How is your exhaust looking? Do you have an cracks in your exhaust manifold? Check for leaks near the 2 downstream sensors, they control the fuel trims on these cars.

 

My scan tool does not show me manifold vac, i could try it with another scan tool when i get my hands on one. Its pretty safe to say there is no more vac leak tho, i checked hose by hose when i was in there found 2 sketchy spots and both was corrected.

 

Exactly what i was thinking, i do have slight crack on the exhaust manifold, do you think i can repair it using muffler cement? it would really suck if i had to take it off and bring it to the muffler shop for welding.

 

It wouldn't. Just pull the line off where the pressure regulator goes and put on a gauge. Princess Auto has them.

Good thoughts on the manifolds being cracked, that is very common on these cars. Not sure if muffler cement would work, I've never had any luck with it. Welding up the manifolds is a specialist thing, cast iron is difficult to weld. The part needs to be preheated, needs a special welding rod, and then peened afterwards. The techniques for it to be a good weld are quite different from welding steel.

I only know because my manifolds are also cracked and I have a good friend who's a Welding Engineer and told me the to procedure to do it right

 

At least you know it's not those things. So did you try a crank position sensor?

 

Update:
After the crushing defeat i had yesterday, it was back to square one. back to the drawing board we go... i spend the night reading through alot of posts and threads. I tried a few different things this morning.

1. Disconnect the secondary air injector ( no change )
2. Swapping the relay behind the RH headlight around ( no change )
3. Inspect the 2 Harness thats right beside it, Upon opening it i see no corrosion, no burnt pins or whatsoever, Cleaned the connections with contact cleaner and made no difference.
4. Cleaned the 2 ground point that was near those harness ( no change )





This afternoon i will try to patch up the cracks on my exhaust manifold with muffler cement to see if it helps, at this point im at a lost... no idea what is wrong..

 

The tach shows 200 rpm while cranking, car turns over everytime but will eventually stall when hot. It leads me to think it cant be the crank sensor? or could it be?

 

Which hose did you pull to test yours? the breather hose on top? or the brake servo?

 

I think it's misleading you. Obviously there is a signal coming from the crank sensor. However if it's a poor signal then the engine will run badly. I have read posts where the car ran badly until the sensor was changed. Yes typically if the car won't start and no rpm when cranking then the crank sensor is broken but that's not the only failure mode. At slow speeds a marginal pulse from the sensor might be enough for the ECU but at faster speeds the pulse width would be a lot less and it might not always trigger the ECU. The ECU might be able to detect the missing pulse and say the sensor is faulty but that's sort of backwards to how it's designed to work. If the ECU knew where the engine was it would not need the crank position sensor. So thinking about this I think it's quite possible the ECU is confused by a poor signal from the CPS.

 

Thank you for the explation! it does make sense thinking about it.Just happens that i have a brand new sensor sitting just incase the one in the car went bad, I will put in the new sensor this afternoon and report back! thank you sir for the knowledge !
If you dont mind me asking, from what you describe the car would stall at high speed/revs due to a slow response from the sensor right? my stall/roughness happens at idle, would a failing sensor cause the issue?

 

I don't have direct experience of this but I worked on electronics for agriculture in the 1990's. I wrote the code for a system which measured the speed of a paddle in a machine pumping grain into a grain store. There was a sensor on the paddle which detected a magnet on each paddle pass. It sounds simple but there are a lot of factors involved in not missing a paddle pass and not counting a double pulse. Sensors don't necessarily provide a clean signal. What might work when turning the machine by hand won't work so well when the whole thing is going at 700 rpm and vibrating and shaking. There was no aspect of synchronization such as firing a spark or injectors after a dwell angle but I can see how reading that sensor correctly would be vital. If the sensor were to send a dirty signal the pulse might be read as 10 pulses if the software were not able to clean it up enough. In fact reading one pulse and not 10 was one of the challenges I solved.

It's hard to say what the exact effect of a poor signal from the CPS would be because there are different ways it could begin to fail and different ways the ECU could try and cope. However if the ECU is taking steps to compensate there maybe a fault code in the OBDII you can read.

 

Thank you again for taking the time to explain things to me, I will go ahead and replace the sensor just to rule out that factor. This car is giving me such hard time, and no code is present.

 

I am finding it really interesting watching your progress. I am really hoping that changing the CPS fixes it first because it's an easy job for you and second because I have been banging on about it so much.

I have seen Jags going really cheap because they don't run right. Potentially bargains for us if it's just this little sensor. I am fortunate that my Jag runs lovely.

 

There was a vacuum fitting on top of the intake manifold, I think it was capped. The fuel pressure regulator has a vacuum line to it, you could always use that. Mine was a VDP, not XJR though.

 

Any news?
I'm betting on the crankshaft position sensor being at the root of this, for no particularly logical reason beyond the fact that the problem has emerged after the car was laid up for a spell. I am struck by the number of threads where a car which was running perfectly before it was laid up either refuses to start, or throws up a seemingly incoherent range of symptoms when it does.
I have been trying to think what could cause there to be an issue when a car is laid up, and one thought I had was that if any corrosion takes hold while the car isnt being used, it could affect the gap between the sensor and the toothed wheel. I understand that the sensors are very sensitive to the gap between them and the toothed wheel, and if this gets out of tolerance, problems will ensue. The act of removing the sensor and replacing it with a new one would restore the gap,and proper running of the engine,but I wonder whether as many sensors are at fault as we think. Touchwood, I haven't experienced this fault, but its frequency is very striking in circumstances where logic would suggest nothing could have happened to the sensor.

 

You did change your >$20 fuel filter just to eliminate it as the problem, right?

Larry

 

Update:

Special Thanks to Wayland and CountyJag, the car is running perfectly after the sensor change. Its been 100+Kms and I can safely say the problem has been eliminated! Sorry for the late update, I was busy with work and i had to run the car to make sure its perfect.

Quick Recap:

-200 rpm while cranking does not mean your CPS is good!
-When the engine heats up, the sensor might produce dirty signal causing the ECM to do weird stuff to compensate for the bad signal
-If your car is stalling at red light/rough idle eventually leading to an stall, if the sensor is never replaced i highly recommend you tackle that first to see if it runs better.

In my case
-Car warm up fine you do notice roughness even when the engine is warming up
-As soon as it warms up it will stumble every few second (Fuel Trim Indicates a Lean Mixture)
-If you keep the engine rev above 2000 rpm for 30 second (Fuel Trim goes back to normal level 0-7% ) as soon as you let go of the gas the car will stumble and dies.
-It would turn over but as soon as it turns over it will die.
-When Unplugging the Coolant Temperature Sensor, the car will turn over and stay running but runs rich as if its cold start (which is perfectly normal because no input from sensor, computer will read the default value which would be -40)
-Removed Everything (Intercooler,Supercharger, Intake Hoses) Took a long time! found 2 spot that looked sketchy and replaced both hose
-Replaced Crank Sensor
-Car Instantly idles better, I notice huge improvement when it comes to fuel trim (10 - 15% while idling, comparing to before where it was 24.2% solid)
-Confirm repair by driving 100kms + just to make sure everything is right.


If anyone in the future exhibit the same symptom I highly suggest you read my post! and double check your sensor first!