1985 3.2 Runs horrible until 3/4 Throttle

[biggtbiggt]

Hello: I have been having a slight bucking problem that I have been trying to figure out. The bucking is noticeable at all RPM ranges but appears to be throttle position related. Today it is raining out and the problem is extreme, (I remember last time it was wet out the problem was severe also). The car has a rough idle and is running terrible and will almost stall out if throttle pushed about 1/8-1/4. It is also backfiring. As soon as the throttle is pushed passed approx. 3/4 it is like a switch is thrown and all clears up. So about 3/4 to wide open is great but less than 3/4 throttle it is bad. The car has no mods that I know of.

[timmy2]

When was the last time it had new plugs, wires and distributor cap installed?

[biggtbiggt]

I just put new distributor Cap and Rotor. New fuel pump, and new fuel filter. It is for sure a misfire but I am not sure why it would not misfire at 3/4 to full throttle??? The plug wires are a silicone aftermarket type wire and look ok but the humidity is making me think they could be the culprit but going back to the throttle position thing I am having my doubts. I disconnected the throttle position sensor and it might make it run better, but it is hard to tell as the engine heat has dried everything and it is not running as bad as it was in the rain.

[inaminit]

check for a vaccum leak.

[biggtbiggt]

I don't believe it to be a vacuum leak as it is intermittent and worse when it is raining? the car has 95,000 km on it.

[plumb4u2]

Disconnect o2 sensor and see if problem persists...I had a bad sensor and was causing similar issues

[biggtbiggt]

disconnected o2 and still the same stumbling. I wonder if it is the TPS??

[timmy2]

TPS could cause trouble if the housing is cracked and it gets wet.
There are some recent posts on how to test it. The Bently manual covers it AFIK.
(I don't have one so I haven't had the need)

[DRACO A5OG]

Does it misfire at idle?

[bigel]

Originally Posted by biggtbiggt

I don't believe it to be a vacuum leak as it is intermittent and worse when it is raining? the car has 95,000 km on it.

Worse when raining seems to point to an electical issue...I would have thought plugs/wires/cap, but it seems you've already replaced. A sensor is also a good posibility...what about DME?

[kodioneill]

Reference and position sensor cht sensor?

[rvidal]

DME relay?. Check soldering joints or replace the unit. Humidity near the relay if not perfectly sealed under the car.
20 minutes repair time. or less...
Look at: http://forums.pelicanparts.com/porsche-911-technical-forum/652375-dme-bad-solder-joint-repair.html

[Chuck.H]

Originally Posted by biggtbiggt

disconnected o2 and still the same stumbling. I wonder if it is the TPS??

Just FYI, the 84-89's don't have a true TPS, they have idle and WOT switches instead. These should be checked but I wouldn't think they would cause this problem unless the WOT switch was stuck.

One thing that can aggrevate spark misfires is very large plug gaps, as the spark will look for easier paths to ground.

Also check your distributor advance springs are intact by gently spinning the rotor - it should move a few degrees and then be pulled back by the springs. Note the rotor advance is only for pointing the rotor at the right terminal, it doesn't actually advance the timing (the DME does that electronically).

HTH,
Chuck.H
'89 TurboLookTarga, 352k miles

[L8Brakr]

Sounds like a typical case of worn wiper arm tracks in the air flow meter. Over the years, your car has spent alot more time at part throttle compared to 3/4 to full throttle. Thus that portion of the conductive material has worn away and is providing poor signals to the DME.

Everything you need to fix this issue is here....

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

You can also check you tube for instructional videos on the repair.

[Rick K]

Originally Posted by biggtbiggt

I just put new distributor Cap and Rotor. New fuel pump, and new fuel filter.

Did it do this before you did this work?

[E Sully]

I would make sure the basic adjustments are made at the throtle body so that you know the micro switch and full throttle switch are set correctly. I doubt the O2 sensor is the issue. I would lean towards the Cylinder Head Temperature sensor or a good vacuum leak. From the sound of it, it runs terrible till the full throttle switch engages and the DME runs on the separately programmed full throttle map.
This is from another thread and shows how you can check the sensors with an ohm meter to try and narrow it down. It also would not hurt to look inside the often neglected distributor to make sure it is clean and weights are moving freely. Put a little oil on the felt pad.

Originally Posted by E Sully

If it were a bad reference sensor the car would not even start. If a bad speed sensor I would think you would have problems throughout the rpm range. They can be checked with a continuity meter, last picture.
It runs good at high rpm, have you looked at the the idle micro switch and the full throttle switch to see if they are set correctly? These affect the fuel and ignition map the DME uses.
The old single wire temperature sensor can cause problems. Later sensors are 2 wire type.
The distributor may need to be looked at. Timing is set by the DME, but it still has centrifugal weights for rotor phasing. If the shaft has play it will affect timing. It is very easy to remove and reinstall, just get a new o ring.
Have you checked it at idle by removing the oil cap to see if the idle drops yet? If the idle does not drop you have a vacuum leak.

[biggtbiggt]

Thank-You to all the replies. It was doing this before the Distributor cap and Rotor replacement. (That is why I replaced) I will take a look at advance springs. Then AFM, and then the DME Solder joints. The problem is consistent from 3/4 down to and including idle. Today I drove car and it had a slight bucking but not as severe. (Dry outside). I drive the car daily to work and back on a 100km/h highway about 30 miles each way so I can get a good feel of what each thing I change is doing. Yesterday it reminded me of a vehicle I had with water in the gas with the slight backfiring and poor running but I do not feel there is water in the gas as it changes with weather conditions and throttle position.

[DRACO A5OG]

I would check the injectors by pulling each connector one at a time to see which one may be faulty.

The other thing would be is to set the dizzy to tdc and pull it out to see if you have lateral slop, if too much play you will get miss fires. If you axial play the dizzy's internal snap in bearings are shot, time for a new used dizzy, a good one would be one that has resistance when spun, if it spins freely, find one that doesn't.

Next would be to crack the afm and inspect the poteniometer membrane, if groved to metal, it is shot, if not it could be salvaged but you do this at your risk. Find the method from the 944 forum, there is also a YouTube video on how to.

As always do the easy stuff first, if you have a 3.2 buddy near by swap the afm if the symptoms can be reproduced.

Jim

[biggtbiggt]

[biggtbiggt]