92 LS400 run time issue
10-05-19, 06:22 PM
#1
Lead Lap
Thread Starter
92 LS400 run time issue
After 15 minutes it craps out and dies. Runs well during the 15 minutes, don't drive it much and this has been an issue for quite awhile now. I jumpered the dianostic connector Fp to B+ and it still did the idle die and did a stumble at around 1200 rpms. The fuel pump resistor gets plenty warm. This timed interval deal is pretty strange.
I didn't time it to the second but its very close to right on 15 minutes when it happens. Anyone else have a problem like this?
I didn't time it to the second but its very close to right on 15 minutes when it happens. Anyone else have a problem like this?
10-06-19, 06:28 AM
#2
Moderator
Is your problem related to the ambient temperature?
If so, I would check the fuel pressure to confirm that the fuel pump is working OK after the heat is accumulated.
If so, I would check the fuel pressure to confirm that the fuel pump is working OK after the heat is accumulated.
10-06-19, 11:02 AM
#3
I had very weird issues with my '91 when the fuel pump soft failed. Diagnostic info in the following thread, but it would help if you can either confirm that circuit diagram (from my '91) also applies to your '92.
https://www.clublexus.com/forums/ls-...ml#post9777959
They did change the circuit design over the years and there is some wrong info floating out there on what exactly jumpering the diagnosis connector does. The circuit tells the story.
In my case, understanding and following all that through logically did not tell me the answer, but it did prevent me from replacing a bunch of parts that were working just fine. Then at the end, basically by process of elimination I replaced the fuel pump and that solved it.
Fuel pressure measurement would have helped, but I do not have the special adapter fitting. Also, since my problem was intermittent, the test might not have been conclusive.
Some even simpler tests you can try: loosen the gas cap so vapor pressure in the gas tank can equalize with the atmosphere. Run with the tank near empty vs. full. See if those affect anything, and it might be a clue.
On the fuel pump resistor getting warm - if the resistor still has the specified resistance, a failing fuel pump could cause it to get warm. The fuel pump will have a pretty low resistance, but when spinning, the back-EMF effectively reduces the voltage the pump sees, reducing current through the pump and also through the resistor. If there is a failure in the pump that causes it to not spin as fast as it should, that back-EMF will be lower, so more current will flow through the pump and through the resistor, making the resistor hotter than it should be. So bottom line - measure the FPResistor's resistance and use that info to decide whether it is good or bad.
https://www.clublexus.com/forums/ls-...ml#post9777959
They did change the circuit design over the years and there is some wrong info floating out there on what exactly jumpering the diagnosis connector does. The circuit tells the story.
In my case, understanding and following all that through logically did not tell me the answer, but it did prevent me from replacing a bunch of parts that were working just fine. Then at the end, basically by process of elimination I replaced the fuel pump and that solved it.
Fuel pressure measurement would have helped, but I do not have the special adapter fitting. Also, since my problem was intermittent, the test might not have been conclusive.
Some even simpler tests you can try: loosen the gas cap so vapor pressure in the gas tank can equalize with the atmosphere. Run with the tank near empty vs. full. See if those affect anything, and it might be a clue.
On the fuel pump resistor getting warm - if the resistor still has the specified resistance, a failing fuel pump could cause it to get warm. The fuel pump will have a pretty low resistance, but when spinning, the back-EMF effectively reduces the voltage the pump sees, reducing current through the pump and also through the resistor. If there is a failure in the pump that causes it to not spin as fast as it should, that back-EMF will be lower, so more current will flow through the pump and through the resistor, making the resistor hotter than it should be. So bottom line - measure the FPResistor's resistance and use that info to decide whether it is good or bad.
Last edited by oldskewel; 10-06-19 at 11:08 AM.
10-07-19, 07:58 PM
#4
Lead Lap
Thread Starter
Yamae the fitting I have to test the pump pressure seems to want to leak on the 92, so I'm going to have to hunt down a better fitting. The one I have in a MAC tool pressure test kit was made by an idiot, it has very little flange area for a gasket to seal on. I tried to veryify the pressure many months ago. This is way down on the list of projects as you can tell. But It would sure be nice to have it be some what dependable if it is needed. Oh and not ambient temp dependent, it does it in 80 plus degrees F and 50's too. But the 15 minutes is very close to right on each time.
Oldskewel and yes good idea normally I would have thought of that one too.
The diagram looks the same, and your correct about the resistor, looks like the wire from ecu to the relay needs to be open to bypass? Or is it powered to bypass?
Oldskewel and yes good idea normally I would have thought of that one too.
The diagram looks the same, and your correct about the resistor, looks like the wire from ecu to the relay needs to be open to bypass? Or is it powered to bypass?
Last edited by dicer; 10-07-19 at 08:09 PM.
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