97L400

bought a second hand 97 ls400 with 187,xxx miles.
owner couldnt get it started,ive replaced fuel pump,plugs and wires,both ignition coils and a brand new battery.re programmed key.......if the car sits overnight and start one shot in the morning,itll fire and drive smooth.......once it gets hot i notice light missfires....ill shut the car off and try to start,itll start and die 3 secs later.....over and over again,i might get lucky and itll start after 50 tries or more,but usually will start once the motor is cool.

No check engine light, immobilizer and ecu must work, what could it be guys.

I did search but my problem seems like a mixture of many topics.ty

CLLEXUSS

so you tackled the usual suspects now you have to search for the unusual suspects, something weird maybe? like a bad connection somewhere, a lose or bad wire on an 02 sensor you could imagine how hot the 02 sensor gets when the exhaust is operating a full operating temperature or any other are where wires are attached to a heat prone place.

or, it could be something like sensor on maybe your throttle body possibly,something like that, the engine sensors can operate differently when a car is cold than how they operate after it warms up, it is a possibility

these engines are also variable valve timing systems, timing can be different when the vehicle is cold than what it is when its warm so maybe something like a variable valve timing solenoid gone bad

i'm just throwing some ideas out there, interesting problem but not a fun one when you're going through it

CLLEXUSS

Heat creates higher resistance in an electrical system so I know it's vague as to whats possibly wrong, but heat does

I am thinking its something electrical or sensor related possibly somewhere something...tough to tell i guess it would help if a person was there and heard the way it was failing as it was failing but yes it's one of those diagnosing trouble shooting brain-storming different possibilities

Yamae

A 97 does not have any VVT system yet. So the OP's problem is not related to it.

I'd rather check the ECT(Engine Collant Temperature) sensor and the ECU. The ECT sensor can be checked easily just reading the resistance or the DC voltage of it. The ECU can be checked measuring the ripple (the AC component) voltage of the ECT sensor. When the engine is not warmed up, the DC voltage of the ECT sensor should be 2.5V or higher and when the engine is fully warmed up, the voltage should become lower than 1V. When you measure the ripple voltage, use a film capacitor 1-0.1uF in series with the multi-meter which is set to AC mode. This makes it possible to measure the ripple voltage and it should be less than 40mV RMS when engine is cold. The voltage varies depending on the multi-meter's accuracy though. A very good multi-meter often shows more than 40mV and cheap one less than 40mV RMS. The best way is to use an oscilloscope.

If the DC voltage is higher than 1V, it simply means that the ECT sensor is not OK and you can't start the engine easily. If the ripple voltage is higher than 40mV RMS, the ECU is not working well and you need to replace infamous QAS capacitors. Bad capacitors can't absorb the AC component of the Vcc line in the ECU well enogh and voltage at the ECT sensor becomes a full of ripple.

97L400

thank you guys for responding quickly, i ordered a second ecu to see if anything changes.

I did not know the ect sensor can do that to the motor,nor a 02 sensor.

I will give it angood look thru and reply here the findings.ty so far.

Sin1UZFE

Mine does this but only when hot...like over 100

97L400

Changed my crankshaft sensor and started perfectly countless times......i tried to remove the oem sensor to check it, but was so brittle it broke to pieces thats why i replaced it.

Scanned and threw 2 codes....p0301 twice

I erased it and started the car.and once again starts and dies seconds later.

New igniters, wires,plugs, ignition coils

I do have a crack in my oem air intake flex tube, gonna seal for the time being.

Efi relay gets hot still, only happens when the.motor.is.heatsoaked.

Where can i order a ecu that already.has capacitors replaced?

FloridaCam

Have you done a compression test? I would if I were you.

Bylan

No VVTi on a 97, sounds like a sensor is failing when the engine is hot. I had a similar symptom on my 98. Crank Position Sensor would fail when the engine bay got hot, caused random stalls and sometimes a hard start, No one will have a definitive answer, buy a multimeter and start testing things!

EDIT: Well I kept reading and sure enough, crank sensor! Good luck with the rest of the car

sdls

dude fix your air leak
an air leak can definitely cause those stalling issues

explanation: I think your air leak is bringing a lot of unmetered and unfiltered air into the engine
when the car is cold, it runs really rich to light off the cars and warm up the car. An air leak wouldn’t be as noticeable then.
when the car is hot, the motor isn’t running rich anymore and the air leak will overwhelm the AF mixture and the car can’t correct for it since it doesn’t know how much air is going into the motor after the air flow sensor

Yamae

Have you tried to bypass the fuel pump resistor?
When it is open, the engine can start but stops quickly.

97L400

Thanks for the response guys

Ill seal that leak up today.

The fuel pump resistor is the relay?

Yamae

No, it is not a relay. It is a big resistor with a heat sink to dissipate the heat.

The purpose to use the fuel pump resistor is to reduce the power to the fuel pump limiting the current. In other words the relay is used to bypass the resistor to make it possible to run the pump fully only for the start up and higher loads such as to run very fast or running up hills. The engine needs the resistor when it does not need the extra fuel. When the resistor is open, the engine can start up but stops quickly after the starting up because there is no power supply to the pump. Other than the inhibiting mode, there is another possibility of the engine halt problem which is caused by the open resistor.