Joe,

Unfortunately, the 1989 and 1990 FI computers (ECUs) are known to be failure prone. Improved, rebuilt units are available, I paid $500 for mine.

Others have said that ECU failure results in a no-start condition - not necessarily true. My car would start, but would not run above idle. The car would idle fine, but I didn't have enough power to get out of my driveway.

DO NOT start replacing expensive parts (especially the AMM) until you know they are bad. Don't allow a mechanic to sell you stuff that he can't prove you need. It is really not very difficult to troubleshoot FI systems. Replacing parts at random is not the most efficient way to fix anything.

Start by checking/replacing the cheap parts:

1. Fuel filter, if overdue.

2. All of the fuses. Disconnect the battery and clean the contacts in the fuse holder.

3. The main FI fuse under the hood. It's a 20-25 amp blade type, located just aft of the battery. Clean or replace ($2 at Radio Shack) the holder.

4. Check for vacuum leaks. Especially check the big pipe running from the AMM to the throttle body - take it out to check, they can crack in hidden places.

5. Check/clean the idle control valve. It lives beneath the throttle body, take it out and clean with carb cleaner spray. They get gummed up just like the throttle body.

6. Tune-up stuff, check the timing (it's not adjustable, but checking will help pinpoint problems).

You would be amazed at the bizarre symptoms that corroded fuses/conectors can cause.

After this, there are two ways to proceed:

1. Swap the ECU with a known good one. This takes about 5 minutes. The most difficult part is convincing someone to let you borrow their ECU.

2. Get a Bentley 240 repair manual (ISBN: 0837602858) and follow the diagnostics for the fuel injection system - ECU, sensors, controllers, fuel delivery, etc. To do this right, you will need a VOM (volt-ohm meter), available at any Radio Shack (wide range of prices, a cheap one will do). Testing the fuel pressure regulator requires a fuel pressure gauge, but the part is cheaper ($40) than the gauge and simple to replace - your call, depending on whether you can justify the purchase of new tools. Check everything, don't rely only on the ECU diagnostic codes - if the ECU is bad, the codes cannot be trusted.

I highly recommend "Bosch Fuel Injection and Engine Management" by Charles O. Probst, ISBN: 0837603005. It clearly explains the theory and operation of the Bosch FI systems. Understanding how FI systems work is a great benefit when troubleshooting.

Fortunately, my '89 and my '90 use the same ECU. But, my wife would not let me borrow "her" ECU until I had gone through all of the diagnostics and exhausted every other possibility. Even then, it took some time to convince her that I wouldn't break anything by swapping the ECUs.