SC engine hesitates when too hot

rockyou · 07-10-2007, 06:23 PM

Hi all,
I have a SC and I have this problem:
My engine runs very well normally, however there has been a few times that it hesitated really badly, it felt like as if few of the cylinders have stopped working. It started to backfire really badly, and loses power. It seems to be related to extreme hot weather, and when there are lots of idling.
Any suggestions or ideas please?
Thanks in advance!

Paulporsche · 07-11-2007, 05:21 AM

Do you have a Permatune CD?

rockyou · 07-11-2007, 08:53 AM

I have the original CDI Module

Paulporsche · 07-11-2007, 11:39 AM

This is the kind of problem that could be caused by many things.

Did this come on w/ recent hot weather? How long ago did you do a valve adjust, change ignition parts? Is the backfire out the intake or exhaust?

rockyou · 07-11-2007, 11:58 AM

I actually just bought the car about 1/2 year ago. The previous owner has done a recent top end rebuilt on the motor (may be few years ago). Before I bought the car, she has been very lightly driven, with only few thousands miles after the rebuild.
The pre-purchase report from the dealer does say the valve needs to be adjusted, but that doesn't sound like where the problem could be...
I've only had this problem twice, both time the weather was quite hot and I was on city roads for quite some time.
In combination with this poping problem, I notice I have a hot start problem too, I suspect I need to replace the cold start valve, but not sure if this is related to the problem here...

Paulporsche · 07-11-2007, 12:53 PM

Hot start problems are often caused by a bad fuel accumulator. The CSV doesn't come into play unless the engine is cold.

Bacfiring out the exhaust is usually a sign of a too rich mixture. Backfiring through the intake usually means too lean. Which are you getting?

Targa4now · 07-11-2007, 02:23 PM

My 1981 SC displayed the same problem. Ultimately the O2 sensor relay under the front passenger seat was to blame.

rockyou · 07-11-2007, 02:50 PM

I think it was from the exhaust...

Paulporsche · 07-11-2007, 06:06 PM

If you get the Bentley manual, it will tell you how to troubleshoot the CIS system, including the O2 relay/oxygen sensor system.

Do you get any other symptoms of a too rich condition, say for example, a fluctuating idle?

rockyou · 07-11-2007, 08:42 PM

I do have the Bentley, but I'm trying to narrow it down to less item to diagnose as I'm only a beginner...
When the problem occurs when it's really hot, when idle, it wants to stall...
another person in another forum suggested that the CDI module may be at fault... how likely would that be?
btw, would you know where does Bentley covers how to change the fuel accumulator?

willf · 07-11-2007, 09:13 PM

have you tried to replace or fiddle with the O2 sensor relay under the passenger seat as Targa4now mentioned? I had similar issues when the weather got hotter; changed the relay and those issues went away... good luck bro

rockyou · 07-11-2007, 09:22 PM

willf, no I haven't, I guess that would be on my list to check this weekend...

TibetanT · TibetanT's Garage

Subscribing for information purpose.
Thanks.

Paulporsche · 07-12-2007, 05:37 AM

The CDIs do fail, but other things like relays are more common. There was a recent thread on changing the accumulator. You should try a search.

Christien · Christien's Garage

Paul, what made you ask if he had a Permatune? I'm asking because I just swapped my old Bosch CD for a Permatune.

rockyou, I noticed the title of this thread sounded unfortunately familiar, so here's my story: I was having some kind of power issues at the track - only when the car was good and hot, it was struggling uphill. I had a local shop owner and racer do a couple laps, and he suggested it was most like the CD box, possible also the coil. He said when these old boxes get hot they don't generate the juice like they should, esp. at higher rpm's. The CD was the original Bosch unit, 35 years old. So I just picked up a used Permatune and swapped it in a couple days ago, and I'm off to the track on Saturday to put the boots to it and see if that got the problem. I realize that a used box isn't necessarily the best choice, but I want to see if it makes a difference. If it does, I'll probably buy a new MSD. I'm getting sick of throwing money and time at this problem (carb rebuild, tuning, fuel delivery, etc.)

rockyou, bear in mind my car is a 1972 with carburetors, so there are many different variables at play, but hopefully this info might help you a bit.

If I remember, I'll post results from Saturday here. If I forget (quite likely!), pm me.

SCWDP911 · SCWDP911's Garage

sub'd

Paulporsche · 07-12-2007, 06:42 AM

Chris,

I asked because there has been a lot of chatter on this BB about failed Permatube boxes, especially ones that are fairly new, and especially when they get hot. I'm not positive, so you should search, but I think this is for the newer, silver ones. I think the old, blue ones are OK(keeping in mind that sooner or later I guess they all will fail).

I've had an MSD on mine for about 2 1/2 yr and it's fine.

Rockyou,

How hot is your oil temp when this happens?

Christien · Christien's Garage

Ok, thanks. I do seem to remember coming across that briefly when I was searching, troubleshooting my Bosch CD. The one I bought is a blue one.

rockyou · 07-12-2007, 09:15 AM

Thanks Christien, have fun on the track.

willf, Targa4now, did your O2 sensor relay went bad when weather gets hot, and back to normal when not as hot?

Paulporsche, I 'think' my oil temp was normal (I should chk again...). I remember one time the problem appeared in a hot afternoon after my car has been parked in my open driveway for a few hours.

rockyou · 07-14-2007, 10:17 PM

Just want to sum up a bit what I've done today on my car.
I found out that the O2 sensor has been disconnected by the previous owner. And then I've read some threads here that says some people actually disconnect it intentionally? Anyway, since the car seems to be fine before without it, that probably isn't the problem.

The engine got the hesitation problem again today after just a few minute drive. The oil temperature was not high at all. I could smell gas, so fuel system probably is not the problem.

I guess the only thing left is the ignition system? I've followed the Bently instruction and tested with the CDI Module 6 pins connector. I got 12v reading for the power, and there were pulse when cranking the engine. According to Bently, this means CDI module or coil are suspect.

Right now I'm still researching how to properly test the coil...
will report back when i have more findings...

Thanks again for all the previous input