Engine runs fine when it is cold, but gets worse as it warms up

When warm and it stalls, does it start right back up?

Yes it starts fine. Today we checked the coolant temp sensor and the air temp sensor and they both seem okay too. Thanks!

Since it starts right back up, chances are it is a sensor. The problem is, which one? Do you have access to a real time scanner that you can view data while the vehicle runs?

Rewind... I forgot it is an 89. The scanner won't help. When it stalls, is it like shutting the key or does it slowly die?

I would say slowly dies. . . it just kind of gets coughier and more spluttery and then quits.

What you described sounds like a fuel related issue. Have you checked fuel pressure or replaced the filter?

It runs great until the temp needle starts to move to the middle of the range. At that point, it starts to idle rough and will stall, but doesn't diesel. We haven't checked the fuel pressure, but we did replace the filter. Using a Robert Bentley service manual, we tested the digifant two module/connectors. We didn't get the right values for the ICU. The airflow sensor had 1 test that was good and 1 that was bad. We didn't really understand the testing process for the O2 sensor. We have gone through all the vacuum lines and we didn't replace them, but they look okay.

If you can allow the vehicle to idle in the driveway till it warms up and dies, put a fuel pressure gauge on it and confirm it doesn't drop off. If it checks okay, the next focus area is the crank sensor.

Let me know what you find.

We tested the fuel pressure. It was 35 PSI when the car started, but as the engine warmed up, the fuel pressure dropped to 20PSI and the car started to run bad. After the car died, the fuel pressure dropped to 18.

If the fuel filter has never been replaced, try that first. It is cheaper. If that doesn't help, most likely, the pump itself it bad.

Let me know if you have questions.