Hmm

Injectors refitted, allowed the fuel system to repressurise and then started her up and it’s exactly the same as before. Whipped out the plugs and ran a compression test and recorded a healthy 14 bar across each of the four cylinders. So, the good news is that the engine core seems pretty healthy. That said all the plugs except that in cylinder 1 (which was just oily)  are black indicating that it is running very rich. So a few more things to try next…

1. Coolant Temp Sensor. Multimeter to read the resistance when cold to hot which should go from 10k ohm (very cold) to 300 ohm (90 degrees) and would make it run very rich.

0 degC: 4800-6600 ohm: 4-4.5v
20 degC :2200-2800 ohm: 3-3.5v
40 degC: 1000-1200 ohm: 2.5-3v
60 degC: 800 ohm: 2-2.5v
80 degC: 270-380 ohm: 1-1.3v
110 degC: negligible resistance: 0.5v

2. Air Temp Sensor. Located on the underside of throttle body, turn it 90 degrees and it pulls out. Has a different resistance scale.

3. Vacuum Reading at Idle.  Gauge connected to the vacuum pipe going to the fuel press regulator should be 18″-20″ hg. With uprated cams this would be 16″-18″ hg for typical 258 deg cams (708s, PT51s, BP270 etc). Cam timing will affect vacuum pressure. A low vacuum makes motor run rich as the ECU thinks it’s under more load than it is. Getting a vac reading can give a good idea what tree to bark up…

4. Check the vacuum pipe to the regulator for blockages. It’s won’t be an air leak as that will make is idle fast (if the air leak is into the plenum) or miss at idle (if the air leak is into an intake runner).

5. Check the ‘o’ ring on the MAP sensor. MAP is favorite with the current symptoms but is also rare. Might try swapping the one from the GTI.

6. Check the cam timing. Just to check to see if it has jumped a tooth…

7. Lambda Sensor. Check voltage on lambda (black wire) with DC meter, connecting the positive wire of meter to black wire on lambda and black wire of meter to earth. Should fluctuate between 100mv and 900mv. If it sits at 800mv+ all the time then it’s too rich for the lambda to correct it.