911SC K-jetronic problems?

[hubosity]

Please help. I have had my 79 911SC for over 20 years now, but I am not too familiar with the fuel injection, which seems to be giving me trouble. current symptoms:

-starts and idles OK
-sputters and occasionally backfires into intake when you give it gas (especially when cold)
-will drive and accelerate moderately using only first 50% of accelerator travel. If I push the pedal down more, it slows down
-at RPM's above 3-4k it seems to run slightly rough
-can get to highway speeds but not enough power to maintain the speed on log hills.
-I ran techron through my last 2 tanks but no noticable improvements.

I live in a small town without a good import mechanic. Any hints or likely culprits will be helpful and well appreciated.

Rich

[fred cook]

Check the distributor. It sounds like the advance weights have frozen in place. Pull the dist. cap and try turning the rotor slightly. You should be able to see the plates move in and out a small amount. If not, the dist. will probably need to be partially taken apart to get to the pivot points.

Hope this helps.

[Nostril Cheese]

I would first check all vacuum connections including the air boot on the top for cracks. Replace all the vacuum hoses if they are more than a few years old.

Replace fuel filter and acuumulator. Check ignition timing, cap and rotor.

[Paulporsche]

When did you last have a tuneup, incl plugs, caps, rotor, wires and fuel filter? Have you checked your fuel and control pressures? I would also check for air leaks, and the operation of your throttle linkage.
Have you ever checked for broken head studs?

[favrok01]

did you fix your problem?

[hubosity]

Thanks for the suggestions. I had no access to internet over the weekend due to someone digging up the phone line down the road.

I guess I assumed the probelm was in the fuel injection...maybe not?

(Paulporsche) The car had a recent tune up with new plug wires, cap, rotor, fuel filter, etc.

(Fred Cook) a couple weeks ago, a mechanic told me the advance on the distributor was frozen, but he says he fixed it. I will check to verify.

(favrok01) I have not messed with the car in a few days (hoping to see some good feedback from this post). It sounds like my next step will be to check for vacuum leaks/cracks, verify the operation of the distributor (although I guess lack of vacuum could cause the same symptom?). I guess I should get the car back to a shop that can check the fuel/control pressures

I have not checked for broken head studs and I don't think the accumulator has been changed in a long time. Are these likely culprits?

thanks!

[Paulporsche]

Did the problem start before or after the recent tuneup?
Have your mech check the fuel and control pressures and the WUR. Make sure your throttle linkage is working properly.
Did he do a valve adjustment? When he does he should be able to check for stud problems.
Check for air leaks.
Have you ever checked compression/leakdown?

[hubosity]

You may be on to something. The problems actually started right after the tuneup when I noticed a general lack of "zippiness" and what felt like a lack of power at higher RPM. It has gotten worse, becoming at one point undrivable. This, though, may have been a fuel problem since it got much better (back to the current situation) after a fresh tank of gas. Before the tuneup it ran great, it just wouldn't idle. Now it idles OK but that's about all.

I didn't get a valve adjustment this time

[Paulporsche]

What did he do when fixing the dist problem? Have you rechecked your timing?
I always try to check the simplest, easiest, quickest, cheapest things first. After that: things like pressures, valves, compression tests.

[hubosity]

Mechanic said the advance was locked up due to what looked like rust in the lower corner. He pulled it out, cleaned, lubricated, etc and said it was working OK.

At the same time, he also said he verfied the timing, fuel pressures and that the engine had proper vacuum - all apparently OK.

I am bringing it back to him this week to check again, as I think that some of the settings may have been complicated due to poor fuel in the system at the time.

I agree (hope) that it must be something simple. Just wish I knew what it was

[Gunter]

Originally posted by hubosity
Mechanic said the advance was locked up due to what looked like rust in the lower corner. He pulled it out, cleaned, lubricated, etc and said it was working OK.

Rust in the lower corner of what? The vac pod? Or the inside of the Distributor?
To properly service (Clean and oil) the distributor, it has to be taken apart.

Go here:
Distributor service (Clean and lube) real easy without removing the pinion gear!

Find out if the mechanic did that.
It sounds to me that there is a problem with the timing-curve.
After the distributor is serviced, have him set the timing to maximum advance instead of going by the standard 5 deg BTDC.

Like this:

Maximum advance for SC 3.0 or ss 3.2

Print out both instructions and take them along.
If he doesn't understand, maybe you'll need another mechanic?

BTW: A real tune-up starts with setting the valves first, then the rest like timing and mixture.

[hubosity]

Gunter - thanks for the advice. I will rely on the extreme ignorance and second hand information defense.
He said the gunk was inside the distributor and asked me if I had driven the car through any deep water recently. The thread you referenced is excellent and I will bring a copy to verify that it was done correctly.

The car was actually running fairly well before I took it in for this "tune-up". The biggest problem was a tendancy to stall at idle, so I choose not to invest in checking the valves at that time (mistake?).

[Gunter]

Depends on how long ago the valves were set?
Long time? Needs to be done!
If you have any doubt about your mechanic, don't go back.
Find a shop that really knows 911s and is able to service the distributor.
Asking if you had driven thru water lately because of gunk inside the distributor wouldn't make me feel comfortable about this expert, especially since it run worse after the "tun-up" Hmm.......
Gunk inside is normal after 25 years and servicing the distributor can only be done by dismantling it.
Sounds to me that the vacuum advance inside was rusted up and that means the same thing: Dismantle, clean, oil. then check the outside vac pod to ensure that the diaphragm is intact.
Hmm.....ask around to ensure that you find the right shop; some Pelicanites in your area may know.

[Mysterytrain]

Check to make sure the vacuum line is connected to the distributor. Also, check that the other end of the vacuum line is connected to the correct port on the intake.

[hubosity]

UPDATE

Yesterday it was hot and dry and the car ran much better (rev to over 4k without problem, accelerate strong but still rough and hesitant if given full throttle. This morning cool and wet and car back to original conditions (lose speed if given more than 50% throttle)

Took my car back to mechanic today:

- verified that distributor was diassembled and cleaned properly. timing verified to factory spec. Vac line is attached and advance is working properly.

- verified fuel pressures within spec and CO adjusted properly


mechanic feels that problem could be related to air flow sensor, especially since the problem seems to be a little inconsistent. Any other ideas? How does one service/adjust/rebuild/replace this sensor if it is a problem? (I notice they are not even available at PP).

[Nostril Cheese]

Thats because the SC doesnt have an airflow sensor.

It has a airflow plate connected mechanically to the fuel distributor.

Check the intake hood on the top the airbox for cracks

[fred cook]

With the car in a dark place, start the engine and open the engine cover. If any of the plug wires are bad, you will see mini lightning bolts arcing to the engine (ground). The coil wire is especially bad about being sensitive to moisture. If the coil wire is bad, you will experience all of the symptoms that you have described.

[stealthn]

Good year!

Start simple, are the spark plug wires in the correct places in the cap, is the cap tight on the dizzy. I would say advance and/or mixture, but track what you do to troubleshoot it.

Check all plugs and wires, then move to fuel.

Good luck and keep us up to date.

[Gunter]

So, still the same mechanic?
Didn't know that there is no airflow sensor on an SC?
Hmm............................
How about finding someone who knows Bosch K-Jetronic CIS before replacing any parts that don't need replacing?

[T77911S]

Originally posted by Nostril Cheese
Thats because the SC doesnt have an airflow sensor.

It has a airflow plate connected mechanically to the fuel distributor.

Check the intake hood on the top the airbox for cracks

WHAT? Q:what is an aiflow plate? A: an airflow sensor

i wouldnt let him go and mess with the airflow sensor as long it freely moves up and down. i would keep looking first.

does this car have a vacuum line to the WUR?