bbullsj

i just recently purchased a 92 sc400. it has around 138,000 miles, and looks to be completely stock. it has a check engine light and was told is was downstream o2 sensor heater code and slow response. previous owner purchased it at auction and was reselling to make a quick buck. he put 5 gallons of 87 into the tank and it ran like crap. then realized it needed premium and put in about 3 gallons and ran better but missed under heavy acceleration. i drove the car home about 40 miles and parked it. would idle great and rev in neutral or park like none other but under accel would bog, miss and stutter. just after purchasing it i filled the just about empty tank with 93 and it seemed to run better when i drove it home. the car looks to have a remote start, keyless entry and a bunch of other aftermarket crap on it with wires going everywhere, (which i will be taking out soon b/c i hate that crap) it usually just causes more problems. also the fuell pump wires are exposed and look to have been tapped into for the security system to cut out the fuel pump if tried to be stolen.

i started the car up 2 days later to let it idle and ws going to take it for a quick spin up and down the road. after starting it i immideatley noticed a miss and was more apparent the more the rpms climbed. i took it down the road anyhow and t would barely take off and when it did try it would stutter and fall on its face and wanted to stall out on me. wasnt sure i was gonna get turned around and make it back home :S

i haven't messed with it much and im not really sure where to go and what to do or test first. i did unplug the MAF sensor and the miss at idle seemed to go away, and when plugged back in started to miss again noticable to the ear. i did not drive it with the MAF unplugged. what would have changed from the time i drove it home and parked it, then started it 2 days later? after a little searching on the net ive realized these cars can be finicky and might think about reselling it if its going to be a huge hassel. im not afraid to put the diagnosis time in but dont have extra cash after buying the car for a lot of repairs so hopefully the people who know these cars best can point me in the right direction and get this thing fixed.

GS4_Fiend

iTrader: (22)

You would never want to drive with the MAF unplugged. PCM will turn on the injectors longer and car will run very rich. I would put a full tank of 91, change spark plugs, spark plug wires, cap and rotor. Start doing those basics first and see if it helps. Then we can start diagnosing after that.

Mike552

First off, I would pull the codes to see what the computer THINKS it is. This is a simple procedure that will take you 3 minutes. All you need is a paper clip. Just google "pulling codes from a sc400" and a link should pop up on how to do it.

Also, something you may want to keep in mind is that the timing belts on these cars are known to skip teeth. Usually, the heads are timed right but the crank is off. Just check out the mechanical timing at all three spots to make sure all is good.

The above steps should get you in the right direction. If not, then I would suggest:

1. Plugs, Caps, Wires and Rotors. $150-200. (Not as simple as an oil change, but completely doable if you've ever wrenched on a car before. There's a step-by-step somewhere online for this.)
2. Fuel filter (probably not the culprit, but for $20, it's an easy DIY and probably needs to be changed anyway...)
3. Ignition Coils. (Each coil is $50 new. The car uses 2 of them. Very easy replacement job.)

Good luck!

MooJohn

You picked the wrong 20 year old car to fix on the cheap!

In addition to all that's been mentioned, try removing the ECU itself (under the passenger floor carpet) and check inside it for leaking capacitors. If the ECU is bad, nothing you do will work until it's fixed.

Since it runs better with the MAF disconnected you may have a missing / leaking vac hose. Replace them all. It'll take 20 mins and maybe $10 in hose -- 4mm is the diameter (I think that's 5/32)

bbullsj

i will start diagnosing next week. picked up some more hours at work. a few days off with nice werather and hopefully i can get it figured out. i will def check the codes first and start with that.

how safe is it to be cracking the ecm open and will a leaking capacitor be easy to see?

besides the 5 gallons of 87 in the tank i filled it up with 93 so the gas mixture shouldnt be a problem.

when replacing the plugs should i use ngk's? or would it be better to go with a denso from a Lexus or Toyota dealership?

are the timing covers easy to pull to check the mechanical timing?

i think its time to buy a repair manual too. might come in handy.

MooJohn

Removing a few 10mm bolts plus pulling the kickpanel & moving the carpet will get the computer in your hands. Just a few phillips screws (some on stupidly tight) will remove the cover and allow you to inspect the internals.

Looking closely at the capacitors will tell you if they're leaking. Sometimes the sealed end bulges & leaks; others will leak electrolyte out of the end that's soldiered to the board. Take your time & look carefully.

Fuel type doesn't matter right now. Go with NGK plugs. The ignition rotors were cracking and close to falling apart on my car. It was a slight PITA to change them but nothing I couldn't do in an afternoon. Luckily they're not too expensive and are stocked at Autozone.

bbullsj

pulled the codes this morning and i got 13, 24, and 29. started it upagain and could hear slight miss in the first 5 seconds it ran but then nothing for the next 30ish. i didnt run it any longer than that.

also found another switch hiding in the door today and when turned on it apparently does nothing (that i can see so far). its so annoying when aftermarket it wired up all over the place :S

bbullsj

any ideas on the codes and which one to look into first?

bbullsj

code 13 was the stupid agftermarket alarm system that armed itself on me! dumb crap. havent had time to pull all the aftermarket wiring yet do to working on my supra. pulled the fuse to the alarm system and now it is driveable. still runs kinda rough so need to work on the other 2 codes. hopefully in the next week ill get some time to work on it.

bbullsj

yesterday i finally had some time to pull everything out and it was a disaster. there was alarm system plus what looked to be a keyless entry system. i took it all out and taped back up and connected wires back together properly how they originally were and no the car wont start i assume it had a stock alarm system but i dont have a fob and im not sure how to reset or disable it. i can hear that the fuel pump does not kick on, and thats while the car is cranking. the aftermarket system was tapped into the fuel pump harness.

anyone know how to rest or get around the stock alarm system?

MooJohn

Turn the key in one of the doors. If the driver's door doesn't do it, try the passenger side.

Unfortunately there is no permanent disable, though some have grounded the alarm's trunk input wire to make it think the trunk is always open and thus it never arms.

bbullsj

well i tried that but my key wont turn in either door. i tried the glove box and it wont turn that either but it will turn in the trunk. does that mean that my door lock cylinder are stuck and i should rip them out, clean them and see if that helps? why would my key not work in the glove box though? the key looks pretty ratty. it is a lexus key but it looks like the top plasic head was broken off and was gorilla glued back together and on. maybe i should get a new key? they are $70 from the dealer and it would take me a few days to get it.

MooJohn

If your key only works the ignition it means someone has replaced the ignition cylinder. I don't know how you'll get a key to the rest of the car unless the dealer can do one by VIN.

I know the "valet" key wouldn't open the trunk but I think it would still do the doors so it rules that out.

bbullsj

i know theres a way to tell if i have a master or valet key by the grovoves in the shank but whats the difference? to get it run run enough to get it do a dealer to have it looked at can i just wire up the fuel pump to a switch and turn the switch on and then start the car so the pump is getting power all the time? it seems to me like that would work.

MooJohn

You can jumper two pins in the underhood diagnostic port and it'll run the pump any time the key is in the ON position.

I think they're marked Fp and B+ but do a quick search to make sure I've got the right ones. They're the front two in the connector.