Revisiting an issue

[LIRS6]

Issue: At cold start-up, engine will idle at only about 1000 RPM's - may wander higher momentarily, but basically at 1000. As engine warms up, when engaging clutch when say coming to a stop, RPM's will drop to about 250-300 - then recovers to about 850 rpm. Once engine is warm, all is good - RPM's stay at about 900, with no near-stall issues.

Warm start - no issues.

Here's a video of typical initial cold start-up:

https://youtu.be/3QSOrRXknik

The car :

'79 SC (USA)

Original 3.0 with WUR ending 045

Temp : 74 degf

WUR ohms : 26.3

Test results :

FP only - engine not running

System pressure: 4.9 bar

Cold pressure : 2.1 bar

WUR Power connected

30 secs : 2.2 bar / 31.9 psi
60 secs : 2.45 bar / 35.5 psi
90 secs : 2.7 bar / 39.2 psi
120 secs : 2.8 bar / 40.6 psi
150 secs : 2.95 bar / 42.9 psi
180 secs : 3.03 bar / 43.9 psi
210 secs : 3.1 bar / 44.97 psi
240 secs : 3.1 bar / 44.97 psi
270 secs : 3.13 bar / 45.5 psi
300 secs : 3.18 bar / 46.12 psi


Residual pressure :

1 min : 2.3 bar
5 mins: 1.8 bar
15 mins : 1.55 bar
30 mins : 1.4 bar
60 mins : 1.2 bar

- WUR holds vacuum - confirmed
- Vacuum at line going into the WUR - confirmed

So the cold start idle should initially be at around 1800 RPM, right? And then back down to normal idle as the engine warms up.

What are the components that control that? - WUR / AAV / AAR ?

Where do I go from here?

Thanks,
Jason

[T77911S]

the AAV is ONLY used when cranking. it is like putting your foot on the gas. once the engine starts, vacuum closes it and its job is done,

the AAR is only used when COLD cranking and RUNNING. it is open when cold and slowly closes after a few minutes of running. it is heated by the same power that heats the WUR. once closed its job is done.

the WUR has several functions. it lowers the CP to richen the mixture when cold for starting AND cold running. as you can it slowly raises the CP as the engine warms to lean out the mixture.
its other function is acceleration and WOT. the vacuum control is used here to control this function. it drops the CP about .8bar to richen the mixture.
you could say there is any function related to the vacuum control.
if you still have the thermo time valve (TTV) it blocks the vac to the WUR when cold for added enrichment of the mixture for cold starting and running. this in only active for about 20 seconds.

you don't really set the cold idle. some will take the AAR apart and "adjust" it. I don't like this as I think it is covering up another issue.

the proper way to go about setting up CIS is to;
do a tune up. plugs, cap and rotor
set timing.
adjust idle, warm
verify no air leaks
check CP's
adjust mixture
adjust idle


when the engine drops down in RPM like yours is doing that is a sign of being too rich.

if it ws running fine and started doing this you need to figure out why it is now rich, if that is the case

[LIRS6]

Top end re-build last summer including plugs/cap/rotor
No air leaks
Idle adjusted warm
Cold Pressure is where it should be

Is there no smoking gun when start-up idle fails to achieve the normal ~1800 RPM?

[SkiVT]

Have you tried starting the car with your pressure gauges attached and video recording the pressures? Why? If the TTV is working correctly, your pressure for the first 20 seconds or so should stay low, or near the CCP without vacuum applied. As stated above, this keeps the mixture rich as things warm up. Once the TTV opens (at around 20 seconds) and lets vacuum take over, the pressure readings should match the readings w/vacuum applied that you took over the 3 minute window. You have confirmed vacuum happens but the timing of when is also important. For example, if the system expects to have 2.1 CCP and for some reason you have immediate vacuum, which typically raises pressure by .6, the system will be working with 2.7 pressure instead of 2.1 which I think would mean it's lean too early. Your video shows rpm recovery suspiciously around the time the TTV should be letting vacuum arrive. I know you tried correctly connecting the vacuum lines and didn't like the results before. My opinion is that doesn't mean it's not pointing out you have another problem.

[SkiVT]

It should look like thus video

https://m.youtube.com/watch?v=x3HXhqlwFMM#

[LIRS6]

Originally Posted by SkiVT

It should look like thus video

https://m.youtube.com/watch?v=x3HXhqlwFMM#

Thx SkiVT, will try that one evening this week and report back.

[SkiVT]

My working theory on your situation given what you have ruled out: you are gong to see an immediate starting pressure of 2.7 instead of 2.1 Only way the car runs as well as it does at that pressure is someone enriched the mixture so it started better, I think. This then causes you to be richer across the warm up, etc. As T77911S states, if the RPM stumbles at the stop sign are from rich condition this could be why. The video will confirm the 2.7 vs 2.1 starting pressures. Rich vs lean you can then confirm by pulling down and pushing up the sensor plate during that first 30 - 40 seconds at startup when it currently stumbles below 900rpm. If pulling down slightly improves the idle, then you know it's too rich. If pushing up a little helps, you are currently too lean and I am incorrect. If it is too rich, you need to do two things: 1) vacuum line from WUR to TTV has to be plugged into the outer plug with center plug going to the vacuum source. 2) lean out the AFR a bit (best to have a CO meter for this). My opinion is your goal should be to keep the startup idle above 950 through warmup. Whether it idles at exactly 1400, 1600, or 1800 as it warms up is less critical.

[T77911S]

Originally Posted by LIRS6

Top end re-build last summer including plugs/cap/rotor
No air leaks
Idle adjusted warm
Cold Pressure is where it should be

Is there no smoking gun when start-up idle fails to achieve the normal ~1800 RPM?

smoking gun would be mixture setting. but its not always a good idea to jump in and adjust it. you could be covering up another issue, especially if it ran correctly after the build and then started doing this or it has done it from the rebuild.

if mixture was never set, personally I would adjust it leaner to see if it fixes it, but then I can get it back to where it was if that is not the issue.
if it ran fine after the rebuild I would check fuse 18, then adjust mixture and try to figure out what made the mixture change to the point that I had to adjust it.

I would also check intake bolts as they can come lose after a rebuild.

you could get a gas analyzer and verify AFR setting.

if you have the TTV I would also verify how long it takes to open when cold

[boyt911sc]

Jason,

What are your control fuel pressures at:
a). Initial CCP with and without vacuum?
b). WCP with and without vacuum after 5 mins.?

No need to run the engine and use a hand held vac pump @ 15~16 inch Hg. Your WCP without vacuum is slightly off spec. Like to see the WCP with vacuum applied looks like. Thanks.

Tony

[LIRS6]

Originally Posted by T77911S

smoking gun would be mixture setting. but its not always a good idea to jump in and adjust it. you could be covering up another issue, especially if it ran correctly after the build and then started doing this or it has done it from the rebuild.
Never ran correctly at start-up IMO - idle was always low. Probably should have addressed with engine re-builder at the time, but near-stall was not an issue initially, and frankly I was anxious to drive the car after 8 mos in shop!

if mixture was never set, personally I would adjust it leaner to see if it fixes it, but then I can get it back to where it was if that is not the issue.
if it ran fine after the rebuild I would check fuse 18, then adjust mixture and try to figure out what made the mixture change to the point that I had to adjust it.
Re-builder initially had issues with idle, was making a lot of adjustments, then found vac leak (air box - was replaced). Unsure of how it was ultimately set up.

I would also check intake bolts as they can come lose after a rebuild.
Will do

you could get a gas analyzer and verify AFR setting. Noted

if you have the TTV I would also verify how long it takes to open when cold I'll check

Comment : This may sound dumb but exhaust certainly smells rich, although cat was deleted so it could be that.

I will check starting pressure this evening as suggested by SkiVT, and as well the suggestions from Tony.

Thanks to all,
Jason

[LIRS6]

Grab your popcorn - SkiVT; you hit it right, just off by .75 Bar or so.

I haven't done any further testing, thought I'd await some feedback to this video :

Cold start up pressure;

https://youtu.be/Zx9Kd_0VMZs

Thanks,
Jason

[pmax]

Originally Posted by LIRS6

Grab your popcorn - SkiVT; you hit it right, just off by .75 Bar or so.

I haven't done any further testing, thought I'd await some feedback to this video :

Cold start up pressure;

https://youtu.be/Zx9Kd_0VMZs

Thanks,
Jason

Record the ambient temperature when you do this test.

Also, given this is a revisit as you say, it will be helpful to summarize what was already tried and done earlier.

[SkiVT]

Can you confirm for us how the vacuum lines are connected from wur to TTV? I think your July thread confirmed the wur pressures, without the engine running, were is spec both with and without vacuum applied. If the lines are not connected correctly, vacuum isnt being held off thus the too high CCP at startup. It seems the solution is correctly routing the vacuum lines so vacuum is held off and CCP at startup is lower/correct. Then tuning the afr so the engine is happy. I too am interested to see what the actual solution is.

[LIRS6]

Originally Posted by pmax

Record the ambient temperature when you do this test.

82F

Also, given this is a revisit as you say, it will be helpful to summarize what was already tried and done earlier.

Post #1 was a summary .

Jason

[pmax]

I agree with SkiVT's observation that you are leaning out too early in general and randomly in your first video.

I would test the Thermo-valve whether it is operating consistently, closed and opening etc as the heating element is plugged in. Eliminate that little CIS elf as a source of the flakiness and go from there. Looks like that part is still available new for less than $100, reasonable by Porsche tax standards ! Your WCP is also a tad high ... that would be next thing to look at pointing to the WUR.

Good luck !

[LIRS6]

Originally Posted by pmax

I agree with SkiVT's observation that you are leaning out too early in general and randomly in your first video.

I would test the Thermo-valve whether it is operating consistently, closed and opening etc as the heating element is plugged in.

How do I go about testing it and what should I expect it to do if it is working properly?

Jason

[SkiVT]

Pull the TTV and cool it down in fridge or freezer, just needs to be 60 instead of 90. Use your vacuum pump to test each port. The outer port doesn't hold vacuum, the center port does. Assuming the center port holds vacuum, you then warm the TTV up and it should stop holding vacuum at some point. Warming it using the electricals on the TTV is best as it mimics what happens while installed.

Alternatively, plug the vacuum hoses in the way the picture from your July thread shows, start the engine with your pressure gauge installed, and watch for the CCP to stay low at your 2.1 for 30 or so seconds, and then watch it jump like in my video when the TTV vacuum opens from heating. I can't recall if you said your engine would actually run with the vacuum hoses connected as in the diagram.

Additional testing details in this link:

TTV test results.......

[LIRS6]

Originally Posted by SkiVT

I can't recall if you said your engine would actually run with the vacuum hoses connected as in the diagram.

When I had previously reversed the hoses (to what the diagram showed), the engine would start but lose revs quickly and die. Time permitting I'll try to address your suggestions tonight.

Many thanks

Jason

[pmax]

^^^Sounds like a plan. Keep us posted.


It would be nice if someone tested one of the new TTV's to be certain of the correct behavior.

[SkiVT]

An FYI: the video I posted early in this thread has a TTV I bought from our host earlier this summer. I also tested that TTV off the car just to confirm which port holds vacuum. I did not test extensivily for opening time at various tempeatures, etc. So not perfect information but the open time around 25 seconds in the video is very easy to see and it was approx 65 degrees ambient out at that time.