82 911sc Running Rough after Warm

[zingerroo]

Interesting one here and have been doing a bunch of research with incredible information however most peoples problems seem to be the reverse of mine; cold start problems/running where mine is problems once warm.
Takeaways and info.
Brief history - have owned the cars for 2 years and love it, no problems whatsoever. Always starts, runs great, etc... the one we wanted and truly enjoy it.
I have a branch office about 60 miles away and take it weekly for a run through the gears and warm up, never fails and enjoy the time spent behind the wheel.
On one of my trips two months ago I was cruising along and a hesitation started, following by a bit of rough running. 60-70MPH, 2000-2200rmp. It felt like it started missing, less power and if I could keep RPM's up it wouldn't be too noticeable however this came with lack of the power I was used to; again best way to put it is a miss. I pulled over and check plugs and sure enough the boot over the forward passenger spark plug was loose. I pushed it down, fired up and cruised home with no issues.

Fast forward two months and everything has been great.
Last week I drive again to the branch office and 10 mins into the drive there is the hesitation and loss of power again. Well I know what this is and check the boot but its secure... Starts back up fine and another 10 mins passes and symptoms come again. Pull over, turn off... wait 5 mins fires right up, no problems. Onward and this pattern occurred until I reached my office. I let it set for 3 hours and took it to dinner, 5 mins away and no problems. I stayed the night, ran an errand in the morning no problem.
I needed to get back so thinking the problem is resolved I start my trip home. Almost identical to the day before everything is fine for about 10 mins, cruising right along and...miss, another, hesitate when throttling, etc... Stopped 3 times, each for about 10 mins and was able to make it about the same time each with no problems until I got home.

A couple more interesting pieces of info now that problem is consistent:
Car always starts no problem whether cold or warm
When cold the car immediately idles great and runs like normal
When the issue is happening and I turn the car off, the car then does not hesitate to start however the rpm's lobe up and down, almost to the point of killing the car when they are low. I need to give it gas to keep it going.
*I went to a gas station I have never visited PRIOR to my trip up to the branch office (as in on my way out of town) and filled up*

Intial thought would be:
Drive existing tank of gas down
Fuel cleaner/octane add, refill with known normally used gas station fuel - determine if noticeable change of performance
Some basic research (again a lot of "cold start" problems) point to replacement/check of:
Vacuum lines
Injector Seals
Cylinder head temperature sender
Fuel pump relay

Not sure if anyone has seen these symptoms or has any additional info but I will update post fuel change and fuel treatment and let everyone know.

[lsf911]

What do you have for a CDI unit? Original Bosch unit? Could be on its way out.

Also could be a vacuum leak as you mentioned.

If you have fuel pressure gauges you should hook them up and check pressures as the car warms up. That will tell you a lot about what's going on if it's fuel related.

[emac]

I second the CDI. Had the same symptoms and one morning no start. I replaced with a MSD street fire.

Ernie. 81sc

[hughc]

For what it's worth, I was having a problem last fall with my 83 sc before I put it away. It ran quite nice but after it warmed up it would start to misfire, especially during hard acceleration.
I reluctantly suspected failing ignition wires and ordered a new set. I say "reluctantly" because those wires were not old at all.

During my engine drop this winter I replaced those wire with new Clewitts and plugs.
When cleaning my shop and putting stuff away I inspected my old wires and separated them from the distributor cap (removed as one unit from the engine).
A closer look at the cap showed one terminal completely black, an indication that there was a misfire on that wire. I guess that wire wasn't completely "home".

At this point I suspect that was causing my misfire . The engine runs and idles very nice but I won't confirm the misfire is gone 'till I give it a good run on the highway.

At this point I would suggest that you check that all wires are well seated. It could be that simple.

Good luck.

[zingerroo]

Thank you all for the replies and the helpful suggestions..
The CDI unit is a replacement Permatune, which I have been doing some research on and getting mixed replies however most seem to not recommend.
I am going to look at the cap this morning after a quick spin, would like to work the new gas through it and a nice morning for a run down to the store, thank you for your input hughc and I hope yours is resolved.
Ernie and lsf, thank you as well. I am leaning at this point to replace the permatune (if problem persists). I need to do a little more research but let’s say the permatune is bad, why is the problem occurring only when warm and not all the time? Is their a control in it that regulates spark differently or is it truly able to distribute spark better when cold then slowly just starts giving out? I guess that makes sense
Regarding going to MSD Street Fire any other items that should be replaced or things to look for if I go that route?
Thanks again, off to the store

[hughc]

Originally Posted by zingerroo

Thank you all for the replies and the helpful suggestions..
The CDI unit is a replacement Permatune, which I have been doing some research on and getting mixed replies however most seem to not recommend.
I am going to look at the cap this morning after a quick spin, would like to work the new gas through it and a nice morning for a run down to the store, thank you for your input hughc and I hope yours is resolved.
Ernie and lsf, thank you as well. I am leaning at this point to replace the permatune (if problem persists). I need to do a little more research but let’s say the permatune is bad, why is the problem occurring only when warm and not all the time? Is their a control in it that regulates spark differently or is it truly able to distribute spark better when cold then slowly just starts giving out? I guess that makes sense
Regarding going to MSD Street Fire any other items that should be replaced or things to look for if I go that route?
Thanks again, off to the store

Hi zingerroo

Just wondering if you made it back from your ride to the store.😀
Did the car run any better?
I'm kind of curious.
Thx

[emac]

Zingerroo
My 81 had the same symptoms and one day while getting ready to leave she wouldn’t start. In my opinion when the unit gets warm/hot something gets weak and then just lets go. I installed the street fire and haven’t looked back. Very easy install.
Ernie

[RedCoupe]

Hey Z,
Before you spend lots of $, I would pull of each ignition wire (and the coil high voltage wire) and use a multimeter and check the resistance terminal to terminal. You could have a wire or terminal faiiling, and a bad wire will show very high or infinite resistance. Also, the infamous "green" wire on the low voltage side is worth checking. I don't have an SC, so I'm no authority on this but there have been many posts regrading this wire.

[Bob Kontak]

Originally Posted by zingerroo

I need to do a little more research but let’s say the permatune is bad, why is the problem occurring only when warm and not all the time?

My guess is that some part of a component inside the unit expands when hot and all the goodness is lost until it cools down and shrinks back together.

I had two Permatune silver box units that failed across a few years in the late 90's. Similar symptoms as yours. Usually took 15 minutes to an hour before restart. Lonnie from Permatune probably spent two hours on the phone with me over several months trying to remedy.

I finally went with the MSD 6AL which is still installed and has been trouble free for pushing 20 years.

I put my box under the drivers seat. It took time to wire with grommets and such.

[john walker's workshop]

O2 sensor relay issue perhaps. Corrosion in the plug to it? Broken wire in the plug? Interior light fuse loose or crusty. Failure goes lean and power loss, like dragging an anchor. Under the passenger seat, check it out. Unplug it when the engine is running and idle should drop and run rough, if it's working.

[zingerroo]

Originally Posted by hughc

Hi zingerroo

Just wondering if you made it back from your ride to the store.😀
Did the car run any better?
I'm kind of curious.
Thx

Thanks for the note!
Car again runs fine and I have worked through the new tank of gas. Will be looking at wires tomorrow and double checking some of the remarks. Leaning towards the MSD Street fight and will give that a run if nothing changes tomorrow. Appreciate everyone’s updates and input, way cool to have the help!

[stlrj]

Sounds like an ignition issue. Try the windex test, the one with the engine at idle while you spray the ignition cables and observe where it misses.

Cheers,
Joe

[Mark83SC]

Originally Posted by zingerroo

Thanks for the note!
Car again runs fine and I have worked through the new tank of gas. Will be looking at wires tomorrow and double checking some of the remarks. Leaning towards the MSD Street fight and will give that a run if nothing changes tomorrow. Appreciate everyone’s updates and input, way cool to have the help!

Hey Z, I have an 83 SC and am having exactly the same issue you described in your post. I am interested to know what the final outcome was? Did you end up changing the CD for a MSD Street Fire? Any feedback on how you fixed the issue would be greatly appreciated.