So i am having a very random and ver intermittent issue.

i will drove the car for a couple weeks with no issue then i will a few starts with some issues

I will stop at traffic lights and then The car will stall.
and when i try and restart is will crank and crank and crank, take quite some time and maybe some acceleration to start. Then it will run.

very occasionally when i go to accelerate it will feel sluggish.

this is a very intermittent issue.

fuel pressure regulator swapped got second hand one recently.
maf swapped for second hand one recently.
exu swapped for second hand one recently.
Tps reset done recently.
fuel pump relay swapped with a few others.
fuel filter changed only bout 6 weeks ago

only things i can think is
1- possibly iacv but idle is standard and does not fluctuate at all.
2 - fuel pump. Annoying to change. So hope not
3 - altenator/voltage regulator. I do get some belt squeal on cold start, and my voltage does drop sometimes at idle. Have not noticed the idle low at the same time it has stalled tho.

 

I had similar very intermittent no start issues. The engine would crank OK but fail to start.
The problem was a failing crankshaft position sensor.

 

would that also cause very occasional random stalling?

had a couple issues 2 weeks ago slow to start. (No stalling tho)
then all good for 2 weeks.
had it stall twice yesterday and slow to crank after the stall.
no problems at all today.

 

I'd go with the CPS as well, FWIW.

The details are long forgotten but replacing the CPS on my X300/XJR solved a mystery stalling condition.

Cheers
DD

 

ordered one just now. for AUD$30 its certainly worth trying.

 

The fuel pump control fuse is # 10 right heelboard fuse box which requires the king relay in the corner of the fuse box to close

You can swap that relay if it is intermittent , but if it was you would expect to see it in all engine ranges

This relay gets a closing ground path / command through the ignition switch , pin 5 a good car frame ground under the dash ?

The CKPS is not used in the initial start sequence but is the Camshaft Position Sensor , unless you unplug the CMPS and it uses the CKPS in the begaining which requires more crankshaft revolutions to light off

So we comeback around to if the ECU is being powered by the 2 relays , small # 9 right engine bay fuse box and # 5 large ECU controlled relay ( this # 5 is also the direct wired power sitting on the fuel injectors )

You can swap the large # 5 with a headlight , fog , or A / C clutch of the same part # 6707 off the top of my head

You can bypass the fuel pump control items ( ECU fuel pump relay control ground path , ignition switch ground path pin # 5 , right heelboard fuse box king relay ) and directly power the fuel pump with socket 3 and 5 of the trunk fuel pump relay , but will drain the battery with the keys in your pocket , use blade type connectors on the jumper wires

You can bypass the fickle inertia / crash switch by jumping the 2 white wires on it's connector , don't take apert the inertia switch as spring loaded parts will fly everywhere

Ask me how I know

I had a connector ( RS 3 on car's right ) that was damaged by the previous owner in a crash that is the one and only ECU ground path wire that closes the fuel pump relay

 

I don’t see how it could be something like inertia switch.
the reason i say that is the times where there has been a lag in acceleration.
i try to accelerate and its like it almost splutters then after a second it accelerates.
this may happen once or twice then it goes back to normal.
this also only happens at lower rpms after backing off then applying the accelerator again.

 

Have you ever done a TPS very very slow sweep with a meter as may be a open spot towards idle ?

You can do it from the ECU connector between socket RED 12 ( Green / Yellow return wire ) and Red 11 ( Blue / White sensor reference voltage feed wire ) as resistance

 

never done.

hardest thing is trying to troubleshoot and diagnose any intermittent fault.
even if you check something and it appears good, you can't rule it out as sometimes it may work and sometimes not.

 

The TPS is a variable resistor feed a reference voltage of precise 5.0 volts DC

it is resisted and comes back to the ECU as resisted voltage value

The return wire ( Green / White wire ) will be at around 0.60 volts DC at the idle stop and will slowly increase toward 5 on throttle up

You can test this with engine not running but key in run position

To do the test on the middle Green / White wire as the TPS connector remains connected , follow the wire as it goes over the fuel rail and nick the insulation

Very very slowly twist the TB open and observe the voltage value very very smoothly increase but no beps or 0 voltage ( bad TPS )

Or you can do the test from the ECU as a resistance test through 2 wires , just press the gas pedal

When there is a bad spot in the TPS the ECU will reject the signal and default to a less that optimum engine regulation and a time lag between switching

 

Will try to get a chance to have a look on the weekend

 

Best way to bench test a TPS is with an analog multimeter. You'll be able to see if the needle makes any dips at all, whereas with the digital, it may miss the dip completely.

I suspect its a failing CPS.

You don't have an EGR Valve in them parts, so that takes that off the list.

Without having any kind of parameter to check/test, this does become a challenge. See if you can make it happen with the engine/battery loaded. Run the AC, headlights, radio, etc.

 

With the TPS signal rejected and you advance the throttle opening the ECU can not compare the TPS signal to the present low RPM and advance the ignition timing as it knows with the TPS signal the engine will be under load or under RPM target

The open throttle butterfly will allow the engine to spool up but at normal no load ignition timing ( less than optimum )

With the alternator loaded down have a meter on the glove compartment light socket and on acceleration see if the voltage drops indicating the alternator is not regulating well enough

May be more noticeable at transmission / engine / alternator speed shifts

 

When you say cps do you mean crankshaft position sensor or camshaft position sensor?

 

I had similar problems in an XJ40 and an X300 stalling for no reason.
Both cases turned out to be oxy sensors

 

replaced those about 1 year ago so "should not" be those.

 

Your engine stalling encounter is somewhat similar to mine - see my earlier post Engine Stalling at Low Rev. My added symptom was occasional high idle, which I suspected was caused by a toasted throttle return spring. The potential culprits as a result of a group therapy here are (1) a bad Idle Air Control Valve (IACV), (2) a toasted intake manifold, and (3) a tired Throttle Position Sensor (TPS), which are not unreasonable because if they have not been replaced before they are as old as the car itself, i.e. almost 30 years. Nevertheless, before I had time changing mine out, my XJR suffered from a sudden lubricant loss leading to engine failure.

Meantime, my other car, a 1990 Mercedes W124 had a similar issue, i.e. difficult hot start, the engine simply stalled a few times before waking up fully and working properly. It might be useful to share a Mercedes mechanic's theory, that the car might be running rich before, and by default, the car treats every start a cold start, hence the over-rich mixture floods the spark plugs and caused a no-start. My Mercedes mechanic subsequently re-tuned the engine to make the mixture less rich, and it works! The W124 hot-start problem has not recurred. For our XJR, if the parts responsible for sending accurate data to the engine ECU are working properly (the temperature sensor, the air filter and the Air Flow Meter...), it would not hurt to replace the IACV, intake manifolds, and the TPS. Among these, only the TPS is more expensive, and you would not trust a used TPS.

 

here is a sample of an issue this morning.
about 3 mins after i left home i stopped at the lights.
while stopped i revved the car a little and noticed the revs go up and then drop then normalise.

so i went to record.
what you see here is acceleration the revs go up then when i release my foot they drop right down and car stalls. It then took me many many many attempts and about 5 minutes to start.
afterwards i drove 20min to work and it ran perfect.

 

I do suspect the iacv is working fine as i have tested it and it does move. At cold start i idle at over 700 and then once warm it idles about 600

 

I had the same experience with my XJR (Rose) and had done all other stuff to rule out fuel supply and ignition issues. Come to think of it, what might cause the engine speed to plunge to zero instead of stabilising at factory idle (BTW, 600 seems a bit low for an auto) if and when an IACV is normal? Would a flat spot in the TPS explain it, or is this an electric electronic issue? I was in total disbelief until the intermittent problem recurred all at a sudden, making me nervous every time I stopped at a traffic light. Unfortunately, my engine died before I had time reluctantly trying to replace the IACV and TPS to test them as a final solution to the intermittent stalling problem.