Ron Minson

I have an 85 Coupe with 150k miles. Car runs like a champ and according the shop the engine and car are in great shape. Just had the car in the shop and they gave it a clean bill of health.

When I start up in the morning I see very little white smoke, just a few puffs but I hear a some knocking in the engine and it sounds a bit ruff. I usually let the car warm up for 5-10 minutes before driving. Once the car has been warmed up it runs and sounds excellent. No knocking and no unusual or concerning noises. If in the afternoons I need to run somewhere, I let it warm up for a about a minute or so and then drive it ever so slowly till the temp warms up. During the warm up drive I never get it over 3.5k rpms. During this time the car seams to run a bit ruff for the first 5 minutes. Applying pressure to the gas does not give the normal response but almost feels like I have a clogged fuel filter. The response is a little jerky. As soon as the temp rises above the first fat white bar, the car runs great.

So the question is, how long should I warm up my car before driving, and is it ok to drive very slow when the car is not warm? In addition is this knocking normal in older high mileage cars? It would scare me it was doing it all the time, but it is doing it only when it is very cold.

Any shared ideas or experiences would be appreciated.

Regards,

Ron

randyrap

Hi Ron.
I'm a newby 911 owner however, I've read that you do not need to have a ''warm up'' period prior to driving. It is said that the car can be driven immediately after starting the engine. The only suggestion is to keep the revs down until the oil has warmed up. I think this is written in the owner's manual too.

My car is very simular to yours. A little ruff at the start but quickly clears out after a minute or two of driving.

Hope this helps ............................... Randy

MRFLATSIX

Hi Ron:
Don't worry about the smoke on start up, most of them do that. It's just oil that runs back from the tank into the engine and gets burned off when you start it. If it smokes while driving, that's something to worry about. Don't sit and warm it up in the driveway. It won't warm up that way. I start my '70 and let it run at about 1800 rpm for 30 seconds or less and then drive off. Porsche wants you to warm it up on the road, not in your garage. If it runs without stalling, that's enough. Most 911s are a little noisy when they're cold and if it smooths out when it warms up I don't think I would worry about it. You said your shop gives it an ok, but If you're really concerned about noise it might be a good Idea to take it in and let them start it up cold and listen to it. Good luck.

911Dave

I have an 86 Carrera and here's what the owner's manual says, page 21:

"Never leave engine idling. When starting engine, be ready to drive immediately. Maintain moderate speed until engine is warm."

And further down,

"Do not let the engine idle to warm it up. After starting, drive vehicle at moderate speeds and avoid engine speeds above 4500 RPM during the first 5 minutes."

And further down,

"Do not stop engine immediately after hard or extended driving. Keep engine running at increased idle for about two minutes to prevent excessive heat build-up before turning off engine."

I found that last bit very interesting, and quite counter-intuitive.

DGaunt

When you understand what is happening in the engine it is less counter-intuitive, I think.

>50% of engine wear occurs in the first minute of a cold start. There are numerous reasons for this including clearances out of tolerance while the parts are cold, to gas condensing on the cold cylinder walls and diluting the oil, to raw gas diluting the crankcase oil.

There is really no way to avoid some of this, but we can reduce the time the engine is operating in this condition by warming it up as fast as is prudent. This is especially true in cold weather with air-cooled engines.

All things being normal, you want to run the engine only long enough to ensure oiling of the cams--the furthest moving part from the il pump, then move off at conservative throttle and revs until the engine warms. In my car, I first watch the oil pressure gage and as the needle falls from 100 PSI where the relief valve pops, I know the oil is warming up and I will give ti a bit more loud pedal. I do not use full throttle until the oil temp is at 180 and the pressure settles at 60-70 psi # 3,000 RPM.

The concept of warming under gentle load applies to any engine be it a Chevy, Porsche or a Lycoming.

The only time I do extend the warm-up just a bit is when the engine has sat all winter. Being too lazy to prime the system after a winter, I run the engine about a minute at 1,500 RPM, just to be sure the oil has made it to the cams, then I move out.

Sorry this is long, but warm-up is probably the easiest and most effective place we can preserve all those expensive shiny bit in or engines.

Ron Minson

It never ceases to amaze me the knowledge this board has. Thank you for your input.

One of the issues that does bug me is when my car is at idle after I have been driving it for a short length of time (5-15 min), sometimes the engine will rev on the idle (go from 750 RPM to 1800 RPM and then back to 750RPM). This will continue until I engage the car and drive. Running it again will sort out the problem and I don’t hear it any more. This is not a regular occurrence maybe 3 times in the last month. I was lucky it happened last time I was at the shop and they stated this is normal for the Carrera. There is a computer chip that wants to define the idle speed during warm up. Shop states to not touch the gas until the idle is stabilized and I won’t have the issue.

Anyone else have this issue or something similar?

Regards,

Ron

911Dave

It's not the warm-up procedure I found counter-intuitive, it's the last sentence in my quote from the manual. That is, that AFTER driving the car and you've parked it in your garage, you should let it idle for two minutes "to prevent excessive heat build-up before turning off engine."

How can letting an air-cooled engine sit at idle possible do anything but CAUSE excessive heat build-up? And not only that, if the point is to cool the engine down, turning the engine off immediately is the easiest and fastest way to do it.

Can anyone explain why this procedure is recommended?

SandyI

Ron-
I have the same up-and-down idle revving situation with my 69 911E during warm-up, only occuring after a couple of minutes of driving from a cold start, while the hand-throttle is still in a partially raised position. Once the engine heats up and I drop the hand throttle to the floor, that up-and-down idle revving stops and the engine idles normally. (I hope that makes sense.)

Barry A. Waters

Since most of the engine's "air cooling" comes from the engine driven fan, Porsche's reccomendation to "Keep engine running at increased idle for about two minutes to prevent excessive heat build-up before turning off engine." probably reflects a desire to keep the fan blowing cooling air over the cylinders to give them a bit of a chance to cool down before the fan steps out of the picture.

Note the words "increased idle". I'd imagine that would be about 1000 to 1100 RPM just to make sure the fan blows more air than a standard idle would provide without putting the engine to too much additional work.

Regards,

Barry

'86 911 Carrera

Barry A. Waters

Folks,

Please be advised that Porsche also makes 'recommendations"...

Just a thought... ;-)

Barry

'86 911 Carrera
and can't spell worth a toot!

911Dave

So, Porsche would have me sit in my car in the garage, with my foot on the gas, for 2 minutes after getting home from a long drive? Are they going to pay for my emergency room bill when I have to get treated for CO poisoning? This procedure is completely impractical for a number of reasons, not the least of which is that NOBODY would have the patience for this day in and day out.

ebsalem

okay, so pause out side your garage let the motor chill out, then pull in the garage. I've done it without incident for the last 6 years. My father's done it since '69, again without incident.

It's cheap insurance. A decent rebuild is $10k in parts and machine work assuming your do the work yourself.

lewis9113.2

in jest......

I guess chucking a garbage can of Gaitoraide over the car isn't a good idea then?

911Dave

But my real point is, why not just turn the engine off immediately? Doesn't it cool even faster that way? I don't understand what the point is of fast idling for 2 minutes.

DGaunt

I think the key here is "hard driving".
My guess is that Porsche is worried more about the valve train than the cylinders. The cooling ability of both the fan and oil systems is proportional to flow which is, in turn, proportional to engine speed. However, heat generation is proportional to throttle setting. further, the cooling system always is a bit behind the heat generation of the engine. So, if you are on the track and have the engine burning as much gas as it can for extended periods then stop it suddenly, the exhaust vales can overheat as can the cylinders. When you shut down hot...I mean track or really fast on the freeway in high ambients, temps in the now stagnant oil can exceed its capability and the vales can coke up, among other things.

This is especially important with turbos as, unless equipped with an aux. pump, the oil stops dead in the turbo bearings and cooks from residual heat. In the case of a turbo, idling for at least a minute after driving is prudent. As long as the oil is flowing, it can tolerate the high (1,000+ temps on the exhaust side of the turbo, but if still, it overheats. I guess an analogy could be touching a hot stove briefly VS holding your hand on it.

My personal solution is to take it easy on the car for a short time when i know i am going to stop, similar to the warm-up procedure.