My 86 is 100% original matching numbers. No 'tuning' or anything else to 'improve'. Has 120k miles when I bought it last year. Overall, I was very happy with the engine behaviour. Easy starting, never quitting, good idle etc.
However there were some absolutely minor things that I attributed to "after all, this is a 35 year old car", like:
- the dyno showed me 225hp (near factory), but it felt as if they were not really there
- once in a while a very slight bogging when leaving from standstill
- the warm up sequence from cold brought rpm up to 1100 rpm (as expected), although with an occasional 'pumping' of 100-200 rpm during a few seconds.
Using my Bentley manual (absolutely great reference tool) I went reviewing the stuff that the DME (ECU) is listening to for making up his mind what to do:
- first the air flow sensor. Many threads describe how the resistor bridge inside can suffer from wear. I opened my unit, and indeed contact traces were clearly visible.
Various threads describe how to 'shorten' the copper slider so it can run along a non-damaged contact path. Many raise concerns that the contact pressure should be respected. What I did is explained in the pictures below. I first measure and draw the angle of the contact wiper assembly as a reference. Then I bent the contacts from 'flat' to 'Z' shaped while respecting the angle of the wiper (distances A and B are identical). This moves the 2 tip contacts inwards, contact pressure remains the same. The carbon contact path was cleaned and the plastic lid glued back on with silicon.
Then I checked the idle switch, the WOT switch as per instructions. Those were fine. As was the Cylinder Head temperature sensor.
Next came adjusting the throttle linkage. I noticed that the butterfly opened only 2/3rd. I checked on the underside of the car, and indeed the throttle linkage 'B' clearance was a couple of mm out of spec. Hence reducing the rotation range of the butterfly. It still does not open to a full 90°, but way better now.
The O2 sensor was checked, seemed ok. However, it clearly had it's age so I replaced it too.
Finally the dreadful Idle Control Valve. I have checked it according to the Bentley instructions, passed all tests. But it did not look right (can’t explain…) and the cold idle pumping was still there even after having restored the air flow sensor contact wiper. A new one went in.
Now ready for a test ride. The car was started, the warm up cycle went immediately to 1100 rpm and then gradually slowed down to a stead idle. No more pumping or surging. Going for the ride, the bogging from standstill was completely gone, the sleeping horses had woken up, torque in the low range was way up. Grin on my face …
The effect should not surprise: when the ecu/dme gets garbage info from the sensors and then relies on crappy output accessories to ‘adjust’ the engine no wonder things are not optimal!
Lessons learned: a methodic (Bentley) approach, more labour ( a couple of days) than money (O2 sensor and idle valve) can bring us a long way
