Hello,

For the past week, my 1996 XJ6 had been cranking, firing, but was struggling to maintain ~800 RPM and would rapidly dive toward 0 and then turn off unless I slammed on the gas, and eventually I would force it to achieve a constant 800 RPM after a minute or so, at which point it would drive perfectly fine until the next time I had to start it up. I removed the air filter, which seemed to fix it temporarily (I only drove it twice without one) and made me think that it was just not breathing properly. I tried to start it yesterday, expecting the same problem, but now it's just cranking without actually firing... without changing anything from yesterday, I tried to fire it up again this morning and it fired a single time then immediately dove toward 0, shut off, and now I'm reliably having the same issues again.

Thus far, I've tried:

1. Removing the air filter
2. Removing the hose from the air intake tube to the engine block
3. Unscrewing the engine gas intake pipe to test if gas is coming out when I press on it... it is and the pressure seems fine, which leads me to believe that the fuel pump is ok
4. Checking all fuses related to starting, they seem fine
5. Checking battery voltage while stationary and while cranking, both normal
6. Checking crankshaft position sensor, I'm cranking at ~250-300 RPM which, from my understanding, means it's fine
7. Locking and unlocking the car to prevent security system issues
8. Cleaning the mass air flow sensor
9. Topping up all fluids

I am mid-way through getting the fuel filter off in order to check if it's clogged, but considering the fuel pressure seems fine up in the engine, I'm not hopeful that that's the issue. I have not checked anything to do with spark plugs yet, but I doubt that they would be the issue given that it isn't a sporadic firing pattern and it would be bizarre if they all suddenly stopped working all at once. The riddle is trying to figure out what the stability issue was, why it able to be solved by giving it gas, and how that issue could have gotten so bad to the point that the engine is now refusing to fire altogether. I feel like it has to be fuel or air related, but I'm honestly pretty new to cars so I don't really know.

Any input is greatly appreciated.

 

When you mentioned removing the air filter completely seemed to help, I wonder if your MAF is having problems.

I would first check the condition of the ECU. Unplug it and see if there is any corrosion on the pins.

I think your CPS is indeed showing that it seems to be working.

Normally I’d look at the 4 sensors required for closed loop 1. TPS 2. Oxygen Sensors 3. MAF 4. Coolant Temp Sensor, but I’m not sure your car is getting there.

 

I had very similar issues with a failed Coolant Temperature sensor. Which was a relatively simple fix.
It also might be worth checking the Intake Air Temperature sensor as well.
Both can be tested with a multimeter for resistance.
I believe if you disconnect them then ECU should use a default reading, so maybe try starting the engine without the suspect sensor.

 

A scan for codes and a look at the live data might be an idea. Dud coolant temperature switches show up with improbable readings, for example.

 

I'm very glad this thread has appeared and will follow very closely. My '97 X300 completely died on the Motorway last Thursday and I have checked as per existing posts suggest,
But nada di nada, cranks but will only just fire up if MAF is disconnected. Mobile Autoelectrician is also confused....

 

Have you tried swapping out the MAF?

 

That's a $20 part and a five minute job. If we're gonna take a stab at anything, I would recommend we start here.

It's hard to really pinpoint the issue without being there, but I'd bet that CTS is original to the car, so it's probably time anyways.

The Intake Air Temp would not prevent the car from starting, it's simply ignored until the car goes into open loop.

 

Yes, made no difference

 

For those with similar problems, I seem to have fixed it by:

1. Jumping the fuel pump relay (I believe it was slots 30 and 87) with a wire to get the fuel pump visibly and audibly pumping (the pump itself was definitely not the issue)
2. Letting it pump for ~20 seconds
3. Cracking the fuel intake line nut in the engine to remove any air and check for adequate pressure
4. Spraying engine starter into the carburetor
5. Starting the engine and revving until the rpm leveled out
6. Swapping the fuel pump relay with another identical relay in the back
7. Verifying that the pump was now pumping on its own, without the need of a jumper wire

Over the course of my testing, I noticed that the relay output line wasn't closing despite the switch line having 12v of power, and that was what lead me to believe it wasn't pump-related. I don't know if the fuel pump relay is activated intermittently or how that whole thing works, but it seems like swapping the relay has fixed the heart of the issue... it's now idling a bit abnormally and the starting feels a little weaker than usual, but it is running seemingly consistently at this point. Tomorrow I will be topping up all of my fluids and going on a test ride, and I may or may not look for a mechanic to do a check-up for me.

I checked the relay against the relay used for the horn to make sure it was operating as expected, and it seemed to perform fine there. Does anyone have any insight as to why both relays might work fine for the horn, but not for the pump?