911sc cold start needs throttle

[Mac-Porsche]

Okay, so apologies up front as I've asked a lot of questions about my CIS! (and I know this is a persistent subject on these threads) - I'll try to be brief!

My US 1981 911sc has rough cold start issues. If not driven for several hours, it cranks many times, then either immediately stalls after firing up, or putters into a VERY low idle (200-400rpm) and slowly works its way to 1400rpm (takes ~10-15 seconds), then goes into normal warm-up routine. Drives fine after that. Settles back to 950rpm idle when warm.

Warm starts are totally fine. Fires up immediately.

Pushing the throttle during cold start does help and doesn't cause it to backfire (as I've read a lot).

Seems like it's starved of either fuel or air (or both)...
Checked fuel pressures:
system - 4.3 bars (I know, its a little low...)
ccp - 1.4 bar
wcp - 3.3 bar
ccp -> wcp time ~2 minutes
checked AAR, it's open when cold, closed when warm
Advance set to 5° BTDC at warm idle (950rpm).
Have a afr meter on dash. Adjusted idle mixture to warm idle = ~13.7.
Low-throttle = ~14.3, WOT = ~12.5 - 13.0 (all values warm, O2 unplugged)

Haven't tried holding sensor plate open when starting. If I did this, and it helps, what exactly does that mean? I've read to try this in several posts, but not sure what that's diagnosing?
I installed 10:1, 3.1L P&C set last summer - has had rough cold start since. Does it maybe need a lower CCP on the WUR (due to higher compression)?

[gomezoneill]

Usually air leaks of some sort.

[fastricky]

My 73.5 does exactly these things! (Takes a few cranks to catch, usually with a backfire, then either immediately stalls after firing up, or putters into a VERY low idle (200-400rpm) and slowly works its way to 1400rpm (takes ~10-15 seconds), then goes into normal warm-up routine. Drives fine after that. Settles back to 950rpm idle when warm. I do find using a bit of foot throttle is better than using the throttle lever. I only use the throttle lever to let the car warm up at a higher rpm once it engages.

Does this indicate an air leak?

[gomezoneill]

Originally Posted by fastricky

My 73.5 does exactly these things! (Takes a few cranks to catch, usually with a backfire, then either immediately stalls after firing up, or putters into a VERY low idle (200-400rpm) and slowly works its way to 1400rpm (takes ~10-15 seconds), then goes into normal warm-up routine. Drives fine after that. Settles back to 950rpm idle when warm. I do find using a bit of foot throttle is better than using the throttle lever. I only use the throttle lever to let the car warm up at a higher rpm once it engages.

Does this indicate an air leak?

It could be, very common.

[Gianni at Pelican Parts]

Just a heads up that we have a bunch of of "How To" Tech Articles available for your car on the site, may help diagnose this issue and definitely will come in handy for future projects!

[Showdown]

I had a similar problem and I found a clogged WUR and a few minor air leaks around the injectors after doing a series of pressure tests and a smoke test. Cleaning the WUR led to almost instant startups when cold.

[Alan L]

Could also be a faulty CSV or circuit - ie non-functioning.
Alan

[T77911S]

check residual pressure too

[pmax]

Originally Posted by Mac-Porsche

Okay, so apologies up front as I've asked a lot of questions about my CIS! (and I know this is a persistent subject on these threads) - I'll try to be brief!

My US 1981 911sc has rough cold start issues. If not driven for several hours, it cranks many times, then either immediately stalls after firing up, or putters into a VERY low idle (200-400rpm) and slowly works its way to 1400rpm (takes ~10-15 seconds), then goes into normal warm-up routine. Drives fine after that. Settles back to 950rpm idle when warm.

Warm starts are totally fine. Fires up immediately.

Pushing the throttle during cold start does help and doesn't cause it to backfire (as I've read a lot).

Seems like it's starved of either fuel or air (or both)...
Checked fuel pressures:
system - 4.3 bars (I know, its a little low...)
ccp - 1.4 bar
wcp - 3.3 bar
ccp -> wcp time ~2 minutes
checked AAR, it's open when cold, closed when warm
Advance set to 5° BTDC at warm idle (950rpm).
Have a afr meter on dash. Adjusted idle mixture to warm idle = ~13.7.
Low-throttle = ~14.3, WOT = ~12.5 - 13.0 (all values warm, O2 unplugged)

Haven't tried holding sensor plate open when starting. If I did this, and it helps, what exactly does that mean? I've read to try this in several posts, but not sure what that's diagnosing?
I installed 10:1, 3.1L P&C set last summer - has had rough cold start since. Does it maybe need a lower CCP on the WUR (due to higher compression)?

What's the temperature up there when you measured the CCP ?

[Mac-Porsche]

The temperature when taking the measurements was ~75° (car stays in garage).

The CSV is certainly one of the suspects. I jumped the starter over the weekend (to see if maybe the ignition solenoid was going bad). In the process of wiring up the remote starter, I disconnected, and forgot to reattach the wire going to the CSV. The car started like it has been i.e. several cranks to low idle. In other words, no apparent difference with CSV unplugged. Was going to try opening the throttle and peering down with a mirror while cranking to see if it's working... Honestly though, I'm doubtful this is the issue... but worth checking.

Would a clogged WUR still give fuel pressures in spec?

I noticed that when I remove the oil cap, you 'hear' the engine drop in revs, but the tach shows barely a drop (maybe ~50-100 rpm drop). So I'm starting to lean towards a vacuum leak.

Residual pressure checks fine i.e. 10min = 1.5bar, 30min = 1.1bar... A bit on the low side, but still in spec based on Bentley manual. Plus, never had a warm start issue.

[boyt911sc]

Make sure your control and system pressures are with-in spec., good ignition, and no significant sources of air leaks and the motor will start and run. You need to test and confirm these basic requirements. Without physically testing the CSV, it is difficult to tell whether it is working or not. Same thing with air leaks. Perform a smoke test.

There are too many variables involve in running a CIS motor and the more you reduce the number of variables, troubleshooting becomes much simpler. Right now, I am preparing a start up for a newly rebuild CIS motor. If I did everything right, I expect the motor to start and run at the first attempt or maybe after a second try.

Tony

[pmax]

Originally Posted by Mac-Porsche

The temperature when taking the measurements was ~75° (car stays in garage).

The CSV is certainly one of the suspects. I jumped the starter over the weekend (to see if maybe the ignition solenoid was going bad). In the process of wiring up the remote starter, I disconnected, and forgot to reattach the wire going to the CSV. The car started like it has been i.e. several cranks to low idle. In other words, no apparent difference with CSV unplugged. Was going to try opening the throttle and peering down with a mirror while cranking to see if it's working... Honestly though, I'm doubtful this is the issue... but worth checking.

Would a clogged WUR still give fuel pressures in spec?

I noticed that when I remove the oil cap, you 'hear' the engine drop in revs, but the tach shows barely a drop (maybe ~50-100 rpm drop). So I'm starting to lean towards a vacuum leak.

Residual pressure checks fine i.e. 10min = 1.5bar, 30min = 1.1bar... A bit on the low side, but still in spec based on Bentley manual. Plus, never had a warm start issue.

Is the WUR 090 ?

If so, your CCP=1.4@75F is low per spec ... looks like it should be around 2.3 according to this chart


from http://forums.pelicanparts.com/porsche-911-technical-forum/893455-1981-sc-cis-low-idle-wrong-wur.html#post8901099

low CCP means it runs rich when cold which is consistent with your observation that adding "throttle" (adding air) improves the cold start.

If you are game, modify the WUR and make the CCP adjustable.

[Mac-Porsche]

It is a 090 WUR. The lower CCP/WCP was actually by design. I rebuilt the engine last summer with 97mm (3.1L), 10:1 CR P&C's. This hard start and sluggish cold acceleration has existed since the rebuild. I originally thought it maybe needed more fuel due to the higher compression, so I had the WUR rebuilt to run at lower CCP/WCP's. Hence the lower values.

During that rebuild, I replaced all vacuum lines, injector sleeves, o-rings (inner and outers), intake boot, and the airbox was replaced with new one (old one had crack on underside). Installed a pop off valve, double-triple checked for air leaks from the glue holding it in.

Given this problem existed since the rebuild, I still thought it could be a vacuum leak. So I ran all the leak tests I could in my garage last summer - propane, reverse vacuum and soapy water, cigar smoke, etc. Couldn't locate anything. Brought the car to a Porsche-specific mechanic who ran a smoke test, also couldn't find any leaks.

Still not ruling out leaks though, as it does run lean when cold (in 15's per afr meter on dash).

Couple days ago I discovered something possibly worth mentioning... When I start the car cold, it cranks and cranks, put-puts slowly to life. As soon as the car is running stable (i.e. ~10 seconds after starting), I immediately turn it off. No time for WUR to lean out or engine to warm up. Then when I restart the car, it fires up immediately.

It's almost as if the fuel lines need to be primed first. Don't think it's the CSV, because 5-10 seconds of running shouldn't warm up engine enough to disable CSV in the second start (or am I wrong about that?). Checked residual pressures few days ago, they're within spec.

[Joe Bob]

CIS diagnostics needs gauges. The fuel pump has a check valve in the outlet neck. Cheap part and a common issue.

[boyt911sc]

Originally Posted by Mac-Porsche

It is a 090 WUR. The lower CCP/WCP was actually by design. I rebuilt the engine last summer with 97mm (3.1L), 10:1 CR P&C's. This hard start and sluggish cold acceleration has existed since the rebuild. I originally thought it maybe needed more fuel due to the higher compression, so I had the WUR rebuilt to run at lower CCP/WCP's. Hence the lower values.

During that rebuild, I replaced all vacuum lines, injector sleeves, o-rings (inner and outers), intake boot, and the airbox was replaced with new one (old one had crack on underside). Installed a pop off valve, double-triple checked for air leaks from the glue holding it in.

Given this problem existed since the rebuild, I still thought it could be a vacuum leak. So I ran all the leak tests I could in my garage last summer - propane, reverse vacuum and soapy water, cigar smoke, etc. Couldn't locate anything. Brought the car to a Porsche-specific mechanic who ran a smoke test, also couldn't find any leaks.

Still not ruling out leaks though, as it does run lean when cold (in 15's per afr meter on dash).

Couple days ago I discovered something possibly worth mentioning... When I start the car cold, it cranks and cranks, put-puts slowly to life. As soon as the car is running stable (i.e. ~10 seconds after starting), I immediately turn it off. No time for WUR to lean out or engine to warm up. Then when I restart the car, it fires up immediately.

It's almost as if the fuel lines need to be primed first. Don't think it's the CSV, because 5-10 seconds of running shouldn't warm up engine enough to disable CSV in the second start (or am I wrong about that?). Checked residual pressures few days ago, they're within spec.

Mac,

I often noticed people not properly doing the pressure tests to locate unmetered air sources. Would you share with us how you did the smoke tests for your motor? How did you isolate the air box from atmospheric condition. Thanks.

Tony

[Mac-Porsche]

Originally Posted by Joe Bob

CIS diagnostics needs gauges. The fuel pump has a check valve in the outlet neck. Cheap part and a common issue.

Have gauges. I posted the pressures. CCP=1.4, WCP=3.3, System=4.3, resid-10min=1.5, resid-30min=1.1. Checked fuel pressures so many times I could do it blindfolded behind my back. Considered check valve, which is partly why I replaced fuel pump, no luck.

As for leak detect attempts:

Plugged the exhaust pipe with tennis ball and duct tape. Disconnected the hoses behind the intake boot and plugged them (made it easier to move the intake boot). I took the intake boot off of the throttle body.

Connected a hose to throttle body, ensured it was air tight connection. First blew cigar smoke into hose (checking for big leaks), no leaks observed. [I don't smoke, so this was quite a nauseating exercise] Then sprayed many components with soapy water and hooked up compressor to hose. Still no observable leaks.

The mechanic went a much more simple route. He just connected the smoke machine to the air intake (i.e. into the air filter area). When no leaks were observed, he then connected to inside the pop off valve. Nothing observed.

I watched some videos of people using smoke machines, they almost always go through the throttle body... So don't 'really' trust the mechanic's analysis. Anyone know of good smoke machines at a decent price? I've seen ones on amazon, but I'm really suspect of the quality. Snap-on has decent ones (what the mechanic used), but they are pricey!

[Showdown]

I bought a cheap-o smoke machine from eBay and it worked wonderfully.

I know I could have made one for less but my time is worth more than $60

https://www.ebay.com/itm/EVAP-Smoke-Machine-Diagnostic-Emissions-Evaporative-Leak-Detector-Tester-/282404917540?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0

[pmax]

Originally Posted by Mac-Porsche

The CSV is certainly one of the suspects. I jumped the starter over the weekend (to see if maybe the ignition solenoid was going bad). In the process of wiring up the remote starter, I disconnected, and forgot to reattach the wire going to the CSV. The car started like it has been i.e. several cranks to low idle. In other words, no apparent difference with CSV unplugged. Was going to try opening the throttle and peering down with a mirror while cranking to see if it's working... Honestly though, I'm doubtful this is the issue... but worth checking.

Hmmm, no change in behavior with the CSV unplugged certainly points to the CSV as being out of the picture completely. There's a thermotime switch attached to the driver side chain cover in the CSV firing circuit which energizes the CSV on a cold start, basically a heat controlled switch. Check if it's in the right initial electrical state during a cold start (I can't remember offhand which is which whether open or closed fires or not, do a search) .

[lsf911]

I had the same cold start problem that you described. Tested everything, had my WUR rebuilt, replaced my check valve, checked for air leaks, pressure tests, etc, etc.

Not saying my fix will be your fix, because as Tony said these CIS systems do have a lot of variables, but mine ended up being the thermotime switch. My CSV was never providing that initial blast of gas to start the cold engine. I had to keep cranking and provide throttle for it to putter to life, only on initial startup. Never backfired either but was always a concern of mine.

[mike sampsel]

Originally Posted by pmax

Hmmm, no Check if it's in the right initial electrical state during a cold start (I can't remember offhand which is which whether open or closed fires or not, do a search) .

Closed initially to allow the CSV to ground. Later the TTS open circuits (on temp, time from current temp) to prevent the CSV from grounding. At least that is the TTS on my SC, first 3 sec or so should allow the CSV to reach ground then energize, then it should open up and prevent the CSV from grounding.

Think mine does not open though ... still checking it
Mine starts up right away up to six days or so after the last start, no throttle. After two weeks, it takes much cranking to start.