Superfast1

Lost connection every time at 2K? That's odd. In regards to your charcoal canister, if it's clogged then most likely from over-filling at the gas station all these years. Most people don't know this & always like to top it off & that's a no no. In the old day, car makers just vented the fuel vapor into the air & that's bad for the environment so as part of the emission control, that's where the charcoal canister comes into play. If your canister is clogged up, you should smell raw gas & based on such a poor gas mileage from what you're describing, your car is running rich so the long-term fuel trims will swing all the way to the negative rails i.e. -18 or -25% so it can't be normal. You s/b getting a Check Engine light so something just doesn't sound right. Take it to another indy shop for a 2nd opinion.

AlaskanLS

The mechanic said the fuel trims were all normal and he also performed fuel pressure test on it, non of the injectors and cold start injector is leaking. He said while driving he did look at the trims which were functioning normal in regards to the acceleration and idling but once the rpm hits 2000 all the communication is lost. You are 100% right about the canister, I do fill it all the way to the top to the last drop, I will stop doing that. The reason I agree with the mechanic on mpg is because ever since I recapped the ECU, my car's mpg went down the toilet. So I am going to replace the ECU tomorrow and then see if that makes any difference. otherwise, I will take to another mechanic. Mechanic did tell me to bring it back for fuel trims inspection once I replace the canister and the PCM.
He said while performing the fuel pressure test, he noticed that after shutting off engine the fuel pressure will go way high but after opening the fuel cap, it went down. Is there anyway to clean the canister?

oldskewel

iTrader: (1)

Not a permanent solution, but as a test to work around the gas tank pressure build up, you can just loosen the gas cap each time you head out for a drive and then tighten it up when you stop. And you can see if that affects your mpg.

Superfast1

There's a way to clean the canister but I've never tried it so don't know how effective it is & whether it's a long-term fix or not. If you want to give it a try then here's how: Remove the canister from the car. Use the air compressor & set it to 50 psi. Once the pressure reaches 50, turn off the compressor then insert the air nozzle into the outer vent control valve pipe on top of the canister. This pipe is larger than the other pipes on top of the canister. Use your finger to cover the opening of the other pipes on top of the canister then squeeze the trigger to force air into the vent control valve for a minute or two. Now move your finger & place it slightly over the pipe at the bottom of the canister & squeeze the trigger again, once you feel the air coming out then you've unclogged the canister. Put it back in the car then drive it w/ a full tank of gas & measure the mpg. If you see no improvement then it's time for a new one or find a good used one from the jy. I've seen people flush the canister w/ WD-40 in an attempt to unclog the valves but I don't think that's a good idea to use WD-40 or any other kind of liquids as it may damage the charcoal inside the canister.

AlaskanLS

I got lucky and found low 65k miles LS400 canister on eBay from Poland. I spoke to the seller and He sold it to me for 43$ including shipping.

AlaskanLS

Ok fellas, after replacing the ECU today, I went a head and refilled the tank and took it for a test drive of around 10 miles. I can tell the car fuel needle stayed steady and didn't move a bit. Before ECU replacement, after 10 miles the fuel needle use to drop 8% right away. I guess I may have fixed the car. I am not sure yet but fingers crossed. The ECU I replaced was bad from the beginning, even though it didn't show any signs of damage on the motherboard but still it was dumping fuel, after replacing the caps, the ECU went nuts and made the car start chugging fuel instead of burning it but it did make the car shift smooth.
Many times I read in FSM, if the 02 sensors and ECT sensor and other components checked good then replace the engine ECU. I wish I had replaced the ECU a head of time, I would have saved a ton of money that I spent on new parts. I will post an average MPG after I drive it for 100 miles.
Thanks everyone for answering my questions and helping me out.

AlaskanLS

One more thing though. I can't find the charcoal canister inside the hood. I did see the diagram location of it on FSM but I didn't find it there. Can anyone direct me where it's located in 91 LS400?