BigBodyAl

Hi All, my first post here on Club Lexus - feels good to be part of the family!

Anyways, I have a 1994 LS400 with 109k. Recently bought it and it was taken care of really well, just not driven enough. Instantly noticed that I was getting 12-13 mpg in the city. Highway was anywhere from 18 to 23 mpg. So I wanted to increase my city mpg and replaced the Coolant Temperature Sensor after reading the post called "All LS400 owners with bad gas mileage READ THIS!" (link: https://www.clublexus.com/forums/ls4...read-this.html)

I expected coolant to NOT leak out when I changed the sensor, b/c it is at the high point of the cooling system, and no one seemed to mention draining coolant in the post so I just went ahead. WRONG!! a bunch of coolant leaked out, and this is one of the culprits that I suspect is not letting my car rev past 2,000 RPM (maybe electronic parts got wet, etc.).

I cleaned the Throttle Body at the same time as changing the Coolant Temp. Sensor. I had to disconnect the MAF and such, so maybe a frayed wire on the MAF harness finally went out after I worked in that area. Another suspect.

After cleaning the TB and changing Coolant Sensor, I topped off the coolant and put everything back together. Car seemed to have more throttle response for sure, especially in low RPM (maybe just placebo effect). However, after running fine for a week, the Lex took a crap on me and couldn't rev past 2,000 RPM. Once I hit 2k on the tach, the car would start sputtering and the check engine light would come on for maybe 5 seconds, but never stay illuminated.

After doing EXTENSIVE research and searching of the forum, I decided that maybe 1 ignition coil went out. Replaced both ignition coils (Denso), installed all new NGK spark plugs and NGK wires, and still problem persists.

Then I read LSCowboy and Yamae's highly informative post on leaking ECU capacitors (link: https://www.clublexus.com/forums/ls4...capacitor.html). I bought all the right capacitors specified in that post and replaced the old ones, and still the same problem! Car won't rev past 2,000 rpm!!

At this point I'm primarily suspecting that my ECU is messed up somehow other than the capacitors. Also maybe could be oxygen sensors , caps and rotors (didn't replace those when I replaced plugs and wires and ignition coils), or the timing belt? And since this all happened a week after I cleaned the TB and changed the Coolant Sensor, maybe its one of them. Maybe something happened with Throttle Position Sensor or I bought a defective Coolant Sensor (I bought OEM part)?

Car ran great before this happened, just didn't have good throttle response and the TB cleaning helped that. It also runs rich, thats why im suspecting oxygen sensors. Anyways, sorry for the long post... just wanted to be detailed. Any info/input is greatly appreciated!

Go Lexus!

LScowboyLS

LS400 ECU's don't typically have issues other than capacitors, although I have seen a couple of burnt resistors (the light blue ones) and a burnt transistor in a GS, both of these are very, very rare.

have you downloaded the stored codes? (just because the check engine light is not staying on, doesn't mean there are no stored codes)

many of the items you have mentioned as suspects, such as a defective coolant temp sensor or O2 sensors, etc, might make it run bad, but wouldn't keep it from revving past 2000rpm

I would do Yamae's ripple test next, to verify your work on the ECU and the engine coolant temp sensor.

How confident are you that some dyslexia didn't slip in and a couple of the caps were swapped (wrong cap in wrong hole) or that all of the polarities are correct?

BigBodyAl

Awesome, you were just the person I was hoping to hear from!! Thanks for responding.

I'll check for stored CEL codes (didn't know they are stored, thought you have to have a solid CEL to get a code) and do the ECU ripple test tomorrow after work.

As far as the capacitor work, I was planning on doing that this weekend. My brother took initiative and did it yesterday out of the blue, and he said he was semi-rushing. So I'll also check over his work... maybe polarity is an issue.

I'll post again once I do all that

LScowboyLS

you need to make a diagram of which capacitors are currently where, or take a pic of the inside of the ECU and then annotate the pic with arrows and capacitor values, then I can verify that they are all in the right place

most people take the caps all out at once, and then it can be very easy to make a mistake on which one goes where, sort of like how people often get two spark plug wires swapped up, and by the way, speaking of which, double check that too!

BigBodyAl

Hey LScowboy and anyone else interested,

I haven't done Yamae's ripple test yet but I'm going to do that tomorrow cuz I finally have a day off. But I did check for stored codes and got only one ... Engine code 41 - TPS signal. As I mentioned, my problem of the car not revving past 2k rpm and bucking as soon as I hit 2k started a week after I cleaned the throttle body and replaced engine coolant temp sensor. When I took off the throttle body to clean it, I obviously disconnected the TPS connector but did not take off the TPS itself from the TB. I used carb/choke cleaner (maybe should've used a bottle actually labeled "TB cleaner" but i read that carb n choke is the same). Maybe I damaged the TPS somehow with the carb n choke cleaner? I want to check the TPS tmorrow but dont exactly know how. I researched and all I found is that its supposed to be getting 5 volts. Does anyone have a tutorial on how to check the TPS?

Most important, does a bad TPS even have the possibility of putting the car in safe mode and not letting it rev past 2k? Or would it just run like crap?

If it actually is the TPS that would b awesome because its a relatively easy fix. But i fear that the ECU took a crap b/c a lot of the threads that I read about Engine Code 41 pointed to the ECU being bad rather than the TPS being the issue. If the TPS checks out fine then I will try to replace the ECU caps one more time, this time more carefully and thoroughly.

Let me know your thoughts on this, and have a good Sunday!!


- oh and btw, do I have to order the 0.47Uf capacitor from digi-key to do the ripple test? Is it specific and hard to find in stock (like ECU caps) or can this one be bought at a regular electronics store? Cus I want to do it tomorrow and don't wanna wait for it to ship. Thanks

Yamae

When the ECU enters the safe mode, the ignition timing is fixed at BTDC 3 degrees as well as the limitation of the rpm up to 2000rpm. A timing light is another convenient tool to confirm this.

LScowboyLS

yes, you should use throttle body cleaner, not carb cleaner, it is a different solvent to prevent damaging certain parts, though that may not be anything to do with the current issues - you can get CRC throttle body cleaner at any auto parts store.

it's a fairly specific type of capacitor for the ripple test, I can mail you one if you PM me your address

if you don't want to wait, then take the ECU and photograph it extensively and write down the numbers on the caps, the ECU is still highly suspect in my mind until you show us a correct job on this.

here is the code41 /TPS section of the factory service manual, there are some tests to do, if the tests pass, the next step is "replace ECM" - so do those tests and you should have a good idea whether it is the TPS or the ECU cap job.

BigBodyAl

As I mentioned in my first post, my LS would not rev past 2k RPM a week after I cleaned the throttle body and changed the Coolant Temp Sensor. Problem turned out to be the TPS sensor. I used carb/choke cleaner to clean the throttle body so I guess that damaged the sensor. I recommend that you always DO USE throttle body cleaner and DO NOT USE carb/choke cleaner to clean your throttle body.

Thanks to LScowboy & Yamae for help and insight!!