911sc stumbling/hesitation

[dntlft]

My 1983 911sc has developed a drivability issue. This occurs when the engine is fully warmed up, as I don't rev the engine beyond 4000rpm when cold.
When Driving at WOT, the engine will exhibit an odd stumble or hesitation, as if misfiring. This will begin around 3500 to 4000rpm and go away usually north of 5000rpm. If I back off to maybe half or partial throttle, normal revving and power seems to resume. Seems more pronounced in 2nd through 5th gears. It has no issues or noticable problems with cold or warm starting, and the idle is smooth and even @950rpm. Thanks to all in advance for responding!
I use 93 octane Mobil 1 exclusively.

[1978911sc]

I experienced something similar with my '78sc, but it only happened under load. It ended up being worn out ignition wires. The shielding had failed and the wires were arcing through the insulation. Once replaced, the problem went away and the car ran great!

[Reiver]

I'd go with that response....I've had similar low power experiences with older wires. One of those things often overlooked as we all go 'worse case scenario'.

[dntlft]

Thanks for your replies. I hope it's something that simple. Is this something I can test for somehow before dropping $$ on parts? Also, I noticed the engine does not misfire or stumble when revving in neutral...

[Reiver]

Not sure, how long have you had the wires in place....one way is to pull the plug caps, shorten the wires and re attach...they tend to wear/burn at the cap connection...or if they are old just replace them.
When in neutral there is no load and even a weaker spark may do fine...a swag tho when at a distance but do the simple stuff first.

[mca]

I've experienced what you are describing. Once it was due to wires (brand new oem) and another time it was due to a bad coil.

[dntlft]

I believe the wires are old, they appear to be the braided steel covered type, should I replace with OEM (expensive) or are the aftermarket type sold on PP's OK?

[Bob Kontak]

One way to check wire issues is to peer into your engine compartment when it is dark. Give your eyes time to adjust - about 5 minutes. You can then blip the throttle, etc to check against your symptoms. You may be surprised what you find.

The Clewitt Engineering wires are nice. I have them on my 81. Favorable price relative to OEM Berus, also. Folks speak highly of Magnecor.

[Rodek]

This is my vry first post and I'm glad to see this thread as I'm having a similar issue on my 78 SC. I'll be checking those plug wires ASAP.

[Linehand]

Guys I’m having the same issue , but it appeared after putting on Brand new wires “clewitt wires, new cap and rotor and back dating the heat. Could I have done some thing to the distributor … it was a tight fit back dating the heat and definitely put pressure on the cap while putting back on the distributor. I just bought a fuel filter going to install that this week.

[Bill Douglas]

I've mentioned this before, so please excuse me for repeating myself.

I thought it was a bit odd the distributor cap was hard to click down on one side. It turned out I had the plastic protector thing on the inside slightly crooked putting the cap on an angle. The SC ran fine when it was cold, and warm, but got a certain missfire when hot and under load.

[Linehand]

I changed the fuel filter no difference. If I hold a grounded screw driver to the distributor cap at near plug 1 it arcs. It does not arch if I hold the grounded screw driver near other plugs. Thoughts? Bad wires causing my problem ?

[Schulisco]

Originally Posted by Linehand

Bad wires causing my problem ?

If no mistake been made on installation - I would say yes, make use of your right of return.

You can measure the cables with a multimeter for electrical resistance about 1k-5k ohms. If one or more wire is broken and if the insulation is broken the spark can arc through the insulation. If the measured ohm value is far higher or infinitive then it's an interruption. But: Even an interruption of a ignition wire must not mean that the spark won't go to the spark plug...therefore most interrutions on ignition wires are discovered only by measruing in case of several symptons...so the sparc can arc but not on some situations...

Thomas

[glewis80SC]

Same here it ended up being a bad spark plug boot and my 80SC.

[kltarga72]

Originally Posted by dntlft

My 1983 911sc has developed a drivability issue. This occurs when the engine is fully warmed up, as I don't rev the engine beyond 4000rpm when cold.
When Driving at WOT, the engine will exhibit an odd stumble or hesitation, as if misfiring. This will begin around 3500 to 4000rpm and go away usually north of 5000rpm. If I back off to maybe half or partial throttle, normal revving and power seems to resume. Seems more pronounced in 2nd through 5th gears. It has no issues or noticable problems with cold or warm starting, and the idle is smooth and even @950rpm. Thanks to all in advance for responding!
I use 93 octane Mobil 1 exclusively.

Had the same exact problem with my 1980 SC. I had replaced plugs, wires, fuel pump and fuel filter plus checking the distributor. Around six months after these repairs I noticed the issues you described, at first intermittently and then more frequently. I fought this issue for the next several months until I pulled into a parking lot and keeping the engine running. I was in the engine compartment listening and looking and not finding anything, I sat in the drivers seat and revved the engine a couple times and let the engine idle. I heard a very soft strange intermittent humming noise coming from the fuel pump area. I drove the 911 home and put it on the lift to gain access to the fuel pump. With the engine running I could put my hand on the pump and feel that the pump was making the noise. That new Bosch fuel pump was the culprit, replaced and problem disappeared. I was told the fuel pump was made in South America.

[Linehand]

I fixed the arcing at plug one. It was not the wires it was a faulty distributor cap. I put a new cap on and the arcing stopped . But I still have the hesitation at low rpm’s . I guess I’ll change the fuel pump and fuel accumulator. I hear loud wining sound coming from where the fuel fuel pump is . Is the fuel pump suppose to make a loud wining noise?

[fanaudical]

Couple of thoughts before you start randomly replacing parts:

- The accumulator is most likely fine unless it is leaking. I have found that cars that run well at load rarely have fuel pump issues.

- Rotor good? Any play in the distributor shaft?

- Is the ignition advancing the timing through the RPM range as it should?

- Wires should be good now. Plugs OK?

- Have you verified function of the injection system? Vacuum leaks?

- Have you checked the spray pattern on the injectors?

[Linehand]

Yes there is play in the rotor shaft.
Not sure if the ignition is advancing through timing properly.
Not sure if the plugs are good.
Vacuum leaks are on my list.
How do i check the spray pattern on injectors?

[Linehand]

Thinking back. It seemed to run a little smoother hot on a 100 degree day. Does that mean anything?

[Linehand]

I will try and run down the check list and see that I can find, thank you fanaudical