Bill.e1

Hello, all

I've owned my 944 for around three months now and this list has been invaluable as I go through the car. Thanks to all. Unfortunately, I have one issue that I can't fix and I can't find in the archives.

I have a lean surge during acceleration. It is most noticable during "hard street" runs- not full throttle but still quick enough to feel good. I have a fuel air meter installed and each power drop corresponds to the lean part of the closed loop swing. For fun, I tried various other maps/settings using coding plugs and the O2 sensor connection. All open loop configurations run lean except for US spec with O2 sensor grounded which show rich on the meter.


Things I have done:

Igniion:
Replaced plugs- first Bosch Platinums (forgot how much I dislike them) then standard Bosch plugs. Replace wires with Accel spiral wound custom fit to distributor cap but without Beru resistance plug connectors. Added MSD 6a and Blaster 2 coil. Inspected and cleaned distributor cap.

Fuel- ran multiple cans of Chevron injector cleaner through the system. Checked fuel pressure.

Control- cleaned and re-tracked airflow meter. Loosened AFM spring one click. Replaced O2 sensor. Added fuel air meter. resoldered computer. Cleaned connections/grounds/relays. Added switch bank to change coding plug configuration connects/disconnect/ ground O2 signal. Checked output of temp sensors. Cleaned TPS assembly.

Vacuum: inspected replaced hoses. Checked function of valves/switches.

Motor- compression check good. Runs smooth and strong except for issue.

Only two changes had a noticeable effect. The new O2 sensor increased the speed of the rich/lean correction cycle but did not cure the surge. I found a hole in my vacuum cannister and repairing it helped performance and eliminated the "solenoids on parade" clicking from the heater.

Any thoughts? The issue seams to be that the computer allows the mixture to go too far before it corrects.

One more possibly related item- I think one of my injectors leaks a bit. The system still holds pressure for quite a while but not as long as it should.

Any help would be appreciated.
Bill

Charlotte944

Early or Late car?

Geo

Unless you're using a wide-band O2 sensor, I wouldn't believe the meter. They are notoriously inaccurate.

Bill.e1

Sorry, forgot the basics. 86 na 114k miles decent shape. The A/F meter is a cheap Sunpro unit. I agree that the the specific reading means little but the swings I see are full scale lean to near-full scale rich. The loss of power happens when the guage indicates full lean. Power returns as the guage reads more fuel. In this case, I think it is safe to say that the loss of power is during a lean condition.

Anyone else have a meter? does it swing full full scale during closed loop?

Thanks again,
Bill

Charlotte944

To get a better idea of what is happening try connecting a Digital Volt Meter to the A/F meter input. Set the meter to read the 2 Volt DC scale. Now you can watch the actual voltage swings.

FWIW, you will always see the O2 sensor voltage go lean when you first open the throttle. This happens because there is a delay induced by the fact that it takes a finite amount of time for air to flow from the AFM, through the intake, through the engine and then out the exhaust.

Just out of curiosity, when you see these lean swings, are you mashing the throttle wide open?

Bill.e1

I've got an inexpensive DVM but I don't think it's fast enough to catch the swings. My analog meter is 20 plus years old- the best ten bucks could buy at Radio Shack.

The AF meter swings full lean to almost full rich most of the time the car is in closed loop. The lean/rich surge happens when I'm accelerating half to 3/4 throttle- quick enough to be felt by not to enough to leave the closed loop mode. The car runs smoothly in that it doesn't miss, it just looses a bit of power whenever the AF meter swings full lean.

Now that it is a bit cooler and I run without air conditioning, the surge is reduced.

Does the Motronic 3 include a "block learn" compensation feature? There doesn't seem to be any. I could see if the mapped mixture was way lean for some reason, the closed loop may be less stable. Just a thought.

Again, any insight would be appreciated. Is this a case for the fuel compensation switch? Should I loosen the air flap another notch?

Thanks,
Bill

Der Rennwagen

Bill, I "had" the same EXACT problem. Installed a new OX sensor and had the SAME problem you did - it exasperated the closed loop condition. This symptom gets worse as the engine warms up which lead me to believe there is one sensor that tells the DME to run richer (and or lean out) as it warms up, the coolant temperature sensor on the head. The lean part throttle would indicate that I've got air leaks. Injector seals, vacuum canister, you name it. My temporary fix was to unplug the sensor connection, pull it out of the head area and then just plug in a good spare temp sensor and hang it over my Carrera Cup air intake so it received cooler air, tricking the DME to think that the engine is still somewhat cold. Cold start enrichment. I'm not sure if the timing is advanced as well (don't think so), but the engine sure runs stronger and the part throttle lean surging has dissapeared. Next step is to repair ALL possible air leaks.

Here is a thread on my solution for now >>
http://forums.rennlist.com/rennforu...ad.php?t=225650

I updated my website with photos of how I attached my temp sensor to stay "cold" until I fix the air leaks.
http://www.thompsonsmithdesign.com << look under "biography/current cars"

Thompson
1985 944.1
Club Sport targa