Don't know where to start

[expat]

I'd rather try sort this out myself than pay someone else, as I would like the satisfaction if I can fix, but I really don't know where to start.

When cruising at any speed, my 87 gets jerky (not the beef type), like small amounts of power on, power off. Its not very bad, but not smooth. Its embarrassing for taking people for a spin in the car because its like the cars out of tune (which a Porsche should not be for those who care enough!!).

Where should I start in tracing down this problem and getting it back to a smooth ride.

PS I've had the car 1 year and it has progressively got worse over that time. Its only driven on w/e but is driven pretty hard during these times. At time of purchase it had just had a service with new clutch. A report (PPI) that came with the car said it needed a new air mass meter to allow adjustment of CO% but I was told that this only affected idle until warm. Is this true and is it possibly a cause of the jerky ride even when warm/hot?

Thanks, Cheers
Mark

[Argo]

First thing I'd do is to pull a plug or two and see how they look; this might help point to whether its a fuel problem or a spark problem. More info would also be helpful; does it start easily? idle smoothly? stutter more under load (going up hill)? fuel filter changed lately? cap and rotor look ok? plug wires look good?, etc.

I'd hit the basics and hope for some luck. Then I'd probably start thinking about a sensor (02, head temp) or a relay (DME).

Argo
88 Targa

[expat]

More info

Starts easily - yes, every time, first time. Sometimes a little smoke on startup but not often
Idle smoothly - yes 950rpm on the button
Stutter under load - no pulls like a truck from and rpm as soon as the throttle is pushed
Fuel filter, rotor, cap replaced, plugs- replaced 5000miles ago - will pull plugs and see condition.
DME relay - new 200miles ago
Plug leads - look ok, suspect they're pretty old though.

Any other thoughts appreciated.

Thanks
Mark

[john walker's workshop]

try unplugging the oxygen sensor and see if it goes away. that will maybe help to point you in some direction. the carreras should have the sensor in the system, so this would just be for diagnostic purposes.

[Peter Kelly]

I'll bet it is the air flow meter. This spring loaded door is connected to a potentiometer and can fail gradually over time. I had one go, and had symptoms very similar to yours.

Both the Haynes and Bently manuals have a test proceedure for the sensor. All you need is an ohm meter.

[ErVikingo]

I had a similar problem which was solved by replacing the microswitch TPS (throttle position sensor). The other symptom mine had was a rough idle.

hope this helps.

[PORSHIN]

It sounds like someone has adjusted the flapper in the air flow meter. If it is adjusted toward the light side, (less spring tension), it will never stay in one position, especially at lower speeds in higher gears and light throttle. If you feel so inclined, remove the meter, remove the cover and remove the air box from it. Turn it upward so that the inlet is pointing upward and look inside. slightly move it up and down as you look in. If the flapper door inside doesn't stay shut when you do that, it doesn't have enough spring tension. If you adjust it tighter, do it only a couple of notches at a time and recheck each time. Stop when you have to shake it abruptly to get it to open. Remember, this is with the inlet of the meter facing straight up. Before doing this though, be sure to check it with an ohmeter. Connect the ohmmeter to the two center terminals, and using your fingers, gradually open the flapper door. Watch for a consistent change in resistance throughout the sweep. The amount of resistance is not as important as it is to make sure it doesn't have any open or shorted spots throughout the sweep.

[expat]

ok you guys are scaring me ohm meter, potentiometer, and microswitch

Sounds like I need to (finally after all these years) actually buy an ohm meter. I'll probably short something or fry myself knowing my luck, but I certainly get one and have a go at your suggestions.

I'll check the O2 sensor too as JW suggested.

Thanks.

Cheers,
Mark

[Peter Kelly]

I highly doubt that it is the microswitch (either the WOT or idle switch).

The air flow sensor can go bad even if no one has tried to adjust it.

your PPI indicated it was bad.

IF an ohm meter scares you then I would advise you take it to a good porsche mechanic, along with al the info you have gathered from this thread.

[HawgRyder]

For this job...an ohmeter of the non-DVM style is what you need.
The digital types are more expensive and will be harder to use...get a cheap one with a needle and scale.
Bob

[jason1guy]

try the Throttle Position Sensor. its like 75 bucks and more than likely that is the problem. when they wear out the car will buck
back and forth when cruising at a steady speed.

Jason
81 SC

[RazorRacer]

Your problem sounds very similar to the problem I am having with my 78SC. You have described your problem better than I described my problem. I am very new to working on cars, much less Porsche's.

The first recommendation was low fuel pressure. Change the fuel filter, fuel accumulator, fuel pump, check valve, etc.

Other possibilities I have been considering are:

Clutch problem: Maybe it's not the engine, but the clutch. I don't think this is the problem.

AAR or AV: It could be that the aux. air regulator or the air valve is stuck open.

Now I am hearing air flow meter, throttle position sensor and oxygen sensor. These are not 78SC parts!

Would anyone care to comment on these possibilities?

[Peter Kelly]

jason1guy, there is no throttle position sensor on 3.2 motronic carreras. There is a WOT (wide open throttle) switch that engages at about 75% throttle which tells the motronic to go to the WOT fuel and ignition map, and there is an idle switch that tells the motronic to go to the idle program.

All points in between idle and WOT are deduced by the motronic by reading the signal from the AIR FLOW sensor, erroneously refered to earlier in this thread as the mass air sensor.

[RazorRacer]

I have been searching for an answer to your problem on my 78SC since I bought it a few months ago. I have learned to drive longer distances by accelerating and coasting, accelerating and coasting... I can't wait to fix this problem so I can put some long distance miles on the car.

Does your car work fine until you travel at constant speed for a few miles? Then it stays rough until it stops (idle or off) for a while? Then it's fine until you travel at constant speed for a while?

If so, we should be looking for something that is similar to a 78 and an 87. Like you, I would rather sort this out myself or have the right information for a pro to fix it. Hopefully someone (Wayne? Johnny Walker?) will point us in the right direction, soon.

[expat]

Thanks everyone for the advice. I will try out the suggestions this w/e and see if I can eliminate a few things.

RazorRacer - I think what your suggesting is the same as what I'm experiencing.

I accelerate (either gentle or at pace) with no drama. Smoooooth!

I get to any cruising speed and back off a little and the feel of the car is a slight jerkiness (such a word?). I actually typed surge instead of jerky but it is not a surging feel, more an uneasy cruising pace. Holding at cruising speed it will vary between smooth and jerky (it is not violent at all). Accelerate or power off completely and the car does exactly as it should (pulls strongly or declerates as is the case).

Passengers would think I was feathering the throttle a little at cruising speed although most of the time the excuse in their head would be more like " Wow, this car really is sensitive to the uneveness in the road!"

Remember, I never get to cruise at any speed for more than 2-3 minutes, living in Hong Kong. (Traffic and nature of roads here!)

Once again thanks all.

Cheers
Mark

PS on second read of your post, no I think we're talking about different things. Mine never runs rough per se, just not smoothly at cruising speed. (Did I just contradict myself?????)

[RazorRacer]

Darn it.

You do have a different problem than mine. If yours ran rough after cruising, then it is the same, but yours is only during cruising.

Best of luck to you.

[Peter Kelly]

Even if your symptoms are exactly the same as razor's, the solution will be completely different as his fuel injection and ignition systems (SC has CIS injection) are completley different from the 3.2 motronic system in your car.