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Anthony
 
Join Date: Jul 2019
Location: Anchorage, AK
Posts: 22
Running very rich after rebuild - air leak?

Just finished a rebuild on an '81 911sc. Ran the engine at higher rpm's for "initial 20-minute" break in. Engine ran pretty hot and had trouble idling (as I mentioned in another very recent post). In that post, I was worried about it running lean.

However, coming to the realization that the engine is actually running very rich... The car starts up immediately when turning the ignition. Has trouble maintaining a constant idle. After ~30 seconds, the idle gets worse and starts surging - goes to almost no idle, then tries to save itself, until it eventually stalls (unless throttle is applied). Exhaust smoke is rather thick, and spews a little gasoline if throttle is applied.

I tried removing oil cap while engine is running and the rpm's did not change. From what I've read on various forums - sounds like a vacuum leak... I've spent all day tightening any connection within reach - no luck.

During the rebuild, I replaced the injector sleeves, O-rings, intake runner sleeves, etc. etc. So I'm in a bit of denial that it's a leak... I tried hooking up an air compressor to the pop off valve opening and sprayed all the various connections with soapy water - couldn't find any leaks that way...

Before I drop the engine to retighten all the lines, valves, etc. Is there anything else I should check/try first?

Any thoughts/suggestions are appreciated!
Thanks!

Old 06-21-2020, 06:25 PM
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al lkosmal's Avatar
 
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The best case scenario would be for you to check the CIS system operation, using the fuel pressure test setup to determine if the system pressure , cold and warm fuel pressures are in spec. Also using an AFR meter to gain an understanding of where the fuel mix is really at,,,is a good thing and I recommend that you invest in those tools and follow those steps....... However, based on your description, you are running very rich....here is a link to adjusting the AFRs without an analyzer......lots of posts regarding this....

http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html


regards,
al
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Old 06-22-2020, 08:06 AM
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You have the opposite problem.
The sensor plate height has been adjusted richer over the years to compensate for all the small vacuum leaks they develop.
Now that you rebuilt it and replaced all the seals, it needs to be adjust back.
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Old 07-09-2020, 08:32 PM
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Whats the model of the '81 engine? A US lambda based one or a Euro/ROW one?
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911 SC 3.0, 1982, black, US model
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All about the 930/16 and 930/07 Lamba based US SC models:
http://www.elfertreff.de/showthread.php?t=26670
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Old 07-10-2020, 04:28 AM
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Anthony
 
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Its a US Lambda... Replaced the oxygen sensor during the rebuild and the headlights still work, so I don't think it's the relay (read the headlights and lambda run off same relay under passenger seat)... So I would assume the lambda is working fine?

Little bit of an update since I first posted this:

Discovered a fuel leak behind the fuel distributor, fixed that. However, when I was restarting the car each time to ensure I fixed the fuel leaks, I noticed fuel actually spewing out of the exhaust pipe...

I purchased a pressure kit for CIS systems as well as an AFR meter. Before I started down the pressure testing route, decided I should really investigate the vacuum leaks one last time, just to be sure... Used air compressor with soapy water... no obvious leaks... Then used cigar smoke - in which I discovered the pop-off valve I installed was not sealing. Problem isn't where the glue is, but the lid and o-ring do not seal... Other than that, when pressing down the pop off valve - appears to be air tight.

So I put everything back together and restarted the engine while pressing down the pop off valve (by the way, engine starts immediately each time) ... Same issue... surging/rough idle, fuel coming from exhaust...

Realized fuel in exhaust could mean fouled plugs... So I started pulling the spark plugs... Each one looks like the picture below and smells strong of gasoline... Kinda surprised it started so easily each time...

Letting the cylinders air out overnight, will buy new plugs in the morning.

Now I have some new questions:

1) Is it possible that the pop off valve leak lead to very rich mixture, which lead to fouled plugs? Hence the fuel in exhaust? Or could something else be happening?... Wondering if I should check/change anything before starting engine with new plugs.

2) Has anyone else had problems with a pop off valve lid not sealing? If so, how did you remedy it?

3) What on earth is that bronze pipe that connects to the throttle body and airbox?! (picture below) It's a little disheartening when you're looking for vacuum leaks and you have this tube connected to everything for seemingly no reason...

Thank you if you're still reading this! I really appreciate the responses and help - this forum has always been a great resource!!



Old 07-10-2020, 10:47 PM
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Isn't that the venturi hooked up to the brake booster?
Old 07-11-2020, 11:17 AM
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Walt Fricke's Avatar
 
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yup - it is the vacuum source for the brake booster.

Air leaks = mixture too lean, not too rich.

All plugs looking the same - black carbon and raw gasoline? - may rule out a bad injector, as they would all have to be bad, which isn't all that likely. But maladjustment of the 3mm adjuster screw, or the position of the flow measuring plate, might do something like this.

Read what Al suggested to get a start on what to try.

If it were me, I'd pull an injector, stick it in a clear plastic or glass jar while attached to its line, turn the key to run (don't start), and disconnect the safety switch which is behind the air measuring plate where you can't see it, and can barely reach it with the engine cabin air heating fan in place. Or jumper the fuel pump relay as shown in various posts and books. Either will cause the fuel pump to run. With the engine off no fuel should come out of any injector. If it does, you have learned something important. If not, it probably isn't bad injectors. I don't think it is the injectors, but this is easy and requires no instruments.

But maybe try the 3mm screw part, per instructions, first.
Old 07-11-2020, 08:35 PM
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Anthony
 
Join Date: Jul 2019
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If you're still reading - thank you! Still trying to resolve surging/rough idle in this 1981 911sc that was fully rebuilt.

I'm at a loss... Have done the following:

1) Fixed the air leak from the pop-off valve, everything 'appears' to be air-tight now. Used air compressor, cigar smoke, and even attached part of a rubber glove to throttle body where I sucked it in (creating vacuum), then plugged line and checked later if it re-inflated. Nothing. Assumed no leaks.

2) Checked the fuel pressures. system pressure is ~4.3 - 4.5 bars (little on the low side, but within spec?). Control pressure builds to ~3.5 bars. Pressure holds at 1.5 bars after engine shut off for good 1 - 1.5 hrs.

3) Pulled all the fuel injectors, placed in jars, adjusted mixture screw until fuel just stops ejecting while fuel pump is running. Then leaned another 1/4 to 1/2 turn. (per baseline setting strategy from another Pelican post).

4) Hooked up AFR meter and started car. Meter showed lean mixture 15 - 16 AFR. Car was no longer surging. Held idle (no observed surging/searching), although idle was high (sat at 1100-1200 rpm). Checked ignition timing with vacuum advance hose plugged, set to 5į advance. However, this was at ~1100 RPM. Tried adjusting idle screw (on throttle) to lower to 950, car did not like this... Very slow to respond - sometimes didn't seem to want to respond at all, and caused drops in RPM/surging? So ended up setting back to where it was. Reconnected vacuum advance hose to ignition distributor. Called it a day.

5) Next day - richened the idle mixture screw about 1/4 turn. Restarted engine. Idle sat a little higher (~1400 RPM). AFR immediately dropped to 12-13, but then began slowly leaning, gradually reached 16-17 after ~60 seconds of idling. At this point, fuel was coming out of the exhaust... which is even more confusing... (This has actually been a reoccurring problem since the beginning of my troubles)

6) Restarted engine one more time to check repeatability, this time it stayed very lean... 17-18 AFR... fuel still coming from exhaust?

Not sure what to do anymore... Starting to wonder if it's been a large vacuum leak from incorrectly connecting the vacuum lines? Or just a big leak I'm missing somewhere... I'm 95% on having connected them correctly (used before/after pictures to aid in connecting process), but these troubles have me second guessing myself... When I did the vacuum test, I took off the intake boot, and had to plug the hose from the right side of the intake boot that connects to one of those saucers - or else injected air would just come through there. Is that normal?

Any thoughts/suggestions would be appreciated...

________________________________
(EDIT - I noticed after posting this that a control pressure of 3.5 bars is rather high - I said that off of vague memory, but I'm going to confirm that number tonight)

Last edited by Mac-Porsche; 07-16-2020 at 10:17 AM..
Old 07-16-2020, 09:38 AM
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I can see why it is driving you nuts. 3.5 is an OK hot CP. Don't think slightly low system pressure would cause these problems, but at 4.5 Porsche would say don't bother adjusting, though that is the lowest level before adjustment.

But the AF measurements and high idle (both indicating a lean condition) and the scary bit about fuel spitting out the exhaust (which sounds like it is super rich) don't make sense. Air leaks are the most common cause of being lean. You can check for those by spraying brake or carb cleaner, of better yet propane from a hose which replaces the burner nozzle, at various usual leak suspects. If the RPMs pick up, you have localized the leak.

You can check how the electronic/oxygen sensor part of the system is working with a dwell meter or various other devices. Lots of information on how to do that. That will tell you if the sensor/frequency valve system are working. However, my experience with installing an SC engine like yours in my ex-2.7 without any of the frequency valve system working or even powered/hooked up/in the car was that it seemed to run fine, but was a bit hard to start. Nothing like what you are experiencing.

If the ignition is doing a poor job of igniting fuel, maybe you get some raw gas? Could that be compatible with a lean mixture, though? Calls for more knowledge of how the sensor works than I have.

The Bosch CD box isn't forever, though usually they work OK up until they crap out. The guys who repair these can tell when failure is approaching, I think by seeing an increased current draw or something. But then something critical inside the box just fails, and it is dead (they can fix).
Old 07-17-2020, 01:23 PM
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If it is spitting raw fuel out the tailpipe...it is likely running too rich.

I have read of instances where you are running so rich, it is causing a misfire and.....you get unburnt fuel, but - when there is no combustion the oxygen coming into the cylinder is not burned so the oxygen sensor in the exhaust (or air-fuel meter) sees what it thinks is lean - excess oxygen.

If I were you I would follow this method to get your Air/fuel mix in the ballpark. .....
:
http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html
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Old 07-17-2020, 01:49 PM
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Anthony
 
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I 'think' I'm starting to head in the right direction...

I tried the carburetor cleaner, no increase in idle. However, I did notice that when the engine was warmed up, I would see a jump in AFR, and a small drop in RPM, when the oil cap was taken off. So hoping that means no leak?...

Replaced with CDI with a classic retrofit CDI+ (had it all along, but wanted to get car running good before I swapped - decided it was time to just do it). Furthermore, noticed the gas canister I used to fill the car had a bit of water in it... So emptied all the gas from car, emptied the accumulator, replaced the fuel filter, and put in fresh new gas. Idle seemed a little more consistent/smooth, but ran the same for the most part.

I toyed with mixture screw a bit more... What I found is that when starting cold, the engine would run at ~12.5 - 13.5 AFR, then after a few minutes of running (warm up to operating temp) it would rise to 15.5 - 16.5 AFR and settle out there. So I played with the mixture a bit more and got it to where it run at ~15.0 - 15.5 when warm. If I tried leaning the mixture screw any more from there, the idle would get very rough. No fuel coming out of exhaust - consistent idle. However, it now idles at ~1200-1400 rpm.

Now that the idle seemed a bit more steady, I mustered up the courage to take it on the road. Drove it on a couple very short drives (1 - 5 miles), varying the rpm's and really pushing it at times. Each time I got back, I immediately hooked up the AFR meter (with car still running). AFR is completely flat at 13.6- 13.8 AFR. No sign of raw fuel in exhaust. It runs and idles pretty good now. Maybe putting load on it pushed some gunk out of fuel injectors or lines?

Only problem now is that it wants to idle at 1200- 1400 rpm and the idle speed screw doesn't seem to have any affect on that? Any thoughts?

In a seemingly unrelated note, the low beam headlights work, but turning on high beams make the lights go dim... When I first turned on the high beams, the rpm suddenly dropped (not sure if that was coincidence, or if the high beams are drawing too much electricity due to bad ground or something...) checking that now.
Old 07-19-2020, 06:25 PM
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Air by-pass.........

Mac,

So even if you fully closed the idle screw, your idle speed remained unchanged @ 1200~1400 RPM? Is the AAR (auxiliary air regulator) fully closed when the engine is warmed up? The AAR should be totally closed after 5 mins. from start-up. Keep us posted. Thanks.

Tony
Old 07-19-2020, 07:26 PM
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Anthony
 
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Okay, so the battle continues...

I followed the advice from Boyt911 on checking the AAR. I plugged both hoses, then monitored the AAR. It is closing, but it looks like it may be stopping a slight hair from closing all the way. If it's not closing all the way, it would be a very, very small opening (is that all it takes?).

Car had a reaally hard time idling cold with AAR hoses plugged. If I held throttle at ~1200-1400 rpm (like it does with the AAR hooked up), then it runs fine. Once at operating temp, it will idle on it's own with some minor surging - AND the idle rpm dropped to ~1100 for the first time in a long while. I then adjusted the idle speed screw and it actually moved with small changes in the screw. Was able to adjust it down to 950, with small turns.... Does this mean the AAR is bad??

At this point, it was making surges during idle. Not enough to stall, but just wasn't idling smooth.

So I was curious what would happen if I unplugged the O2 sensor and it immediately stalled. Decided to let the engine cool down and call it a day.

Last edited by Mac-Porsche; 07-21-2020 at 10:05 PM..
Old 07-21-2020, 10:03 PM
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CIS troubleshooting.........

Quote:
Originally Posted by Mac-Porsche View Post
Okay, so the battle continues...

I followed the advice from Boyt911 on checking the AAR. I plugged both hoses, then monitored the AAR. It is closing, but it looks like it may be stopping a slight hair from closing all the way. If it's not closing all the way, it would be a very, very small opening (is that all it takes?).

Car had a reaally hard time idling cold with AAR hoses plugged. If I held throttle at ~1200-1400 rpm (like it does with the AAR hooked up), then it runs fine. Once at operating temp, it will idle on it's own with some minor surging - AND the idle rpm dropped to ~1100 for the first time in a long while. I then adjusted the idle speed screw and it actually moved with small changes in the screw. Was able to adjust it down to 950, with small turns.... Does this mean the AAR is bad??

At this point, it was making surges during idle. Not enough to stall, but just wasn't idling smooth.

So I was curious what would happen if I unplugged the O2 sensor and it immediately stalled. Decided to let the engine cool down and call it a day.

Mac,

If you plugged the vacuum hoses for the AAR , you are choking the engine when it is cold. AAR works as an air by-passed for metered air for a cold start. On warmed engine, the AAR is totally closed. You need to check your fuel pressures and air leaks. Could you vary the idle speed by manipulating the idle screw? BTW, the AAR should be closed before 4 mins. from a cold start up with a high engine RPM, then gradually goes to an idle speed of 900+/-(50) rpm. Make sure you donít have unmetered air going into the system because your AFR meter reading will give you a false information.

Tony

Old 07-22-2020, 06:11 PM
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