Ignition problem or fuel/air delivery?

[Alan Lindquist]

I’m chasing two problems, and they may or may not be related. I have a 1980 SC in stock configuration with 300K miles. I’m thinking its ignition related, but I’m open to ideas.

The good – The car starts quickly and willingly when hot, warm, or cold and immediately settles down to a firm and steady 950 RPM idle.

The bad - When accelerating lightly to moderately or coasting in gear and using very little throttle I can feel either an ignition miss (multiple misses that continue until I accelerate harder) or something related to fuel delivery that would cause multiple momentary lapses of power delivery. This is happening in the 1500 RPM to 3500 RPM range, as far as I can tell. My butt dyno says that under hard acceleration I’m not feeling it, but my butt dyno also says that I may be down a bit on power.

Under no load, I can rev the motor and notice the condition by hearing an irregular exhaust pulse above 2K RPMs. Note that Cyl #2 is likely going along for the ride at this point and not contributing to horsepower much.

The ugly - Occasionally under HARD acceleration (especially when engine is over normal operating temp for daily street driving) I can hear detonation at about 4,000 RPM. It seems to happen pulling hard in 4th gear, occasionally third, I never hear it when accelerating hard in second gear. When I back out of the throttle the pinging goes away.

Here’s some current conditions:
Car just passed smog with flying colors. Mixture was leaned for the test, car passed with plenty of margin, then mixture was enrichened. This was done by the mechanic I use when I can’t work on my car, and his business is exclusive to Porsche and especially air cooled, so he knows what he’s doing. The problems I described above precede and follow this service and test.

A week ago I replaced a faulty WUR with the correct replacement. Our Pelican friend Tony Donato (boyt911SC) shipped it to me and it was tested for correct pressures by him. The problems I described above precede and follow this replacement.

A month ago I adjusted the valves, replaced the plugs with NGK BR8EIX gapped to .28, changed the oil, cleaned the K&N air filter and set the timing to 5 BDTC. Ran the RPMs up to 5K and watched the timing mark float about 25 degrees to the advance – but I don’t know how to mark this to be sure of how much advance I actually got. The problems I describe above precede and follow this service.

Several months ago I replaced my distributor (severe axial play) with a known good used distributor. I don’t recall that the problems I described above were happening with the worn distributor and I’m wondering if the mechanical advance and all other aspects of the distributor are working correctly.

About 8 months ago I replaced all fuel injector sleeves and seals, epoxy filled a crack in the air box, and repaired a junction for the main vacuum line for the brakes. I confirmed no more vacuum leaks with a positive pressure test with soapy water and a shop vac blowing up the tailpipe with the throttle body blocked. At the same time, I baby bottle flowed the injectors simultaneously and the pattern looked good with even flow across all six injectors.

Several years ago, my coil gave out and was replaced with a Bosch silver (chrome) unit. I don’t know how to test this.

The spark plug wires are very likely factory wires with 300K on them. They are the shielded kind. I got the car with 120K miles, and I haven’t changed them in the last 180K miles. Is there a simple/trustworthy way to test each wire individually?

With all that to consider, would you start by chasing an ignition problem and investigating the distributor or start getting fuel pressure statistics to evaluate the running condition?

[timmy2]

From all the excellent information you have provided, I would first confirm the fuel pressures are all within specifications and them take a good look at the distributor.
A replacement coil is inexpensive if you are willing to try the MSD high vibration coil model 8222.


Sent from my iPhone using Tapatalk

[wwest]

Originally Posted by Alan Lindquist

I’m chasing two problems, and they may or may not be related. I have a 1980 SC in stock configuration with 300K miles. I’m thinking its ignition related, but I’m open to ideas.

The good – The car starts quickly and willingly when hot, warm, or cold and immediately settles down to a firm and steady 950 RPM idle.

The bad - When accelerating lightly to moderately or coasting in gear and using very little throttle I can feel either an ignition miss (multiple misses that continue until I accelerate harder) or something related to fuel delivery that would cause multiple momentary lapses of power delivery. This is happening in the 1500 RPM to 3500 RPM range, as far as I can tell. My butt dyno says that under hard acceleration I’m not feeling it, but my butt dyno also says that I may be down a bit on power.

Under no load, I can rev the motor and notice the condition by hearing an irregular exhaust pulse above 2K RPMs. Note that Cyl #2 is likely going along for the ride at this point and not contributing to horsepower much.

The ugly - Occasionally under HARD acceleration (especially when engine is over normal operating temp for daily street driving) I can hear detonation at about 4,000 RPM. It seems to happen pulling hard in 4th gear, occasionally third, I never hear it when accelerating hard in second gear. When I back out of the throttle the pinging goes away.

Here’s some current conditions:
Car just passed smog with flying colors. Mixture was leaned for the test, car passed with plenty of margin, then mixture was enrichened. This was done by the mechanic I use when I can’t work on my car, and his business is exclusive to Porsche and especially air cooled, so he knows what he’s doing. The problems I described above precede and follow this service and test.

A week ago I replaced a faulty WUR with the correct replacement. Our Pelican friend Tony Donato (boyt911SC) shipped it to me and it was tested for correct pressures by him. The problems I described above precede and follow this replacement.

A month ago I adjusted the valves, replaced the plugs with NGK BR8EIX gapped to .28, changed the oil, cleaned the K&N air filter and set the timing to 5 BDTC. Ran the RPMs up to 5K and watched the timing mark float about 25 degrees to the advance – but I don’t know how to mark this to be sure of how much advance I actually got. The problems I describe above precede and follow this service.

Several months ago I replaced my distributor (severe axial play) with a known good used distributor. I don’t recall that the problems I described above were happening with the worn distributor and I’m wondering if the mechanical advance and all other aspects of the distributor are working correctly.

About 8 months ago I replaced all fuel injector sleeves and seals, epoxy filled a crack in the air box, and repaired a junction for the main vacuum line for the brakes. I confirmed no more vacuum leaks with a positive pressure test with soapy water and a shop vac blowing up the tailpipe with the throttle body blocked. At the same time, I baby bottle flowed the injectors simultaneously and the pattern looked good with even flow across all six injectors.

Several years ago, my coil gave out and was replaced with a Bosch silver (chrome) unit. I don’t know how to test this.

The spark plug wires are very likely factory wires with 300K on them. They are the shielded kind. I got the car with 120K miles, and I haven’t changed them in the last 180K miles. Is there a simple/trustworthy way to test each wire individually?

With all that to consider, would you start by chasing an ignition problem and investigating the distributor or start getting fuel pressure statistics to evaluate the running condition?

"Epoxy filled"

Air boxes generally don't crack in just one place from a BACKFIRE...

Positive pressure within the intake manifold might not have the same leakage effect as the a high level of VACUUM at idle or nearby.

Get rid of the shielded plug wires that have a detrimental effect on the HV risetime and install one of the newer style airboxes with the internal fuel distribution manifold.

Buy a bit of insurance by installing a pop-off in the new airbox.

[Joe Bob]

CIS is all about fuel pressure. Get some gauges and check pressure. Also could be a clogged fuel filter.

Also a good test on plug wires is to fire it up in the dark, look for arcing....it will be obvious.

[Alan Lindquist]

Originally Posted by Joe Bob

CIS is all about fuel pressure. Get some gauges and check pressure. Also could be a clogged fuel filter.

Also a good test on plug wires is to fire it up in the dark, look for arcing....it will be obvious.

Thanks JB. I have the Harbor Freight fuel pressure kit. I guess I'll try it. The VERY detailed threads are here to be searched so I will dig them up and go through the pressures tomorrow. I heard that HF fuel pressure gauge is not to be trusted but it doesn't hurt to see what I get.

Do you think shielded plug wires arc? I wouldn't imagine so. I'm seriously considering replacing the coil and wires. The wires are long over due, and it would be good to have a spare coil around anyway, even if it doesn't solve the problem.

[wwest]

Originally Posted by Joe Bob

CIS is all about fuel pressure. Get some gauges and check pressure. Also could be a clogged fuel filter.

Also a good test on plug wires is to fire it up in the dark, look for arcing....it will be obvious.

According to Ops indications/symptoms the fuel pump provides adequate flow/pressure under hard acceleration circumstances.

HV plug wires will ALWAYS have corona discharge but in this case maybe not visible due to the shielding. But here again the ignition firing is under the most "stress" with the engine under hard acceleration, effective CR at the maximum.

[wwest]

Originally Posted by Alan Lindquist

Thanks JB. I have the Harbor Freight fuel pressure kit. I guess I'll try it. The VERY detailed threads are here to be searched so I will dig them up and go through the pressures tomorrow. I heard that HF fuel pressure gauge is not to be trusted but it doesn't hurt to see what I get.

Do you think shielded plug wires arc? I wouldn't imagine so. I'm seriously considering replacing the coil and wires. The wires are long over due, and it would be good to have a spare coil around anyway, even if it doesn't solve the problem.

Other than the detonation in 4th gear pulling hard (***) all of your symptoms seem to point to LEAN mixture except with the engine with a heavy intake fuel flow. Exactly the expected result with a low level vacuum leak.

*** "lugging... engine RPM to low..?

If you have CDI I would converting to the more appropriate coil.

[Targalid]

I agree with comments above regarding lean mixture, however, test the plug wires anyway. Total darkness, engine running, engine lid open. Just have a look. I had the original metal braid shielded wires and the light show was spectacular. Replaced with Magnecors from Pelican and rough running went away.

[Alan Lindquist]

[QUOTE=wwest;8181979]Other than the detonation in 4th gear pulling hard (***) all of your symptoms seem to point to LEAN mixture except with the engine with a heavy intake fuel flow. Exactly the expected result with a low level vacuum leak.

*** "lugging... engine RPM to low..?

If you have CDI I would converting to the more appropriate coil. /QUOTE]

Interesting you suggested a lean mixture. When I did the valve adjustment last month the Bosch plugs I pulled were all indicating lean. When I took it to my mechanic to lean it out for the smog test I asked him if it seemed lean and he thought it did. When he richened it up following the test, he took it farther to rich than it was originally and commented, "That should take care of it." The problem is less pronounced, but still there. What's the harm in richening some more? Potential plug fouling? I am running almost the coolest plug NGK makes.

I will confirm pressures first. It's free and it is baseline information I can start with to consider the problem with a bit more information.

Just to comment on some things mentioned above - I do have a pop off valve, my 1980 intake is less prone to backfires. My fuel filter is around 15,000 miles old. Since I run it daily, I'm not suspecting a crappy fuel tank or bad filter so I'll leave that until I've exhausted other things to look for.

[Alan Lindquist]

Originally Posted by Targalid

I agree with comments above regarding lean mixture, however, test the plug wires anyway. Total darkness, engine running, engine lid open. Just have a look. I had the original metal braid shielded wires and the light show was spectacular. Replaced with Magnecors from Pelican and rough running went away.

I will test the wires tonite. I can close the garage door about 7/8ths, turn out the lights, and hand throttle it at the throttle body to about 5K RPMs. This is a quick and free thing to test. Thanks for the idea.

[wwest]

Originally Posted by wwest

Other than the detonation in 4th gear pulling hard (***) all of your symptoms seem to point to LEAN mixture except with the engine with a heavy intake fuel flow. Exactly the expected result with a low level vacuum leak.

*** "lugging... engine RPM to low..?

If you have CDI I would converting to the more appropriate coil.

Originally Posted by Targalid

I agree with comments above regarding lean mixture, however, test the plug wires anyway. Total darkness, engine running, engine lid open. Just have a look. I had the original metal braid shielded wires and the light show was spectacular. Replaced with Magnecors from Pelican and rough running went away.

Even without the braid the corona discharge dark night light show is impressive. I would expect the level of corona discharge to be substantially greater with arcing directly to the grounded shield/braid rather just to atmosphere but am still surprised, having never seen same, as to the visibility.

The grounded shield acts well as one side of a capacitor, suppressing the risetime of the HV spark somewhat, thereby making it less likely a slightly fouled plug will fire.

[The cap'n]

Why are you using a spark plug that's not specified for the car? The heat range for an NGK should be 7, not 8. I would ditch the old wires and replace them with the same variety. Lotsa bucks, but they'll last another 2-300K miles. I'd also suspect a patched airbox.

If you're gonna use a non-spec. coil, talk to Loren before doing so. Bosch maintains that using a coil that's not the OE type can take out the CDI, and Loren has a fix for that.

The Cap'n

[boyt911sc]

Alan,

Could you post a picture of your ignition coil? If you have the silver ignition coil made from Brazil, I strongly recommend replacing it for the test. Use the old black OEM coil and see if there would a difference in running performance. I could lend you one of my ignition coils but they are not for sale just a loaner to use for your test. PM me it you need one.

Tony

[Alan Lindquist]

Tony,
Here's a few photos of the coil. The label has worn off. Last nite I revved the motor in the dark and there was no light show. So for now, I'll hold off on ordering new plug wires and check a few other things first. I thought I would have time to work on it yesterday, but my daughter's car absorbed all my time. Today, family is in town so I don't think I'll get much done today.

Alan

[morrisjm1]

Alan,

I had similar issues with my '78 SC. It had maybe 210,000 on it at the time. I did pretty much everything you've done, still had the problem. It had the original wires, they look indestructible, so I looked else where. I was checking the plugs again one day and I pulled the #4 plug wire and it came apart in my hand. Replaced with Magnecor wires and it hasn't missed a beat in 8 years.

Mike

[wwest]

Originally Posted by Alan Lindquist

Tony,
Here's a few photos of the coil. The label has worn off. Last nite I revved the motor in the dark and there was no light show. So for now, I'll hold off on ordering new plug wires and check a few other things first. I thought I would have time to work on it yesterday, but my daughter's car absorbed all my time. Today, family is in town so I don't think I'll get much done today.

Alan

Just as I thought, proposed, you cannot see the corona discharge because it is shielded to view by the braid. On the other hand maybe the shield's capacitive effect suppresses the HV risetime enough to abate the corona discharge.

It was/is my understanding that the braid was a misguided attempt by the factory to solve the problem of airbox explosions from backfires. The belief was that the HV was jumping/arcing from a plug under compression to a nearby HV wire not under compression and with the intake valve still open.

I would not hesitate in replacing those HV wires and installing a new airbox.

[Alan Lindquist]

Originally Posted by morrisjm1

Alan,

I had similar issues with my '78 SC. It had maybe 210,000 on it at the time. I did pretty much everything you've done, still had the problem. It had the original wires, they look indestructible, so I looked else where. I was checking the plugs again one day and I pulled the #4 plug wire and it came apart in my hand. Replaced with Magnecor wires and it hasn't missed a beat in 8 years.

Mike

Thanks Mike. I'm not ruling them out and given their age it's easy to suspect them. Does anybody know a good test for a wire? Shouldn't I be able to test both ends for resistance (ohms)?

And back to my original question, am I dealing with one problem or two? I have both hesitation (low RPM bucking) and occasional high RPM detonation.

[morrisjm1]

Hmm, detonation is usually caused by timing too far advanced and/or mixture too lean. Maybe it isn't the wires after all.

[morrisjm1]

Or too low and octane rating for you engine.

[Alan Lindquist]

Just answered my own question. Went to YouTube. Saw a short video on checking ohms. Apparently 4,000 to 5,000 ohms per foot is acceptable. So I will check mine and report back. I'm also going to re-check timing for 5 degrees BTDC at idle.