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Sc400 Running Bad


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The victom is a 92 sc400. I have had problems for a while now but seem to have the dying fixed. Right now it is running very poorly she has the sniffles.

At idle while working on it, it has a dead miss on 4 and 6 cylinders. Compression on both is 145 psi. Plugs were totally black when pulled. Cleaned plugs and run a couple minutes both plugs were wet. It is getting fire to plugs, both are sparking out end of wire when pulled no carbon tracks on side of plug wires, even swapped wires to check and nothing. Switched plugs and still nothing. Cylindewrs 1,3,5 spark plugs are red looking. I would say from looking at them they ahve been so hot it is unreal, cermic around electrode looks brittle.

Plugs are a couple months old, I cleaned idle air valve (that fixed dying problem). It has a fairly new fuel filter. Pulled rear seat this week and checked fuel pump connections all looked good, tank and sock are perfectly clean. Guy that lookeda t it with me tonight is a very experienced auto mechanic and it has him puzzled also. Anyone have any ideas?

Thanks

Josh

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I guarentee you its the mass airflow sensor...my car had the exact same issues. like issue for issue. no codes came on right away, but it was misfiring like you wouldn't believe...so i replaced the MAF and it was magic.

Bad MAF = terrible fuel / air mixture so the engine just dumps WAY more fuel into the engine than it needs so it just misfires...

my advice is this..it was like magic to my car.

Derek

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I guarentee you its the mass airflow sensor...my car had the exact same issues. like issue for issue. no codes came on right away, but it was misfiring like you wouldn't believe...so i replaced the MAF and it was magic.

Bad MAF = terrible fuel / air mixture so the engine just dumps WAY more fuel into the engine than it needs so it just misfires...

my advice is this..it was like magic to my car.

Derek

Can I unplug the maf and see how it runs then? I have heard you can do that.

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Yes you can. However it has bad side effects. During all this time when i struggled with this problem, one time i forgot to plug the sensor back in...both the TRAC OFF light and CEL will stay on permanantly with it unplugged. and also you will probly smell gas like i did since it will run rich without it.

this is a good way to test if its the MAF. because if the engine runs beautiful without the sensor plugged in, then thats what it is. even though it runs rich and stuff without it plugged in, it should RUN seamlessly...definetly don't drive it like that as a permanant fix but just as a pretty un-deniable test for whats wrong.

let me know. and oh btw ebay has good MAF for cheap money probably around 100 dollars

Good luck ;)

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hmmm...very interesting

i guess that can only mean the MAF is functioning properly. this confuses me plenty because those were the exact same symptoms my car experienced...im sure you have done this already but have you considered the basic tune up procedures?

hopefully its something small man. best of luck

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Timing is correct.

I done some more work this weekend. I now know 4 and 6 are both dead. I used a new spark plug wire and found I do have spark but still have dead cylinders. Compression is good on both cylinders. I have switched plugs and still dead. Plugs on those 2 holes are always black when I pull them. Mileage has went down to 13-16 mpg and always smells very gasy.

A couple things I have noticed is the cel does not come on when I turn the key on, but the light is good because I see it come on once in awhile while driving. It never has logged codes, I cannot pull them up myself and I know I'm doing it correctly because it reads trans codes. I have also noticed that it seems to run decent at about 80 mph +. This tells me I am dumping tons of gas in those cylinders and it can't ignite it until it gets way up in rpms and needs that much fuel.

With all of this going on I have just about decided I have lost the driver/s in the ecm that opens the injectors for those 2 cylinders and it is causing them to just dump fuel all the time at way to high of rates.

Has anyone had this problem? If I can find a cheap ecu I think I will buy it, unless someone has a better idea.

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my car seems to run rich also, but it does fire on all cylinders. when i reset the ecu all my engine codes go away and it runs beautifully for about 2 days until it pulls up those codes again then it starts running like *BLEEP*. you dont think my ecu could be done too? i just replaced the cam sensor and the o2's, and like i said after that it ran good for 2 days then the cel light came back on and it runs like crap.

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  • 12 years later...

     I know I am about 10 years too late on the conversation but I have a 92 SC 400.  230,000 miles on it and runs great and is  well taken care of.    Until last week, while I was coming home from a less than 10 mile drive home with my daughters.  All the same symptoms you all have been describing.   Big loss of power,  like running on less than 4 cylinders.  rich gassy smell.  made it home with less than a mile left from when it happened before a red light.  Parked it in the garage and its be parked there until I get off of Work this coming weekend.   I'm thinking Fuel Injection problem... But I have been having an issue with my battery light coming on from time to time.  I checked the battery on the ohm meter and its charging.  The Issue started when my Car Stereo fuse started blowing and I could not find out why....  Ran an Independent Power line to the Amp under the CD player and everything was fine for about 3 months.   Fuse didn't blow until about 2 weeks ago so I have been doing without music for a minute.  Dealer wont look at it with my Independent Line.  so I am in a funk.  Anyone else experience anything like this?   I think its the classic power steering fluid weeping into the alternator and getting it to not charge all the time.    Anyways...  I'll get this.  Wiring Diagram books are out....  

  • Like 1
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I would definitely say that if you are leaking fluid into the alternator then there is the first place I would go to sort the issue.
You could do a great deal of damage if the alternator goes high volts which can pop ECUs and cook the battery.

Let us know how you get on with resolving this issue

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  • 3 months later...

Similar issue with my 92 sc400 265,xxxmiles. New plugs, wires, caps, rotors, coil packs. The misfire issue comes up around 3k rpm after the car is fully warmed up and then the misfire trickles down in the rpm’s till it almost stalls out when at a stop(but hasn’t yet died!) previous owner did some sort of fuel pump mod to the wires under the drivers quarter window. There is also a surge in my electrical system that pulses with the lope of the motor. My power steering pump looks like it has been leaking for a while. Could this be causing the voltage regulator located on the alternator to intermittently send bad current to the ignition system? Should I start by just replacing the alternator or getting it checked? I can’t tell if the misfire is lack of fuel, spark, or air. 
 

NOTE: The power surging doesn’t get much better or worse with air conditioning or headlights or anything else electrical running. 
         : Doesn’t seem to have a misfire at idle when parked. 

Edited by Reinsc400
Correction on conditions
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My goodness, I love the dance, the bob and weave, the could it be toss of spaghetti against the wall.  The replacement of every single part except the door hinges.

YOUR INJECTORS ARE BAD.  My company rebuilds injectors, is rated 5 star across the board for the last 12 years, I own a 1990 LS 400 which now purrs like a kitty, and I have seen MANY sets of these injectors that leak down and have bad sprays.  Surging, gas smell, melted cats, etc are CAUSED BY THESE BAD INJECTORS.  They are fairly easy to remove -- save yourself time and effort and bad words and pre order the throttle body and upper intake manifold gaskets from Rock Auto.  Pull the intake of course, makes reinstallation much easier. 

I'm not shilling for my biz, we got lots.  I'm in Wheat Ridge, Colorado, so look us up if you want this fixed.

By the way, don't ignore the cold start injector.  Under the intake.  It clogs solid and makes cold starts take a while.  Later LS 400 models upgraded the software so the cold start was eliminated.

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