I had a problem whereby my 1994 AJ6 4.0 XJS (US spec) had a slight 'miss' or hesitancy around 25-30mph.
I pretty much went through everything and with the help of a local Jaguar specialist tracked the problem to the EGR system. I replaced the EGR valve, the blue Denso solenoid (and O2 sensor) but really it was still there. I found that disconnecting the vacuum tube at the EGR valve removed the 'miss' and the car ran smoothly.
There is a strong vacuum being drawn if you remove the tube from the back of the Denso solenoid that is connected to the intake. I assume the solenoid is working because the other tube connected to it that runs to the EGR isn't drawing a vacuum at idle. As I understand the EGR system, when the car is at full temp and driving the solenoid must get a signal from a sensor (located under the throttle intake area?) which must open the solenoid and allow the vacuum to pull the EGR valve open and burn off the Nox etc. There is another rubber tube T'eed off the intake vacuum take off point, and that disappears through the firewall on the passenger side to ?????
I can't figure out why the miss comes back when the system is properly connected up? Maybe when the car is moving a bit and under load the EGR is opening too early? Perhaps the sensor is bad? Where does that tube disappear to?
Any EGR system experts out there?
Thanks

 

What led the specialist to conclude that there was a fault with the EGR system? Did you have a DTC 37 or 39?

When you changed the EGR valve, did you clean the EGR inlet port to the intake manifold? This gets clogged up with carbon deposits over time which can create problems.

A few things that might help.

The EGR system is not designed to function at idle so you can't test it that way. The ECM has an EGR strategy that requires the following conditions before it will trigger the EGR solenoid.

1. Coolant temp must be above 140F
2. Engine speed must be between 1000-4000 rpm
3. Engine load (from the MAFS) must not be idle or full load
4. Transmission must be in gear

Here is a diagram of the EGR system in the 94

EGR diagram.pdf

There are some tests you can do.

1. With the engine off, check that you have 37-44 Ohms resistance between pins 1 and 2 of the EGR solenoid. If you don't, it may indicate a faulty solenoid.

2. with the engine off but the ignition on, check that you have battery voltage between pin 1 and ground on the solenoid. If not check for a short or open circuit between the EMS relay and the EGR solenoid.

3. Raise the rear of the car onto jackstands start the car and bring it up to temperature. Then put the car in gear and run it up to 1200 rpm or so. When the EGR operates there should be 2 volts on pin 2 of the solenoid. Be VERY careful when you do this and absolutely ensure that the car is safely on strong stands. If the car is an automatic you can have someone just sit in the car with a foot firmly planted on the brake and run the car up to speed in gear.

If any of these faults are present you should get DTC 37 and your CEL on.

Once the EGR valve has operated, the ECM looks for an increase in temperature at the EGR temp sensor at the inlet manifold. If it does not see it or the measured temp is outside the limits, it will flag DTC 39

 

I should have added that one way to check for a blocked pipe is with the engine idling reach under the EGR valve with two hands and push up on the disc. This will let exhaust gases run to the intake manifold. If the inlet port is not blocked the engine should cough and splutter and just about stall. If it doesn't, the pipe is blocked.

 

Allan
Thanks for the detailed info.
Before I took the car in, I had done the following:
New
Spark plugs
Coil
Plug leads
Rotor
Had all fuel injectors cleaned and volumes matched
EGR valveFuel pressure checked

Made no difference

At the shop
Tried another EGR they had on hand
New oxygen sensor
New EGR solenoid
They thought it was the ECM unit. Amazingly Jaguar Heritage had an NOS in stock for about $300 correct for a US spec 5 speed. Fitted it, no difference.
Then they bypassed the EGR system and it stopped the 'miss'. I took the car back after that. I couldn't think of what else to do.
With it idling, if you raise the EGR by hand it does cough and splutter.
I have wondered if it was the EGR temp sensor. With the EGR hose connected and from cold at revs around 1500rpm it 'misses'. Its really noticeable in 3rd gear at about 25-30mph on a light throttle.
When you put your foot down, it gets up and goes, no hestiation. Fuel consumption per the onboard computer averages around 19mpg with a mix of highway and local around town driving. I run the car on premium without ethanol.
So, since then I have driven it with the hose to the EGR disconnected.
I've been at a loss as to what it is.
I will try your electrical tests though.

 

Well it does seem to point in the direction of a wiring fault as replacing the component made no difference. Do the electrical tests for sure, also check the white 20 way engine management connector, it's in the front righthand side of the engine bay. This connector is known for having issues with build up of dirt/grease/moisture and that can cause all kinds of odd performance gremlins.