Misfire and plug fouling

keynsham1 · 05-18-2016, 04:07 AM

My 1978 911SC runs beautifully except for one thing. If I drive along for more than 5-10 minutes it starts to misfire. I suspect this is plug fouling as if I drive it harder and higher revving, the problem clears. This would mean I need hotter plugs, which relates to a lower number. Does anyone have any experience of what to use? I have standard plugs at the moment which I believe are Bosch WR3CC which in the NGK numbering system would be a 9. This is a very cold plug.

Does anyone have a recommendation for a plug type to solver this issue?

bpu699 · 05-18-2016, 04:36 AM

Do a leakdown test and make sure your ignition is working properly (car/rotor/wires/etc)...

Look at your plugs, what do they look like? Start with that. Is it oil soaked? Carbon covered?

Is your alternator working well? Fuel injectors flowing? AFR good?

A misfire on a stock SC is rarely plugs, unless you are running something odd...

Leebag · 05-18-2016, 04:53 AM

Quote:

Originally Posted by keynsham1

My 1978 911SC runs beautifully except for one thing. If I drive along for more than 5-10 minutes it starts to misfire. I suspect this is plug fouling as if I drive it harder and higher revving, the problem clears. This would mean I need hotter plugs, which relates to a lower number. Does anyone have any experience of what to use? I have standard plugs at the moment which I believe are Bosch WR3CC which in the NGK numbering system would be a 9. This is a very cold plug.

Does anyone have a recommendation for a plug type to solver this issue?

If I'm not mistaken. I thought WR3's are use for turbos ??

RSTarga · 05-18-2016, 10:01 AM

Ngk bp7eix

Targalid · 05-18-2016, 12:05 PM

It could be your engine warm up system also. That the engine miss happens reproducibly at 5 to 10 minutes is a clue. When the engine is fully warm then it seems to run well? That indicates to me that your fuel mixture is too lean too soon. First place to look is fuel pressures with a test gauge. You can monitor the fuel pressure gauge and the engine miss simultaneously if you set it up properly.

keynsham1 · 05-18-2016, 12:21 PM

I may have been a bit misleading in my question here. There is nothing wrong with any system on the engine as it has recently been fully rebuilt by me and all fuel and ignition systems fully checked. The problem occurs when I drive for a period of time at low revs, say in a 40mph speed limit in 5th gear. It happens after the engine is fully warmed up and can be cleared with a good blast through the gears. I have really already come to the conclusion that this is plug fouling due to having too cold plugs. I was just wondering what plugs others use?

Bill Jennings · Bill Jennings's Garage

Why would you drive around at 40mph in 5th gear?
I'd say you are lugging the engine when doing that!
Not sure if that would foul plugs but I cannot think that is a good thing to be doing.
Cheers
Bill

Jonny H · 05-18-2016, 02:10 PM

I can't say I ever use fifth gear below 70mph! You should be in third at 40mph. It's not a taxi.

SCadaddle · 02:45 PM

Quote:

Originally Posted by Jonny H

I can't say I ever use fifth gear below 70mph! You should be in third at 40mph. It's not a taxi.

Yep. If you are driving around under about 2200 rpm then you're in the wrong gear. My 82 SC manual suggests:

Minimum upshifting points

1st to 2nd 15mph or 2500 rpm
2nd to 3rd 25mph or 2300 rpm
3rd to 4th 40mph or 2700 rpm
4th to 5th 48mph or 2600 rpm

and

Maximum downshift points

5th to 4th 118mph or 5300 rpm
4th to 3rd 95mph or 5100 rpm
3rd to 2nd 63mph or 4600 rpm
2nd to 1st 36mph or 3600 rpm

As for spark plugs in my 82SC it's NGK BP7ES for my tropical climate at sea level. Install them bone dry---no anti seize.

RSTarga · 05-18-2016, 03:14 PM

The plugs are still too cold. The BP7EIX is a much better heat range.

keynsham1 · 05-19-2016, 04:16 AM

Quote:

Originally Posted by Bill Jennings

Why would you drive around at 40mph in 5th gear?
I'd say you are lugging the engine when doing that!
Not sure if that would foul plugs but I cannot think that is a good thing to be doing.
Cheers
Bill

I was always under the impression these engines were supposed to be torquey, but they aren't are they? Actually for a 3 litre engine, the on road torque characteristics are extremely poor! If you cannot drive around in a high gear at low speed, as you can in almost any other car I have ever driven, then that would seem a bit weird to me? Having to drive a 3 litre car over 2200RPM al the time would suggest an engine problem to me! I certainly expect more from my car.

Bakester · Bakester's Garage

Quote:

Originally Posted by keynsham1

Having to drive a 3 litre car over 2200RPM al the time would suggest an engine problem to me!

For me, its torture to keep it under 3k for the warm up.

theiceman · 05-19-2016, 04:46 AM

I guess you expect what you expect from your car ... but it doesn't change the fact if you drive around at 40 in 5th gear on a too cold of a range plug you are gonna have issues ..

wayne robson · wayne robson's Garage

Hello, why would you use spark plugs that are for turbos ,go to your owners manual
and check, i think the correct plug for your year is Bosch WR 8 DC, or as RS targa
says NGK bp7 especially in a cold ,wet area you live, .Also 40 mph in 5th thats another
story that others have mentioned regards.

RSTarga · 05-19-2016, 10:59 AM

Quote:

Originally Posted by keynsham1

I was always under the impression these engines were supposed to be torquey, but they aren't are they? Actually for a 3 litre engine, the on road torque characteristics are extremely poor! If you cannot drive around in a high gear at low speed, as you can in almost any other car I have ever driven, then that would seem a bit weird to me? Having to drive a 3 litre car over 2200RPM al the time would suggest an engine problem to me! I certainly expect more from my car.

You bought the wrong car, maybe a Corvette would be a better fit for you.

keynsham1 · 05-19-2016, 11:36 AM

This is an interesting thread now. Interesting because we all put up with these cars and their issues and still love them. Most other makes with inherent major issues such as failing head bolts, failing con rod bolts, exploding airboxes, excessive oil leaks, quirky gearboxes, etc, etc, would be avoided like the plague. Imagine if your nice new modern Ford or Chevrolet needed an engine rebuild at 75k miles! Would you buy it?

gomezoneill · gomezoneill's Garage

How about a photo of the plugs pull them all out number them to the corresponding cylinders let the guy's see what they see. This is all just unsubstantiated conjecture so far. What is your co% set at? Is oil level to high are the plug wires new? Are the injectors spraying correctly do you have a vacuum leak is the distributor wire cracked is the CDI working correctly? on, and on the point is it could be a hundred things start with the basics wires, cap, rotor, valve adjustment tune up.

RSTarga · 05-19-2016, 01:46 PM

Quote:

Originally Posted by keynsham1

This is an interesting thread now. Interesting because we all put up with these cars and their issues and still love them. Most other makes with inherent major issues such as failing head bolts, failing con rod bolts, exploding airboxes, excessive oil leaks, quirky gearboxes, etc, etc, would be avoided like the plague. Imagine if your nice new modern Ford or Chevrolet needed an engine rebuild at 75k miles! Would you buy it?

Still a hell of a lot more reliable than a British car!

On this site you only see the problems, there are thousands of cars that run perfectly everyday for years. From personal experience, I can say that the Porsches I've owned have been way more reliable and cheaper to fix than any of the other cars I've owned.
Mercedes, Jaguars, A Rolls, VW's, (water Cooled). Ford, and a Range Rover.

Of course you have to use the proper parts and do proper maintenance. Then drive them like they are supposed to be driven.

Jdub · 05-19-2016, 06:58 PM

Have you ever wrested the leaking, misting heater box out of the center of a '69 MGB, or seen the horror of the rusted box frame rails your TR-4a is hanging its swinging arms from, or had the kickstart of a Norton Commando swing around on your shin thanks to that Portuguese layshaft bearing?

Frankly the 911 was a revelation to me.

stlrj · 08:42 PM

Quote:

Originally Posted by keynsham1

I was always under the impression these engines were supposed to be torquey, but they aren't are they? Actually for a 3 litre engine, the on road torque characteristics are extremely poor! If you cannot drive around in a high gear at low speed, as you can in almost any other car I have ever driven, then that would seem a bit weird to me? Having to drive a 3 litre car over 2200RPM al the time would suggest an engine problem to me! I certainly expect more from my car.

I agree.

Most 911s are poorly set up which is why most need to keep their revs up to mask underlying problems due to poor ignition and mixture. I personally feel very comfortable cruising along in 5th gear at 25mph on my 87 Carrera (it's quite torquey) and never feel like I'm lugging it as my foot is barely touching the throttle.

Even my domestic car, a 96 Chrysler Concorde, has a fairly high performance 24 valve 3.5 liter V6 that puts out a respectable 214hp at 5800rpm and yet spends 90 percent of it's life loitering between 1500-2200rpm with lots of torque at any rpm.

My advice, since it's your car, is to drive it the way it feels comfortable to you. I think it could take it.

I also think the hotter plug suggestion is a good idea.

Cheers,

Joe
87 Carrera
96 Chrysler Concorde
03 Grand Caravan