83 SC troubleshooting/ spark plug changing adventure

[REVerend]

A couple years ago I bought my 1983 911 SC from a friend who is fairly mechanically able but also didn’t worry about a lot of stuff. Examples being tires that still had plenty of tread but were a decade old and had cracks. The oil leak it had was resolved by using a new aluminum O ring when I changed the oil.

Overall the car runs pretty well but recently developed a bit of stumbling at mid RPMs when under acceleration. It doesn’t do it in first gear or really second, only 3-5. From the little bit I know about distributors and looking at it that seems ok. I’m suspecting it could be spark plugs due to running a bit rich as the O2 sensor is disconnected. Really didn’t see any point as it has a cat bypass and Supercup exhaust that I put on it last year.

In any case I figure it won’t hurt to replace the spark plugs and it gets me more familiar with the engine. Only problem at this point is that I can’t seem to get the #6 cylinder plug out. I have the tool from the kit which works really well. I also have tried sockets but for whatever reason it doesn’t seem to want to get in deep enough to engage the #6 spark plug.

Any ideas? I have searched around at a bunch of old threads and have found a lot of good info, jsut not anything that addresses this particular issue.

[ahh911]

Have you looked into the hole? I used a boroscope the first time round, found a spring in there as well, bonus. I've never had an issue with 6, best to go visual if you've already put in so much effort and something doesn't seem right. Probably something stuck to the plug.

Phil

[Alan L]

You have an air bypass tube or something that gets in the way of #6. Have you tried moving that? It is tight in there, but with the right tool it should drop right in. Another possibility is that something was left behind by a previous spark plug tool - like an inner rubber boot. A torch and small mirror may help.
I really like the magnetic spark plug tools
Alan

[REVerend]

The tool seems to drop right in, it just doesn’t seem to want to engage on the spark plug.

[REVerend]

Turns out I didn’t think it was engaging but it in fact was… I just didn’t think it was because the plug was turning so easily.

I can’t seem to find my spark plug gap gizmo but just eyeballing them number 6 (on left) looks more than a little out of spec. The rest are also not looking great but the electrodes aren’t nearly as short.

If anyone can offer further diagnosis I would love to hear it.

[REVerend]

Another shot… the plugs are WR5 so at least they are correct per Bentley

[REVerend]

Curiously enough I fished this little washer out with a magnet when I when I was pulling out the #2 plug. No idea what it’s from… it was just sitting down there next to the plug. Really glad I caught it as it would have easily fallen into the cylinder.

After that I went around and used a vacuum tube and Magnet to try and find any other mysterious little bits around the other plugs

[Alan L]

Those plugs are quite worn. Hard to tell by eyeball, but the gaps also look too large.
But since you are trying to diagnose a miss, may be time for new plugs. One of them the center electrode is near gone anyway. They seem to indicate a reasonably tuned engine with no obvious issues showing from the plugs.
Stumbles in the rpm range can be many things. Airleaks are an obvious - they tend to show up under low throttle openings - so may be more obvious in the higher (cruising) gears.
But new plugs would be a good start point.
Alan

[walt]

Almost no electrode remaining on the left plug a few others have excessive wear. You may want to check your injector. Any idea how long they been in there.

[REVerend]

Thanks for the comments… fortunately I have an auto parts store nearby and was able to pick up four new plugs with the other two coming in tomorrow morning.

My feeling is that the car is tuned reasonably well, it seems to make good power and it has only been recently that I have noticed a stumble developing. There could be some vacuum leaks but I don’t think it’s anything too bad. It does idle a bit high at around 1,200 rpm but I believe that to be due to some stickiness in the throttle assembly. The previous owner had overfilled the oil at some point and decided to just drive it leaving a big smoke trail until the excess had burned off…

I don’t know how long the current spark plugs have been in place, all were Bosch copper but two were made in Brazil and four made in Russia which I thought was rather curious.

In any case new NGK Iridium plugs are gonna start going in as the old ones were all off spec (.28-.32 per Bentley). Cylinders 1-5 are all around .37 with #6 around .78

[PopsRacer]

if its running rough and any of your plugs are loose you probably found your issue. It happens! Put a little dab of dielectric grease inside the boot too, keeps it from corroding.

[REVerend]

Since I had things apart I went ahead and removed the cruise control unit from the engine compartment. The cable was a bit screwed up and it didn’t work, having read about some of the issues I decided to pull it entirely out to troubleshoot at a later time. There were two vacuum hoses which I plugged and secured out of the way.

Got everything put back together and fired it up… it gave me a couple backfires and then settled into a low idle that seemed a little rough. Normally it will idle around 11-1200 RPM. Something is definitely off. Took it for a short drive and it’s still running rough and is backfiring occasionally if I let off the gas.

The exhaust doesn’t smell like fuel so my guess is that it’s firing on all cylinders. The more telling indication is that the RPM only changes barely if at all when I remove the oil filler cap. The gasket is fine in the oil filler cap so it’s not leaking air from there.

Initially I’m wondering if I shouldn’t have capped off both of the vacuum lines that went to the cruise control? Doesn’t really make sense but maybe one isn’t actually a vacuum line?

Also possible that I disrupted some other vacuum line in all the wrestling around getting the spark plugs out?

[REVerend]

Spent about half an hour looking around at all the various hoses and tried spraying some water various places with the car running and no indication of where the issue might be. I’m tired of trying to figure it out and calling it a night in the garage.

Just for kicks I did try pulling the tubes off the distributor and the RPM sped up to more normal range. I’m not nearly familiar enough with how that works though.

In recap… I changed the oil, spark plugs, and removed the cruise control unit from the engine compartment and car now runs terribly. Current hypothesis is that it’s a vacuum leak as that’s what it’s acting like and there is no discernible change in RPM like before when I remove the oil filler cap.

[REVerend]

Pulled all the air ventilation hoses out just to open things up and went over the air box and vacuum hoses again just to make sure I wasn’t missing anything.

After reading about half a dozen different ways to look for vacuum leaks I tried the easiest one I had the equipment for. With it running I put some propane around various places to see if it changed the RPM, no results.

It backfired a couple times on start up which isn’t something it has done before this spark plug change so I’m also wondering if it isn’t something with the plugs.

I’m gonna see if one of the local auto parts places has a smoke machine as a loaner tool.

[PeteKz]

I seriously doubt that the spark plugs are causing the miss, especially since you replaced them.

Your car has the CIS fuel injection system. Being a 1983, it should also have the oxygen sensor system (OXS). CIS is very sensitive to air vacuum leaks. It sounds more likely that you have several of these and will have to chase them down. A smoke tester is the best tool for this job. They are not terribly expensive, about $80 for the cheaper ones, so I recommend that you buy one rather than rent (I doubt your auto shop will have ones to rent). This is what I use:

https://www.amazon.com/AutoLine-Pro-Automotive-Diagnostic-SMK-1/dp/B06XHRBKTM/ref=sr_1_1_sspa?crid=30H5C00CEMVLN&dib=eyJ2IjoiMSJ9.Bf9jRFCmdydw0-R34GZzZ2xGMYJdvAngmoIpC1udLlOquUlwA52FqPlgqWYPLxUj WsAC1fGvsNJYgpEwj8tRu2oA9GOCvtpURGUGRqIDbiflId2jPe gIN0tWheKAQVRS7K8LDMu1m4h96XdrPohG2qxAbb4YXqHmkPt6 EFUnbKr168lpJOc-Zx7vvyVJFWgfHUV0fTsiyrJymCIjar5BA6n1VRvFg-VEYuTrnl2FLZb0aqnBbpxzEJ5pUAU1RuoT0Td56jj9r4ozcZXF zgsWVImGDVGSWKvQkiRUupYbbTg.1KPTPc-9Fmx1qAxlH7cF0fyrgDPNCCzIafBTEEeEfig&dib_tag=se&keywords=smoke%2Btester%2Bautomotive&qid=1735451515&sprefix=smoke%2Btester%2B%2Caps%2C255&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

When you disconnected vacuum lines, did you plug them? When I'm dealing with a difficult to diagnose vacuum leak or rough running, I will disconnect the vacuum lines from the airbox and throttle body and plug them, to eliminate some of the sources of leaks from my troubleshooting. Another possibility is a leaking power brake vacuum line or booster. See the latter part of this thread too:

https://forums.pelicanparts.com/porsche-911-technical-forum/1170007-wideband-data-3-2-getting-10-mpg-city-what-do-you-think-5.html

[REVerend]

Thanks… I really appreciate all the suggestions and advice. I ordered up the smoke tester, seems like a must have item for only $80. That will be here Friday

The disconnected vacuum lines for the cruise control I did plug with some round nylon rod stock and a hose clamp to ensure it wouldn’t go anyplace. Once the smoke tester arrives I will try selectively plugging off the various lines. That thread about the power brake vacuum line gives me another thing to check out.

Now having said all that I’m pretty sure I found the problem… going through all the spark plug wires checking the resistance of them all it appears I mixed up the #4 and #5 plug wires. Really hoping that I didn’t mess up anything with this screw up and it resolves the issues. I’m not exactly an engine expert but I’m guessing that the symptoms the car was showing, particularly the backfiring, would be explained by crossed wires.

The whole thing was confusing at first. Going from the Bentley book I was expecting the firing rotation to be clockwise but after the #1 wire the resistance just wasn’t right and it then appeared a bunch of my wires were wrong and I knew I couldn’t have messed up 2 and 3 because the wires simply weren’t wrong enough. Then I notice the little sticker on the car has a pictogram showing counter clockwise rotation and it all started lining up, just that I had #4 and #5 reversed.

The resistance on all the wires is fine, they seem in good shape. There is a bit of crud inside the distributor cap on the contacts and I’m going to hit them lightly with a dremel polishing wheel for now.

[PeteKz]

I doubt you messed up anything with the #4 and #5 wires crossed up. It must have been running pretty rough, because it was missing on 2 cylinders.

[REVerend]

Sure enough the entire problem was self induced with a crossing of the #4 and 5 plug wires. I rechecked the wires today and put things back together and it fired up with zero drama and idled smoothly at the normal RPM.

Took it for a short drive and things are operating normal, the new plugs seem to have resolved the higher RPM stumbling I had been experiencing.

Overall it was some frustration and I’m annoyed at myself for screwing up the plug wires but I learned a fair bit am more comfortable working on the engine now.