87 911 with poor acceleration and backfire on deceleration

[roger911]

Hello all,
I have an all original US 87 911 Carrera with 70k miles and an acceleration and deceleration problem. When I take off from a dead stop, there is lack of power under load. I have to ride the clutch and apply more gas until the engine reaches 1500 rpm. Once moving, acceleration seems to be fine. On deceleration or downshifting, there is at least one backfire. When idling, the rpm varies from 820 to 880 rpm about every second.
Some of my fault finding actions were: check for vacuum leaks, replace suspected lines, have a garage do a vacuum system smoke test, retorque the intake manifolds mounting bolts, bench test the O2 sensor, replaced all plugs, plug wires, rotor and distributor cap.
I do not believe the Motronic ROM is original. It is a 24 pin chip with the following writing: SGS, 88749, M2732A-4F1, 84-87 911.
I am new to Porsche vehicles and would appreciate any advise or suggestions

Thanks

[livi]

Welcome, Roger! I will start off the respond chain so that others with more knowledge follow suit.

[JeremyD]

What does your cylinder head temp and reference sender look like? [driver's side front of engine] When were they last changed? Do you have a spare DME relay you can try?

[rick-l]

can you pull the connectors off the injectors one at a time and see where there is no rpm drop?

[roger911]

Livi - Thanks for the welcome.

JeremyD - The CHT and reference sensors measure within spec. I believe they are the original sensors. I can get a DME relay and will be trying it this afternoon. I'll let you know what the results are.

rick-I - The fuel injector electrical cables were disconnected one at a time and a slight drop in rmp was heard. Somehow I was expecting a more significant change in rpm. What is a typical drop in rpm when doing this test?

This reminds me that the rmp does not drop when the oil cap is removed.

Thanks for all your help.

[ischmitz]

The oscillating RPM at idle could point to a lazy O2 sensor or faulty AFM.

I wonder how you would bench-test an O2 sensor? The only reliable test (I know) is to observe the output while it is at temperarture in the car. You want to see it changing its output voltage between 0.1 volt and 0.9 volts several times per second and react to blipping the throttle.

If you heat it in a propane torch flame this only proves static operation but could still mean that it doesn't react fast enough to changes in O2 concentration. And this causes hunting.

Ingo

[kodioneill]

Could be a dead spot in you AFM potentiometer. Do a search "AFM rebuild".

[stlrj]

It might be helpful if you could tell us when these symptoms first appeared. Were they the result of some recent work or an unusual event?

Difficult to diagnose a problem without first going into it's historical background.

[SpeedracerIndy]

I had the EXACT same symptoms with my dad's 88 Carrera. I was banging my head against a wall trying to figure out what the cause was. I think my problems were a combination of things causing it including a bad O2 sensor, bad plugs, clogged CAT and worn spot in the AFM. Every time I replaced or fixed one thing, it would get a little better but not go away completely. You should also check your cylinder head temp sensor. Just because it bench tests ok it might not be. If it's the original 1 wire type you could be having grounding issues on the aluminum cylinder. The replacement is a two wire type with the additional wire as a ground. Good luck.

[roger911]

Jeremy D - Replacing the DME relay with a new one did not improve the poor acceleration.

Ischmitz - After a good warn up, the oscillating rpm seems to settle down.
The O2 sensor was bench tested using a propane torch flame. Once heated, the output voltage would change from .9 volt to .1 volt and back again within a second of removing or applying the flame.
A faulty AFM could be the problem. I had to replace the AFM on my VW Passat VR6 to solve a lack of power on demand problem.

Kodioneill - Checking out the AFM is my next priority.

Stirj - The problem has been there since I purchased the Porsche two years ago. My son purchased an 85 911 ROW about the same time. After driving both, it became obvious that my 87 911 had an acceleration problem. A good description would be "dead spot" on take off.

SpeedracerIndy - The final solution may be a combination of differeny actions. I have decided to replace the O2 sensor only because it has 70k miles on it. All the ignition parts have been changed. My next task is to check out the AFM.

Roger911
87 911 Carrera

[rick-l]

http://www.the944.com/afm.htm

lots of info here
http://www.the944.com/

[Goth]

Hi Roger,

Here is my 2 cents on what may be two issues:

Low off idle power: There is a micro switch on the backside of your throttle body. At closed throttle the micro switch is activated. This tells the computer to hold the RPM's at 850 or so. This micro switch (if properly positioned) should deactivate at the first crack of the throttle. If it stays activated longer, it will try to hold the RPMs down causing what you have described.

Occilating idle: This indicates a rich idle mixture. Someone may have adjusted the idle mixture to the rich side to try to overcome what they thought was an off idlle stumble from a lean condition.

Hope this helps,

[stlrj]

Originally Posted by roger911

Stirj - The problem has been there since I purchased the Porsche two years ago. My son purchased an 85 911 ROW about the same time. After driving both, it became obvious that my 87 911 had an acceleration problem. A good description would be "dead spot" on take off.

You could be experiencing a rich bog off idle. The solution might be as simple as having your mixture adjusted.

Cheers,

Joe AKA stlrj