83 SC HOT muffler on the right side

[mhackney]

This is one of the stranger things I've tried to diagnose and need some collective wisdom.

I drive my 83 SC Cab a lot. This week I put 1000 miles driving to Rochester, NY - Canada and back home to the Boston area. I do the Rochester commute about twice a month all of last summer/fall. This was my first major trip this year. Fresh oil/filter before departing and the trip went smooth. This car runs great, starts on the first click, all good.

I got home late Thursday and pulled into my garage. I always “drive down” for the last 1/2 hr or so to let the engine cool down after a long drive. On Friday after work I thought "I should do a nice little upgrade and install those cool red Clewett spark plug wires I got this winter." It has been down hill since then! I'll tell the entire story so please be patient as there might be a clue in there somewhere.

I pulled each plug wire one at a time to make sure I routed the new wire to the right place on the distributor cap. I did have to remove the cap to make sure the back 2 wire were plugged in tightly. The job went quickly. I started up and it fired right up but I noticed the idle speed was much lower than normal 950 RPM. The engine was dead cold at start. I typically idle right at 950RPM, now it was idling around the 500RPM tick. But the engine didn’t sound bad, no hesitation, etc. So I started to drive out of the driveway and experienced stuttering and low power. I drove to a neighbor and turned around. At around 2500RPM the engine ran smooth(er) but was still noticeably under powered.

When I returned I checked all the wires. I pulled one of the wires from the plug and when I looked at the plug end I realized that the Clewetts were designed to snap on the little screw-on nipples that come with the plugs. I looked at my old wires and realized that they were designed to snap onto the threaded stud on the plug and that you have to remove those nipples.

So, I had started the car and drove for 5 minutes maybe with the plug wires basically just sitting on top of the plugs without a good mechanical/electrical connection. I suspect that this might be the root of the problem and caused some damage somewhere - likely the coil.

Once I realized this, I pulled each plug and replaced it with a new, gapped plug with the nipple removed. I did notice that the old plugs were worn and had a significantly bigger gap. This time I could feel the Clewetts snap securely on the plugs. But again, low idle and hesitation. It was getting dark and starting to rain so I went in for the night to lick my wounds.

Next morning (yesterday) I checked the HV wire between the coil and distributor cap as there are reports that the Clewetts do not seat well. Sure enough, the coil end was probably just barely making contact as I was able to push it into the coil a good 3/8” or so. I fired up again but still had the problem.

So, I removed the Clewett cable from the coil and replaced it with the original. This time when I started up, the idle speed was looking good so I backed out for a short ride. But, unfortunately the problem still existed. I had driven maybe 1.5 miles. When I got home and popped the lid, smoke came pouring out the very rear. Not oily or fuel smelling but a sort of dry metallic smell. I discovered that the entire right side of the rear heat shield and even the painted cowl and tail light were very hot. No IR thermometer handy so I couldn't measure. I then noticed that the red lead to the Thermo-timer was disconnected (probably during the wire install by accident) so I connected it up and thought that might be the root of the heating issue.

When idling, I also noticed an arcing sound and when I closed the hood and looked through a crack I could occasionally see an arc from the center HV boot on the coil. I reseated the wire but still heard the arcing. I removed the coil and inspected it but didn’t find an issue. That’s when Chris suggested measuring the coil resistance. The terminal 1-15 resistance was good at 0.5 ohms and the terminal 1-4 resistance was .78 K Ohms. I found a post from EarlySMan who was helping a guy with an 83 SC and he said the secondary should be 650-790 ohms, so mine is actually in spec.

I’ve since reinstalled the original set of wires. I removed each plug and removed its nipple. I made sure each wire was well seated on both ends and the coil wire was properly seated, This should have put things back to the way they were but with fresh plugs. But again, low idle at startup and hesitation/low power. I also do still hear the occasional arcing but it does not seem to be as persistent.

So, I suspect that when I installed the Clewetts and they were not snapped securely to the plugs - and also the center terminal of the coil was not secure - that “something happened”. Could the coil still be the culprit? I suspect it could be if it blew a path for the arcing but did not destroy a coil. I believe I found some posts that commented about this. Another thing that could have changed is perhaps the distributor twisted slightly when I reinstalled the cap - throwing off the timing. I have not had a chance to check if the distributor clamp is loose, I’ll do that tomorrow.

A friend dropped off a coil, CDI and some other parts to test with this morning. I installed the coil (an original black Bosch like mine) and fired up from dead cold. Again it seemed to idle correctly at 950RPM so I went for a spin around the block again. Again I got stuttering so the coil was not the issue.

I popped the lid and immediately felt the heat rise. No smoke though, it was not as hot as yesterday. I had my IR thermometer handy this time and measured the heat exchangers and cat and muffler. Everything was normal EXCEPT the muffler. The middle and RIGHT side of the muffler were very hot - 260°C (550°F) and the left side dropped off to 138°C (280°F).

So I suspect that whatever is causing this heat issue is probably the root of my problem and is probably unrelated to the original changing of the spark plug wires - but I've been wrong before!

Anyone with any ideas on what could be going on?

Cheers,
Michael

[DRACO A5OG]

Remove and check for fractures. I recently found several fractures and even holes. Had a muffler shop weld them right up. It is SS so best to let them weld it.

[mhackney]

Thanks. Did you have a similar symptom (hot on one side)?

[DRACO A5OG]

Not that I noticed. I was just agasp seeing the holes and fractures I wanted to fix them right away.

My understanding, is a leaking the exhaust system will generate more heat at the leaks not to mention possible back fires and louder exhaust (not so bad, i guess)

[ischmitz]

Not sure how hot an exhaust is supposed to get. It should not burn the paint though.

One idea could be un-burned fuel due to miss-fires igniting in the muffler maybe. Just guessing here.

Did you check timing? Did you check that nothing else inadvertently got unplugged when you installed the pug wires?

[mhackney]

No backfires or obvious misfires. I have not checked timing but will tomorrow. This is not like anything I've seen before.

I have checked for unplugged things multiple times. It's odd, the car starts right up strongly, idles at 950 and then idle drops after a minute or so. No evidence of overheating anywhere else - just the right side and middle of the muffler. And it gets much much hotter than it has ever been. As I said, this car gets about 12k miles a year and has been rock solid. I don't even know where to begin on this problem. The cat is on the left and does not overheat, the outer muffler shell literally has a temp gradient right to left and is way overheated on the right.

My local 911 gurus Chris Bennet and Shaun Budka are stumped too.

[pete3799]

Rotor seated good? Dist. cap cracked? Does the idle drop when you remove the oil cap?

[mhackney]

Thanks Pete.

Yes, triple checked rotor seating.

Distributor cap good but I also borrowed another to test with from Shaun. Also made sure cap is seated properly.

I'll have to check idle drop with oil cap off. It is idling so low that I'm not sure what will happen. I'll check this tomorrow.

[trond]

re-check spark plug cables connected to correct cylinders on 3-6 ?

[tirwin]

This sounds like a 'puzzler' (for anyone that uses to listen to Click and Clack).

A thought that occurred to me was that your idle should be higher on a cold start and then settle to 950. That could be a red herring to this issue though. I would look into it later or if nothing else helps.

If the timing is retarded, the result of that is normally a hotter combustion. This is the principle behind the idle retard vacuum on the distributor. I would suggest checking the timing both at idle and full advance.

Did you change the rotor too? Maybe check that it is seated properly.

As trond suggested, double check the plug wire order.

Plugs gapped correctly?

Assume you are using Bosch CDI and coil? Swapping with a known good coil and CDI is a good idea.

Thinking out loud, could the cat be partially clogged?

[mhackney]

@trond - I'll add that to my list of things to verify today, thanks

@tirwin - yes, C&C fan here!

I plan to check the timing today. I did not check the rotor. Both the rotor and cap have about 6000 miles on them as I changed them in the fall. I have removed, cleaned and reseated the rotor though so I'm sure it's correctly seated.

Plugs were gapped correctly and double checked as I had to remove them to remove the "nipple" when I reverted back to the original spark plug wires. .75mm (.030") gap - mid point of the .7-.8mm range. I should say that the old plugs were worn and the gap was significantly larger at about 1.04mm (.041") - that's a pretty big difference. I'm wondering if I've been running very rich for a while (but there has been no backfires, etc like I would expect running very rich).

Bosch CDI and coil. I have spares that Shawn lent me to test. I've already swapped the coil and the same overheated muffler thing happened.

I am beginning to suspect the CAT and or muffler. The out of control hot temperature is isolated to the right side and middle of the muffler. Nothing else I measured (CAT, exchangers, engine block, valve covers, etc) were out of control. Since the car was only driven 1.5 miles around the block, the engine hadn't even started to warm up and the oil temp gauge hadn't really moved.

Doing some more research here, I found this:

How close did I come to burning it down?

This is the closest thing I've found to my symptom. See post #14 in the thread by the OP:

So I pulled the exhaust down, rather cut it down, and found the muffler must be partially collapsed inside, hear allot of rusty stuff moving around in there, so that would explain why it got so hot.

Can I cause any harm by running around the subdivision with just no muffler and cat, just HEs?

Adam


So this is really looking like a problem inside the muffler - but it could be the CAT. I don’t think its the CAT because when the muffler gets so hot the CAT is still at a reasonable temperature. If the CAT was at fault I would expect it to be pretty darned hot too.

[mhackney]

I have a laundry list of things to check out today now.

• recheck timing
• rubber mallet muffler
• verify wires into distributor cap are ordered correctly
• remove oil cap at idle to see if idle decreases

On top of these, I plan to focus taking temperature readings from dead cold at all the key areas UNDER the car BEFORE it gets so hot that the cowl is overheated. Specifically, the exchangers front and rear, CAT inlet and outlet and 3 spots on muffler (left-mid-right). Perhaps I can see where the temperature starts to rise first. I suspect it will be the muffler right side.

[tirwin]

I thought .028" was the stock plug gap. .041" would normally be expected with something like an aftermarket CDI such as MSD. That seems not normal off hand but maybe there is an explanation.

[mhackney]

I have the stock CDI.

However in checking the wire order on the distributor cap it looks like I don't have a Bosch distributor (or things are very wrong!). Looking down at the cap from the rear of the car the #1 is above the housing notch as it should be. But the firing order goes COUNTERCLOCKWISE 1-6-2-4-3-5 as shown in this photo.



I can't get a good look at the # on the distributor but here's what I can get:



What I can read:

0 23730
PGFU6

There is also a stamp in the oval on the lower left by the "PG" number that has OSO in it.

I also tapped all over the muffler with a rubber mallet and heard no loss rattling. It seems solid and secure.

[mhackney]

Ok, mystery solved on the distributor with a little image enhancement. The number is:

0 237 304 016
PGFU 6 ------>

It is a 1980-83 Porsche 911 SC dizzy. The arrow means it rotates counterclockwise so my timing order is correct.

[mhackney]

Ok, I have a bit of chicken & egg problem! To check the timing I need to have the engine warmed up and idle speed set BUT the idle speed is significantly lower than it should be and I can't warm the engine up without a muffler melt down. I suppose I could start and idle for 5 minutes, shut down and let the engine heat soak. Maybe do this a couple of times to get in range. But that won't solve the low idle speed issue. Any recommendations appreciated!

[tirwin]

I see your concern now.

I wish I was closer. I have a stock muffler and open cat sitting on the floor of my garage that would be perfect for ruling out the exhaust problem.

May be a crazy idea. Could you take the rear bumper off and then stick something over the muffler to make a kind of heat shield?

Or it may be easier to pull the cat and muffler and see if there is any glaring issue. Is it possible you've got a little rodent bbq happening in the exhaust?

[mhackney]

Thanks tirwin. I don't think I have anything organic in the muffler. My highly calibrated snout would pick that up. The sequence of events and the symptoms on this are driving me nuts! I'm a very methodical person. The only think I would do differently is drive the car last Friday before swapping the spark plug wires but that's such a simple job I didn't think it necessary.

I have done some searches and found a photo of a cut open muffler. Nothing jumped out on what could be going on. So now I am going to do an experiment as soon as I'm off work. I made this matrix:



I'll take temp measurements at each of the spots every 30 seconds or so. Maybe that will tell me something about how the heat evolves. If the car is running very lean, wouldn't I expect to see the heat exchangers get pretty hot first?

[mhackney]

’m ready to call professional help - if I knew who to call. Here’s what’s new…

Before doing anything I went over all the hoses and electrical connectors. I pulled the airbox cover and use a light to really see things. I found a crack along the OUTER side of the air box:



All of the screws inside the box on the left side were pulled out a few mm. There are no cracks inside the box so that part is sealed. I don’t know if this is just a mechanical fastening or if this would allow air to leak. I did reseat it (I had opened the crack for the above photo, it wasn’t nearly that open) and screwed in the screws. They were mostly stripped but did hold enough to keep the box together.

I then got my temperature measurement chart and started to test. I did the first 6 minutes at idle - which was very low, 500ish RPM. What I found was that the right side heat exchanger gets much hotter at the FRONT than the back whereas the left side is pretty consistent along its length. I got in and got the idle speed up to about 1600RPM for 2 minutes and then remeasured.

From my grid:

1) 215°F muffler right
2) 265°F muffler mid right
3) 197°F muffler mid left
4) 167°F muffler left
5) 300°F CAT
6) at back: 152°, at mid 154°, at front 156° left heat exchanger
7) at back: 186°, at mid 220°, at front 300° right heat exchanger
8) 153° engine block

Also, there is a faint but perceptible knocking but I can’t pinpoint it. It is idling ver slow - 500ish RPM. If I had to guess, I would say it is coming from the LEFT side, not the hot right side.

I’m completely stumped and have no idea what to do. The cracked airbox needs some research. The knocking I don’t know about, perhaps it will sound familiar to you. The really hot front section of the right heat exchange is perplexing. I did not drive the car so I don't know if it would ultimately make its way to the muffler to get it as hot as I've observed, I suspect so.

[ischmitz]

How about a little carb or brake cleaner towards the crack of the air box. This will tell you quickly whether the crack is causing a leak or not. If it does there is the first reason for the mixture to be off and your erratic idle. The crack looks real and it would explain why one cylinder is running hotter (leaner) than the others.

From your last post it sounds like your idle is back down to an unhealthy 500 RPM. I was under the impression you had that addressed when re-seating the plug wires, maybe not. So something isn't right there and it wasn't the plug wires. All points towards mixture being off.

For the timing check you really just want to get a an idea if you are in the ball park or way off. So it doesn't really matter if the engine is close to operating temps or not.

I think you are at a point where you either need to get the proper tools (CIS pressure gauges, wideband O2 sensor) or have someone take a look. Chances are your mixture is off because of a vacuum leak and this causes the low RPM at idle. Once that is addressed you need to look at the muffler. Maybe it is rotted out internally.

Ingo