STILL running rough

[surfdog4]

I am getting seriously frustrated. My '81 US 5-speed stutters and stumbles under acceleration. Here's my list of things I've done and checked

+ Replaced timing belt (for other reasons)
+ reworked intake manifold (new hoses, vacuum lines, etc)
+ new spark plugs
+ new hoses for charcoal canister, fuel tank, etc
+ new decel valve
+ checked all vacuum items for leaks (decel valve, diverter valve, purge valve, pressure regulator, etc)
+ checked timing (it was off, but now in spec)
+ checked vacuum advance (working)
+ checked distributor cap/rotor (not new, but looked OK)
+ temp sensor II OK
+ idle and WOT throttle switches OK
+ new fuel filter
+ cleaned out fuel tank

It idles fine at 750rpm and runs at higher rpms fine while sitting in the driveway, but when you try to accelerate it stumbles and bumbles.

What should i check next? Could the distributor cap/rotor be bad but pass visual inspection? seems like that would affect idle as well.

Any help would be appreciated

[vdubr928]

Cap or rotor would affect performance across the board in my experience. Mine misses a little under slow acel but if I stomp it no miss. Pulls hard and smooth. Traced mine to the number 8 fuel injector not opening at the smallest hits. in other words it dirty and only lets fuel flow when it's open wider. Here is an outfit that several ppl recomended to me.

http://www.witchhunter.com

I'm going to send mine in next week or so. I'll post when I get them back in.

[MPDano]

Mine did just that upon acceleration, but smoothed out once the rpms were up. I cleaned the plunger on my fuel distributor. Smoothed it right out. My 81 Euro is CIS. Not sure what an 81 US uses.

[surfdog4]

'81 US uses L-jet, so no plunger

[psychII]

is it stumbling at hard acceleration, soft acceleration (like in a parking lot), or both?

Mine stumbled at low throttle, like in a parking lot, but would clear under hard acceleration. I ran a couple bottles of Techron and cleaned the MAF (which I'm not sure yours has??) and two years later no problems.

good luck.

[surfdog4]

i suppose it is more noticeable at launch than higher speeds, but it's happening some at all range....as part of of redoing the intake manifold i cleaned everything involved, including the spider, the thottle..the airflow sensor was pretty clean

[vdubr928]

He does have MAF but he is saying he cleaned. I think what everybody here is asking is:

When you stop hard on the gas off the line does it miss?

If you ease on the throttle off the line like regular city driving does it miss?

If it's worse on the latter than it sounds like dirty injectors. You can try some injector spooge. (STP or the like) but you will get best results if you sent the injectors off to be cleaned. (or replace them all) and while your at it buy rogers underhood fuel line replacement kit (less than 100$$) and replace all the bad rubber. You can also clean the electrical contacts at the injectors. Those fuel injectors are 20 some years old. Since you have already done most everything else it's worth a couple hundred more.

PS does it do this when hot and cold? I had a bad coil once but it only missed when hot.

Originally Posted by psychII

is it stumbling at hard acceleration, soft acceleration (like in a parking lot), or both?

Mine stumbled at low throttle, like in a parking lot, but would clear under hard acceleration. I ran a couple bottles of Techron and cleaned the MAF (which I'm not sure yours has??) and two years later no problems.

good luck.

This is what mine does. Traced to #8 injector. I can pull number 8 injector wire and the thing doesn't act much different under soft throttle till about 1500 where it would normally clean up. With the wire connected if I floor it off the line I get no miss.

Dano.... Could his be the green wire problem? I guess we really need more info from him. When does it miss. How bad is it missing. More than one clynder or just a single clynder lope?

[surfdog4]

-It happens any time i try to accelerate, worse from standing start than underway, but present every time i change gears or mash the gas.

-Also happens on light throttle, just not quite as badly

-At a standstill, i can rev it up and hold rpm without noticing any problems, and it idles fine

-I've run a couple of bottles of Techron through it at high concentration

-Does it hot and cold

I did not check the electrical connections on the injectors...at one time i had idle problems that i traced to corrosion on the Temp Sensor II contact

[Niles]

Have you checked your spark plug wires - at night to see if there is arcing? Did this problem come on gradually, or did it appear right after some recent maintenance? Since even light throttle causes it, I'm thinking it's not a fuel starvation problem, but electrical - coil, cap, rotor or wires.

[lizard928s]

I would rebuild the injectors next.

[Normy]

I second Niles' thoughts.

First- at night, open your garage door about 2 feet. Open your hood, then get in the car and start it up. Get out, and then look at your engine bay in the dark. Do you see little flashes of light? This is arcing. If you have this, change your wires.

NOW. Bosch L-Jetronic fuel injection, which fuels your 928, has a thing known as an air flow meter. This device measures air flow, and sends the information to the computer, a metal box on the right side of the passenger floorboard area.

My '85 928S2 uses an updated version of this system. My car has a true MAF, but that small distinction isn't important right now. What you need to know is that my '85 did exactly what you are talking about, and I worked for years to figure out the problem. What I eventually discovered is that the connector on my MAF was subject to corrosion. Literally, every 10 weeks I have to take the MAF out and clean the connections with Deoxit. It is bizarre...but this works!

That is what I would suggest you do. The air flow sensor is under your airbox, and has a strange shape: It looks like a metal rectangular box with 1/4 of a circle added to one side.

Here's what to do: Use a 13 mm socket wrench to remove the two bolts holding down your airbox. Try lifting the thing away. It SHOULD come away with the air flow sensor intact. Now, using a very small flat-blade screwdriver, pry off the connectors for the sensor. My LH-Jetronic car has a second sensor in the air box that senses intake air temperature, so there may be another small connector a few inches toward the drivers' side of the car to pry off.

When you get the airbox-sensor assembly loose...use Deoxit [available at any Radio Shack; this is a miracle substance that I highly recommend!] to clean the connections. Just put in on, let it sit for 5 minutes, and then put it back together.

-Dude- this makes SO much difference in my particular car! It is night and day. The gentle Doctor Jekyl turns into screaming, wailing 12.9-seconds to 100 mph Mr. Hyde after I clean my MAF contacts.

Bosch fuel injection from the 1980's was good, but it is now over 20 years old.

I'd try cleaning your contacts. Not just the air flow sensor, but all of them- the individual injectors. Pull each one off, spray it with Deoxit, and then take the car for a drive.

If that doesn't work, then you need to find a Porsche dealer that has the suitcase computer that can diagnose your system. Don't put too much faith in this last; They had one for my car, but it could not find the problem with my MAF connector.

Good luck!

N

[surfdog4]

thanks folks...
Niles: before i starting working the intake "stuff" it ran, but not great, so that got me started looking for vacuum leaks, etc.
as expected, all the lines were rotten, missing, or misconnected. After cleaning and replacing everything it ran much better..the"stumbling" seemed to occur after it had been sitting for a period. soo....

I could certainly believe a corrosion story..a couple of times after the car sat for a while (it lives outside under a cover) it wouldnn't idle at all...after some investigation i found out the Temp II contacts had corroded just enough to lose contact....i would clean them up and it would fix the problem..so a neighbor gave me something called BO-Shield, apparently developed by Boeing for airplane contacts..seems to work better than WD-40

while doing other things, i had pulled the air box and air sensor out several times, but I haven't pulled them in quite a while so quite possible those contacts have corroded, and i never pulled the injector wires, so that gives me a couple more things to try, along with the spark plug wire..

Reading in other places i'm starting to suspect the injectors as well..perhaps all my "cleaning" broke something loose..but wires are easy so i can start there

Thanks again

[surfdog4]

well i cleaned all the injector contacts and the air flow meter connections...nothing

[Danglerb]

I would double check the vacuum routing, with special care on the vacuum line to the distributor.

Engine controls have different goals depending on the throttle setting.

Idle is about smooth running and low emissions.
Part throttle is about emissions and economy.
WOT is about power.

Throttle body vacuum fittings activate for each of those throttle settings, and things need to transition smoothly. Bad actuators, leaks, or misrouting vacuum lines will mess that up.

[Normy]

Is it possible for the air flow meter to be effected by dirt?

The throttle plate in the 928 is a single piece design. That is, one single butterfly opens when you push the pedal down. Many cars have two smaller butterflies, so that the sudden change in intake vacuum isn't as sharp and the injection has a chance to keep up.

I don't know much about the L-jetronic air flow meter, but that might be something to test. Good luck-

N

[surfdog4]

seems like dirt would be a gradual problem, but you did make me think perhaps in all that handling of the intake the air flow meter, spring, or circuit may have broken..guess i'll check the manuals on how to test it..thanks!