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Car running diesel rich…black smoke

I just love my 930. When it runs well, its a joy. And, thats most of the time.

1 week ago I hooked the decel valve back on as the PO had taken it off. No problems. Ran great.

The PO had one of those small circular airfilters attached to the intake. Days later I replaced it with the original intake to get closer to stock. Ran great for a day…

Next day, wouldn't start. Gave up after 15 tries.

Following day started right up. Nice idle at 1000 rpm or so. Then within 1 minutes the idle start to bog down to 400 or so. Its never done that before.

Took it for a ride, it bogs down getting out of the drive. At 4000 rpm it stutters, with the car just jiggling back and forth at 20 mph for a second. Above 4000 rpm, when the turbo kicks in, runs like a bat out of hell . At steady speed, also no issues, unless below 4000 rpm.

Odd. Had a buddy looking at the engine while I gave it gas, and he commented that it was "smoking like a diesel." Lots of black smoke, though I can't see it out the back window.

Now, I know there are dozens of things that can make it do this. I have founds threads that mention air leaks, boost leaks, bad coils, bad CDI's, bad WURS, bad relays. I saw threads about checking fuel pressures.

Before I go nuts, is there anything I could have screwed up by hooking the decel valve or putting the airbox back on? It ran fine for a day after I did this… but still.

Any simple tests to do before I go buy an air/fule mixture or fuel pressure gauges? Temporally, I suspect I must have inadvertently done something, and need to undo it.

One interesting point… my oil pressure is now better than ever. Its 1/2 a bar higher than before… for no good reason. Does this suggest some whacky electrical issue? I moved the the relays around, no change.

Should I adjust the idle misture, to see if that could do it? Could the air filter screws have anything to do with it? (Could they be pushing the airplate down?). Any hvac hose that could have been dislodged and cause this?

As always, any help appreciated. Some of the problem solvers on here are amazing.

Bo


Last edited by bpu699; 07-21-2014 at 03:36 PM..
Old 07-21-2014, 03:34 PM
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Crotchety Old Bastard
 
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Quote:
Could the air filter screws have anything to do with it? (Could they be pushing the airplate down?).
Great place to start. Make sure the plunger isn't hanging up due to installing the new air filter assembly.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 07-21-2014, 07:57 PM
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I appreciate the help. Unscrewed the air cover screws most of the way, no change.

Disconnected the oxygen sensor, no change.

Tweaked the idle mixture screw several degrees one way and then the other. No change.

Car starts at an idle of 1000. Drops to 400 in 10 seconds. Then slowly decays until it shuts off, unless you press the gas pedal down.

Restarts easily.

Unplugging the relay for one of the fuel pumps made it run better, suggesting it was rich. Unplugging the other one, while replugging the other, made it die. Go figure.

Any thoughts on next steps? Could the aar valve do this?

Thank you

Bo

Last edited by bpu699; 07-22-2014 at 03:33 PM..
Old 07-22-2014, 03:29 PM
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Crotchety Old Bastard
 
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Don't change any settings. Whatever has happened has nothing to do with tune so you need to leave that baseline alone in order to get back to it and know the problem is solved.
Have you checked the air meter plate? Could something have lodged in it impeding movement? Could you have left a rag in the intake? (not a stupid question it happens)
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Old 07-22-2014, 07:57 PM
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Will do it this weekend. Thanks again for the input. The car was in storage for 3 years, and just started running it this year. So, lots of stuff could be acting up.

I will post further this weekend. Thanks!
Old 07-23-2014, 05:08 AM
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Make sure your electrical plug on the control pressure regulator is still plugged in and didn't get dislodged from the air cleaner install
Old 07-23-2014, 05:45 AM
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Good idea. I did jiggle it, and it seems ok. But willl look at more closely once the airbox is back out.

Giving it more thought, I think it may have been doing something similar before the airbox was put on, but it only did it briefly. For example, when cold, backing out of the driveway it would bog down and I had to take it out of gear and rev it, then it would go back to normal. I attributed that to lack of use, and figured it would go away with some fuel injector cleaner.

It only lasted several seconds, whereas now its pretty much constant.

I will look at the air meter plate, and the connections. The intercooler was also removed of couse, so will check that too.

Bo
Old 07-23-2014, 06:10 AM
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If the assumption is a rich condition is causing these problems you need a way to monitor AFR and fuel pressure real time. Completely different problems can have similar symptoms such as a failing fuel pump or electrical short feeding the WUR. If you can hook these gages up to monitor a cold start and warm up that will tell a lot.
Old 07-23-2014, 09:51 AM
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Quick update.

Took off the air cleaner. No change. All the connections seem fine.

Adjusted the mixture 1/4 turn lean at idle. Worked great. Idle went from a lumpy 300 to a solid 900.

Went to get my shoes, and by the time I got back it was running like crap again.

Ordered an afr monitor, to go in next week. Want to purchase the fuel pressure monitors too... But the budget won't allow both for now...
Old 07-27-2014, 11:52 AM
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Not to send you on a wild goose chase but do you have your emissions system still hooked up and what year car?
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Old 07-27-2014, 12:47 PM
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Odd you should mention that. Last week I disconnected the oxygen sensor, and it made no difference. Now, I just did it, and disconnecting it fixed the problem for now.

I bought the plx afr meter, will connect next week.

Would a flaky Bosch oxygen sensor do this? The prior owner had it disconnected.

Note that I did have to lean it out a quarter turn either way.

I do wonder if after years of sitting something in the fuel system was constricted, and it just opened causing a rich condition...
Old 07-27-2014, 05:40 PM
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Take a voltmeter and check the voltage in the plug going to the control pressure regulator. It should be 12 volts to heat up heater element on the bimetallic spring inside the CPR.
Your good cold start followed shortly with very rich mixture symptoms sound like it isn't getting it.

Also check the rear fuel pump relay. It's the one closest to the windsheild on the relay panel. That relay powers the rear fuel pump and the heater elements in both of the mechanical CIS cold start devices: the control pressure regulator and auxilliary air valve if thats the right name for it.
If the rear fuel pump relay isn't working the car will run on only the front pump and it will be running too rich because the CPR or WUR or whatever you want to call it will stay in cold start mode.
Old 07-27-2014, 07:10 PM
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Quote:
Originally Posted by bpu699 View Post
Odd you should mention that. Last week I disconnected the oxygen sensor, and it made no difference. Now, I just did it, and disconnecting it fixed the problem for now.

I bought the plx afr meter, will connect next week.

Would a flaky Bosch oxygen sensor do this? The prior owner had it disconnected.

Note that I did have to lean it out a quarter turn either way.

I do wonder if after years of sitting something in the fuel system was constricted, and it just opened causing a rich condition...
Disconnecting a working Oxy sensor/Lambda system as i have done just for test purposes on my '87 will make the engine run lean around 17.5 AFR. Reconnecting it back up will take it back to 14+- on my car, just right. Maybe someone adjusted the air fuel mixture screw with the Oxy sensor disconnected (made it richer) but then with the Oxy sensor installed back you will run too rich. I feel an AFR gauge is a must on these cars, JMHO
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Old 07-28-2014, 07:14 AM
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Thanks guys. Will check the voltage and await the AFR guage.

Just curious, how much sway in the AFR does the oxygen sensor have?

Can it change the ratio from an 8, or 20, back to normal? IE. How much of an influence does it have? Does it have an effect only at idle? Or, also at running RPM?

I thought the effect was minor.

Bo
Old 07-28-2014, 09:55 AM
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Like Jim stated above - - your WUR may not be getting power. There may be an issue with the Electrical Plug on the WUR. Mine had gotten a short in it and I simply used the Connector for the AAV. It's the same size; has the same voltage at the same time; but it's a different color.

Since my AAV was removed, I just continue to use the AAV plug for my WUR.
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Old 07-28-2014, 10:03 AM
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Quote:
Originally Posted by bpu699 View Post
Thanks guys. Will check the voltage and await the AFR guage.

Just curious, how much sway in the AFR does the oxygen sensor have?

Can it change the ratio from an 8, or 20, back to normal? IE. How much of an influence does it have? Does it have an effect only at idle? Or, also at running RPM?

I thought the effect was minor.

Bo
Here is a good thread on the oxygen sensor.


Question about Lamda Oxygen Sensor
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Old 07-28-2014, 10:47 AM
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And another.

oxy sensor klambda problem
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Old 07-28-2014, 10:48 AM
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Thanks guys. I haven't had a chance to check the voltage or install the afr meter.

I did drive the car for 45 minutes, and it drove great. Until...

...I was on the highway and it just died. Pulled the clutch at 65 mph, and tried to restart. No luck. Coasted to safety...

Tried starting for several more tries. Nothing. Crap.

2 minutes later, it started just fine.

Drove 2 minutes. Died again while cruising at 4000 rpm. Crap.

Same process, after 10 tries it restarted, ran fine, and I babied I home.

Odd thing is, the stumbling and smoking issue seem better.

I am at a loss as to tie this all together.

Bad Cdi? Causing rich running and intermittent dying?

Bad coil?

Can't see how a bad wur would cause this. Could the connector not working cause this?

Can't see how a bad ground would do it...

I suppose it could be 2 completely unrelated issues to.

This weekend, will install the afr, check the wur voltage, and change the coil.


As always, ongoing input and puzzle solving appreciated.

Bo
Old 07-28-2014, 03:34 PM
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A bad fuel pump can cause all of those symptoms.
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Old 07-28-2014, 05:46 PM
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+1 on possible bad fuel pump... if WUR had no voltage, your car would still run...

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Old 07-28-2014, 05:58 PM
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