Poor cold start + knocking problem

[scotsman60]

I have a problem with my '90 911 C2. When I open the throttle at lowish revs (<~3000) the engine knocks. I haven't experienced the problem at high revs and I've only ever experienced it when the engine has warmed up.

What can be the cause of the knocking problem?

Some additional information,

I always use 92 octane fuel. My mechanic said he suspected carbon build up on the valve stems and performed an aggressive top end flush followed by an oil change but this didn't appear to help.

I also experience some cold start problems. Each morning when I start the car the engine fires but then dies on the first try but then always fires and runs on the second try. After starting, when I pull away with low revs the engine is flat, almost no power, for about 3 or 4 seconds and then it runs great.

Any ideas, suggestions will be gratefully received by this first time poster.


Doug

[Bob W.]

Sounds like it's starving for fuel or a lean mixture.
Bob W.

[Rod Walter]

Doug, as far as the engine knock goes you are describing an almost classic case of pre-ignition due to some kind of engine deposits. If carbon or some other material is stuck in the cylinder or low enough on the valve it gets hot and becomes a source of pre-ignition or "knock." It takes awhile for a deposit to become and stay hot enough to function as a sort of unwelcome spark plug so that would explain the warm engine aspect. As the rpms increase and the spark advances the spark plug fires at the same time as or before the deposit can ignite the mixture and that would explain the knock disappearing at higher rpms. You might try a bottle or two of Techron followed by an oil change again even though your mechanic already tried to clean the sytsem. Your use of high octane gasoline will help to control pre-ignition because higher octane fuel is intended to resist uncontrolled combustion (as in an explosion) in favor of a very rapid, top-down burn in the cylinder.

Having said this, are you certain that you are hearing "knock?" This type of problem has become somewhat rare in cars as young as yours because of the improvements in fuels and mixture control.

Bob's idea of a lean mixture makes sense too, partly because a rich mixture would help keep engine deposits somewhat cooler and a little bit less likely to pre-ignite. Since the engine management computer controls the mixture for cold starts you might check out your cylinder head temperature sensor and any other inputs to the computer that assist in cold starting.

Sorry I can't be more specific. My car's a couple of years older than yours.

------------------
Rod Walter
'88 911 coupe

[Doug Zielke]

Doug,
Your car's motor is equipped with the (infamous) twin-spark distributor. There is a small toothed belt deep inside the distributor that can fail, leaving you with only 6, instead of 12, spark plugs firing.
This can lead to the symptoms similar to what you're experiencing. Have it checked by a shop experienced with the 3.6L. This problem can damage your motor, as it did on a '91 my friend used to own.

------------------
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

[Doug Zielke]

And one other thing I forgot to mention....
There was an upgrade kit made available to the dealers, to help with the service life of the rubber belt. It's a small hose that takes air from the fan shroud, and ducts it into the distributor body. This helps dissipate the build-up of ozone in the distributor, therby prolonging the life of the belt. Most early 3.6L motors had this upgrade done at sometime in their service history. Make sure yours has it.

------------------
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

[scotsman60]

Doug,

After reviewing this board yesterday I came to the conclusion that I probably had a broken belt in the dizzie so last night I took of the caps and tried to ratate the secondary distributor shaft - it wouldn't turn. I removed the rotor arm and shroud and checked out the belt (my god it's very lightweight!!!) and it looked to be in good condition - no visible cracks or anything. I didn't check to see if the vent kit has been installed - I'll do that tonight.

I think the next thing I'm goimg to try is to check if the mixture is lean - I'll probably do this as part of an engine tune-up.

How much should an engine tune-up run me?

Is it best to get this done at the dealer?

I've got 54,000 miles and had all the plugs, filters et.c changed at 45k - 10k miles ago. Should I get them all renewed before the tune-up?


TIA,

Doug

Originally posted by Doug Zielke:
And one other thing I forgot to mention....
There was an upgrade kit made available to the dealers, to help with the service life of the rubber belt. It's a small hose that takes air from the fan shroud, and ducts it into the distributor body. This helps dissipate the build-up of ozone in the distributor, therby prolonging the life of the belt. Most early 3.6L motors had this upgrade done at sometime in their service history. Make sure yours has it.

[scotsman60]

Bob,

You say it could be starving for fuel - are you saying this is the cause of the cold start problem? It runs fine at high revs and WOT when it needs most fuel. What could cause fuel startvation the first time it's started each day only?


Doug

Originally posted by Bob W.:
Sounds like it's starving for fuel or a lean mixture.
Bob W.

[scotsman60]

Rod,

I'm pretty sure this is pre-ignition. The very first time it happenned I thpought that it might be a heat shild vibrating or some other ancilliary rattling in the engine compartment but it consistently appears only when the engine is hot and under what I would call "high vacuum" conditions from my Mini days.

How good is Techron? - I've ever used it before.

I almost always use Mobil gas - it's the cheapest 92 Octane where I live. Should I consider using another brand of gas?


Doug

Originally posted by Rod Walter:
Doug, as far as the engine knock goes you are describing an almost classic case of pre-ignition due to some kind of engine deposits. If carbon or some other material is stuck in the cylinder or low enough on the valve it gets hot and becomes a source of pre-ignition or "knock." It takes awhile for a deposit to become and stay hot enough to function as a sort of unwelcome spark plug so that would explain the warm engine aspect. As the rpms increase and the spark advances the spark plug fires at the same time as or before the deposit can ignite the mixture and that would explain the knock disappearing at higher rpms. You might try a bottle or two of Techron followed by an oil change again even though your mechanic already tried to clean the sytsem. Your use of high octane gasoline will help to control pre-ignition because higher octane fuel is intended to resist uncontrolled combustion (as in an explosion) in favor of a very rapid, top-down burn in the cylinder.

Having said this, are you certain that you are hearing "knock?" This type of problem has become somewhat rare in cars as young as yours because of the improvements in fuels and mixture control.

Bob's idea of a lean mixture makes sense too, partly because a rich mixture would help keep engine deposits somewhat cooler and a little bit less likely to pre-ignite. Since the engine management computer controls the mixture for cold starts you might check out your cylinder head temperature sensor and any other inputs to the computer that assist in cold starting.

Sorry I can't be more specific. My car's a couple of years older than yours.

[scotsman60]

Doug,

One other thing - I checked the centrifugal weights in the secondary distributor when I had it uncovered and they seemed to be in good condition and were well lubricated - they moved out easily with a screwdriver. I couldn't do the same check on the primary though because there is some circular device on the primary distributor shaft located just below the rotor arm. What is this device?


Doug

[Bob W.]

I'm making the assumption that you are experiencing detonation.I can think of a few causes. Some of the basics.
1.Too high of a cylinder temp for the fuel used. Because the engine is cold and the fuel has a high enough octane, it shouldn't be this.
2.Too much ignition advance. Could be this, but you say the distributor is operating correctly.
3.Mixture is lean. A lean mixture is more prone to detonation (pre-ignition). when the engine is cold it requires more fuel to run because the amount of atomized fuel normally required is condensing against the cool metal surfaces. The fuel metering system must compensate for this by richening the system when cold. This extra fuel when cold will over come a lean setting. This extra fuel is not available when warm so the engine has a lean mixture detonation. At higher speeds the system normally goes richer and may be rich enough to compensate for the basic lean setting, so the engine smoothes out. The same lean mixture when cold may be what is causing a reduction in power when you start up in the morning. Because the engine is not up to normal operating temperature you don't get detonation-only a reduction in power.
Bob W.

[This message has been edited by Bob W. (edited 08-09-2001).]

[Rod Walter]

Doug, most people on the board seem to give Techron the nod for keeping a fuel system clean compared to similar products. I'm going by these and other recommendations rather than technical specs in this case. It's the same additive as found in Chevron gasoline but in a more concentrated form.

I don't think that there is anything wrong with Mobil gas. But if it were me I would still experiment with another brand after running the tank down nearly to fumes just to see if another refiner's additive package helps at all. Still use premium, though - 92 or 93 octane.

------------------
Rod Walter
'88 911 coupe

[scotsman60]

Rod, Bob,

I'm going to try changing gas brands to see if this helps.

I'm also considering trying Techron - I'll need to plan a road trip

After that I'll get the car tuned, check the mixtures and timing and see how things look then.

I'm just worried about any ongoing damage that the knocking might be causing.

Thanx for your input!!!

Doug

[This message has been edited by scotsman60 (edited 08-09-2001).]

[scotsman60]

Originally posted by Doug Zielke:
And one other thing I forgot to mention....
There was an upgrade kit made available to the dealers, to help with the service life of the rubber belt. It's a small hose that takes air from the fan shroud, and ducts it into the distributor body. This helps dissipate the build-up of ozone in the distributor, therby prolonging the life of the belt. Most early 3.6L motors had this upgrade done at sometime in their service history. Make sure yours has it.

Doug,

I checked at lunch time today and can see no sign of the vent kit. I'm assuming that it should be pretty obvious. Does this have to be installed by the dealer or can I do it myself?

Doug

[Doug Zielke]

The kit looked pretty simple on my friends car. But his was installed by a dealer.

------------------
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

It probably hasn't got anything to do with your problems but as far as I know a 964 is supposed to run on at least 95 octane.

Maybe you should look in your owners' manual to see what it recommends. (I'm not sure if US 964s are different from euro ones).

[scotsman60]

Doug - I'm going to order the kit from Wayne and install it myself

Mikkel - 92 Octane is as good as it gets here in California and that's fine for CA spec cars.

Thank-you both for your input