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Bogs down at 3000 RPM in most gears, and now backfiring.

I few weeks back I picked up a 911 SC and it's been driving like a charm.

A couple days ago I experienced the car having a hard time revving over 3000rpm in 2nd & 3rd and subsequently not being able to really accelerate over whatever MPH I could get at 3000rpm based on the road. Almost as if the car was being held back.

I pulled over and took a look at a few things, nothing seemed obviously wrong, started it back up and everything was working fine.

Today, same problem. Revved to about 3000rpm and it does not want to go further than that. Along with that I also started to hear some backfiring.

Going to jack it up this weekend and start trying to fix the issue, whatever it is.
Any advice or pointers on things to check would be much appreciated as this is my first aircooled 911.

Thanks

Old 11-08-2019, 07:51 PM
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Plugged cat?
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Old 11-08-2019, 08:06 PM
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That could be numerous issues....a bad electronic ignition box...so erratic spark signal.
Poor gas flow...did the SC set for a long time before purchase.....rusty tank or gunk in the lines gumming up the works....could simply be bad spark plug wires too.
So, check for solid spark and fuel delivery.
Is the lambda in place or not?

You did not mention the year...there are some differences and whether US or ROW

I'd take John's suggestion too...disconnect the cat and run it.
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Old 11-08-2019, 08:09 PM
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Thanks for the insights John & Reiver.

Let me add some more details.

It is a stock US 1981

The first time it happened it did it when it was pretty cold. Both other times and today the car had been driven for a while.

The car has sat for a while before purchasing. I've driven about 1200 miles on it so far and this issue is just cropping up.

I'd driven about 150 miles between the first time it happened and today when it started additionally backfiring.

Is it possible for the cat to get blocked then unblocked?
Old 11-08-2019, 08:21 PM
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Backfiring when...on start up or decal or accel? Have you had the AFR (air fuel ratio) adjusted...lean running can lead to some issues and backfiring but prob not to the symptoms you are observing....but that sounds like a fuel starvation deal to me....or an intermittent issue with the electronic ignition.

I have a great indy Porsche shop in the Phx area and if I have a teaser like this call Jack up and kick it around...we've solved issues like that.

Do you have a source nearby? You should find a good indy Porsche shop/wrench with a good solid rep.
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Old 11-08-2019, 08:33 PM
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@Reiver

Today the bogging happened for about 15 seconds it the morning and never came back. On my drive home it bogged for about 30 seconds and went back to normal. 5 minutes later it came back and the bogging continued for about 30 minutes. During this 30 minutes it backfired about 15 times while decel and accel.

I don't believe the AFR has been adjusted. It passed smog a few weeks back with no problems. Hopefully that helps.

I am in San Jose, CA and will look for a shop, but would prefer to try to resolve the myself.
Old 11-08-2019, 08:43 PM
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fuel delivery issues. Do the usual CIS testing.
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Old 11-08-2019, 09:07 PM
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Could be the oxy sensor relay under the passenger seat, or it's plug or wires in the plug. When that fails, the engine leans out severely because the frequency valve doesn't work.
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Old 11-08-2019, 09:09 PM
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I was thinking the same given it comes and goes intermittently.

Fizh,
The O2 relay is the small silver box under the passenger seat. Try reseating the unit.

I have a spare if you want to try it out.
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Old 11-08-2019, 09:23 PM
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I'll put the oxygen sensor first on the list of things to check. Seems like a good potential match for a culprit as I believe I have some items moving around under and behind that seat. Did not realize there was anything under that seat.
@John do you think the symptoms I am experiencing are a match for this sensor turning on and off intermittently?
Old 11-08-2019, 09:57 PM
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@pmax - thanks! I'll take a look at it tomorrow and let you know. I appreciate the offer.
Old 11-08-2019, 09:59 PM
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Do the interior lights in the headliner above the door windows work? They act on a rocker type switch, i.e. push on one end of the light then the other to make them turn on. IF they will turn on and you can see them on during the day (otherwise go for a ride at night) , take a drive and if they suddenly turn off or erratic on and off you might have found your problem, as the power to the Lamda box (large cigar box shaped unit) with the O2 relay next to it is on the same circuit.

I can tell you from experience, IF the fuel mixture in a Lamda controlled SC is properly set with the Lamda system functioning correctly, and the system goes out, the engine will run lean to the point of backfiring and no power. The rear end of the car sitting idling will also look like a paint shaker.
Old 11-08-2019, 10:56 PM
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@SCadaddle - Apologies in advance for the Friday night ramblings, here is an answer and some comments in response...

The interior headliner lights do not work and I have yet to diagnose the cause. But, since those do not work and the oxygen sensor, as you mentioned, is on the same circuit it seems like it could be plausible. My only hesitation to the theory is that the interior headliner lights have not worked since I've owned it (1 month) and this deceleration issue has come to light in the last 4 or 5 days. BUT... to help the theory... a quart of oil came rocketing through to the pedals the other day and I was not aware it was under my driver seat... I wonder what may be under the passenger seat that could moving around erratically because of my curvy 50 mile ride to work every day.
Old 11-08-2019, 11:12 PM
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Might check the interior light fuse and wiring to/from it.
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Old 11-09-2019, 03:56 AM
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@fizh let me know, pm or email.

To check if the lambda's working, see if you can hear or feel the FV located "at the back" of the engine. It should be buzzing or vibrating.
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Old 11-09-2019, 11:35 AM
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engine bogs down

Another quick way to verify that The Lambda control system is working is to momentarily - with a warm engine - disconnect the relay under the passenger seat next to the Lambda control box. If the system is working you should immediately notice a pronounced difference in the engine idle. The fuse for the interior lights and the lambda system is # 18 counting back from the front of the car - 5 amp.

Dave
Old 11-09-2019, 11:50 AM
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Looks like the Lambda system is "working" or at least the sensor.

I unplugged the oxygen relay sensor under the seat and there is a very noticeable difference immediately while idling.
Old 11-09-2019, 04:48 PM
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If you can, try that when it misbehaves. Also, try running with the O2 sensor itself unplugged for a while, or forever.
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Last edited by john walker's workshop; 11-09-2019 at 06:51 PM..
Old 11-09-2019, 04:53 PM
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Gotcha, yeah I will try to unplug it when it is slowing down. See if there is a difference.
Old 11-09-2019, 04:59 PM
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The issue seems to be happening more frequently at this point.

I unplugged the sensor relay while the problem was occurring and it resulted in some backfiring.
Seems like the sensor relay is operating.

Old 11-09-2019, 05:50 PM
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