My SC detonates, even with low advance.

Gogar · 01:56 PM

I've been having an ongoing problem with timing advance on my 83 SC.

I know that "proper" advance is 5 BDTC at idle, and "not more than" 25 BTDC at 4K rpm, vacuum lines unhooked and plugged.

Ok.

My SC has started 'pinging' under load at lower rpms (2000-4000). FWIW I'm using the best gas I can get here (91.)

I checked my timing, which was right around 20 BTDC at 4000 rpm. To make the engine not 'ping', I have to reduce the (high RPM) advance to about 12-15 BTDC or even LESS .

This makes my idle timing right around 0 (TDC) with the retard vacuum unplugged.

I'm just worried that I'm losing some performance by having the high RPM timing so tame., or compensating for something else that is not right.

What can I look for? So far I have checked:

Idle mixture: is fine, about .8 to 1.0

Vacuum leaks: I can't seem to find any vacuum leaks. In addition, when I remove my oil cap, the idle changes (as I've heard it should.)

Distributor? I thought my distributor may be dirty or advancing improperly, so I borrowed a known good, recently rebuilt late SC distributor from my wrench and installed it, with same results. It seems my distributor is functioning ok.

Lambda: I had my Lambda checked, and it is reported to be functioning as it should.

My most recent guess is that I'm not getting proper fuel flow, maybe I have dirty injectors or something? (causing a lean condition and the pinging?)

I'm right about at the end of the things I know how to check, so any input would be appreciated.

Thanks!

911Freak · 10-27-2010, 02:28 PM

When was the last fuel filter change?
[edit: have you checked your fuel pressures?]
How much gas do you have in the tank?

I had my 911 run lean when tank was below 1/2 full, ended up being a bad fuel pump, above 1/2 symptoms were nill

YMMV

Gogar · 10-27-2010, 02:59 PM

Quote:

Originally Posted by 911Freak

When was the last fuel filter change?
[edit: have you checked your fuel pressures?]
How much gas do you have in the tank?

I had my 911 run lean when tank was below 1/2 full, ended up being a bad fuel pump, above 1/2 symptoms were nill

YMMV

Thanks, Freak.

Hot and cold pressures are in spec. Fuel filter is 2.5 years old, Tank is full.

longhornchris04 · longhornchris04's Garage

I'd swap the fuel filter out. 2.5 years and it could be clogged, especially if you got a few bad batches of fuel or ran some old fuel through it. This (potentially) wouldn't reduce the flow enough to affect idle pressure, but cause it to fall at high power.

I'm also assuming you are running premium.

Question, is the pinging only when fully warmed? If so, you may have some carbon buildup causing hot spots that are causing problems.

If it were me, I would run a some [engine cleaner of choice] through there for a tank or two, see if that helps. Just remember to change your oil after you've run the tank out as the fuel cleaner will get into your oil and accelerate its breakdown.

Personally I've used Techron with decent results. I've heard others swear by Seafoam, but I haven't tried it myself. Perhaps said others will chime in.

rsscotty · 10-27-2010, 05:41 PM

What heat range spark plugs are you using?

We recommend a Bosch W4CS if you are having detonation issues.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by Gogar

Thanks, Freak.

Hot and cold pressures are in spec. Fuel filter is 2.5 years old, Tank is full.

Gogar,

Do you know the values of your fuel pressures (control & system)? Both cold and warm conditions too. Have you done a vacuum test for the system? Keep us posted.

Tony

Gogar · 10-27-2010, 07:18 PM

I can't remember the values, it's been a few months. Perhaps it's time to check again and if so I'll post my numbers. Thanks!

9dreizig · 10-27-2010, 07:50 PM

Jeremy
Do you have an AFR meter? You need to check them I have an lm2 you can borrow also have a set of fuel pressure gauges too
A programmable MSD would ultimately help
Call me if you need to borrow anything

psalt · 03:42 AM

Gogar,

This is not unusual, I've seen it in several high mileage SC's over the years. It is caused by a combination of factors, but not vacuum leaks, O2 sensors, fuel filters, or preignition. Ultimately, detonation is caused by heat, and higher octane fuel just has a higher heat tolerance. The later SC is designed to run on 87 CLC fuel, but as the engine wears, vacuum drops, sensor plate deflection drops, ring seal degrades, temps go up, the distributor springs wear advancing too quickly, and the octane requirement goes up. Besides the octane label on the fuel pump, several other factors effect the detonation resistance of the actual charge in the cylinder. Intake air temperature is around one RON per 10 C and oil dilution has a big effect. Here is what has worked for me. First do a compression test to determine if your ring seal is bad. Use Normal Bosch 5 plugs. Disconnect and plug the breather hose at the top of the oil tank and fit a fresh air vent. I use a foam sock air filter and a pipe with two 90's to allow drain back of blowby oil. Disconnect and plug the vacuum retard hose to the back of the distributor. Rest the idle speed to 950 and the timing to 5 BTDC. Warm up the car by driving, connect an analog dwell meter to the test port, set the mixture to 30-40 dwell. You also want to make sure that the oil cooling system is working properly, the cooling fins are clean and the fan belt is not slipping. If you have the factory exhaust, make sure the heater plumbing is intact and air is going through the exchangers. If you want to isolate the problem at the source, you need to monitor cylinder head temperature, and having the stock heater with the fan on lowers it. You should be able to reduce or eliminate it with the optimum tune up, if not, you may have another issue. What is your altitude up there ?

Flat6pac · 10-28-2010, 04:13 AM

The lambda relay under the seat, its aluminum usually and has a VW mark on it. When this is defective it acts like a faulty warm up regulator which is a pressure control.
Bruce

T77911S · 10-28-2010, 05:09 AM

detonation can also be caused by build up of carbon on the pistons. it gets hot a creates an ignition for the fuel. it is also a source for preignition which is worse.

in my case, my cams where advanced 15 degrees. anytime i floored it, i got detonation and i could not go above about 4k. the more i retarded my timing, the better it got. but, i bought my car like this, so unless someone has messed with your cam timing, this is not your problem.

Gogar · 10-28-2010, 06:15 AM

Thanks for the replies.

Psalt, thanks; I I suppose I need to add that this engine has a fresh top/bottom about 14K miles ago. also in your post you mention to use Bosch 5 plugs, but I think SCs are supposed to take 7s? Right now i have NGK BP7ES plugs in there.

Flat6pac, I DID have a faulty lambda relay last spring, which made startup and cold running really poor. Replaced that and everything got better.

psalt · 10-28-2010, 06:35 AM

Psalt, thanks; I I suppose I need to add that this engine has a fresh top/bottom about 14K miles ago. also in your post you mention to use Bosch 5 plugs, but I think SCs are supposed to take 7s? Right now i have NGK BP7ES plugs in there.

Gogar,

No, 83 US SC spec is WR5DC, the higher compression ROW is WR3CC. NGK has a different heat range number and higher is colder. I like NGK's in many other engines, but have had pinging problems with 911's, especially with the EV plugs. If you want to understand the problem, I recommend a compression test, swapping in the correct plugs and doing everything you can to lower cylinder head temp. If you are at a high altitude, the octane requirement should be lower. Was the engine rebuilt with stock parts, what is the compression ratio and what is the oil and air temp when you have this problem ?

MotoSook · 10-28-2010, 06:44 AM

Its getting cold in the mountain, Jeremy. I'd check your CO level with a meter or richen it up a bit using the field method to see if the pinging goes away. A fuel filter is cheap, I'd replace that too just because. If it's really clogged, you may be fine at low RPM, but the car may lurch at higher RPM (even at low throttle opening).

I'd pull the plugs and see if there is maybe just one or two cylinder is leaning out. You can also pull the injectors and get a six pack Corona bottles (I'm sure you won't have a hard time emptying them out). Put an injector in each bottle, then spray fuel into them to see if they are all injecting the same level of fuel.

Gogar · 06:52 AM

Thank you

I'm at 5300 feet. The engine was rebuilt with stock parts, and 9.3/1 pistons.

Oil and air temp don't seem to matter as to 'when' i can make it happen, but in the spirit of that idea I feel that this car runs too hot (long drives = 230F on many days), even though I have installed a 28-row cooler in the front to try and help that out.

MotoSook · 10-28-2010, 06:50 AM

How's the cold idle and start up?

Gogar · 10-28-2010, 06:53 AM

Start up is easy, with a little bit of idle bounce for about 2 minutes, then no prob. The car does not idle high then settle.

MotoSook · 10-28-2010, 06:56 AM

Without much effort, I would let it reach operating temperature, then open the airbox, remove filter and just lightly push the airplate up to see if your idle increases from 950 or so. If it can stand a little bit more fuel without going high, I would add more idle fuel and see if things change.

psalt · 10-28-2010, 07:04 AM

Unfortunately, an air cooled engine's octane requirement is extremely sensitive to heat. In my opinion, you just answered your own question. Are you still running the vacuum retard ?

Gogar · 10-28-2010, 07:20 AM

Quote:

Originally Posted by psalt

Unfortunately, an air cooled engine's octane requirement is extremely sensitive to heat. In my opinion, you just answered your own question. Are you still running the vacuum retard ?

No, right now my idle timing is 2-3 BTDC, retard line removed and plugged, idle adjusted back to 950. Which makes my 4k timing 12-13, ish.

Maybe you're right, Paul. I'm just kind of stuck because I can't figure out if I have a lean problem or a heat problem, or a lean problem that's creating a heat problem, and my limited knowledge just tells me to buy the highest octane I can. I'm trying though.