ECU Substitution?, Using 1974 ECU in a 1973 Options

[ThinAir]

My stock 1973 2.0L has developed a problem. Intermittently, the engine will start stumbling and then die. Sometimes reseating the 14 pin connector/harness on the relay board will get things going. Other times I have to just let it sit for awhile. I've noticed recently that when this happens it also smells like it is flooded and when starting it I have to floor the accelerator.

A long time ago I had a similar problem with my son's 1970 that ultimately was resolved by replacing the ECU. Hence the following question:

I have available a working 1974 ECU. I'm wondering if I can swap ECUs with no other modifications to see if the problem goes away and stays away. Brad Anders' FI page states that the 1973 should use only a particular ECU, but in his chart of the preferred setup he states to use the 1974 ECU. Would it be harmful to any component to swap the ECUs and test my theory?

This problem came up just before leaving for a summer road trip and I won't be able to work on it until just before Labor Day, but I'm hoping to figure out a direction now so that I can jump on it when I get back home and get the car ready for the RRC.

[JamesM]

No wont work (well, it will run but it will be REALLY rich), the 73 system is sort of an oddball and very specific about what parts you use. If you want to use the 74 ECU you need to swap out the 73 specific Head temp sensor and MPS along with it.

Verify which setup you have though as the later 73 cars use the 74 setup.

Honestly its probably not your ECU though as that is the component least likely to go bad. Relay boards and wiring harnesses on the other hand tend to be weak spots.

[JeffBowlsby]

Its hard to understand why folks even contemplate intermixing critical parts to a functioning FI system. The 73 2.0L ECU is the most common 914 ECU available, recommend the correct part be located.

That said, 'stumbling then dying' sounds like it might be a clogged fuel filter or a failing coil.

If it smells like it is flooded, then the need to 'floor it' to start, those are opposing conditions. If its getting too much fuel, look for other sources - leaky injectors or cold start valve. Use a fuel pressure gauge to see what fuel pressure is doing under different engine operating conditions.

It could also be running rich causing the hard starts = failed/failing MPS diaphragm.

The simplicity of the D-Jet FI is that each of the these discrete components can be isolated and their function verified to find the culprit.

[JamesM]

I've noticed recently that when this happens it also smells like it is flooded and when starting it I have to floor the accelerator.

That sounds like it could be your MPS or your head temp sensor/connection to your head temp sensor. Either of those failing can cause behavior like that. Open circuit on the head temp sensor causes an extremely rich condition as does an MPS that wont hold vacuum and either one of these can vary/be affected by heat and or motion. If moving the harness around sometimes resolved the issue though I would be very suspect of the wire/connection to the head temp sensor.

[ThinAir]

Thanks for the help guys. I was aware that Brad Anders' recommended setup with the 74 ECU also included other parts that were different from 1973. Your experience helps me avoid doing something dumb (other than asking the question!). You've given me some good directions to explore when I get back home. Thank you very much for your advice.

In the meantime I've got exploring to do in Wyoming, Montana, Idaho and I'm not sure where else.

[ThinAir]

I've noticed recently that when this happens it also smells like it is flooded and when starting it I have to floor the accelerator.

That sounds like it could be your MPS or your head temp sensor/connection to your head temp sensor. Either of those failing can cause behavior like that. Open circuit on the head temp sensor causes an extremely rich condition as does an MPS that wont hold vacuum and either one of these can vary/be affected by heat and or motion. If moving the harness around sometimes resolved the issue though I would be very suspect of the wire/connection to the head temp sensor.

You get the gold star, @JamesM! See this post for details.