1981SC Pinging

Nicks911 · 09-22-2008, 06:24 AM

Hi all,

I'm just back from Gabon (in Africa) and have taken the 911SC out of storage. I enjoyed reading the forums while in Africa, but didn't participate much as no porsches other than the SUV in Gabon. Anyway 911 came out of storage and everything seemed fine. Its started right up and ran great. Now after daily driving it for a month it has started to ping and its getting worse. details below:

Cold does not ping at all

Timing is dead on factory spec

Ignition system in general seems to be in good shape cap rotor plugs etc all are new or replaced within the last 5K

Car surges slightly at idle maybe 50-250 rpm

Car does not fast idle. Its starts right up, but is un-drivable due to back fires and missing until it has warmed up for 30 sec or so

As the car warms up pinging starts at first its 4K and higher. As the car further warms ping gets lower and easier to provoke. By the time the engine has warmed to the point the guage is out of the lower box it will ping at 2K.

I tried retarding the timing as much as 5 degrees below factory spec. this had no effect on the ping, but made initial acceleration sluggish.

I beginning to think that this maybe a lean ping caused by a CIS fault. Maybe the same fault that is giving me the poor driving at start up. Could it be that the car is running rich enough when it is first started to stop the ping, but not rich enough for running well when cold? Then it leans out too much when warm giving me the ping?

Thanks for the help

Nick

mca · 09-22-2008, 07:00 AM

Besides checking the timing, you need to check the mixture.

Like you suggested, if it is too lean it will cause detonation - pinging.

Get a gas analyzer on it and let us know what you find.

Paulporsche · 09-22-2008, 03:17 PM

After you check your gas quality and mixture, if the problem persists, check your control pressures (WUR).

ischmitz · ischmitz's Garage

I would start with simple things first: Vaccum leaks. If you have one chances are your mixture is lean and that causes problems. Try using starter spray and check if the idle changes when you hit parts of the intake. Have a fire extinguisher near by and be careful.

Next what others suggested. Control pressure - there are tons of threads on how to do this. Just don't start playing around with stuff without the proper tools (pressure gauge, gas analyzer) since it is easy to move away from the optimum.

Cheers,
Ingo

Nicks911 · 02:48 PM

Well I think I have an answer. Hooked up a pressure tester and had a look. At 20c or 70 f control pressure was 3.5 bar. from the way I read the charts it should be around 2.0 bar. I guess its time for a new WUR.

Other test also performed:

sprayed starter spray at vacuum hose connections no change in idle

fuel pressure with tester valve closed 65 psi

residual pressure after 20 min 1 bar

Engine hot 60 psi or 4 bar

Other info:

Stored with brimming tank and stabil. Its been through about 5 complete tanks since then as its a Daily Driver at the moment.

Nick

Daviboy · Daviboy's Garage

Nick sorry to hijack,
"I'm just back from Gabon (in Africa) " is there another one, sorry just joking. What were you doing in Gabon? I worked there a few years ago in Port Gentil (oil industry) but also spent a lot of time in Libreville. I think it is a great place myself but then having worked in Nigeria, Equatorial Guinea and a few other spots Gabon is a virtual paradise.

Nicks911 · 10-06-2008, 06:58 AM

Worked for US Embassy Libreville. Had a great time. Built up another type of hot rod for those two years. Defender 110 with all the goodies. Amazing 4 cylinder diesel. Very efficient and powerful. Drove to Franceville Mayumbia Gamba Lope etc. Fantastic. Terrible roads though especially in the rainy season. Also Went to Sette Cama via the Schell concession. Very interesting drive through there. It was fun. next post is Quito Ecuador. The Defender is heading there and we are in DC for the next year for training and our first child.

On another note I'm think about doing a rally setup to my SC to ship it to Ecuador as well. It would be a blast to have had it in Gabon as long as you could get enough ground clearance.

Nick

Paulporsche · 10-07-2008, 01:38 PM

Before you try a new WUR why not try some of the WUR fixes available, like making sure the filter screen @ the top is not clogged, ensuring the bimetal strip gets current and warms up, and knocking the plug to lower the cold cp?

KobaltBlau · 10-07-2008, 04:06 PM

Quote:

Originally Posted by Nicks911

On another note I'm think about doing a rally setup to my SC to ship it to Ecuador as well. It would be a blast to have had it in Gabon as long as you could get enough ground clearance.

Good idea - 911 SC/RS replica perhaps?

TRE in LA did a really wild "beach 911" with no windows for someone, there's a picture on here somewhere. it's super high.

KobaltBlau · 10-08-2008, 07:32 PM

Here's an awesome thread for inspiration for your Ecuador 911. The TRE car I was talking about is in the thread, pictured below.

what's involved in building a rally car?

Nicks911 · 10-09-2008, 12:51 PM

When check the Control prerssures a check the screens and they were clear

12 volts at the WUR and resistance

Not sure if the the trick for adjusting the Control pressure would work in this case, but I guess I may as well try . . .

Nicks911 · 10-27-2008, 08:51 AM

Changed the WUR on Sunday and now Control pressures are all where the should be and the car is running much better. No backfires when cold smooth etc. However the pinging has not gone away entirely. Instead of starting at 2K it starts at ~4500. Anythoughts on what this could be or what to look at next. Could the pinging sound be something else? Something that sounds like pinging, but isn't.

Nick

T77911S · 10-27-2008, 09:14 AM

check timing, fuel mixture too lean.
vacuum leaks will make it run lean,= pinging
advanced timing=pinging.
low octane fuel can also cause pinging, seems that some can use 87, i have to use 92+.
i have tried setting my car up to run on 87, with the exception of reducing my timing, without any luck.

JMH82SC · 10-27-2008, 11:36 AM

I had the same pinging you describe and did everything including rebuilding the CIS fuel injection, checked WUR pressures (spot on), colder plugs, wires, running open and closed loop with CO correct for both as well as a distributor over haul. No pinging when cold but as soon as it warmed up pinging started and nothing seemed to make pinging go away. My next check was head studs and I discovered I had 2 broken one on the left bank and one on the right. I dropped the engine and replaced the lower studs with steel ones and wile I had it apart I took the heads to have a valve job done. I also replaced the rings and all related gaskets and put it back in 3 weeks ago and can say ALL PINGING IS GONE.

Nicks911 · 11-05-2008, 07:35 AM

Spent some more time with the car and a timing light. If I set the idle to factory spec I get too much advance and pinging at 6K. If I set the timing so the advanve at 6K is correct I'm about 10 degrees more retarded then factory spec. Cra runs OK just hestitates a bit at low RPM. Thoughts?

Nick

mca · 11-05-2008, 07:43 AM

Tell us about your dizzy. Does it have the vacuum advance and retard? If so, are you pulling and plugging those lines when you set your timing?

What figures are you using as "spec" for full advance? Is your advance vacuum line pulled and plugged when you are taking the measurements?

psalt · 11-05-2008, 07:59 AM

Hello Nick,

Pinging in these engines varys considerably with heat. What is your oil temperature and outside temp ? If you are unfamiliar with CIS, there is a jiggle pin in the injectors that can be mistaken for pinging under WOT, but you should be able to hear the difference with a cold engine. The US 81 SC was detuned to run on 87 CLC, but most engines with normal wear need higher octane fuel. If your engine has high mileage, crankcase blowby and oil dilution can also cause pinging. The heat and oil vapor lowers the mixture's octane considerably. You can check this by disconnecting and blocking the hose to the oil tank fill and fitting a road draft breather. Is there any evidence of oil burning on the spark plugs ? If your CIS lambda is functioning correctly, you can check the mixture and full load enrichment with a dwell meter.

Nicks911 · 11-05-2008, 02:05 PM

I'll try and answer all the questions

dizzy has both vac lines disconnected and plugged when readings were taken.

as for spec I'm using 5 degrees advance at idle and 20 degrees at 6K

Outside temp ~mid 50s engine temp just out of white box at the bottom

Oil consumption is about a quart every 800 miles 131,000 on the motor never been opened up

Fuel is 92 octane

Haven't pulled the plugs but they are old so maybe I should replace them . . .

Lambda sensor is three years old ~ 10K miles

Nick

mca · 11-05-2008, 02:31 PM

I suppose you are seeing 30 degrees advance at 6000 with the advance line plugged?

Did you ever check your mixture?

Hopefully someone else will chime in. My first inclination would be to try a different dizzy of you have one available. Or, inspect it for worn components and dirt.

I feel for you. There are so many variables to consider.

psalt · 11-05-2008, 02:56 PM

Hello Nick,

Max advance should be set to 25 BTDC. The usual pattern for a worn out distributor is too much advance at idle when the springs wear and too little high speed advance at the false idle setting. Maybe your distributor needs cleaning and lubrication. Go ahead and replace the plugs with BOSCH normal WR5. Hook up a dwell meter to the green white stripe wire in the test plug under the plastic cover in the left side of the engine compartment. Cold open loop reading should be 58 dwell (4 cyl scale) around 65% duty cycle. After a few minutes the system should go closed loop and the needle should fluctuate between 40-55 dwell. Adjust the mixture screw with an allen wrench to 30-40 dwell. The system should go open loop after 35% throttle if the TPS is working. Leave the vacuum retard plugged and set the idle to 950 rpm. Go for a test drive and see if the pinging goes away. If not, at 131 K, blowby is probably the issue. You can try a separate breather, these engine love cold air. Good luck.