. I have made adjustments to fuel mixture since I have installed an O2 sensor I can take readings. That seems to be okay.

I truely believe the problem lies with the vacuum advance line for some reason. With it disconnected and opening the throttle it starts pulling vacuum just like its supposed to. With the vacuum retard disconnected its pulling vacuum while the engine is at idle, just like its supposed to. So why when they are both connected it wont rev, it will only idle????? Also, if i remove the vacuum retard while the engine is idleing the motor idles really low and wants to die. It will just barely stay running. I would have thought it should be the other way around. Pull the vacuum retard at idle and revs should come up, but it doesnt. It works opposite.

I will put another 8 hours or research into it tomorrow, so hopefully someone here can give me another avenue to search down.

I have to smog it in two months at a smog only station in California, not looking forward to that. I highly doubt the inspector will let me smog it with a mityvac hooked up under the hood.

Thanks again. I know I have posted this subject before but someone out there must have had a similar problem. My car cant be that unique. or can it...

Thanks

ViribusUnits

A question.

Have you followed the service mannual on adjusting the timeing, and trouble shooting that sort of stuff?

What happened when you switched the lines around? ie. put the advance line on the retard nipple, and the retard on the advance nipple?

V-Fib

Curt, My 79 only has one vacuum line to the distributor (advance). Sorry I can't be of more assistance.

Anthony Tate
79/928 Silver Metallic

Dennis Wilson

Curt,

Have you measured the vacuum for each side? The retard line should be 15 to 20 inches at idle. The advance side should peak near the same reading at an off idle rpm. If you have a low retard vacuum, this would allow the timing to advance too fast. BTW did you check for vacuum leaks at the gasket of the lower plenum?

Dennis

Dennis Wilson

Curt,

Just had a look at both of my 78's since the schematics aren't alway realistic. The retard vacuum line goes to the front of the throttlebody at the connection coming up on the right side of the idle adjustment. The advance line goes to the far right connection at the rear of the throttlebody. BTW by right I mean the passenger side.

Dennis

Jim V

Curt,

Sorry to hear that you are still fighting this.

How about getting a US-spec distributor or just the vac. unit from a US spec and do away with the retard function?

Dennis may be onto something too; when I got my '79 (as weak memory recalls) the PO had compensated for a CIS problem by opening the throttle blades to acheive a useable idle speed. The idle adjust screw did nothing. Sooo...

As I recall, there is an adjustment on the blades for stop position ONLY and ALL actual idle speed adjust ment should be done with the proper screw at the front.

I would assume that the vac signals for the adv/retard are taken from very near the edge of the throttle plate and if someone has mucked with the adjustment and opened the plate rest position too far that would make the retard signal weak, and the advance signal strong right?

Just skimmed the manuals and couldn't find how to check this out but I know I've read it and then adjusted it a couple yrs ago because my idle speed screw works now.

928FIXER

In reality it should almost run fine with both lines unhooked.The retard line is for emissions and the advance is for a little(very)extra fuel mileage at highway cruising speeds.Have you checked the mechanical advance to see if it is working??Set the timing as per the shop manual?Set the air fuel ratio with a C.O. analyzer?By keeping a vacuum pump on the retard side all you are doing is retarding the timing about 10 degrees.Maybe the harmonic balancer ring has "slipped" and the timing is off but a light shows thats it OK.Is this a euro 79....USA cars do not have two vacuum lines going to a dist....at least mine didn't until I changed the advance can to one that did for better fuel mileage.

Scott M.

FWIW, I have the 'exact' problem w/ my 80 US auto car (it also has adv & retard lines). Started immediately after a t-belt job. Also replaced all vac. lines at that time as well. I've done 12 t-belt jobs so I know it is not an installation issue.
I tore apart the distributor, it operates perfectly, timing is set to spec. Played w/ dist. different ways. I replaced the dist. green wire. Got good compression, haven't checked leak down yet. It feels like a lean surge. Fuel pressure is fine, haven't checked volume.
I really haven't put any time into diagnosing yet, maybe 2 hours total.
Car runs better cold, as it warms it gets worse. I will next look at the Temp sensor for correct ohms. Did find a few injectors not seated correctly. Maybe an injector or two clogged and not firing adequatly. My car did sit for 3 months before I did the belt job.
Let me know what you find. I won't be able to get back to it for a few weeks.

Dennis Wilson

Curt,

When you set the A/F mixture did you leave the smog pump operating? If you did, you are running too rich as the smog pump on our OB's dump air upstream of your O2 sensor. Try removing the belt and reset the A/F mixture with your O2 sensor. Another manual method of setting it without a sensor is available. Let me know if you want to try it.

Dennis

Scott M.

Dennis;

He shouldn't have an O2 sensor as its a CIS car. If he does have one its only for an AF meter.

Scott

WallyP

Once more - sounds like a timing problem.

1) Remove #1 spark plug. Insert a three-foot wooden dowel so that it touches the piston - keep it as vertical as possible, and keep it from binding in the plug threads as it moves.
2) Rotate the crank by hand until the dowel tops out. (This is a touchy operation, and is easier with two people.) When you are certain that the dowel is as far out as it will go, check the "0" mark on the crank pulley.
3) When you are sure that the "0" mark is actually TDC for #1, replace the plug.
4) Attach an inductive pickup timing light to the wire going to #1 cylinder. Remove both vacuum lines going to the distributor.
5) Crank the engine and set the timing to 31 degrees BTC @ 3000 rpm.
6) Allow the engine speed to decrease as you watch the timing marks. The advance must smoothly decrease as the speed declines (it is not a linear relationship, however) until the timing is near 0 degrees at idle.
7) Attach the retard hose only. The timing should retard to 4-5 degrees ATDC at idle. Rev the engine - the timing should smoothly advance.
8) Attach the advance line - the timing should not change at idle. The timing should advance slightly more (usually about 10 degrees) than it did with just the retard line hooked up.

Scott M.

Wally;

Then do you think that it is purely a timing issue?
In you experience, do you know of any electrical gremlins that will cause the same symptoms?
As you may have read above, I have the exact same problem w/ my 80 L-jet automatic. I have essentailly ruled out a fuel problem, have pressure at spec and have volume. Though, I haven't had time to look at injectors.
More advice is always appreciated (especially since I've hammered the timing question everyway from tuesday.

Curt

Thanks for all the replies. Unfortunately I had problems with my daily driver this weekend and had to devote about 7 hours to it. So I had to push the Porsche back untill sometime this week.

I did print out all of the above and will be checking into everyone.

To answer some of the above questions:

Jim V. The car was running extremely well until I had the green wire on my distributor go bad. At that same time is when this other problem started. Sounds like Scott M. and I are trouble shooting the same thing. Could have something to do with the green wire going bad. I even have a spare computer that I swapped. It did nothing.

As far as Wally's post about timing: I have checked the timing many times but have never measured the piston. I have removed the plug and looked at the piston. Looked like it was at the top but I will do the "stick" method of measuring and find out for sure.

I installed a cat. off a 86 or 87 S4 that is how I have an O2 sensor. I have it hooked up only to read A/F mixture for tuning. I did the A/F mixture adjusting with the smog pump disconnected as per the manual.

Again, first thing I will check this week is for the timing to be at 0 degrees with #1 piston at the top.

Thanks again for all the posts and I will update later this week with all my findings.

Curt

Got a chance to look at the timing issues wally brought up.

Set timing to 0 degrees, cam gear markings line up with pointers on both sides, rotor is pointing in direction of #1 on distributor cap and the most important thing is the #1 piston is dead on at the top. I rotated the motor over several times to double and triple check. All timing markings line up.

The car starts up and runs perfectly with both vacuum lines hooked up. but as soon as throttle opens car wants to die. I unhook the advance and leave the retard line on. Again car idle's perfect as soon as you start to open throttle car wants to die. Connect vacuum pump to retard and hold 5-10psi car idle's perfect and you can rev up to infinity, well almost... Leave vacuum pump on retard with 5-10psi and reconnect advance, car idles perfect but when given any throttle wants to die.

Thats the low down. Same as before.

Also, I was able to set timing with vacuum pump on retard, then reving motor to 3,000 rpm, then unhook retard line and set timing to 31 degrees. Actually timing was still correct from last time I set it.

When I pull off the retard line while car is idleing the car wants to die, it doesnt idle smoothly. In fact the car should idle faster without the retard connected, but it doesnt. I may try to adjust the mixture screw while the retard line is off to see if I can get it to idle. Maybe the mixture is just way off, however the readings from my O2 sensor dont indicate this.

Dennis Wilson

Curt,

If the throttle plate was partially open it could skew your vacuums. May want to make sure the throttle cable isn't too tight.

Dennis