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1991 Ls400 Starts For Split-second And Then Dies!


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#1 Raymond Chen

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Posted 07 January 2008 - 06:50 PM

Hello everyone, I just finished replacing the fuel pump on my 1991 LS400. However, I am still having a problem starting my car. I know the fuel pump works because I can hear the fuel surging under the hood every time I turn the key to crank the motor. I even bridged the B+ and FP pins in the diagnostic port under the hood to make sure the fuel pump is working, and that I am getting fuel into the engine. However, the engine will not stay running. Whenever I crank the motor, it will come on for a split-second, rev to around 2000 RPM and then die. Could someone please tell me what could be causing this?

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#2 smooth1

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Posted 07 January 2008 - 07:01 PM

Are you getting spark? It's a 91? What else have you done to the car? What propblem were you having with it before that led you to replace the fuel pump? Does it idle ok, and then die if you go over 2k rpm? or does it just wanna start and then die?

#3 Raymond Chen

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Posted 07 January 2008 - 07:07 PM

Are you getting spark? It's a 91? What else have you done to the car? What propblem were you having with it before that led you to replace the fuel pump? Does it idle ok, and then die if you go over 2k rpm? or does it just wanna start and then die?




How do I test if the car is getting spark? I replaced the fuel pump because I found the original one to be bad. It would not run at all. The engine will not idle at all. It will just start and die simultaneously.

#4 smooth1

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Posted 07 January 2008 - 07:27 PM

wow, man, to be honest, the boards aren't the place to determine your issue. there could be a slew of diagonstic and check points to go over. And if your asking me how to check for spark, then this could be a really long post. I suggest you get someone with enouph mechanical experience under the hood to help you in person. It could be everything from timing, to spark issues, to throttle body, to vacuume hoses to all kinds of things.

#5 Raymond Chen

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Posted 08 January 2008 - 06:55 PM

Ok, today I worked on the car a little bit more. The engine will now occasionally run for a little bit longer, more like around 3 to 5 seconds, and then it will die again. This happens every time I use the deflooding procedure, by cranking the car a few times with the gas pedal all the way down on the floor. I also did some diagnosis, and I got code 13 from the check engine light, which has to do with "RPM signal 2." Could someone please help me with this problem?

#6 R.Holmes

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Posted 08 January 2008 - 07:48 PM

Ok, today I worked on the car a little bit more. The engine will now occasionally run for a little bit longer, more like around 3 to 5 seconds, and then it will die again. This happens every time I use the deflooding procedure, by cranking the car a few times with the gas pedal all the way down on the floor. I also did some diagnosis, and I got code 13 from the check engine light, which has to do with "RPM signal 2." Could someone please help me with this problem?

just a couple of questions. When it starts, can you give it a little gas to keep it running? When you let off the gas, does it die?
If you can do this, then my dianosis is the IAC Valve which you can clean and it will work like new.

#7 Raymond Chen

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Posted 08 January 2008 - 07:56 PM

Ok, today I worked on the car a little bit more. The engine will now occasionally run for a little bit longer, more like around 3 to 5 seconds, and then it will die again. This happens every time I use the deflooding procedure, by cranking the car a few times with the gas pedal all the way down on the floor. I also did some diagnosis, and I got code 13 from the check engine light, which has to do with "RPM signal 2." Could someone please help me with this problem?

just a couple of questions. When it starts, can you give it a little gas to keep it running? When you let off the gas, does it die?
If you can do this, then my dianosis is the IAC Valve which you can clean and it will work like new.




No, stepping on the gas pedal doesn't keep the engine running at all. Sometimes, it seems like stepping on the gas pedal actually kills the engine even faster.

#8 mc teg

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Posted 08 January 2008 - 11:38 PM

I had the same problem you're having now. Sounds like you have an air leak. Try checking the MAF sensor, disconnect the MAF sensor plug and start the car up. Reply back when you try this.

#9 Raymond Chen

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Posted 09 January 2008 - 02:24 AM

I had the same problem you're having now. Sounds like you have an air leak. Try checking the MAF sensor, disconnect the MAF sensor plug and start the car up. Reply back when you try this.



I tried starting the car with the MAF sensor unplugged, and then with the MAF sensor plugged in, and during both times the engine would still shut off immediately after it started. Also, code 13 seems to come up intermittently. Every time I clear the codes, code 13 doesn't come up immediately.

#10 eatingupblacktop

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Posted 11 January 2008 - 05:09 PM

Are you getting spark? It's a 91? What else have you done to the car? What propblem were you having with it before that led you to replace the fuel pump? Does it idle ok, and then die if you go over 2k rpm? or does it just wanna start and then die?




How do I test if the car is getting spark? I replaced the fuel pump because I found the original one to be bad. It would not run at all. The engine will not idle at all. It will just start and die simultaneously.


How did you determine that the pump was bad? Was that your assessment or from a shop? As Smoothy said, it could be any number of causes. Simple quick checks before replacing anything would be spark/voltage starting at the plugs and going back from there, fuel delivery/pressure, vacuum leaks. Any shop could test these for you for a reasonable cost. If you're not sure about the cause and aren't comfortable to find it for yourself, have it checked out and spend a little before you spend a lot.

#11 Jerry Bransford

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Posted 11 January 2008 - 07:08 PM

You sure it's not a security key thing? Does the '91 have a security key icon on the instrument cluster so you could tell?

#12 Raymond Chen

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Posted 13 January 2008 - 07:10 AM

Ok, I pulled the relay under the hood labeled "Main Engine Relay," tried to start the car without it, and it did the exact same thing. I was wondering, since the car displayed the same symptoms without this relay, if this might be the problem and the relay is bad. Also, the EFI relay (not the fuse) seems to get pretty hot whenever I try to start the car. Is this normal?

#13 Raymond Chen

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Posted 13 January 2008 - 08:09 AM

I was also wondering if the problem could be a bad alternator? Every time I try to start the LS400, it has been by jumping it with my other car, because the battery in the LS400 is dead. The guy from whom I bought the car said the battery was new, but it has been dead since when he first showed me the car. Would a bad alternator cause the symptoms that I have described previously?

#14 Leadfoot

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Posted 13 January 2008 - 02:20 PM

I was also wondering if the problem could be a bad alternator? Every time I try to start the LS400, it has been by jumping it with my other car, because the battery in the LS400 is dead. The guy from whom I bought the car said the battery was new, but it has been dead since when he first showed me the car. Would a bad alternator cause the symptoms that I have described previously?


All the electronics in the car are going to need a stable voltage to work properly. When you are jumping from the other vehicle have you checked you are getting 13.5 - 14.5 volts on the battery terminals?

#15 eatingupblacktop

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Posted 13 January 2008 - 03:09 PM

To check your charging system I would start with fully charging the battery, to see if first it will accept a charge and second if it will hold a charge. Check the voltage at rest/under load. Once your satisfied the battery is good, test the alternator. Check the output voltage at rest/under load. Once you know your charging system is operating properly, as Leadfoot says, you'll know there's a reliable source for all the other systems in your car.

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