RPM dropping and shuts down after starting

930 porsche · 02-23-2010, 07:21 AM

1978 930 start normally and idles at 1000 RPM then a few minutes later RPM drops and the car shut down but both fuel pump still running. If when RPM drops down I accelerate to keep it up, it at first chock then it hesitate and RPM goes up. I cleaned the WUR, auxilary air valve, fuel filter. It takes longer for the RPM to drop but still the same problem. I need some help. The problem is probably staring right at me and I do not see it. What are your thoughts. Thank you.

A930Rocket · 02-23-2010, 07:31 AM

Vacuum leak? Check all your o-rings, hoses and snug down your intake bolts.

equality72521 · 02-23-2010, 07:38 AM

Check the hose on the back of the AAV that is shaped like a "U". I had mine rub a large hole in it and it did that. It acted like an AAR during warm up then the idle dropped off after. Check all other hoses as well as mentioned above.

9dreizig · 02-23-2010, 07:38 AM

Still sounds like a WUR issue to me. Have you done a complete pressure check on the fuel system ??

mark houghton · 02-23-2010, 09:35 AM

Quote:

Originally Posted by 9dreizig

Still sounds like a WUR issue to me. Have you done a complete pressure check on the fuel system ??

+1, probably the first place to confirm. It sounds to me that your WUR cold enrichment control pressure isn't raising up as the engine warms. So, as your AAR slowly closes as it warms up and begins choking down the extra bypass air, you could find yourself way too rich if the WUR hasn't adjusted the control pressure up proportionally....and the rpms would drop and/or the engine would die.
Just one theory, anyway. Get out your gauges and check the control pressures.

930 porsche · 10:05 AM

I changed a new hose for the AAV. On my car, is there a fuel pressure switch or air pressure switch or regulator? It almost feels like it is loosing fuel or air pressure when RPM goes down. I cleaned up the WUR and the lf hole was very dirty with deposit so I cleaned it and use compressed air to make sure that it is debris free. The diaphragm is still in good shape. I was just wandering on the AAV, when it is cold it is 1/4 open is it normal or is it suppose to open completely? As you know there is a little slit opening weather it is open or closed. If it continues I will have to do a fuel pressure check.

JBL930 · 02-23-2010, 10:03 AM

It could be that you're not getting power to the WUR, which would mean the bimetallic spring isn't warming and raising control pressure. That would be a simple check first, then have a pressure gauge on there with the ignition on to see whether the WUR raises control pressure once current is applied.

930 porsche · 02-23-2010, 10:12 AM

JBL930 I will try that. Can I just turn the ignition to see if there is power on the WUR before doing fuel pressure check?

mark houghton · 02-23-2010, 10:16 AM

Quote:

Originally Posted by 930 porsche

I was just wandering on the AAV, when it is cold it is 1/4 open is it normal or is it suppose to open completely? As you know there is a little slit opening weather it is open or closed. If it continues I will have to do a fuel pressure check.

Generally the auxillary air regulator is 3/4 to fully open when cold, and when warm all you can see is that little vertical slit.

JBL's advice is right on. But I believe you need to have the engine running before you'll be able to measure any voltage at the WUR connection. Just turning on the ignition won't put any voltage there.

JFairman · 02-23-2010, 10:32 AM

"I cleaned up the WUR and the lf hole was very dirty with deposit so I cleaned it and use compressed air to make sure that it is debris free."
Can't tell what you mean by this and if you put more than 16psi into the boost signal line.. it you probably blew up that diaphram and killed it.

"The diaphragm is still in good shape."
Which one? the S.S. fuel metering diaphram disc at the top under the fuel line connections or the boost enrichment air pressure diaphram at the bottom?
With all your questions one wonders how do you know that, did you take it apart?

Do you know the overall condition of the ignition, timing, and cold start timing if the hardware is still there and functioning?

930 porsche · 02-23-2010, 10:46 AM

Mark, the auxilary air regulator does not open all the way but 1/4 of the way. It feels like there is a spring. When I take a small skinny screwdriver and try to open it up, it does it fine but then comes right back to closing itself 3/4 of the way. Does anybody have a picture of it open while cold? If it does not open all the way, does it mean that the spring is worn out? Is there anyway to retighten the spring?

JFairman, I did take the WUR apart and noticed that the bottom diaphragm was in good shape. blowing compress air to clean it very carefully without ripping it.

JFairman · 02-23-2010, 10:53 AM

How far the TB bypass AAV slide valve opens when cold depends on the ambient temperature so you need temperature info to go with it.

930 porsche · 02-23-2010, 10:59 AM

JFairman, in my garage it is about 70-75 degrees. How cold should it get to open up completely? Are you telling me the AAV is good? If you say that it has a lot to do with the ambient temperature then why don't we try to put it in the refrigerator and see if it opens up completely before I buy another one?

JBL930 · 02-23-2010, 11:49 AM

The feed for the WUR is probably the same as the fuel pumps. Reach behind the fuel head and disconnect the cable there, with the ignition on (engine not running) the fuel pumps will then run, my bet would be that you will also get power to the WUR, not certain about that though, so check with a wiring diagram. You could run a separate source to the WUR just to see if the WUR works and sorts your problem.
Make sure you reconnect the cable to the fuel head though, it's a safety device, if you crash with the ignition on and the engine cuts, but you've ruptured a fuel line, the pumps will still fire fuel everywhere.

A good test for your AAV is to remove it and put it in the fridge, leave it for a while and observe the position. Then put it in the oven on a low heat for a while, then again observe the position... Simple check like you would with a thermostat

930 porsche · 02-23-2010, 12:00 PM

JBL930, great idea for the AAV. I will try that.

JFairman · 02-23-2010, 12:02 PM

I check the WUR and AAV heater functions and control pressures with the ignition on and pumps running because they are on the fuel pump circut.

With the engine off I've also run the battery down doing that because the 2 fuel pumps draw around 13amps and loosing battery voltage while running them completely screws up fuel pressure readings so hook up a 10 amp minimum battery charger to the battery while doing that.

930 porsche · 02-23-2010, 12:25 PM

JFairman, good point.

mark houghton · 02-23-2010, 12:34 PM

From what you've described, I would say that your AAR is working correctly, but you can always do the oven and refrigerator thing to confirm full range of movement.

Your problem lies elsewhere, probably in control pressures is my bet.

930 porsche · 02-23-2010, 12:39 PM

Mark, I am thinking just like you. But where else can we look at? What else is controlling any sort of pressure? Switch, relay, coil?

mark houghton · 02-23-2010, 12:55 PM

Quote:

Originally Posted by 930 porsche

Mark, I am thinking just like you. But where else can we look at? What else is controlling any sort of pressure? Switch, relay, coil?

That's the point...there isn't much else in a 1978 (such as emissions stuff to go wrong), at least I don't think you have an O2 sensor and Lambda system to affect fuel mixtures. Nothing you've described makes me think it's ignition related, so I would continue to focus on fuel delivery. Typical faulty AAR symptoms are either a lack of high idle when cold, or a high idle that won't go away when warm. You don't have either symptom, so look elsewhere.

If your car is idling at only 1000 stone cold....especially with a working AAR...then you definitely have it running too rich. Try adjusting the air correction screw in the back of the throttle body....open it up a good half to full turn and see if you get a higher idle. You may be able to tune around the car stalling, but still need to find out why.

Do four things as least: (1) confirm that you have power to the WUR; (2) confirm that the WUR reacts to the power (i.e., control pressure increases as it warms up); (3) check all control presssures and the system pressure against specifications; (4) check for vacuum leaks in the intake plumbing (IC O'rings and the like). And #5...let us know what you find.