1on1

Hello,

I've been a long time lurker since I've been able to resolve a lot of my issues by searching. However, I am running into two issues at this time and would like to address them since there hasn't been any updates about the titles:

1. I've noticed when I am driving my car that it does not have any issues until my car is running hot or above normal temps. What happens is my car seems to lost power/stutter when I am in traffic. After inspecting the issue when I am on the side of the road, I noticed the fuel pump relay, MAF sensor and fuel pump resistor (silver box) gets very hot which causes my car to jerk or do nothing when I press on the gas pedal and tends to shut off. I have gone to the junk yard and replaced the MAF sensor and the fuel pump resistor. It seemed fine so far but after taking a drive back to norcal on a hot summer day (this was back in July), the issue came back. For me to continue the drive, I had to cooldown the car for at least 20-30mins in order for me to continue driving. Since then, I've never bothered to drive the car outside of San Diego, CA unless the outside temperature is cooler than 80 degrees (I have zero issues driving at night or cooler temps).

I've read many articles about the fuel pump resistor is the main cause for this problem so when I tried to purchase a new one, nobody online has it in stock since the product is discontinued. My question is, do I keep replacing this part with a used one from the junkyard or is there a permanent fix for this?

2. Currently, my transmission started to act up recently with a few small jerks/stall when I am driving to and from work. The jerk/stall doesn't feel as bad as when my car is having an issue on a hot day but I want to fix this before it gets worse. I've read every transmission problem and suggests that I should replace my solenoid, however, I do not have any check engine lights nor any codes stating I have an issue. In addition to the slight jerk/stall, I noticed my transmission doesn't shift correctly.

Will performing a transmission flush fix this issue or should I also replace the A3140 shifter solenoids too? Any other recommendations or tips?

Thanks in advance!

dicer

So is the transmission slipping or acting like its falling out of gear ? What gears does it happen in? Does it do it just cruising or during acceleration?
The fuel pump resistor seems to be blamed all the time, I don't know for sure if it is the cause or not. I know of one LS that does that acting up when hot, especially when the AC is on. With the AC off it doesn't seem to have the problem, its a 91. Maybe someday I or someone else will run a wire to that resistor and monitor what the voltage does on the hot days with AC on. Not running good when hot can be a vapor locking deal too, and low pump pressure would add to that, meaning if the resistor drops pump voltage too much then the pressure will go down. And there are stories about how changing the fuel pump fixes the problem as well, again worn pump lower pressure, but another thing to consider would be the pressure regulator. I guess to really figure out what is happening would require running a long hose with a fuel pressure gauge, and the wires to monitor pump resistor voltage and maybe voltage at the pump too. It seems only the first gen LS's have this problem as far as I know.

Yamae

Sounds like your fuel pump is consuming too much current. Use and think in accordance with the Thévenin's theorem, you'd notice why the fuel pump resistor becomes too warm. I think your fuel pump is close to the end of the life.

oldskewel

iTrader: (1)

I've had similar issues to your fuel problem, on my '91. Just a couple of EASY debugging things to try next time it acts up. If anything changes, these may get you closer to a good diagnosis:

loosen the gas cap - if the gas tank vapor pressure is not properly regulated due to a blockage, releasing the pressure will temporarily fix this, perhaps fixing your fuel-related issue

jumper the fuel pump - if you remove the gray rectangular diagnostics port cover on top of the engine (near center), and put a paper clip jumpering the pins at the two rear corners together, that will bypass the fuel pump resistor and the fuel pump relay, providing full power to the fuel pump.

Nothing beats finding the actual problem and fixing it, but as we know that can be easier said than done sometimes. These may help you get there.

If you search this list you'll find many many cases of similar problems and very few solved cases. Here's one of the few:
https://www.clublexus.com/forums/ls-...ml#post5850122

1on1

Thank you for your response. I believe the gearing is faulting out. For example from a stop, I would go and I believe the transmissions "skips" 2nd gear and goes into 3rd gear and slowly gets up to speed. Second, I noticed sometimes when it's in third gear, it would drop down to second to accelerate normal and then shift normal. I don't run AC since it needs to be recharged. While cruising on the freeway or going up hill, I notice the transmission feels like it's going to stall but does not and keeps going fine. This is why I plan on flushing my transmission fluid since I haven't done it since I bought the car back in 2011 or 2012 (plus, I dont know the history from there). On an upside, I the transmission fluid still looks red but I'll double check tomorrow.

Thank you for your response. I was thinking about this too since I'm going to assume the fuel pump was never changed. I've searched online and i've seen fuel pumps run from $20 to $300 and I know "you get what you paid for..." My question to you as it was unclear to me, would running a walbro 255lph be fine with the stock fuel pressure regulator (if not, which fuel pump would you recommend)? I've read once that I might need an adjustable FPR if I goes this route, is this true? Lastly since we're on fuel topic, would a dirty fuel filter cause this issue too to where blockage in the filter would cause the fuel pump to run harder? If so, I can definitely try it since it's a cheap part to replace.

Thank you for your response. The gas cap makes sense and I take it, I should replace this? I've read about jumping from the "relay" and not the diagnosis port. In addition, I noticed there was a wire sitting under the cap which makes me think now, the previous owner had this issue before he sold it to me. As stated in my previous response and reading your response, I'll be changing the fuel pump. After reading the link above, it seems I should purchase the fuel pump off rockauto but I will wait until tomorrow to give you and the other poster a response to this.

Once again, I thank you guys enough for your input and now it's on me to resolve this. I will take full consideration of your responses and definitely fix my issues as soon as possible and will definitely respond once I've made changes. I will also take any future responses that can help me figure this out.

oldskewel

iTrader: (1)

On the gas cap - no I'm not suggesting to replace it. If the car is running rough, then loosening (or removing) the gas cap would allow any vapor pressure to equalize, removing that as a potential problem. Whether loosening the cap makes it run better or not will just give you information for diagnosis.

The following is all for my 1991. I don't know how long this circuit design lasted after that. It is likely that later models are similar but different.

The diagnosis port appears in the circuit here, labeled "Check Connector":


It is often said (even by me, sorry) that jumpering at the diagnostic port will command the fuel pump relay to bypass the resistor. But I looked into it more carefully today, confirming that diagram vs. my '91, while testing the fuel pump relay and fuel pump resistor. I now believe that jumpering at the diagnosis port will bypass the "circuit opening relay" and nothing more. Just as shown in the diagram.

The "Circuit Opening Relay" looks like it is supposed to energize the fuel pump when the key is in STA (starting the engine), and when the engine is running or just powered ON, controlled by the ECU. So the ECU uses this relay to pressurize the fuel system for 2 seconds when you first turn the car to ON, then disables fuel until you actually start the car. It also will disable fuel when the engine stops, for example after a crash when fuel lines are ruptured (so it's a safety feature).

The fuel pump relay, shown in its rest state in the diagram, will power the fuel pump without the resistor. That actually makes sense from a reliability viewpoint, which Lexus seemed to value in their EFI design.

So jumpering at the Diagnosis port bypasses this CO Relay, causing the fuel pump to run whenever the key is ON, but it does not affect whether the resistor is used or not. So you'll hear fuel flowing with the engine stopped, but it is not giving any extra power to the fuel pump.

With that Diagnosis-port jumper in place, the ECU is still in control of whether the resistor is used. To use it, the ECU will still need to pull down the FPR line, energizing the coil in the fuel pump relay, and throwing the switch over to the resistor outlet.

BTW, spec on the resistor is 0.67 - 0.79 Ohms, which mine measured (0.7 Ohms) , and my fuel pump resistance, measured at the fuel pump relay is 1.0 Ohms, which is within the wide spec of 0.2 - 3.0 Ohms too. That spec is actually for measurement at the pump, but I measured from the fuel pump relay socket to the battery negative terminal. EDIT - after replacing the fuel pump, I confirmed that measurement of the pump in hand is 1.0 Ohms, so measuring from the fuel pump relay is a valid procedure if it indicates a valid resistance (but if it indicates an open circuit, something else may have failed and the pump is still good). My fuel pump relay coil measured at 79 Ohms (there is no spec on this, but it triggers nicely) and internally things looked very good.

So the jumper would make the fuel pump run even if there is a problem with the "Circuit Opening Relay" (from the diagram) and in some cases, the ECU too. But a problem with the ECU, Fuel Pump Relay, or the Fuel pump resistor could still cause problems with power to the fuel pump.

If you want to bypass everything (circuit opening relay, fuel pump relay, ECU, resistor) and power the fuel pump, here's what I would recommend:

- you'll still rely on the EFI Main Relay, so it will run only when the ignition switch is ON

- jumper at the diagnosis port to bypass the Circuit Opening Relay, so you get +12V to the Fuel Pump Relay socket +B terminal, which is in the lower right corner when looking at the rectangle with the tab on top.

- measure that with a voltmeter to confirm +12V. It should be +12V when ignition is ON, 0V when ignition is OFF

- measure the resistance from the "FP" terminal (mistakenly labeled "FR" in that figure) in the Fuel Pump Relay socket, which is diagonally opposite the +B terminal, to ground. You should get the fuel pump resistance (0.2-3.0 Ohms) plus any wiring resistance (hopefully near zero).

- jumper from that +B socket terminal to the "FP" terminal. When ignition is ON, that will directly power the fuel pump, and you should hear fuel circulating when the engine is quiet and not running.

Both of those jumpers will be carrying a lot of current, so a thick wire would be good. I use 12AWG solid copper (Romex), with the ends flattened like a spade connector so they can fit in the connectors.

Once the fuel pump is spinning, the back-EMF will significantly reduce the current, but if there is a problem with the pump and it does not spin like it should, the current will be high. Don't burn your fingers on the jumpers.

I think considering fuel pump replacement is a good idea if these tests support it. I may do the same thing myself, although my problem is _very_ intermittent. EDIT - I did swap in a new fuel pump, and it seemed to improve and fix everything related to fuel.

Good luck. Looking forward to see what you figure out and how this gets solved.

j2b4o

If it happens on hot days with a low tank of gas it may just be the fuel pump. The fuel cools the pump so on a low tank the old pump can overheat and cut out intermittently. That's what happened to mine. I replaced the fuel pump with a Denso from rockauto and have had no problems since. The tranny may be reacting to a fuel irregularity.

1on1

Thank for your guys response. I've jumped the gun and ordered a SAP fuel pump with kit and ordered the fuel tank gasket off rock auto so I look forward to working on this issue when I get the parts; I am hoping on or before Saturday.

As for the gas cap, I will consider and noted it on my phone that if this issue happens again, after changing the fuel pump and fuel filter, I will release the pressure from the gas tank. Also noted jumping the pins from the diagnostic port.

As for the transmission, I hope it's related to the fuel pump when I have the jerk/stalling issues. However, I am hoping that flushing the transmission fluid will help with the shifting issues.

oldskewel

iTrader: (1)

Even though I have not had any of these running problems in several months, your questions here prompted me to do some tests on my car today since I know there is a problem. I carefully checked the resistor, fuel pump relay, and some other things. I updated my post above from a few days ago with the circuit diagram, in case anyone else is interested in this line of testing.

I basically found nothing wrong with my resistor or fuel pump relay, so I'm inclined to swap out the fuel pump. I'm at 192k miles, replaced the filter a couple of years ago, and I figure it's worth a try at this point due to the high mileage. I do not have the fitting to do a pressure test (although I do have a fuel gauge set), and the symptoms (heat related, tank empty) point toward a fuel pump.

I may wait to see what the OP finds with his fuel pump choice before buying something. From a quick look online, Toyota pumps cost $300 and China brand pumps (that look OK) cost one tenth that. Hmmm.

1on1

Update as I have flushed the transmission and it was overdue. Fluid looked like engine oil that has not been changed for over 20k (black and brown). So far, the transmission shifts a lot better and I can feel the acceleration throughout each gear. I am unsure if it has helped with the jerk/stall but I won't find out until I swap out the fuel pump.

As for the fuel pump, I have not changed it yet but tomorrow it will be my project after I get off work (changing fuel pump, fuel pump gasket and fuel filter). I will update once I changed everything and currently my fuel gauge is reading between 1/4 to empty tank.

Losiracer2

For the longest time, I had a shudder that would happen when cold and got progressively worse. Initially it didn't trigger any light, so I really didn't know what it was, just thought it was a Fast Idle valve not running as it should, since my RPMs used to dip down to 200rpm when coming to a stop. After 6-7 months, I eventually got a P0770 Shift Solenoid CEL. I determined that it was my lockout solenoid not unlocking the torque converter when coming to a stop, so it was basically lugging the engine down and finally when the speed went to 0 it would unlock, creating this bad shudder.

I did a drain and refill 3 times and swapped in a new tranny filter, which didn't help. So I eventually took the pan back off and did the solenoid swap. It typically runs about $300 at the dealer but I put in a JY one, to try it out and sure enough, it solved the problem completely.

This is what it looks like:



There's a mechnical part inside there that gets broken and starts clicking. If you take yours out and shake it, and it makes a clicking noise, then its for sure broken and needs replacement.

oldskewel

iTrader: (1)

Very good to know, thanks for posting.
Based on the condition of the ATF you drained, do you think dirty fluid may have contributed to this problem (I know you said changing fluid did not fix it, but maybe it was too far gone at that point)?

1on1

After a few nicks, i managed to swap out the fuel pump. Comparing the new fuel pump vs the old one, it seems it was never changed. Once i got the new one in, it seems my engine revs a lot faster than before. I got too excited and hesitant to change out my fuel filter and will be doing it now.

New SAP fuel pump vs old
The old fuel tank gasket

Losiracer2

I don't think my fluid helped a ton. It was a little dirty but I could still see a darkish red color. The previous owner said they changed this solenoid out about 30k miles ago, so I was in denial thinking that it was something else, when it was this all along.

1on1

Thank you for your response and I was hoping I did not have to change this out or the other two solenoids. However, after driving today up and down hills, I am still having some jerk/stalls as I am driving up hill. I going to head to the junkyard this weekend since it's been raining here in san diego. I'll update once I change this part out.