Choppy acceleration/deceleration

[Tommy Z]

Hello All. 79' SC with 125k on the clock. Lately I am noticing what can best be described as choppy acceleration and deceleration. This only really occurs up until 3,000 RPM's and then it's fine and smooth. Same goes with the deceleration, from 3,000 down it has a choppy somewhat jerky feel even if I'm feathering the pedal in traffic. These symptoms are especially noticeable when the engine is cold and less so when warm but still there. I have also noticed in the last few weeks that it's a little difficult to start when cold. It turns over just fine but doesn't seem to be getting fuel right away. Car starts like a champ when warm. Wondering if it's a fuel pump issue?

Full disclosure I've had the car for only about about 4 months and I do not know the last time the plugs, wires and cap were replaced and it's on the list. I'm going to try and tackle that one myself. I've no experience wrenching prior to owning this car but have jumped into it with both feet thus far and I'm loving every minute of it. Any advice on what to start looking at to solve this one would be greatly appreciated. Cheers!

[darrin]

you might want to try throwing a bottle of techtron in at your next fillup -- can do a good job cleaning out any "gunk" in your injectors -- could be a lot of other things, but couldn't hurt

Wheels are looking NICE

[991gts6s]

do you hear the pump buzz or whine when you turn the key...

misses are usually ignition ... plugs wires cap etc..

[Tommy Z]

Thanks Darrin, They look nice in photos but not perfect under scrutiny. I'm actually thinking about going back to black.

991 - I haven't noticed any pump noise except after busy traffic days in high Florida heat. The pump audibly whines under those conditions. I'm sure it's time for an ignition tune. That will be a first for me as well. Never done anything like that before but I'll have to just suck it up and go for it.

[FrontGarage911]

I had a similar issue after doing a valve adjustment and spark plug change on my 1987 Carrera.

It sounded a bit throaty at idle, but I knew something was wrong as soon as I shifted to second gear and the car felt "heavy" or sluggish.

After removing and gapping the plugs to spec along with changing the cap and rotor, the hesitation went away.

[SCadaddle]

Similar symptoms with my SC, then accompanied at idle by a distinct "crunching" noise, which was isolated to the distributor. Unclipped the distributor cap and lifting up the rotor while still on the shaft it would move up almost 1/4". Should be much much less, and once the distributor was removed it was apparent that the fiber washer/shim at the base of the distributor was completely gone. Refitted a proper distributor until time to rebuild the old one and problem solved.

[racer]

Does sound like a fuel issue or even a WUR issue given the low speed/ hard start issues. Can't hurt to baseline ignition just to be sure though.

[Tommy Z]

Thanks Racer, sorry to be so ignorant but what does that mean, "baseline ignition"?

[racer]

Originally Posted by Tommy Z

Thanks Racer, sorry to be so ignorant but what does that mean, "baseline ignition"?

If you aren't sure how many miles are on or how old certain items are, it might be worth checking timing, replacing plugs, checking spark plug wires, check distributor etc.

[Flat Six]

Sure sounds like an ignition issue to me . . . .

[jpnovak]

Most likely sticking advance mechanism in your distributor. This will help.

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

[kach22i]

Originally Posted by jpnovak

Most likely sticking advance mechanism in your distributor. This will help.

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

I remember that thread now.

My post on page #9 shows some wires that were worn and shorting on my car.

Get in there with a flashlight and look around for possible similar problems.

Also note that DME relays can get burs on the contacts, replace with new.

You can open up the old one (cylindrical type) for inspection (cap may come off base) - keep it as a spare once de-burred.

[jpnovak]

Originally Posted by kach22i

Also note that DME relays can get burs on the contacts, replace with new.

No DME relay on a 79SC. CIS car. Not motronic.

[kach22i]

Originally Posted by jpnovak

No DME relay on a 79SC. CIS car. Not motronic.

I guess I stand corrected, the cylindrical ones I described are called "Multi-Purpose Relay" not DME relay.

Example:
https://www.pelicanparts.com/More_Info/91161510801.htm?pn=911-615-108-01-OEM&gclid=EAIaIQobChMI0eqWhNzb7QIVSrzACh2iPAomEAQYBCAB EgKjovD_BwE


Great article on the topic.

https://philipraby.co.uk/porsche-dme-relays-explained

DME stands for Digital Motor Electronics..................

A relay is essentially an electromagnetic switching device that permits a low-current circuit to control (to switch on and off, in other words) a relatively high-current circuit (in this case those feeding the fuel pump and the ECU). It consists of essentially four parts: an electromagnet (a coil of ultra-thin copper wire with a moveable iron core inside it), an armature, a spring, and a pair of electrical contacts.

Like I said, at one time (12 years ago) I had to clean off the burs on the contacts until a new part came from Pelican.

I've seen cars running without the protective cover cap on, sometimes they just come apart. Other times the base and top cap are plastic fused together real good.

More info:
Multi purpose relays

The question was can you use a red relay in place of the black relay or vis-versa.

So..I purchased a red and black Porsche relay (whey were not expensive at the time) and took them apart. The terminals and numeration are the same for both relays. 30 goes to 87a when not energized and 30 powers up 87 when powered up (86-85)

The only difference in the relays is the diode..used on the red relay to avoid voltage spikes when the voltage collapses.

The relay with the diode must be used whenever a solid-state device is attached/connected to the relay armature coil, as is the case with the fuel pump circuit. The diode "damps" the voltage spike that the armature coil's induction would otherwise generate.

More correctly, the diode in this application prevents the back emf from re-energizing the relay by blocking the reverse path (during the decaying time constant). Also known as ringing or 'chatter'. It only lasts milliseconds if that, and the spring helps a lot too.

The spike of concern would only come from the relay contacts re-contacting - in the unlikely event that happened - again for milliseconds.

A 'clamp' is used to prevent voltage from exceeding a certain level and is typically a zener, and this one isn't a zener. This one is known as a snubbing/snubber or squelching diode, and is used to block the reverse polarity and help the voltage decay to zero.