The issue: 1990 S2 85Kkm
Hesitation during loaded acceleration throughout the rev range... most noticeable at lower RPM, once at highway cruising speed the car 'pulses' slightly and persistently.
Here is the twist... with a cold start (first start of the day) the car runs @ 100%. Then after the car has been turned off and restarted the problem will continually be apparent until the next cold start.
Since the problem came about I have replaced and checked the following (note the problem has now been boiled down to the above description, so some of these attempts might seem out of place now, but at the time it seemed to fit)

1) Fuel Filter
2) Fuel Tank Strainer
3) Fuel
4) Spark Plugs (Cap / Rotor OK)
5) Oxygen Sensor
6) Temp II Sensor
7) AIT Sensor (OK)
8) AFM potentiometer (OK)
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So I am thinking that there is a sensor loop that is resetting after the cold start, but am not sure what exactly kicks in on second start that overrides the otherwise hard pulling engine.

 

It sound like your engine is misfiring however in most cases it should misfire after a cold start too, once the engine reaches operating temperature. So this is tricky one. In any case I think it some sort of electrical problem. As you replaced most possible parts, here are some more.

A dry contact in the engine management computer. Swapping it with a known working one just to test the theory is a good choice if you can get one. The ignition amplifier may be a problem. Also the sensor at the back of the engine not sure the exact name, (Crank Position Sensor?) may be acting up. Also check the condition of all the contacts and wires. A loose contact or bad connection can cause symptoms like this.

 

Yea this is a tricky one... I doubt its a misfire problem since it runs perfectly up until about 30 minutes of driving. I am thinking about swapping the computer with my friend.(if he lets me haha) To see what that might do.

Does anyone know if and what sensor loops cycle? Or where I could obtain the information.
Also... the problem seems to be self rectifying as its become less apparent... its so odd.

 

Well, I'm not going to be much help, but I've been chasing a similar one of these gremlins for a number of years. For me it only happens when it's very cold outside (<10°F/-12°C). Might read this for some ideas:

https://rennlist.com/forums/924-931-...tough-one.html

Not listed there is a new AFM (measured ok, but since this is an intermittent problem and I had nothing left, I went after that - kind of guessing that fuel was cut out because the AFM way lying to the DME about how much air was going through...). Made a big difference overall, including better emissions readings, but really only tested it once at low temp (drove the C4 almost exclusively all winter...).

I started by measuring everything mentioned in the DME-Diagnosis/Troubleshooting Guide for S and S2 in the FSM, and am still not sure if it is air, fuel, or spark (pretty discouraging, I know).

Three things that come to mind that that happen when you warm up are 1) the cold enrichment ends, 2) the O2 sensor heats to temp and the DME starts to pay attention to it, and 3) the ISV sets the idle to ~800rpm instead of 1050rpm.

Good luck.

 

It does sound more like an electrical contact. I will first definitely disconnect all connectors spray in WD40 and reconnect. Also I would check all ground connections and do something similar to ensure the connections is good.

 

Alright, ill give that a go.... WD40?... I was thinking dielectric.

 

Good plan to clean connections. I'd use contact cleaner, not WD40, and in particular, I'd clean the AFM connector. For the grounds, contact cleaner plus scotch-brite or emery cloth, etc. Then protect those with dielectric grease.

 

Illl give the car a good run through later this week, as I have exams to study for... I hope that the problem turns out to be a simple connection.

 

Like already suggested, the flywheel reference sensor may need a whack and a re-positioning.