Shudder, Stutter and Sputter

[lukeh]

My 88 has just started to act up. When I'm running under 3000rpm the car stutters when I try to accelerate. It's worse in the higher gears and even worse when the car is under load. For example, when the car is high gear around 2250 or 2500rpm going up a hill and I put the gas to the floor the car won't go any faster but will instead maintain speed while just sputtering, stuttering and emitting a muted pooping sound from the exhaust.

On flat surfaces the problem lessens and seems to disappear completely once the car gets over 3000rpm. But once I drop below 3000rpm and try accelerating again it starts the hesitation/sputtering all over again. Any thoughts on where to start looking at a fix? Thanks.

[PatrickB]

Sounds like a bad fuel injector...
However, you need to keep the revs up a tad more!

[lukeh]

Originally Posted by PatrickB

Sounds like a bad fuel injector...
However, you need to keep the revs up a tad more!

Revs up?!?! Haven't you seen the price of gas?

[Flieger]

+1 on keep the revs up. My E21 BMW did the same thing. It was running rich (10x gross polluter rich) and would not like to start, idle, or getup 'n go. (What else is there?)

Got a full head rebuild, tightened valves, fixed vacuum leaks, leaned the mixture. Runs like a champ now. It has a sweet spot at about 4000 rpm. I keep it there for fast highway cruising and hills (though I am in 5th gear on the freeway).

Porsches need revs. If you lug them, the plugs, combustion chamber, and vlaves will carbon up. It also is bad because you are putting a lot of torque stresses through the pounding of the connecting rods on the crank. With lower revs, there is less oil pressure and less lubricating oil to protect your bearings.

At higher revs, you don't stress the motor as much. It is not making as much torque, has good oil pressure, and is breathing better and cleaner.

You are having a lot more fun, too.

[WI wide body]

Had an '84 that was doing the same thing and changed both the DME relay and the Head Temp. Sensor (told those were the first things to suspect) but it ended up being the O2 sensor that goes into the catalytic converter that fixed it. Go figure.

[stlrj]

Originally Posted by Flieger

+1 on keep the revs up. My E21 BMW did the same thing. It was running rich (10x gross polluter rich) and would not like to start, idle, or getup 'n go. (What else is there?)

Got a full head rebuild, tightened valves, fixed vacuum leaks, leaned the mixture. Runs like a champ now. It has a sweet spot at about 4000 rpm. I keep it there for fast highway cruising and hills (though I am in 5th gear on the freeway).

Porsches need revs. If you lug them, the plugs, combustion chamber, and vlaves will carbon up. It also is bad because you are putting a lot of torque stresses through the pounding of the connecting rods on the crank. With lower revs, there is less oil pressure and less lubricating oil to protect your bearings.

At higher revs, you don't stress the motor as much. It is not making as much torque, has good oil pressure, and is breathing better and cleaner.

You are having a lot more fun, too.

I dissagree. Porsches are cars like any other.

If you have to drive them at high revs just to have them run right, at least that's what the used car salesman told me, then there's something seriously wrong that needs your attention.

My 86 3.2 will easily putter around town at 1500-2000 rpm all day and then scream to 6000 in an instant when asked without hesitation.

But if you don't mind the extra wear and tear and visits to the pump, then by all means, rev to your hearts content.

Besides, I never let anyone tell me how to drive my car and you need to hear the other side.

[Flieger]

Drive how you like

Just keep to the right as I fly by at 6000 rpm

I only go out for short runs on the weekend with my 911 so I don't put many miles on it. It is a little harsh, like a track car. The gas mileage (MFI) will never be good and as little as I drive, I want to have fun.

[Flieger]

I did not intend to ruffle any feathers.

Does the 88 go uphill smoothly at high revs? (can you try it once just to help diagnose the problem?)

If it does, it might be a dirty fuel injector.

The problem sounds like an overly rich condition, though. It would be masked by greater airflow at higher revs. Putting the gas to the floor causing the engine to stumble more sounds like too much fuel, though.

[jurhip]

stlrj is correct.

Poor running below 3K is a sign a sensor is likely gone, which others have addressed. The mark of a good engine is smooth power and clean burning from idle to redline. Porsche typically makes good engines.

I see a lot of this "Porsche engines love revs, they need it to stay clean." That s true for poor running engines. Like Flieger said, though he avoided the problem by keeping revs up, his car was running rich - i.e. out of tune.

Don't believe myths.

[jurhip]

+1 on Flieger.
Another question:
Does it idle smoothly, and at the correct rpm. If so, an injector is not likely the issues as you would expect to see it here.

Under load the Motronic unit will inject more fuel (to accelerate obviously). However, it could be too rich (usually o2 sensor) or timing advance issues. My guess is o2 sensor. if you pull it and gve it a little shake, you can sometimes detect a bad one by listening for a rattle. also, it the o2 sensor has a lot of black soot on it, it is probably bad. They were installed in the car to keep AFR near stoich as to not foul the cat. when they go, the car goes to safe mode to ensure it does not run too lean.

[Flieger]

What happened to the thread starter?

[jurhip]

hopefully his car did not leave him stranded

[Flieger]

I wonder what happens when he starts up the hill at high revs and then lets off some pressure on the throttle, letting the engine revs drop to the 2500 range. Does it still sputter?

I am concerned about him going uphill in high gear, as he said. Low revs with high mechanical advantage (e.g. first gear) are not bad. It is when you are asking for lots of torque at low rpms. The bearings dont like it. The 911 engine is much better than the roller bearing Fuhrmann Carrera engine, though.

I don't mind low revs without loads or downhill.

The O2 sensor replacement on my BMW helped, though not enough. The head work was great. The O2 sensor did make a noticable improvement on the acceleration under load.

[jurhip]

Lugging an engine is not good for it for reasons that do NOT include the bottom end/journal bearings (on a well designed engine)

Modern engine journal bearings are designed for max engine loads and the associated journal bearing loads/reversal angles. The carrera 3.2 has the added benefit of journal sizes based on much higher torque outputs of the 3.3l turbo. This is typically why the bottom end on a 3.2 seems to last forever.

If you look at a dyno, your max driveline load will always be a highest torque output in 5th gear. On a 3.2, this occurs fairly high up the rev range. The engine itself will always see max torque at full throttle in any gear, unless there is wheelslip. At lower gers it just doesnt stay there as long.
What you are basically saying is a Porsche cannot run flat out up hill at max torque (around 4800 rpm) for any length of time. That is not true.

Though the oil pressure at lower revs is lower, it is not low enough to reduce lubrication properties at the bearings unless you have sludge build-up or restricted passages.

[Flieger]

I was referring to the torque needed to drive the car forward.

For a given speed, with aerodynamic drag, hills, and other friction, there is a given power required to maintain the speed (or accelerate).

Torque * RPM / 5252 = Power

So, if your engine is turning slower, you need to make more torque to maintain the speed.

If the engine does not make that amount of torque that low, it needs to speed up to the point where it makes enough power.

Correct me if I am in error

[jurhip]

No, your equation and theory are correct, but an engine is limited to the maximium rated torque at a given rpm. It cannot produce more, therefore, the maximum bearing forces will be seen at max torque. For a 3.2, this is around 4800 rpm. If you are lugging at lower rpm, the max torque will be whatever it is at that engine speed. Being lower than the max, the forces on the engine bearings are lower.

If one looks at road load (the sum of the items you glanced upon) one see's:
engine torque output > road load = car accelerates
engine torque output < road load = car decelerates
engine torque output = road load = car maintains steady speed.

To simplify, say you are starting to go uphill at 3000rpm in a toyota. torque required to maintain speed up this steady grade is 40 ft-lbs for your particular car. You have the throttle open 1/2 way in 4th gear and your engine is producing 20 ft-lbs (i.e. ~11.4 hp). Obvously you start to slow so you open the throttle wide open. But your engine has a max power rating of 18.3hp at 3000rpm (32 ft-lbs). - you slow down until your torque output matches the road load. or you change gears.

So, you can see that there is really no way that engine loads can ever be higher than the max torque output of the engine (perhaps very very slightly if you are decelerating at full throttle)

[Flieger]

It appears that lukeh has gone on vacation.

I wonder if it could simply be carbon on the valves from his driving style. Maybe he needs hotter plugs.

[Flieger]

Or maybe his gas went bad with the great fuel economy he gets puttering along