Taking All Bets: 1998 Lexus SC400

[insightbrewery]

Just trying to generate more sections under the Lexus heading.

So I got in this gorgeous 1998 SC400 that has a cold start stalling issue. Only on cold start, it tries to die several times, and sometimes succeeds. Runs great once warmed up (closed loop).

I got it solved. But for the sake of discussion, taking all bets on what it was.

EDIT: feel free to ask questions to get logical, like 20 Questions.

 

[insightbrewery]

Aw, no takers? Fiiiine.

Common problem on gen1 SC and MKIV Supra is that the fuel pump ECU goes out. Its job is to drive the fuel pump at either low-speed (9V) or high-speed (12V) operation depending on the engine's fuel demands. Same system whether 2JZ or 1UZ. When it goes out, it gets stuck at 9V when it should be at 12V; fix is to cut and splice together the + output to the fuel pump and the 12V wire from the EFI relay; defaults the pump to 12V operation, like, well, every other Toyota.

BTW, he's selling the car if any of you are interested in an ultra-clean 1998 SC400 (light blue exterior, black interior) with barely 101K on it. Immaculate inside-and-out.

 

[71Corolla]

That's an odd design choice why did Lexus do that? Is it similar to some Toyota's where they have a resistor inline with the pump? Try to make the pump last longer? For the record I would not have guessed the cause.

Aw, no takers? Fiiiine.

Common problem on gen1 SC and MKIV Supra is that the fuel pump ECU goes out. Its job is to drive the fuel pump at either low-speed (9V) or high-speed (12V) operation depending on the engine's fuel demands. Same system whether 2JZ or 1UZ. When it goes out, it gets stuck at 9V when it should be at 12V; fix is to cut and splice together the + output to the fuel pump and the 12V wire from the EFI relay; defaults the pump to 12V operation, like, well, every other Toyota.

BTW, he's selling the car if any of you are interested in an ultra-clean 1998 SC400 (light blue exterior, black interior) with barely 101K on it. Immaculate inside-and-out.

 

[insightbrewery]

TBH, since the LS (prototypical application of 1UZ) was designed/reverse-engineered/meant to compete with the S-class (yeah, doesn't come close), I think they were trying to be more "German," i.e have tighter control of all the support systems.

Pontification time: somewhat related, default-map engine control (open loop) on BMWs is waaaaaay tighter than on the average Toyota. It's why on a lot of Toyotas, if you disconnect the MAF the engine won't start, or will die; but on Bimmers, you can disconnect the MAF and it'll throw a code, but switch to the default map and you won't be able to tell that anything is wrong (good for troubleshooting MAF/IAT issues).

That's an odd design choice why did Lexus do that? Is it similar to some Toyota's where they have a resistor inline with the pump? Try to make the pump last longer? For the record I would not have guessed the cause.

 

[71Corolla]

Pontification time: somewhat related, default-map engine control (open loop) on BMWs is waaaaaay tighter than on the average Toyota. It's why on a lot of Toyotas, if you disconnect the MAF the engine won't start, or will die; but on Bimmers, you can disconnect the MAF and it'll throw a code, but switch to the default map and you won't be able to tell that anything is wrong (good for troubleshooting MAF/IAT issues).

Interesting. But on a German car you need all the diagnostic help you can get, in the limited work I've done on them their electrical systems are flaky and unpredictable. I know someone with an X3 that gave up trying to repair it, the dealer failed he took it to a specialist they failed as well. Car throws codes and sometimes dies without warning.

 

[insightbrewery]

Haha, you'd be surprised how similar classic (pre-'00's) German cars are to what we normally work on out here. More stuff, sure, but not really more complex.

Now '00's and up German cars, they went somewhat overboard...I've got BMW ISTA and the factory hardware to interface with these cars that have 35-65 computers on board. It all comes down to understanding the logic...the reality is, as much as they're rolling computers, there's very little glitching. They're still cars, the software controls mechanical implements, as knowing this I've never failed to figure out a German car's problem. Probably what keeps me in business LOL

E83 X3's and E53 X5's suck.

Interesting. But on a German car you need all the diagnostic help you can get, in the limited work I've done on them their electrical systems are flaky and unpredictable. I know someone with an X3 that gave up trying to repair it, the dealer failed he took it to a specialist they failed as well. Car throws codes and sometimes dies without warning.

 

[insightbrewery]

Welp, turns out I was wrong - while it seemed to run great after the fuel pump mod, the guy said the same thing happened two days ago. It'll be back tomorrow.

 

[insightbrewery]

ACH! Finally figured this one out. After $750 worth of red-herring parts.

The cause is the Metromile tattler. This is the guy's fourth car, only takes it out on dates and such. This and his 2017 Mustang GT, he insures through Metromile, cheaper since they barely get driven. The Mustang has no problems, but the SC400 doesn't play nice when you plug in the Metromile OBDII dongle.

What I suspect is that with the PCM probably having been designed in the 80's, the CPU is very low-powered and doesn't have enough clock cycles to devote to responding to the flood of data requests from the Metromile. Also probably not much (if any) of a priority system implemented, which is why it was dropping its engine-management duties to respond, and thus dying out. The Mustang probably has a much more powerful CPU and can handle the same requests.

Or the Metromile tattler is defective or just plain sucks. I mean, my go-to Actron scanner can display all the live data PIDs available, updated every 1-2 seconds, and no issues.

In any case, this is one for the books. When in doubt, remove aftermarket crap first. BTW, this car is for sale if any of you are interested. 1998 Lexus SC400, 101K, all maintenance up to date and it is nearly immaculate...only two modifications:

- aftermarket deck
- resonator delete (sounds pretty great)

 

[71Corolla]

When in doubt, remove aftermarket crap first.

Hells yea. Many techs won't touch a car with aftermarket crap, don't blame them.

 

[insightbrewery]

A couple of my mechanic buddies down the street, if they get a car with aftermarket alarm that's having problems starting, they won't work on it unless the customer authorizes alarm removal first.

Hells yea. Many techs won't touch a car with aftermarket crap, don't blame them.