The 2.6 in our 1989 190 is having a few issues.

- It does not seem to be making as much power as it should once warm

- If driven before it gets up to 80c but over 50c it misfires at full throttle and can barely do 80mph

- If between 50 and 80c it is very hard to start and will misfire on one or many cylinders for the first few moments of operation

- Once warm it does start easily and run smothly. Just slower than 160hp should be in what is now a 2,400lb car. Occasionally has a sudden surge of power right before redline.

- I have no reason to suspect a mechanical failure such as jumped timing or burnt valves with the engine as it idles fine and seems to be happy but slow once up to temp.

Things we have done:

- Swap OVP with a known good one

- Ran it with the ECU unplugged and it runs about the same. Maybe its stuck in some sort of open loop. ABS computer seems to still function.

- Replaced 02 sensor before all this was happening

- Exhaust is far from stock with no cats and was done this winter. This may be part of the problem but I remember it doing this before we parked it for the winter and built the exhaust.

- Does not seem to be a vacuum issue

The current theories are that its running in open loop or the engine thinks its at 120c or something. What is a good way to go about testing if its in open loop and what could be causing that?