Just out of curiosity, what were the trouble codes?

I don't believe the problem is in the ECU. I had exact the same problem with my '89. The reason for running lean can be :

Clogged are broken fuel-injectors

Fuel pressure regulator (pressure to low)

AMM out of spec.

There is a simple (and very cheap) way to make your engine running richer.

Dismount the AMM, take off the inlet screen. Inside the AMM, you see a very thin wire in a smaller tube. This is the measuring wire. Don't touch it !!!. The mass of air that's going to the engine is measured by that wire. If you put some foam between the small tube and the outer tube in the AMM, more air is forced in to the smaller tube, so the AMM gives a higher output to the ECU. You have to experiment a bit how much you gonna clog this outer tube to get the right mixture.

I did this 2 years ago, and the foam is still in there, no more check engine lights since then !!

Good luck !

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.

You should do a quick check for a faulty AMM. With KEY OFF, disconnect the electrical connector on the AMM. This will force the FI ECU to ignore most of the sensor inputs and operate the engine in "limp home mode". Start the engine. It may not run well (especially at higher rpms), but if it runs better than before then a bad AMM is a likely culprit. If that turns out to be the case then you should also investigate the themostatically controlled warm air flap in the air filter box (see the FAQ and/or archives for more on this).

When LH2.4 ECU's (of the failure prone 1989 vintage) go bad the symptom is usually a complete no start condition. Your problem is hopefully elsewhere. You don't want to know what a new ECU costs.

Know anyone with an 89-91 (92?) who will let you swap ECU's to test? This is why you have to have at least three bricks (240's) in your driveway. Interchange parts and see if things clear up.

How about checking the fuel pressure at the injector rail. Maybe the regulator has gone bad. You could also check into the MAP sensor and AMM to see if they operating within spec.