My fuel economy has dropped from 27 to 21MPG after replacing a ICM the died when it got hot. Engine no longer dies since replacing the ICM and going through 3 tanks of fuel. I got the timing set to 10 degrees BTDC as instructed on the label on the desk lid... ALDL was jumped across A-B and vehicle in drive. Idle is rock steady and no pinging from engine. Vehicle runs like a brand new car with the exception of the fuel economy.

ICM installed came from Cardone distributor. Could this ICM be the reason my mileage sucks compare to the mileage with the old ICM that died?

I had a new Cardone 84-1633 on hand when the original ICM died, so I used that ICM.

I'm pretty sure that timing should be adjusted with the vehicle in neutral. "Vehicle in drive" indicates you have an automatic, so make sure your torque converter is locking up at highway speeds. An easy way to tell is to try to accelerate a little while driving at highway speed. Engine RPM's should not increase unless the car's speed increases. If the engine RPM's increase without the car going faster, your torque converter is not locked up, and that will cost you MPG.

I do have and automatic. 4t60 to be exact.

Torque converter is locking up at highway speeds. My rpm is about 2300 at 75 mph.

Is there a difference in ICMs. Ex. Performance vs stock?

I am not one for throwing parts at a problem, but in this case I would replace the O² sensor, because:

A. It's cheap, and it can't hurt
B. If it is unable to detect small changes in fuel mix in the exhaust, the computer will not send less fuel to the injectors....which is what you want.
C. I can't figure out a way to trouble-shoot low mileage.

edit: I have not heard of any variation in ICM construction, but a good one might send much stronger signals than a bad one.

[This message has been edited by Gall757 (edited 04-10-2015).]

quote Originally posted by My88RedGT: I got the timing set to 10 degrees BTDC as instructed on the label on the desk lid... ALDL was jumped across A-B and vehicle in drive.

jump AB is good but in drive? no.
Reset timing in park/neutral.

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quote Originally posted by theogre: jump AB is good but in drive? no. Reset timing in park/neutral.

The GM sticker writers disagree.

I'll replace the O2 sensor I guess... It was replace 2 yrs ago due to failure.

I connect my Actron diagnostic scanner and no codes are stored. I did run a real-time analysis on it with the scanner and the O2 sensor is alternating between rich and lean. The system is going into closed loop, the running temp is 210˚F . MAP sensor voltage is approx 1.35v according to the scanner. I already replace the thermostat with new 195˚F and IAC. I pulled #1, #3 & #5 spark plugs and they look normal. Plugs and wires are about 2 years old with about 8k miles on them. I replaced the coil since I had a spare too.

I called a trusted mechanic friend of mine and he is baffled as well.

Engine does not ping even with 87 octane.

I'm usually not one for throwing parts in hoping to fix an issue, but I'm coming up with nothing else to try.

My emissions need testing here in May and with economy sucking, I hope it will pass this time.

Guess it's time to get a AC Delco ICM and try that.