79' SC CIS rough on cold start and high idle when hot

[gregoryp]

I've been going through my new 79' SC. I initially replaced the injector seals, sleeves and sleeve seals. That made a significant improvement in idle quality. Now I have the following issues:


When cold the car starts fine, but then runs rough for about 2 minutes and if I apply any throttle I will get some stumbling and popping from the intake. After this time period it runs fine.


The other issue is that I get high idle when the car is hot, sometimes worse than others, but will idle from around 1100 rpm to 2000 rpm when you let off the gas and come to a stop. It will eventually idle back down to about 900 rpm if you sit for a few minutes.


When it is hot and sits for about an hour it can crank over quite a bit before firing, but for shorter periods than that it starts fine.


Here is what I've done:


Checked fuel pressures per 911 CIS primer results below, but they look good from what I can see.

Fuel pump on, pressure gauge valve closed and WUR electrical connection unplugged = 4.6 bar

Pressure gauge valve open with cold engine = 1.9 bar, outside temp 90F

Plug in WUR electrical connector with pressure gauge valve open, pressure stops rising at 2.7 bar

Checked cold engine pressure holding and was at 1.4 bar after 20 min.

Fuel pressure with engine hot, pressure gauge valve open and engine runing = 3.2 bar

Hot engine pressure holding, pressure gauge valve open 1.2 bar after 35 min.

Next I made a smoke tester using a paint can, a soldering iron, some Tiki torch wick and baby oil. Worked really well.




I didn't find any leaks, so that appears to be good.

When I was replacing the injector seals I took the opportunity to look at the injector spray patterns and they looked good. I also checked that the fuel delivery was even which it was. I did notice that the injectors seemed to drip a bit, but they all did it, which made me think the injectors weren't faulty since they were all doing it?



I also checked the operation of the Auxiliary Air Regulator and it was open when cold and then slowly closed all the way with battery voltage, so that seems to be working.

Anyone have any recommendations for next steps?

Also what is normal operation after a cold start, this is my first CIS car, so not sure what I should expect from it?

[allaircooled]

Our resident CIS experts should be chiming here in a few, but I am interested in hearing more about that smoke tester.

[NYNick]

For the hot start problem, I'd replace the fuel accumulator. Easy peasy.

[gregoryp]

The smoke tester was very simple. I combined a few designs I found online, mostly to keep it as simple as possible.

Materials List:

* Metal 1 gallon paint can from Home Depot
* Weller soldering iron, cheapest one I could find.
* Tiki torch wick
* Some 3/8" ID x 1/2" OD hose from Home depot
* Some barbed fittings for the hose and a quick disconnect fitting for my air compressor and some misc washers and threaded connectors.

To assemble it I cut a small notch in the rim of the can for the soldering iron cord to pass through and put some silicone around it to seal it.



Then I drilled two holes in the lid for the fittings and inserted them with fender washers on each side. The nuts to hold the pipe thread fittings in the lid were made by cutting a brass pipe threaded connector in half and using each half on one of the fittings. I did have to tap them deeper so they would thread on all the way due to the tapered pipe thread.



To make it work I slid the tiki torch wick over the end of the soldering iron and soaked the wick in baby oil.



Close up the lid, plug in the soldering iron and add some air.

I used the regulator on my compressor to drop the pressure down to about 3 psi.

I connected the outlet from the smoke tester to the brake booster connection on the air box. Mine had a short hose coming off with a connector in it, so I just disconnected it and pushed the smoke tester hose in, fit perfectly.

I initially wrapped the air filter in saran wrap and put it in the airbox, but did find a leak between the airbox and the fuel distributor housing. That would just cause unfiltered air to leak in, so I removed the throttle body air boot and stuffed a rag into the throttle body with some duct tape to seal it of for final testing.

[gregoryp]

My next area to look at was going to be the Auxiliary Air Valve, not to be confused with the Auxiliary Air Regulator as it seems to be frequently. I've put some descriptions below for clarification and borrowed the descriptions form the CIS primer on RarelyL8's website and Jims baesment workshop

911 CIS Primer - Index
911 CIS Primer - Components

Auxiliary Air Valve

Provides bypass air around the throttle when the engine is first started. When the intake manifold vacuum reaches 5 - 6 inches, the valve closes. The closing is independent of engine heat. Part number 0 280 160 400 pulls in at ~8" vacuum. This regulator supplies air to the intake manifold during warm and hot starts when the auxiliary air regulator is closed. This was added in '76.

Failure mode: When not closing properly, can cause high idle speed , or idle speed to oscillate.



My valve has some dents in it and based on the description above was thinking this could be my problem. I do recall seeing where this valve could be compressed in a vice to fix some issue, maybe that is why mine had the dents in it? You can see where it is a bit smashed on the lower left side in the picture.



Is there a way to test it on the car? Could remove it and rig up a large hose adapter for my Mityvac to see what it does, but was hoping for a simpler test.


Auxiliary Air Regulator

Provides extra starting air (bypassing air around the throttle body) with a cold engine. The valve slowly closes with heat, provided by an internal resistor, and to a lesser degree, from heat from the engine itself. For part number 0 280 140 200 (from a 1976), the cold resistance of the heater is about 17 ohms. This device was added in '76.

Failure mode: May remain partially open in cold ambient conditions due to open resistance element. Idle speed stays high after warm up. If it stays closed due to mechanical malfunction, the car may be hard to start.

[timmy2]

Decel valve may be acting up or has a vacuum leak on the lines to and from it.

[gregoryp]

Originally Posted by NYNick

For the hot start problem, I'd replace the fuel accumulator. Easy peasy.

The previous owner said he did that at some point, so I was looking elsewhere. My understanding of the fuel accumulator was that it was a spring loaded diaphragm, that basically held fuel pressure on the system so that on a hot restart the system didn't have to build pressure. Would a failed accumulator show up on my pressure test above? It held pressure for 35 minutes and looked to be in specification.

[gregoryp]

Originally Posted by timmy2

Decel valve may be acting up or has a vacuum leak on the lines to and from it.

I think I saw a way to test that by pulling the vacuum line to it and plugging the removed line to see if the problem goes away. Any experience with that test method?

I ran out to the garage and put the Mityvac on it and it holds vacuum, so it seems the diaphragm is good.

Picture of my decel valve for reference. I did do a temporary plug on the fitting below it as the nipple for the cruise control got broken off.

[gregoryp]

[boyt911sc]

Test if your decel valve could hold vacuum. If it does, how much it could hold. Post the value in inch Hg. BTW, measure the control pressure versus time. For example:
@ Zero seconds (start) and the WUR elec. plug is not connected. Plug the elec. connector on the WUR and start the count.
@ 30 sec.
@ 60 sec.
@ 120 sec.
@ 3 mins.
@ etc. no more than 5 mins. after a cold start.

This would give you a better picture how the WUR reacts with heat. No need to run the engine. Just the FP with a fully charged battery. I can not understand how your control fuel pressures are all over the chart (?). Keep us posted.

Tony

[boyt911sc]

Originally Posted by NYNick

For the hot start problem, I'd replace the fuel accumulator. Easy peasy.

Nick,

Why would you replace the fuel accumulator? If it is tested defective, replace it. What if it is not the cause of the problem? Replace another CIS component till you get it right? I can not follow the logic of your recommendation. Could you elaborate on it? Thanks.

Tony

[Moozak]

I'll second the Decel valve testing idea. I had the same issue on my 77. My decel valve needed adjustment so that it wasn't opening at too low of pressure. Bring a couple pencils on your next drive and plug the decel valve when it is idling high. If the idle drops when plugged, then likely your decel valve is out of adjustment. It's pretty easy to remove and test with a vacuum tester too. Mine was around 15" HG, then set it too 22" and problem solved. The top is adjustable. Tony was the man who suggested this to me so I would follow his advice to the "T"

[NYNick]

Originally Posted by boyt911sc

Nick,

Why would you replace the fuel accumulator? If it is tested defective, replace it. What if it is not the cause of the problem? Replace another CIS component till you get it right? I can not follow the logic of your recommendation. Could you elaborate on it? Thanks.

Tony

No data per se Tony. Seems to be the easy fix on these SC's with the hot start problem. Worked on mine!

[SCadaddle]

My SC would randomly "high idle", then one day after working on the engine with it idling at a nice 900 rpm we closed the decklid and it immediately went to 1600 rpm or so. Turned out the random high idle was due to the A/C condenser in the rear decklid pushing down on the cruise control cable that was pulling the throttle arm. Backed way off on the cruise cable adjustment (as I don't use the functioning cruise anyway) and problem solved.

Just throwing that out there, sometimes it can be the really simple stuff.

[T77911S]

Originally Posted by NYNick

No data per se Tony. Seems to be the easy fix on these SC's with the hot start problem. Worked on mine!

you tests things bedfore throwing money at it and just replace things.
you got lucky.

[T77911S]

great work greg. its nice to see good info.

WCP with engine on looks low. think that should be around 3.5bar. it could be running rich unless somone has adjusted the mixture back down.

decel valve as others have mentioned.
get a hand vacuum pump. you can pull a vauum on it to test it. you can also check to see at what vacuum it opens and adjust it if needed. (compared to idle vacuum).
you can remove the DV if you dont want it. you can remove it to see if it fixes your problem.

verify where vac connection to WUR is connect at throttle body (TB). it needs to be on manifold vacuum. this is actually Teed with the DV.

a hand pump will also check the AAV. its a little tricky. you will have to cover one hole with your hand to get it to close. jsut make sure it will stay closed and hold the vacuum.

pull a vacuum on the brake booster hose. that can crack and leak air.

whats with that little screw in the pic above

[boyt911sc]

Gregory,

I went over your data and here are my observation:

System pressure @ 4.6 bar (66.7 psi)..........within the specification range value but on the low side. Adjust it higher and set it between 70 - 72 psi. I seem to prefer the upper value.

Cold control pressure @ 1.9 bar (27.6 psi) @ 90 F. I don't have the chart with me but this is low if my memory serves me right. This won't be a problem and I like to keep the CCP lower than the spec.

Warm control pressure @ 2.7 (39.2 psi)............ Was this just the FP running?
Warm control pressure @ 3.2 (46.4 psi).............similar to the system pressure, the warm control pressure is with in the spec. range but on the lowest end of the spectrum.

Residual fuel pressure is good based from your 2 data point. What is the Bosch ID number on your WUR? Is it a -045, -069, or a -089? What resistance value of the heater? BTW, excellent pictorial and illustrations. Keep us posted.

Tony

[gregoryp]

I tested the decel valve. With the engine running and my Mityvac connected to the top of the decel valve I get an increase in idle speed when the vacuum reaches 24-25 in Hg. When I bleed the vacuum from the valve the idle drops back down around 22-23 in Hg.

Here is a picture of the test setup and the vacuum reading where the DV opens.




To see if this was in the operating range of the engine vacuum I switched the Mityvac to the pressure setting,so it was then reading engine vacuum to the DV and connected the hose to the DV vacuum line.

With the engine at idle I was getting a reading of 18-20 in Hg of vacuum (DV would be closed per above readings) and if I revved the engine and let off the throttle the vacuum would initially drop to about 9 in Hg and spike up to about 28 in Hg when the throttle butterfly closed.

Here is a video showing the vacuum reading as the engine is revved:

https://youtu.be/jz_4gMmTrIA

This operation seems ok to me as the DV would be closed and not allow any air to bypass the throttle plate at idle and during acceleration, but at a sudden close of the throttle the vacuum would spike to 28 in Hg, which is over the 24 in Hg and the DV valve would open holding the idle up slightly.

This was on a cold engine, so might retest when the engine is hot. Will have to do that when the rain stops, everyone knows Porsches and rain don't mix, especially targa's!

[gregoryp]

I followed the CIS primer test procedure shown below. Again for reference purposes this was taken from the web pages listed above.




Then I compared my values to the chart below. I have the .045 part number on my WUR




Step 1. On a cold engine with fuel pump on, engine not running with WUR unplugged and FP valve closed I got 4.6 bar. Chart says 4.5 to 5.2, so I'm on the lower end.

Step 2. Same as step 1, but with the FP valve open I got 1.9 bar, chart says 1.8 bar at 10 deg. C, not sure how the ambient temp changes this reading.

Step 3. Same conditions as step 2, I plugged in the WUR electrical connector and watched the pressure slowly increase until it reached 2.7 bar. Tony to answer your question this was just with the FP running not engine per the test procedure.

Step 4. I looked at the pressure with the engine running and up to temp, with the FP valve open I got 3.2 bar. Then I shut off the engine and watched the pressure slowly drop down to 1.2 bar after about 35 min.

All this seems to match up with the the chart above fairly closely. The FP gauge is not calibrated, so not sure of the accuracy.

[gregoryp]

Here is a picture of the car just so everyone knows what I'm working on