Hi Dave
Great news on getting your car back up and running I am pleased for you. Like you I am an old distributor and carburettor man and used to fixing my own car so know how frustrating it is to have to hand it to someone else. But, as you say you can at least take comfort in knowing that without the resources available to BHM you would have ended up there anyway.

When I got my present Morgan I decided I would try and learn something about how this computer control stuff works and how to fault find with it. I found a book called “Automotive Diagnostic systems” written by Keith McCord. This is very informative and together with this, and bits and pieces on the internet got a better understanding of what goes on. The following is a bit of a simplification but basically as I understand it the computer has to deal with two things. (1) inject the right amount of fuel to get the correct mixture for different engine loads and RPM's, thus replacing the carburettor, (2 ) set the correct ignition timing thus replacing the distributor. It does this with a combination of referring to tables stored in its memory that we refer to as the engine map and measuring engine parameters with sensors.

In the case of the fuel mixture it starts by injecting the amount of fuel for a given rpm and engine load that is stored in the table but it has the ability to trim this value by as much as 25% in either direction leaner or richer. If it attempts to trim outside this range it will throw up an error code saying trim out of range and in what direction.

So with my basic understanding I have been trying to think up a story that would fit the facts in the problem that occurred in your case.

For what its worth here it is.

First there were no error codes and that must mean all the sensor values the computer was receiving must have been within their accepted ranges even if they were wrong.

John said that your MAF also contains the intake air temperature sensor or IAT so when the MAF was changed so was the IAT. The purpose of the MAF is to measure the weight or mass of the air entering the engine but what the computer really needs to know when trimming the fuel is the weight or mass of the oxygen contained within that air mass. That will vary with air density the hotter the air the less dens and the less oxygen. To calculate the density the computer needs to know the intake air temperature. Suppose your IAT was within range but was reading higher than it should,. that would be interpreted as the air being less dens than it actually is and cause the fuel trimming to reduce the amount of fuel injected and you would be running too lean. Also the hotter the air the hotter the engine and to protect against detonation the computer will retard the ignition. As you drive along the engine bay will of course get hotter and this will increase the intake air temperature making the situation even worse this is in keeping with what you experienced. Under these conditions you would be running too lean and with ignition too far retarded and no error codes because every thing was in range. Remember this is my fairy story with absolutely no experience of chasing these faults but based on what I have read but may have miss interpreted.

I am still trying to learn more about this subject so if anybody reading this can find holes in it I would love to hear.

By the way, following the progress of Monty and his deep breath on this forum I have found it to be quite educational in regards to engine mapping and computer control. Thank you Montegue.



Sorry about the long post


Regards Tony