denSC300

Hey everyone, I have a 95 sc300. I little while ago I posted about a rough idling issue. Turns out it was a big vacuum leak. I found out what the issue was (I think) and I proceeded to fix it. After putting everything together properly and checking to make sure all vacuum lines are in good condition I took it for a test drive. Before I put everything back together properly, when I tested it out there was excessive engine shake and as a result I could hear the exhaust shaking very loudly. Anyway, I took it for the test drive after the idle was more steady at around 900-1000 rpm, and I didn't hear the exhaust shaking anymore and everything sounded fine again, however I noticed a huge reduction in power compared to when I drove it a while ago. I floor it and it barely accelerates. I'm wondering what could cause this issue? if anyone has some insights that'd be greatly appreciated.

PS: Spark plugs, wires, valve cover gaskets, Throttle body gaskets at the top of the intake, and oil have all been changed within the last month and the car has barely been driven.

Arsenii

Hello,

You wrote that you are in the process of resolving the Vacuum issue that caused rough idle, how bad is it currently?
Was there anything at all done to the distributor in the car recently?
Was the exhaust rattle sound anything like in the video below?
Does your car have and OBD2 port?


One very important aspect of JZ engines is that while the car can adjust Ignition Timing automatically, the base setting must be done by hand, using the Timing Light, like this one. If the distributor was moved or taken out completely, the timing will have to be readjusted. The video linked above shows the sound of detonation, if you hear that, stop driving the car until an issue is resolved, since it can ruin your engine.

If that is good, try resetting the ECU by disconnecting the battery for a few minutes, since the car may have learned to run on Lean mixture from the recent vacuum leak, and now that it's gone, it may be trying to run on settings that are no longer actual.

Next thing to check would be the recent repair, if the issue wasn't present even with the big vacuum leak that you had, it is likely something else that is causing it. You wrote that you didn't assemble everything completely, sorry if it sounds obvious, but is there any chance you missed something when you were putting the car back together?

If all that is good, and you don't have an OBD2 port, you may have to create a tool like the one described here to access Engine Live Data, and start checking all the sensors that the tool can provide. The only downside is the 1 second delay between refresh, so to test O2 sensors for example, you will need something like a tool linked below.


Hope this helps and best of luck!

KahnBB6

iTrader: (5)

I haven't seen your previous post and what you did other than your recap here but depending on their age you may want to install new front Denso O2 sensors (CA-spec and Federal-spec cars have different types for 1995) and I would ask if you have ever had your ECU serviced with brand new OEM-brand and OEM-spec capacitors?

The ECU re-capping service with the correct type (and brand) capacitors is something we generally recommend as baseline maintenance today for all SC300's and SC400's.

Since you've already gone through your vacuum lines and have corrected the bad leak I am just going to assume that you have corrected that aspect entirely.

Additionally it is *possible* that your 1992-1995 Karman-Vortex style MAF sensor may be going bad but this is not usually the case and I would still recommend tuning up the rest of the engine bay *and* getting the ECU serviced first before looking into that. Also just FYI, DO NOT EVER CLEAN YOUR MAF SENSOR ON AN SC300/400!!! As tempting as that may be that action will destroy it and 100% require finding a replacement. As of right now the best bet for a replacement is a good working used OEM sensor unit from eBay (just the sensor itself).

There is also the possibility that one of your OEM cats has clogged up but again this tends to be rare. You can determine this with a simple and relatively inexpensive infrared spot-temperature sensing tool that uses a laser beam.

There is also the possibility that your OEM Throttle Position Sensor isn't functioning correctly any longer and/or needs a recalibration done in accordance with the procedure in the Lexus service TSRM (not that hard to do but a little tedious, just involving a cheap electrical multimeter and some alligator clip electrodes to set the optimum sensor deflection point back within range).

...

These are, out of order mostly, all that I can think of for you to look into in addition. However I would prioritize getting the ECU recapped (by Tanin Auto or SIA Electronics) and replacing the front O2 sensors.

Arsenii

ECU is certainly a known issue on those cars, with that, any kind of intrusion, especially of that kind, comes with a certain amount of risk involved, and can only be done once or twice before traces on the board will start burning off. While it is a possibility that the ECU is getting bad, I would reserve re-capping for later, until you verify that everything is assembled properly and there are no further visible issues.

O2 sensors can very well result in said issue, but simply replacing them is $200 off, so I would suggest diagnosing them first.

Catalysts usually clog up only if the engine is running incorrectly for a long time, especially if it is running rich or burning oil. If the engine operates within normal boundaries and doesn't consume oil, catalysts can fail, but won't clog. With that, it won't hurt to check them.

100% on Karman-Vortex sensor, do not try cleaning it, it has a very precise light that reflects from a mirror, the latter must be very precise, and usual cleaners tend to leave a lot of residue on said mirror, rendering the sensor worthless. With all their drawbacks though, durability is not one of them, those tend to last a very long time, longer than regular Hot Wire types, I only know a handful of cases when they actually failed, outside of people trying to clean them that is.

Here is how to calibrate the TPS sensor, check it first before loosening the screws, because the calibration is usually set for the lifetime, unless there was any service done to the throttle body. Setting it incorrectly can result in bad idle and issues with transmission, it was one of the causes for some havoc back in the day, people were setting it incorrectly and their transmissions would start jerking, causing some panic in the eyes of owners and concerns about failed transmission.

Hope this helps and best of luck!

denSC300

I went to diagnose my O2 sensors, I watched a youtube video saying to use a multimeter and check if there is continuity. However, the video showed that the sensor clip had 4 wires, and I've seen other threads where other SC owners also have 4 wire clips. I'm confused because my front O2 sensors only have 1 wire. Are the front sensors only supposed to have one wire? Also, how do I go about diagnosing them? I understand I should probably replace all of them since they're all probably the same age. But I would still like to know how to diagnose them for future reference. Any insight at all would be greatly appreciated

Arsenii

Huh, it's been a good while since I've seen those..

First O2 sensors only came with one wire, being grounded through the exhaust, but due to some known issues that can visit any exhaust, namely rust, the signal would get distorted over time, so later models gained a second wire.

For the sensor to start working, it needs to warm up quite a bit, and to do it faster, ECU can utilize a heater element built into said sensor, that is how later models gained another 2 wires, and that is what most people suggest to diagnose the sensor. With that, heater circuit is only half of that system, other half is the readout to the ECU. Here is a thread with information on the subject of testing O2 sensors, and catalysts with them, but since your car doesn't have an OBD2 port, you will need some kind of an oscilloscope like the one linked below to get its readings, since OBD1 protocol simply lacks the precision needed.


Since you have the simplest model sensor, they are not as expensive as I thought, NTK one (the same company that makes NGK spark plugs) go for about $43, here is where you can get one. With that kind of price, you may as well just swap it out, considering the age..

Hope this helps and best of luck!

scsexy

iTrader: (1)

only the front 2 have any affect on driveability. anything after the cats is just for emissons

Arsenii

It's a pre-facelift SC300, there should be only one Upstream sensor.. But yes, anything after catalysts is just for emissions.

Supra Dr

iTrader: (14)

With out reading anyone's posts, gut feeling the the catalytic converter is clogged. The rattle was the insides of one of the converters rolling around inside the pipe. It stopped rattling because it is now stuck inside the pipe outlet & clogging things up. Extreme lean conditions will melt the converter substrate & cause all of your issues. Usually it's the front converter that melts because it is closest to the manifold. Try removing the upper / top oxygen sensor & see if you get your power back. A back pressure gauge is the proper way to test the system for clogs.