scotsman60

My '90 C2 is pinging. In fact it's been pinging for a while but I can drive around the problem. This week we had 90+ temps in Southern California and the pinging is getting to be annoying enough for me to do something about.

Here's what I've got and what I've done so far,

1) I'm running 91 octane gas - it's the best I can get here.

2) I've run a bottle of Techron through the system and it didn't make any noticeable difference. (I changed the oil afterwards)

3) New plugs, new leads, new distributor caps, new rotor arms and new coils - No difference.

4) I'm getting a spark on all plugs - distributor drive belt checks out OK


I've got some questions,

1) If the knock sensor detects pinging and adjusts the timing at any time should I get an engine warning light? (I haven't got any lights)

2) I'm thinking of hooking my DVM up to the knock sensor wire. Anybody got any ideas about what kind of signal I might expect to see and how it should change when pinging occurs? (I'm going to assume to start with that I'll see constant voltage and then a change in voltage as pinging is detected)

3) It's possible that the knock detectors are either faulty or incorrectly attached. How do I access them? If I remove the fan and the alternator can I get access to them? If I'm in there I'll probably replace them anyway - any special things to know about this procedure?


Any and all ideas and suggestions gladly accepted......






Doug

PC2

Doug,

If everything checks out ok then I believe the problem is the California gas we're putting in. Have you tried different brands of gas? Different brands may yield slightly better results. My car tends to do better with Cheveron brand. A bottle of premium octane booster can usually confirm that the pinging are results of low octane fuel.

My car will ping more during summer months because of all the additives California is adding into the summer gas.

scotsman60

Peter,

Thanx

I've tried switching gas brands. I settled on Chevron because it seems to suit my car best (least pinging and best mpg).

I'll try the octane booster - someoine told me that there's a 76 station with 100 octane race fuel somewhere close to my home (Aliso Viejo) so I might try a mixture too.

Any idea if the knock sensor will cause an engine light or a fault code if it detects pinging?



Doug

PC2

I'm not sure on the engine light but faulty knock sensors will generate a fault code. If your car is '91 and newer you will be able extract fault codes with the check engine light. Bill Wagner has a nice write up on this topic. http://members.rennlist.com/billwagner/

Randall G.

The closest station I'm aware of selling 100 octane is the 76 station by Fashion Island. Try about 5 or 6 gallons with a fill-up, and see if that gets rid of the pinging:

http://www.76.com/76locator/Distribu....asp?Add=92660

Engine pinging seems to come up more often on the 993 board. I don't quite understand this, but it appears that fully functional knock-sensors do not necessarily prevent audible pinging. Not that you shouldn't verify your knock-sensors are okay. My mechanic checked my knock-sensors by hitting the heads with a (edit) screwdriver (or something similar), and watching the timing retard.

schubee

There is a 76 off of the 5 that has 100 octane, try La Paz exit.

Randall G.

Really? That would be great.

Randall G.

Would it be this one:

http://www.76.com/76locator/Distribu....asp?Add=92653

But, the link says no "Racing Fuel" (100 octane). Perhaps the website is incorrect.

scotsman60

Bill,


Mine is a '90 - so I can't use the check engine light to recover the fault codes. I suppose I could go to a dealer and have the codes read.....

Randall,

Thanx for the 100 octane info. I drove past the 76 station at MacArthur and PCH in Corona Del Mar this evening - unfortunately before I read your post....

I'm surprised that the knock sensors would react to a blow froma rubber mallet - I always understood that the knock sensors only responded to particular frequencies and that these frequencies would be much higher than you would get by striking th eheads with a rubber mallet (Metal I could beleive but I wouldn't want to do that......) I'm going to hook up my DVM to the knock sensor and see if I can see any response at the onset of audible pinging. I may try the rubber mallet approach too - I'm assuming that I can identify timing changes using a simple timing light right?


John - I'll swing by La Paz tomorrow morning - I usually get onto the 5 at El Toro but it's a small diversion to make. Thanx for the info.




Doug

John Boggiano

Have you tried a DME reset (e.g. battery disconnection for 15 mins)? If a knock sensor 'fault' was logged, I think they can be taken out of service until you do a reset.

Adrian

Dear Doug,
This is a common problem in CA and it is the fuel. You have to get the octane rating up. The fuel quality is also a problem.
On the knock sensors.
A failed knock sensor system will give you an engine light however a working system will not.
The knock sensor system also has a limit built in. If the knock sensor system is required to retard the timing more than 9 degrees on 2 or more cylinders combined total it will switch itself off, bring on the engine light and retard the timing to a fixed value of 6 degrees on the crank.
If anyone in CA is getting a knock sensor failure I doubt they are failed. It is the fuel causing excess knocking which is outside the system tolerance.
I can assure you that the only solution is to raise the octane rating of your fuel.
Ciao,
Adrian
964C4

scotsman60

John,

I've done the DME reset (several times) - no change.


Adrian,

Thanx for the reply. I'll certainly try to increase the octane rating - unfortunately however this is both inconvenient and expensive as a long term solution.

If I interpret your post correctly, a normally operating knock sensor, when it detects knock and retards the timing, will not cause an engine light to come on. If the knock sensor fails then it will cause an engine light to come on.

I'm confused by your description of the operation of the knock sensor. I thought that there was one knock sensor for each bank of cylinders - two in total. How can a knock sensor affect the timing of a single cylinder? (I'm having difficulty believing that these devices can identify the actual cylinder that's causing the problem and then be able to identify that cylinder to the DME). Also, if I'm getting audible pinging then I'm assuming that the DME has reached/passed the timing retardation limit of 9 degrees and I should get an engine light - but I'm not getting any light.

Am I misunderstanding something?





Doug

Lorenfb

You should not have to increase the octane number beyond
91 or 93 even in CA. The DME with its' knock sensors should
compensate for pinging unless a non-standard chip is in
the DME (performance chip). The 9 degree retard should
provide enough for octane problems.

The DME correlates when the knock occurs with the last
cylinder to "fire" and thereby knows which cylinder to
retard the spark on. The DME only knows if the knock
sensors provide faulty signals and don't test them for
functionality as the later DMEs do. Therefore, if the sensors
are not functioning, the DME would never know.

Adrian

Dear Doug,
A failed knock sensor, 2 failed knock sensors or the knock sensor control system inside the DME failure will bring on the check engine light. Maybe I rephrase should bring on the light. If a sensor fails and the signal input is lost the DME will log it as a failure. I cannot see a circumstance that a failed knock sensor is not logged but Loren seems to think there are so I have to defer to him on this. The system descriptions I have from Bosch and Porsche clearly state that if it fails it will log a fault. However like with everything of this vintage there will be exceptions.
Loren explained the operation so that is fixed. Loren also states correctly that the knock sensors should take out the knock. The 964 engine was not designed to run on 91 octane fuel but was tested by the factory with this fuel octane. The Turbo engine was also tested on 91 octane fuel and passed but Porsche also say that excessive knocking can still occur on hot days. This testing was carried out way back in the early 90s. Fuel has changed since then. Knocking has been reported many times on many Porsche forums by owners in CA using 91 octane fuel. I have heard of the odd company offering a DME tweak for this but I have never investigated it. It is well known that the fuel quality in CA is not the best and is far below what we have here in Europe. I am not sure it is worthwhile spending large sums of money chasing a possible fault in your 964 when you have not tried increasing the octane or resistance to knocking first. It is my belief that CA, 91 octane fuel and hot weather exceed the ability of the knock sensors to retard the timing sufficiently to stop the knocking.
Ciao,
Adrian
964C4

Randall G.

>Knocking has been reported many times on many Porsche forums by owners in CA using 91 octane fuel.

Yup, I agree.

>It is well known that the fuel quality in CA is not the best and is far below what we have here in Europe. I am not sure it is worthwhile spending large sums of money chasing a possible fault in your 964 when you have not tried increasing the octane or resistance to knocking first. It is my belief that CA, 91 octane fuel and hot weather exceed the ability of the knock sensors to retard the timing sufficiently to stop the knocking.

Agreed again....Though, I still find it odd that the knock-sensors don't prevent audible knocking.