Out of ideas - bad cold start

[Mac-Porsche]

Sorry for this topic, I know it's been discussed a million times on this forum (I know this because I've read every past thread 10 times over).

1981 911sc, stock CIS... Currently have a bad cold start. When cold, the car will fire up then immediately die. Only way to keep it running is to manually crank the idle setting way up (turn the idle adjustment screw way out). At which point, it will start up, but run REALLY rough for 1-2 minutes until settling into its respective idle speed (which is initially high until I readjust the idle screw back down).

This really started after I did some work... I noticed the wire connecting to the ignition coil was brittle and cracked (showing exposed wire), so I replaced the entire wiring harness from one I purchased from PartsKlassic - the harness worked great, perfect fit/connections. Installed new fuel lines from HEL (got rid of the rigid steel lines and installed the 'older' style braided lines), and replaced all the fuel injectors (previous ones were pretty old and one was bent).

The car has always run rich, so I decided to completely reset the fuel mixture by turning the 3mm screw in the fuel distributor all the way out (CCW) and slowly richening until fuel just started to come out the fuel line holes - per videos/recommendations seen online. Confirmed correct setting by having injectors in jars. Also, once running, I set the AFR to ~13.7 and confirmed the fuel mixture with a CO detector that showed 2% CO (all without O2 sensor hooked up).

** Everything above and below is without the O2 sensor hooked up and the AFR set to 13.7 / CO set to 2%)**

To clarify, there's no 'natural' increased rpm timeframe for 1-2 minutes, then settling into warm idle (like a properly running CIS). It just runs really rough at low rpm's until it gets to its warm engine rpm. Which screams WUR... However...

I've tried just about EVERYTHING....

1. WUR Diagnostics: System Pressure = 4.5 bars, CCP = 1.8 bars, WCP = 3.3 bars, time to get from CCP to WCP is 2-4 minutes (seems normal). Screen looks clear of debris.

2. Fuel Injectors - All brand new Bosch injectors. Tested spray patterns and volume, looks good.

3. Vacuum leaks - Tested with smoke machine three different times. Connected to line that connects to throttle body, putting rubber glove over throttle body. Increased pressure of smoke machine until it popped the rubber glove. Tested at higher pressure by bleeding pressure with pop-off valve every so often.... Could not find any leak whatsoever... Furthermore, RPMs do drop (200-300) when removing oil cap (for what it's worth - I know that's not a conclusive test of the vacuum system).

4. Cold start valve - Checked that it's getting good voltage - checked good. Don't think this is the problem, as the engine will fire up. It's the idling after initial start that's the problem.

5. Spark plugs - Replaced them all. Used the iridium BPR7EIX, tried going back to the previous WR5's. No difference between them.

6. Leak Down / Compression Test - All cylinders ~99% and within 5-10psi of each other (I rebuilt the engine a few years back, so makes sense they're in very good range).

7. Timing - Double checked timing of distributor with timing light (without vacuum line attached). Timing is good (set at 5° BTDC at 950 rpm idle). While I know it's better to check at higher rpm, I figured this was good because it's the cold start that's my issue.

8. Oxygen sensor relay - Replaced with new, no difference.

9. Ignition cables - Systematically pulled each cable from distributor while running, car got much rougher with each one pulled (I know it's not a good test, but checked anyways).

10. CDI box - Bought the Classic Retrofit years ago when I rebuilt the engine. Swapped between the two when diagnosing this issue, no difference.

11. Oxygen sensor plate - Checked and it looks exactly as it should from the Bosch Fuel Injection Handbook / Porsche manuals. Plate moves freely when car has been off for a while.

Is there 'anything' that I'm still missing?!
Any help would be appreciated...
Mac

[AndrewCologne]

a) Did you check whether the auxiliary air valve is working properly?
b) Did you check the ECU’s duty cycle during cold start to see if it provides the correct 65%?
Also check here: https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/

[Mac-Porsche]

Thanks for link, very helpful!

A) Yes, I forgot to add that in my list. If you’re referring to the half-moon shape opening that closes when warm. I confirmed that it’s open when cold and closed after getting warm. I even liberally sprayed carb cleaner in there to ensure it’s not sticking.

B) Could this be a culprit even without the O2 sensor plugged in? If so, I have not checked this - something I need to learn more about. I’ve actually suspected the frequency valve (I assume they’re related), because even when warm, the engine doesn’t idle as good as it used to (will misfire here and there).

[AndrewCologne]

Originally Posted by Mac-Porsche

Thanks for link, very helpful!

A) Yes, I forgot to add that in my list. If you’re referring to the half-moon shape opening that closes when warm. I confirmed that it’s open when cold and closed after getting warm. I even liberally sprayed carb cleaner in there to ensure it’s not sticking.

Yes, check that one as well (Auxiliary Air Regulator), but I meant the one at the back of the engine on the right side (Auxiliary Air Valve).

B) Could this be a culprit even without the O2 sensor plugged in? If so, I have not checked this - something I need to learn more about.

Yes, even without a sensor connected, the duty cycle should be 65% with a cold engine and 50% with a warm engine.

I’ve actually suspected the frequency valve (I assume they’re related), because even when warm, the engine doesn’t idle as good as it used to (will misfire here and there).

Check the OX relay — it seems your engine is running too lean, which could be caused by a faulty OX relay.

By the way, all details are mentioned in the link above ...

[JackkTheMackk]

Cold start troubleshooting

[JackkTheMackk]

Hot start lambda troubleshooting.

Since yours is only cold start then the lambda system isn’t coming into play but here’s this for reference just in case.

Turn your key to accessories, disconnect the plug on the backside of the afm plate, and confirm that your frequency valve is buzzing. Also, can you upload a photo of the diagram on the relay you installed? Trying to confirm you that your relay is a dual 87 or 87b and not an 87a relay.

[Mac-Porsche]

Sorry, have been real busy this last weeks, so today was my first chance to get back into it.

I attached the picture below of the relay, I actually have three that I’ve tried. The one on the left is the original, one in the middle is from a 1982 911sc, and one on the right is the new one I ordered. All three give the same outcome, so unless all three happen to be bad, I don’t think that’s the issues.

I checked the duty cycle by hooking a DMM to that little green/white plug wire in the left side of the engine bay (by the CDI box). It reads 69.5% with the O2 unplugged and engine cold (which is odd because it’s supposed to be 65%). Checked several times (while unplugging/plugging in various things like the thermotime switch) and it was exactly 69.5% every time.

I removed the ECU and looked inside to see if it was damaged. Looked good, no obvious signs of electrical/water damage. After reinstalling, the frequency valve stopped working. I panicked a bit until I noticed the OXS fuse had blown. I was playing with the dome light at one point (it’s always been left off). My guess is the wires to the dome light shorted because I inadvertently left the light in the on position before reinstalling the ECU (and I’ve read that’s common with the dome light).

Just to be clear, the frequency valve ‘was’ working prior to this, could hear the buzzing and everything. It was only after the fuse blew that it stopped.

‘Could be’ related to issue, but I kinda doubt it… I’m going to buy some fuses tomorrow and recheck the duty cycle. Will probably also hunt down why it shorted and try to fix that too (probably door jam switch or dome light itself).

As far as the rough cold idle goes, still trying to think of what to do next…

[Ltanderso]

A few ideas come to mind that you may want to check:

1) Have you confirmed the cold start injector is spraying fuel during the cold start? There may be some better ways to test it, but I’d put it in a jar and watch the fuel spray pattern during a cold start. It could be opening but you have some kind of blockage (or wrong connection) preventing fuel flow.

2) It doesn’t sound like a vacuum leak is the issue, but have you checked for smoke in the footwell and around the brake booster during the smoke test?

3) I’d check the thermo-time switch and temperature switches. I think ‘81 has the switch on top of the oil breather tower. Pray that this one isn’t faulty. I think that the cold start valve receiving power verifies that these are in the correct position when cold, but they could still be defective causing your idle/fuel settings during running conditions to be improperly adjusted to compensate.

4) If you removed several vacuum and fuel lines, I’d triple check that the are in the correct locations. For example, the advance/retard vacuum lines on the throttle body and the fuel lines on the back side of the fuel distributor.

I’ve been working on some CIS issues recently, so I’ve spent a lot of time studying how it works.

[Mac-Porsche]

Thanks for the response! I hope your CIS journey is going better than mine.

1) I have not checked the CSV for spray. One day I decided to try until I quickly realized my car has a steel rigid fuel line that connects to the CSV, so not sure how I can remove it and check without dropping the engine (or bending the heck out of that steel line). If I end up dropping the engine (which im getting closer to every day…). I’ll probably just swap it out with new one to rule it out.

2) That’s really not a bad idea, I used the brake booster line for the smoke test, but did not test in reverse (I.e. test hose going out to the brake booster). I’ll give that a shot today.

3) I’ve tried two different thermo-time switches. No luck. I did check that the CSV is getting voltage when cold and it is. Like you said it ‘could’ still be bad. But the car fires up just fine, only to then rapidly drop in rpm, “chug” a couple times, then stall. I replaced the thermo-time switch in the oil tower when I did my rebuild in 2020. Not saying it didn’t die already, but hopefully that’s not the case.

4) I’ve triple, quadruple, 5x, 6x checked the vacuum lines… I cross-referenced with multiple Porsche books and the forums. Don’t get me wrong, a good comment! I’ve just exhausted that to death.

I did replace the OXS fuse and turned off the dome light. As I figured, I’m back to where I started. Frequency valve is working again and still showing 69.5%. I think the wires are grounding/crossing in the dome light, which led to the fuse blowing, a problem I’ll worry about another day.

[Mac-Porsche]

Well I’ll be damned… Borrowed an ECU from a friend that has a 1982 911sc. Car fired right up and idled better than it has in a long time. Did exactly as it should, went immediately to ~1200 rpm and then settled down to 950 after 2-3 minutes. So I’m thinking I discovered my issue.

I honestly just assumed those ECU’s were indestructible, but I guess you learn something new everyday.

I opened the ECU and it “looks” fine… I attached pictures, maybe old electronics buffs can spot something I can’t. Admittedly, the right side of the first image looks a little discolored, but all the soldering looks relatively fine.

(Sorry, the quality of the pictures after uploading looks scaled down, so not ideal for spotting problems)

[zuch]

Sounds similar to my problem. I have an 85 so the computer is different. Check out my "need help with vacuum leak" post I made today.

[mysocal911]

Originally Posted by Mac-Porsche

Well I’ll be damned… Borrowed an ECU from a friend that has a 1982 911sc. Car fired right up and idled better than it has in a long time. Did exactly as it should, went immediately to ~1200 rpm and then settled down to 950 after 2-3 minutes. So I’m thinking I discovered my issue.

I honestly just assumed those ECU’s were indestructible, but I guess you learn something new everyday.

I opened the ECU and it “looks” fine… I attached pictures, maybe old electronics buffs can spot something I can’t. Admittedly, the right side of the first image looks a little discolored, but all the soldering looks relatively fine.

(Sorry, the quality of the pictures after uploading looks scaled down, so not ideal for spotting problems)

Looks can be deceiving!

[pmax]

Originally Posted by Mac-Porsche

...
Increased pressure of smoke machine until it popped the rubber glove.
...

Haha, sorry, that is funny. Go easy there with the smoke

Originally Posted by Mac-Porsche

Would be good to post closer pics ... of the discolored area and the 7013 transistor on the left with the resistor on the side. What model box is that ? It looks similar to the 006 in my 80.

I'm speculating your FV hasn't been in play due to the fault. When warm, the mixture probably has been enrichened so much that it still "runs fine".

[mysocal911]

Check here they repair those Lambda ECUs; https://www.systemsc.com/products.htm

[Mac-Porsche]

Thanks for the link and the observations! It's a -037 box "blue label" (the car is a 1981 US 911SC). The box has a sticker on the inside that has a hand-written "96", makes me wonder if was replaced in 1996, or if that's referring to something different entirely.

Anyways... Interesting update... When I made that last post, I had started my Porsche cold and it fired right up and idled very well - but only let it idle for a bit in my garage, hadn't driven it yet.

The following day, I started again (cold) and it fired right up and idled great. All still seemingly good... However, about five minutes into driving it it suddenly dropped to 'very' low idle (~500 rpm) and ran terribly with significant loss of power and backfires. Obvious very lean condition. Then while turning around trying to get back home, it instantaneously cleaned up and ran great... then... instantaneously started running terribly again - as if something was turning off and on.

I'm guessing the frequency valve operation was going in and out. The question is why... I tested the voltage going to the plug that powers the FV and was seeing 0.2 - 0.4 volts, is that normal?

My current theory (after some quick searches through these forums), is that the plastic plug next to the left rear strut in the engine bay is corroded and not getting good contacts. I'm going to get some electrical contact cleaner and small brushes and see if I can clean it up.

Overall - I'm still more excited than I've been in a long time, because I feel like I'm actually getting close to a solution.

One other thing to note, I noticed that the fuse to the OXS relay (one that I replaced the other day) was previously a 16a fuse - however, the owner's manual says it should be 5a. I corrected it. It was my first time replacing a fuse, so likely from a previous owner. Only mention because I'm worried that could have caused a short somewhere else?

[Ltanderso]

The FV going in and out should show up when you check the frequency at the test port. Are you sure that you’re testing that correctly?

[Mac-Porsche]

So… Made yet another (rather embarrassing) discovery. While swapping relays/ECU’s, trying to figure out why they were intermittently working, I noticed the ground wires to the relay/ECU were very loose (the bolt was barely on). I either forgot to tighten it years ago when I reupholstered the interior, or it loosened from vibrations…

Nonetheless, tightening that down fixed the intermittent issues and very rough idle. Which is great. I think I was mistaken in believing it was the ECU because simply the act of swapping it was moving the ground wires and providing ground contact, only to separate again while driving.

That being said, I’m still back to a reluctant cold start which I’ve been battling for years. The ECU I’ve borrowed seems to eliminate that issue (don’t know if that operationally makes sense?…). Spending the week experimenting each day cold starting with both (only get one try a day and trying to build a sample pool) - before making a conclusion.

[mysocal911]

Originally Posted by Mac-Porsche

So… Made yet another (rather embarrassing) discovery. While swapping relays/ECU’s, trying to figure out why they were intermittently working, I noticed the ground wires to the relay/ECU were very loose (the bolt was barely on). I either forgot to tighten it years ago when I reupholstered the interior, or it loosened from vibrations…

Nonetheless, tightening that down fixed the intermittent issues and very rough idle. Which is great. I think I was mistaken in believing it was the ECU because simply the act of swapping it was moving the ground wires and providing ground contact, only to separate again while driving.

That being said, I’m still back to a reluctant cold start which I’ve been battling for years. The ECU I’ve borrowed seems to eliminate that issue (don’t know if that operationally makes sense?…). Spending the week experimenting each day cold starting with both (only get one try a day and trying to build a sample pool) - before making a conclusion.

A warm running engine will also indicate a bad/good ECU! You are aware of that, right?

[Lammers]

Have you tried simply disconnecting your oxygen sensor, worked miracles for my 85.

[Mac-Porsche]

Yes, I’m aware a bad ECU can affect a warm running engine. I guess what I’m wondering is whether it’s ‘possible’ that a bad ECU could affect ‘only the cold start’ operation and not the engine at temp? I know someone may point out it’s highly unlikely, but I’m really just curious if it’s possible…. Because what I’m experiencing is the other (055) ECU I borrowed is allowing the car to start much better cold than the original (037) ECU. They both run the same warm. Trying to determine through systematic testing if that’s actually happening, and asking all of you, is that possible or am I crazy?

Answer to the other question is, yes, the oxygen sensor has been unplugged this whole time. Despite that, it runs in open-loop which still requires a functioning ECU/relay to hold the duty cycle / frequency valve constant.