cficraig

Greetings,
I acquired a '91 LS400 3-4 years ago. It ran fine for a year and a half then started misbehaving. The first incident I was driving about 50 mph, slowed to make a sweeping left turn, hit the accelerator and it went flat, like fuel or ignition cut out. It then bucked a little as it seemed like the fuel and/or ignition came alive and it was fine for several seconds. It then transitioned from no-power falling flat on its face to running as if all was well the rest of the ride home. At one point I pulled over and let it sit for 15 minutes. From that point it ran fine for months.

I then drove it from philadelphia to miami with a job transfer. One time during the drive in heavy stop and go traffic it showed this same personality for several minutes then it went away.

It ran fine again for several more months. Down here in Florida it has been extremely temperamental and not drivable. Where it would transition from running normally to this on again off again fuel/ignition thing it now just doesn't want to idle.

It will start every time and if I keep my foot in the accelerator I can usually keep it running but if it gets near idle speed it will stall.

I've done a few things to it to troubleshoot or otherwise maintain it.
Before I drove it to Miami I changed the air oil and fuel filter, oil and serp belt. Recently I did the following:

volt checked MAF, it won't stay running for me to check the HZ output to the ECU
changed plugs
ohm checked each coil, not sure of what is required in resistance but they both are the same so I figured they are good
wire jumped B+ and Fp on the diagnostic block on top of the manifold. no change.

I searched all over these forums (fabulous) and other sources for help. Read about the fuel pump resistor, ECU, IAC, and the stickys here.

Before I start throwing parts at it, iac, ecu, maf sensor does anyone have any other troubleshooting advice? I will check for vacuum leaks if I can keep it running while outside the vehicle. Like I said, I can start it and keep it running by pumping the accelerator or just maintaining a high (~2000 rpm) but when I let go of the peddle it usually stalls. Having said that, yesterday I got it to idle on its own for several minutes then depressed the gas peddle and it stalled. ugh.

I love the car, 84000 miles, cream puff. My uncle has a 95 that has over 300,000 miles and it runs like new.

Any help, advice or comments are greatly appreciated, thank you.

YODAONE

Check for trouble codes. If everything else in perfect tune, then Mass Air Flow Sensor or Throttle Position Sensor are likely suspects

An FYI...pre - 1995 LS400 ise a MAF is destroyed using MAF cleaner...

See:

http://www.lexls.com/tutorials/

cficraig

Thanks for the reply. I tried jumping B+ and Fp (again) on the diagnostic block. I turned the key to run and heard the fuel pump running. I started the car and it ran at idle for a short time. With the pins still jumped and the car running I no longer heard the fuel pump running. Then as the car was about to die I heard the pump buzz to life again and the engine came back to life. Then it repeated this cycle. I thought that when the pins on the diag box were jumped the pump would run non-stop regardless of the of the engine ECU.

I don't know that if by jumping these pins there is still something else controlling voltage to the pump? I was going to wire it (pump) directly to the battery and see if it repeated this behavior.

Does this just sound like its nothing more complicated than installing a new fuel pump?

Thank you and to anyone else that reads this, cheers.

The car has 84,000 original miles according to the odometer and the overall condition.

cficraig

So a little more testing. I held a voltmeter to the blue lead on the pump cover and to ground. With B+ and Fp jumped i started the car. The voltage to the pump was cycling on and off and of course the car was responding in like until it eventually died. So I am thinking the pump is okay but something is cycling the voltage on and off to the pump and making the car run/die. I will attempt to bypass the relay and see if something is still cycling the voltage. Thanks for reading.

cficraig

Another update. I pulled the fuel pump relay switch and jump wired two of the terminals and its alive. Running like a champ. I then pulled out the paper clip jump from B+ to Fp. It is still running great. Hopefully this is it.

There has been so much informative content here. I'll post an update when I replace the relay and report on the result.
cheers