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Discussion Starter · #1 ·
For quite a while know I have been researching this mysterious 84-85 N/A lean (as in fuel mixture) running condition that I and others suffer from. This causes the car to:

1. Idle poorly before reaching operating temperature
2. Rubberband effect - you push down on the gas, the car hestitates for almost a full second, then jumps forward. It generally struggles from the lower RPM's until you are through about 4K RPM. This also disappears at operating temperature.

I had recently replaced to no avail (after ECU codes flagged):

1. CHTS and CHTS harness
2. O2 Sensor

After a reply to a post from Slicktop from z31.com I bugged him further and he explained that this "lean surge condition" was caused by "the computer thinking the engine is warmer than it actually is."

He continued to tell me that there are to ways to correct this condition. The best way is to replace your current ECU with an 87 NA ECU. The alternate method involved substituting the CHTS signal for the FTS signal (Fuel Temperature Sensor, which is attached to the Fuel Pressure Regulator) until the car was at operating temperature. A 3-way switch could be installed in the cabin to switch over once operating temperature is reached.

I replaced my 85 N/A ECU with an 87 N/A ECU and viola - SOLVED.

I wanted to post for anyone else who may be suffering from the same problem.

If this is old news I apologize. I had been searching for a solution for quite some time and am thrilled to be finally putting this behind me.

Side note: This also cured the problem I had with my Fuel Pump not stopping after 5 seconds when the Key was in the "On" position before starting. The problem did not occur when the car was off.
 

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thanks for the heads up Stu85na
 

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if you put a 87na computer in, did you have to put a new O2 sensor in as well? I heard the O2 sensor must correspond with the year of the ECU...
 

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Discussion Starter · #4 ·
Sorry I was so slow to respond!

phxz31 - if you use an 88 or 89 you do have to make adjustments for the 02 Sensor but if you use an 87 ECU the 02 sensors will match.

I was reading a post of yours that is from today (October 10th) and you mentioned having problems similar to the ones I was having before I implemented this new ECU. If the symptoms are identical, I would consider the swap. If it doesn't work, you can always resell the ECU. You can get an ECU fairly easily on z31.com by putting out an "Items Wanted" request. I picked mine up for about $50 shipped.
 

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I just swapped my 85 ecu out with an 87, i noticed quite a positive change. my pickup in lower gears is almost doubled and my hesitation is practically gone. i think the slight hesitation right now, for me, is the fact that my fuel temp sensor is gone... my buddy broke it off accidentally. Im going to replace it, but I cant seem to find the part number.
 
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