Here is a short story of my 91 LS400 ECU. It's dumping fuel for some reason, I have replaced the caps and the mileage became worse, before recapping it, I was getting 11 mpg after recap it went down to 7 mpg. After recapping the ECU my car became more responsive and would shift gears at right rev range but my mileage became worse.
I have read FSM almost 10 times and on some O2 sensor diagnosis it recommends that if O2 sensors check alright then replace ECU. I wounder what chip is responsible MPG. My car's original ECU number 89661-50042 with trac control.
I ordered another ECU from eBay just to test if it is going to improve my car's MPG but it's number is 89661-50041, The one I ordered from eBay also has trac control and is also for 91 LS400. Hopefully the ECU I ordered is compatible with my car.

 

To answer to your question, yes, it can be. The acid from the old caps when leaked, can destroy the PWB (Printed Wiring Board) which made out of fiberglass epoxy resin & the copper (conductive) signal traces on the board. Most of the time, this type of damage is visible to the naked eyes & if the damage has caused an open circuit then the board can be repaired by putting metal jumper wires to re-connect the broken signal traces. However, if the damage has caused a short circuit then some of the associated circuitry i.e. active components (IC - Integrated Circuit) w/ the broken cap(s) may have been fried & if it's too extensive then it would be deemed beyond repair. Also, when you're dealing w/ electronics, proper handling is essential as some of the electronic components on the boards are very sensitive to ESD (Electrostatic discharge). This type of damage is extremely hard to diagnose. It may caused an instant catastrophic failure which is much easier to troubleshoot & repair. On the other hand, a latent damage will cause the affected components on the board to degrade over time, some of the functions may not perform & work properly & eventually they'll fail at a later time. Most of the time, the damage is invisible to the naked eyes until you x-ray or cross-section the part(s).

 

Btw, how come your mpg is so bad even before & after capacitors replacement? You should be getting like around 17, 18 mpg for mixed driving & around 24+ for highway/freeway driving as that's what I'm getting on both of my 97 & 00 LS400s.

 

I wish I knew the answer to that question. But I noticed my car's MPG got worse with new caps. There is no damn OBD1 scanner with many mechanics where I live, to find the real cause of this problem. I have been throwing arrows in the dark ever since I bought this damn car and hoping that It will hit the target.
There was no acid damage to the board, I should have left it like that but after caps my car's shifting improved a lot and got much smoother. I got few more things to test if those turned out to be positive working fine then I am screwed. I have spent soo much money on this car and I am pretty sure I am not going to get half of that back but I am gonna get rid of it. My next plan of attack is doing leak down test on injectors and rechecking the o2 sensor's. These cars are soo heavy and it's very hard to find a good spot for jack stands under the car that's the reason I am waiting for summer to get under it. It almost slipped out of jack due snow outside in my drive way that's I am not getting under it and I don't have garage to work on it.

 

Did you replace the caps yourself? If so, did you replace them w/ the equivalent or better parts? These caps aren't the regular caps w/ just capacitance value & voltage rating. These are electrolytic caps so you have to pay attention to the ESR (Equivalent Series Resistance) & also the polarity. If you accidentally installed them incorrectly meaning having the positive (+) & negative (-) leads of the caps connected to ground & power, respectively instead of the other way around as it should be then you've just defeated the purpose of having it in the circuit & eventually it'll shorted out. In some cases, it may even explode & splatter acid all over & cause permanent damage to your board. Here's the thing: Today's microprocessor-based systems require power source that can deliver high current & ultra-fast transient/switching performance with tighter voltage regulation that's why the designers incorporate them in their designs so make sure to get the correct replacements & double-check your work before you put everything back & power it up.

 

Of any injector leak I would suspect the cold start injector. These engines have 9 fuel injectors, not just the normal 8 that you can think of.

 

To give a closer data point, my '91 gets about 2 mpg less than your numbers here.

 

I replaced the caps myself and bought the recommended caps with the help of this website from recommended seller, and I made sure all the caps are of right voltage when I was desoldering and soldering them.

 

Dicer- 9 fuel injectors? Where's the location of the 9th one?

 

AlaskanLS- All vehicles sold in the US after Jan 1, 1996 are equipped w/ OBDII so it's much easier to monitor emission control electrical parameters & retrieve codes. Since yours is older, it'll have to be done by putting jumpers on the OBDI connector & count the # of Check Engine light flashes to determine the code(s) or take it to a shop & have them check the short-term & long-term fuel trims & other parameters for you. I suspected 1 or more of your fuel injectors may be leaking (running rich). You may also have vacuum leaks (running lean) so check all the rubber hoses as they're all dried up & some may have cracks by now if you haven't replaced them. With the engine running, also check the fuel pressure regular by pulling the small rubber hose attached to it & see if you see any gasoline squirting out of the regulator. If so, the internal diaphragm had been ruptured so you will need to replace it. Any of the mentioned above can contribute to your poor gas mileage. Good luck.

 

He is talking about cold start injector that is under intake manifold, I have checked that and the switch the turns the cold start injector on and off but I will check again if I find something wrong with the fuel pressure. The new ECU is coming on Friday, before I increase my headache by checking other things, I am going to swap the ECU first and see if it makes any difference.

 

I did some researches & found out the pre-93 has the cold start injector. Mine are 97 & 00 that's why I wasn't aware of it. Anyway, if yours is faulty, that could explain the poor mpg so you may want to check it again to make sure.

 

Faulty cold start injector could also caused hard starting & engine performance issue. Good luck.

 

I found a mechanic that is going run fuel trims test on my car tomorrow. Tomorrow, I hopefully will be able to find the culprit. He is going to do fuel injector leak test, vacuum leak test and check the fuel trims too. No more throwing parts at my car. I get 140 miles a tank that's horrible and worse than the diesel pickups I drive at work.

 

Ok, all the fuel trims turned out to be normal, the mechanic was amazed with the performance of the car but he did find two problems with it. He told me at 2000 rpm the car loses communication with the scan tool. He connected it to the scanner while driving. He said when he floors the pedal and every time at 2000 rpm ECU disappears from the scan tool. Second problem is charcoal canister is clogged and not venting and it creates a lot of pressure in the fuel tank.
I can't find new charcoal canister on Lexus parts now website, it shows two parts number for charcoal canister and I chatted with them online and they told me both are discontinued.