911sc warm running issue
 

New issue. First experienced it after a 70 mile drive home. After a few mins tried to restart the car in the garage but no go. Doesn’t take long to stall and is 100% recurrent.

Fires up cold to “high” idle (2000) does some hunting, settles around 1200, and as it warms RPMs begin to slowly drop. As it gets around 1000 it begins to audibly stumble and progressively gets worse over the next minute or so until it dies. It will not restart again until cold, and the exact process begins again. It is not drivable.

Data thus far:

1978 US 911sc

WUR 045 - 26.7 ohm (26)
.5 bar with no spring in WUR

Residual Pressure
10min 1.6 bar (1.3 bar target)
20min 1.4
30min 1.3 bar (1.1 bar target)

System Pressure (4.5-5.2 bar target) 4.8 bar, 70 psi

Control Pressure:
COLD @ 80F/27C - (2.25-2.7 bar target) 2.5
WARM (target 3.2-3.6)
2.9 WUR 12v plugged in, engine not running
3.5 engine warm running w/ intake vacuum then drops to 3.0 when stalled.

When I open the oil cap, the RPMs drop.

New parts due to general maintenance:
Fuel pump
Fuel pump relay
Fuel accumulator
Fuel filter

Suggestions on where to look next?

Based on your post you have tested the CIS fuel delivery system. What have you done to verify that you do not have any vacuum air leaks. False air getting into the mixture system will add to some of the issues you are seeing. Read through CIS for Dummies and individually test the different auxiliary parts of the CIS to make sure they are functioning properly. When the engine is warmed up and you remove the oil cap does the engine stall? If it does the you have false air getting into fuel mixture from air leaks in the vacuum lines. You should perform a smoke test of the CIS to see where you may have vacuum air leaks.

Great question. Short answer, not enough.

Longer answer, I’ve only done what I mentioned above and that was to open the oil cap and the RPMs would drop further (500ish?) on an already stumbling engine, which would bring it closer to death if not kill it. But I’m not sure why it’s stumbling to begin with. It definitely could be unmetered air as the car has never had any rubber bits replaced. However my test is a bit iffy as I cannot really get the engine warmed completely up, so I have to do it AS it’s warming up. Too soon and the cold start system won’t allow it to have an effect, and too late and it just dies. I agree that a smoke test is in order. Cuban or Swisher Sweet?

Please see the attached thread I posted several years ago when I had a blown air box. It has info on using a smoke machine. I had the engine out of the car but you can do the testing with th engine in place. I would recommend a partial drop to make it easier to get access to the peanut cover.

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html?highlight=Smoker

The following YouTube will show you how to build a cheap reliable smoke device.

https://www.youtube.com/watch?v=tixozqgSwN0

Do a search for CIS for Dummies to find all the tech info on testing the CIS.

Wow, that was a lot of work. A couple of questions. I understand that if you have an unmetered air leak that you will get a high idle, which I don’t have. And also that the RPMs would not drop when the oil cap was removed, which mine do.

I’ll do some more research on smoke testing in place. Thanks for the link!

CIS Troubleshooting…………
 

Observe and record your cold control pressures versus engine RPM during the time it is running until it stopped running. Find someone to help you read the gauge and tach at a given moment. These data will give you a better picture what is going on with your CIS. Keep us posted. Thanks.

Tony

In my case, I might be wrong but before I repaired all of my vacuum leaks, the engine would stall when I removed the oil cap. My view is that the oil breather line from the oil tank to the air box was impacting the fuel/air ratio causing the rpm’s to drop to a point where the engine would stall. Normally, with a warm engine at idle and removing the oil cap will lower the idle from let’s say 950 rpm’s to 600~700 rpm’s. The engine will not stall it will just run a little ruff at a lower rpm. After fixing my air box and reinstalling my engine I could not get the rpm’s lower than than ~ 1200 rpm’s with the idle screw fully clockwise. That’s when I did the smoker and found several vacuum air leaks. Once the leaks were fixed I was able to adjust the idle to 950 rpm’s with at least one clockwise turn left on the idle adjust screw. Naturally each of the individual components were self tested and fuel system was checked for all the correct pressures and the mixture was adjusted using a dwell meter for a lambda CIS system with a functioning oxy sensor. Some of my testing would not apply to a 78SC.

Hi Tony. It starts at 2.9/3.0 with just the fuel pump running before engaging the starter….and when started it jumps to 3.5 and stays there solidly (even though the motor is hunting and RPMs are changing significantly) until the final revolutions of the dying motor. Once the motor dies it reverts to 3.0 and holds since the FP is still running.

Ambient temp 88.5/31.4c. Target 2.4-2.9 cold, 3.2-3.6 warm

I should add that the initial 2.9 reading doesn’t involve vacuum since the motor isn’t running, and the 3.5 involves engine manifold vacuum.

Twaldron,

Do the test with a stone cold motor. Start the car and observe the engine RPM and your control fuel pressure. How long before it comes to stop? Since we are not there, we could only rely on your feedback. Thanks.

Tony

This was with a cold motor. The ambient temp is because I’m in Texas. :) The RPM fluctuations make no difference in the pressure reading of 3.5. It takes about 2 mins to start stumbling and 3 mins to completely die and the 3.5 bar reading doesn’t move on the dial. A video I took of the problem is 3.5 mins including a mis-start

Do you still have spark when it stops running? Simple, free and easy to check. Could possible be a bad coil. I wouldn’t just go swapping it out, but it’s certainly worth checking for spark to see if your problem is a failing coil. A failing coil often works when cold but fails when warm. If you’re south in Austin anytime soon I have a spare you could try.

It is still firing as it’s dying but not cleanly and possibly not on some cylinders. When those coils fail with heat do they fail 100% or cause misfires?

Could you attach the video? I would like to see the CCP to WCP transition profile from start to end. Thanks.

Tony

I’m not sure I can post anything other than still pics here. Essentially there is no slow transition. The needle bangs from 3.0 to 3.5 when started and doesn’t deviate. Then bangs back to 3.0 when it dies. It does not rise slowly during the 2 minutes. It’s instant.

I can shoot a lower rez video tomorrow AM and send a link here.

It can be either. I’ve had them fail both ways. Typically when Ive had them failing they got worse as more heat was introduced. Maybe start it up when it’s cold and see how strong the spark is. Then compare it as it starts to struggle. Easy enough to check, I’ve heard of some people cooling them off in the fridge quickly after misfires to see if it temporarily helps. Not sure if this is a legitimate testing method or not as I’ve never tried that. I just verify spark cold and warm as it starts to misfire.

I haven’t reshot the video yet since I tried this experiment and I’ll wait until AM to start cold again…

Ambient 87f

Cold (unplugged WUR) 2.7
Warm (plugged WUR no engine start, hence no vacuum) One minute to settle on 3.0 (gradual)
Warm (plugged WUR, engine started) Bangs instantly straight to 3.4

I’m not sure why it is a gradual change unstarted, but an instant change when the engine is started. It should rise slowly with engine warming? Or is that a function of vacuum?

Please advise if you care to.


http://forums.pelicanparts.com/uploa...1685744443.jpg
http://forums.pelicanparts.com/uploa...1685744443.jpg
http://forums.pelicanparts.com/uploa...1685744443.jpg

Warm up regulator……….
 

TWaldron,

Once you start the FP, with the WUR plugged-in the WUR starts to heat up from CCP to WCP. Therefore, it would take hours to get back the WUR to CCP status unless you chilled it. Secondly, check your TTV (Thermotime valve). It looks like it is OPEN @ START. Test and confirm.

Tony

Thank you Tony. I will look into the procedure for testing the Thermotime SW.

Also, I understand the need to cool down the WUR between tests, but in the test above the WUR goes UP from the slow (unstarted) warmup. It jumps from the unstarted “warmed up” status of 3.0 to a started bar of 3.5 in a millisecond. I understood that the electric only warmup and the started warmup should achieve the same bar level.

TTS is closed to start and opens when heated, closes again when cooled.

While the car is cranking, and the yellow wire is disconnected, the yellow wire reads a mere 10v during cranking.