edmundprieto98

Hey guys, so my 94 lexus sc400 idles around 700rpm and when I put my foot on the gas pedal, the rpm drops down to about 200rpm then jumps back up to 700/800rpm. Sometimes when I slowly press the gas, it lets me reach up to 3000rpm but most of the time it gets stuck around 1000/1200rpm and sputters. She only goes up to around 10mph. This started happening right after I replaced the catalytic converters but I wasn’t sure it was the cause of the problem so I put the old cats back on but it still runs like crap. I have no CEL with the original ECU and I already tried with a different ECU (realized later it was the wrong part # and it was giving me code 71 for EGR system but I think its because the 92-94 have different EGR systems.) Could it be the fuel injectors? Fuel pump? Fuel pump ECU? Vacuum leak? Fuse/relays? IDK help please..

Parts I already replaced since I’ve had this car:
-spark plugs
-ignition coils
-distributer cap and rotor
-MAF sensor
-upstream o2 sensors
-idle air control valve
-air filter and 1 fuel filter
-fuel pressure regulator

Here’s the video: https://m.youtube.com/shorts/ACCTJYUUuec

Thanks in advanced for any help

Arsenii

Hello,

How long has it been since you replaced all the listed parts?
How the Spark Plugs look immediately after you turn the Engine off?
What Brand parts did you use, especially for MAF and A/F Sensors, as well as Coils?
Are there any smells coming from the car, something like unburnt fuel, especially when it sputters?
It does sound like there is a leak in the Exhaust, was it present before you started, or it is an aftermarket exhaust system?
While the exhaust was off, did you notice any obstruction in the system, better yet, did you try running it without the exhaust at all?

It sounds like there is a considerable obstruction in the exhaust preventing it from working correctly, from the fact that half of the sensors were replaced, and that repairs to the exhaust specifically prompted the issue, so it may be useful to take another close look at the exhaust, or try running straight from the headers, making sure to come up with a list of excuses for the neighbors coming for your soul with pitchforks.

Another thing that can cause similar issues is the Spark, even though less likely. Here is a tool to check Spark Strength, a good coil should be able to spark through 7 - 9mm of air, if yours is less, it's a good reason to revisit the Spark System.

While it may not lead anywhere, still worth a try - press the Throttle 1/4 way down and try Starting the engine, see if the RPM would go up momentarily, and then what happens next without releasing the throttle. If you still have OEM parts from the list above, I would try placing the old MAF sensor, unless you bought a New OEM unit, and maybe the Pressure Regulator, as it is relatively rare to fail.

Hope this helps and best of luck!

edmundprieto98

Now that I think about it, when I first had the original cats removed they were completely hollow as if the “catacombs” inside the cats were burnt off or something. I should’ve removed them myself since now I’m not sure whether the mechanic took the catacombs and sold them off or they were burnt off and pushed down the exhaust system. It’s possible that the latter is true since my SC was running rich for a while before I had the MAF sensor replaced. Luckily though I think all the catacomb fragments should be piled up before the resonator right? Thank you for your advice! This really got me thinking! I’ll check it out when I have the time.

Arsenii

I am not sure on the system you have, some 1UZ cars came with a 3-Way Catalyst system, with two of the ones up front, and another one way in the back where the two pipes merge, at least that's how it was in later LS models if I understand correctly, if that's the case, all the remains of the catalysts could have piled up in front of the mesh of the last catalyst, causing your issue.

Either way, I would start with taking the exhaust off completely to see if it runs without it, and go from there, now that the exhaust was serviced recently, it shouldn't be too much of a problem that it usually is.

Hope this helps and best of luck!

edmundprieto98

UPDATE:
So I checked the exhaust system and seems to be clean; I don’t think the problem has anything to do with the exhaust. When I jumped Fp and B+ on the diagnostic port with a paper clip, the car finally runs normal so I thought it was a fuel pump ECU. I went ahead and replaced that but that did’t fix it. That leaves me thinking it could be an electrical problem or the fuel pump itself. What do you guys think?

Arsenii

Try measuring the Voltage on the Fuel Pump, place Multimeter Leads between Pins FP and E1 in the Diagnostics Connector, it should be close to 12V on startup, and should then drop to about 9V when at Idle, jumping back to 12V when the Load on the engine is Increased. If the engine at Idle runs with 12V supplied to the Pump, but not with 9V, it could be that the Fuel Pump is beginning its descent into the afterlife, not providing enough Flow when the ECU commands it to run at Lower RPM, you can bypass the Fuel Pump ECU for the time being, but be prepared that said Fuel Pump may give out at any point.

Hope this helps and best of luck!

edmundprieto98

Thank you so much for your help! I really appreciate your asnwers!

edmundprieto98

Ok, so the fuel pump ECU and the fuel pump itself has been replaced. My SC400 still only runs smooth with FP and B+ jumped. Could this actually be a TPS problem? I just hope its not an electrical problem.

Arsenii

Read the previous reply a bit more carefully, the idea was to test the Voltage between Pins FP and E1, not to replace the Fuel Pump right away, and that is still what needs to be done. There is nothing else that connects to Pin FP other than the Fuel Pump itself, B+ is going straight to the Battery through a Fuse, shorting one to the other won't change anything but supply more voltage to the Pump than either Engine ECU deemed necessary, or the Fuel Pump ECU is willing to output.

By measuring the Voltage, you will know what exactly the Fuel Pump ECU is supplying to the Pump, if the voltage is low, considering that the Fuel Pump ECU has been replaced, it may suggest that the Engine ECU is not providing the correct Signal, you will have to check the Signal Voltage supplied to the Fuel Pump ECU, but it will happen later, right now see what voltage you have on the pump as it is described.

Hope this helps and best of luck!

pishta

my 1992 issue>>>>>remove the idle control valve from front of motor and take apart 6 screws to reveal the stepper motor and 2 sealed skateboard sized bearings. Feel if the bearings are locked up. Mine were almost seized and that explained the 1800 rPm idle (I drove the freeway without even giving it gas and had to torture it with the brakes in N to get surface streets home) These could be seizing up at random travel and casue your idle to freak out. This would not cause a 200 RPM idle but it sure F'd my UZFE up. I lubed the bearings and put it all back together and it started on first bump and immediately settled at rock steady 600 RPM with lights off and on and even when going into gear. Even turning the wheel didnt change idle that much.