Results of CIS pressure testing

[azasadny]

I was able to hookup the pressure testing gauge today. The car has been idling too slow, stalling out and "pops" from the exhaust and feels like the engine doesn't have full power. I'll make a test drive Saturday AM and see if it's imporved...

Results of CIS pressure testing on 8/17/2006

“cold” pressure with valve in “open” position 30psi (control)

“cold” pressure with valve in “closed” position 60psi
drops to 25psi after 15 seconds

“warm” pressure with valve in “open” position 60psi

“warm” pressure with valve in “closed” position 25psi


With engine running, the pressure stays at 30psi and is steady


With idle set to 1100rpm, pressure is at 38psi and holds steady

I'm not really sure how to interpret these numbers. Do they look right for a '74 with 2.7L CIS? Thanks!
With engine off, pressure stays at 24psi



Pushed down slightly on control lever and the idle increased to “normal” and ran better, adjusted the mixture CCW 1 full turn and it holds at 1000rpm

[ruf-porsche]

Hey Art

Don't have the spec for a 74 2.7 psi but for my 82 USA model 3.0 CIS engine these are the spec

Control Cold pressure at 20 degree centigrade 1.95 bar to 2.4 bar (28 psi to 35 psi)
Control Pressure warm 3.4 to 3.8 bar (49 psi to 55 psi)
System Pressure 4.5 to 5.2 bar (65 psi to 75.4 psi)
Leak pressure after 10 minutes 1.3 bar (18.85 psi)
Leak pressure after 20 minutes 1.1 bar (15.95 psi)

Your is probably different

1 bar is equal to 14.50 PSI

Did you hook up the wur per my last post? Find the problem with the alternator?

How is the engine running now?

[1982911SCTarga]

I'll try to help and hopefully others will pitch in.

Art, your system pressure seems slightly low, but maybe it's correct for your year. On an SC, the cold system pressure should be between 65 to 75 PSI.

Also, ambient temperature is an important factor, too, along with your pressure readings. If the temp was say 70 to 80 degrees F, you might be low on your cold control pressure as well.

Your residual pressure looks OK, but it should "hold" for a long time (above 16 PSI after a half hour).

Your warm control pressure looks low, too.

You need a control pressure graph for your '74 that tracks ambient temps and recommended cold control pressures.

Stay with it. You're making progress.

Brian

[ruf-porsche]

Art

If you can find a copy of the technical specifications for your year 911 it will give all the info you need.


EDIT

Our host have it on their website

Porsche 911 Factory Spec Book (1974)
Brand: Genuine Porsche
WKD-422-121-OEM $48.00

[azasadny]

Thanks guys! I'll have time to do more investigation on Saturday. I had to hook everything up today and took awhile. I'll drive the car and see if there's any change. I did lean out the mixture 1 full turn CCW and that raised the idle to 1100 rpm. I am still feeling a hesitation and stumble at 3500rpm... almost like the engine isn't getting full power.

[1982911SCTarga]

Just to show how these cold control pressures can vary depending on the vehicle model and type, I'm looking at a VW chart that indicates cold control pressures from 21 PSI at 68 degrees F to 28 PSI at 86 degrees.

Some worthy person should collect the control pressure/temp charts for all Porsche 911 CIS years. They could then be housed and archived in the tech articles section of this Web site.

Brian

[1982911SCTarga]

Art, don't be in rush to adjust, tweak, fiddle and solve everything in one shot. Take notes as you go along of what you've changed and what the settings, readings, etc. were before you changed anything and what they are afterwards. Work patiently and with forethought.

Brian

[ruf-porsche]

Originally posted by azasadny
I am still feeling a hesitation and stumble at 3500rpm... almost like the engine isn't getting full power.

Did you check the advance on the distributor?

[al lkosmal]

Art,
Per the "How to tune and modify Bosch Fuel Injection book, a 1974 911 should spec out as follows.

Cold Control Pressure (valve open, engine cold)
17psi@50F, 32psi@75F, 45psi@100F

System Pressure (Valve closed)
65-75psi

Warm Control Pressure (valve open, engine warm)
Idle 41-44psi, Part throttle 49-55psi, Full throttle 38-44psi

Rest Pressure (shut off engine and observe pressure over the next 20 minutes. Should not drop below....

16psi

[azasadny]

Brian, Al and Ruf,

I don'tknow how to check the distributor advance, but mt mechanic checked my timing and said it's spot on. I'm taking notes and only changing one thing at a time. I didn't check the gauge when accelerating or at full throttle, so I still have to do that. It seems my system pressure is a little low (60) and my warm control pressure is low, too...

[ruf-porsche]

Timing an engine is different from the advance system in a distributor use to advance the timing when the engine speeds up. Your timing could be spot on, but if the advance is not working, you will not get any power ar high rpm.

Some distributor have vacuum advance as well as mechanical advance. Vacuum advance is easy to check with a vacuum pump and a timing light.

Connecting the vacuum pump to the distributor and holding a timing light to the zx see if the mark on the pulley advance when the vacuum pump is pumped. Vacuum advance will not work properly if you have a vacuum leak.

Mechanical advance is a little harder to check. You will need to bring it to a service station that still have a distributor machine.

I don't remember if a 74 cis have vacuum retard, but on my 82 911SC there is also a vacuum line to retard the ignition timing, sometime people switch the two lines.

Originally posted by azasadny
Brian, Al and Ruf,
It seems my system pressure is a little low (60) and my warm control pressure is low, too...

You could have the reading backward, warm control pressure is when the valve to the WUR is open to allow the wur to regulate the pressure.

Warm system pressure is when the valve is closed so the wur is no longer controlling the pressure.

Check your CIS gauge and valve setting again.

[azasadny]

Ruf,
I'll recheck everything on Saturday. Thanks!

[toolman]

Hey Art. I have a vacuum tool that you can borrow, and a good timing light. The advance should go up to about 20 or 30 degrees BTDC at about 3500 RPM and should be easy to test with just a timing light.

[azasadny]

David,
Thanks! I'll use my timing light and check for the advance on Saturday. I'm also going to recheck my CIS pressures. So far, my cold and resting pressures as well as wamr pressure are all on the low side of normal or below normal...

[azasadny]

I'm getting a good, steady idle speed with warm pressure at 60psi at idle and stays at 25psi for 30min after engine shut-down. I'm getting a strong backfire through the exhaust on acceleration and the car seems to be missing power... something's wrong...

[ruf-porsche]

Art

What is your idle speed set at right now?

Verify that all the cylinders are operating. Buy an insulated spark plug wire plier and pull one wire at a time from the distributor. Each time you pull a wire from the distributor the engine rpm should go down. A cylinder that does not lower the rpm when you pull the wire is not operating correctly. Check the wire, spark plug and fuel injector for the cylinders that are not operating correctly.

The back fire through the exhaust is from unburnt gas from one or more cylinders. Did you ever get to check the timing advance? No advance in the timing at higher rpm means less power.

Another source of backfire in the exhaust is an exhaust leak between the heat exchanger and cylinder head or heat exchanger and muffler.

[azasadny]

ruf,
I'm idling at 1000rpms now and it's steady and doesn't sound like it's "missing". I'll check everything tomorrow as I'm out of time tonight. I'll try to figure out how to check the vacuum advance. I need another set of hands/eyes to look at the timing light while I get the rpm's up to see if the vacuum is advancing. What should I see when I do this? Thanks for helping me out...

[ruf-porsche]

Connect a hand vacuum pump to the distributor (remove the vacuum line from the manifold and plug the end of the line). Connect a timing light to the engine and start the engine. With the engine ideling, pump the vacuum pump while aim the timing light at the zx mark. The pulley mark should advance when the vacuum pump is pumped.

disconnect the vacuum pump from the distributor and start the engine again (without the vacuum line connected to the distributor and still plug.

Aim the timing light at the zx mark and rev the engine to about 4000 rpm. Again the pulley mark should advance. This will confirm that the mechanical advance is working.

The backfire, does it occur when the engine is reving or while the engine is declerating after you have reved it?

If it occurs during the deceleration portion of the rev, you have an exhaust leak

[azasadny]

Thanks, I'll try that tomorrow AM. Enjoy your Friday PM!!

[ruf-porsche]

Hey Art

Another thread on timing a 2.7 engine. When your mechanic check the timing did he also check it at 6000rpm or did he just check/set the timing at idle?

Timing a carbed 2.7