Hello,

I've been trying to track down an issue with my '92 XJ6 for over a year now.

The most noticeable problem is stumbling upon deceleration; for example, when closing the throttle to idle when a car pulls out in front of you at slow speeds (under 25 mph). The engine will misfire and stumble, and my volt gauge drops a bit until the engine evens out.

Also, when coasting down a long descent at idle, doing less than about 50mph, the engine will surge rhythmically, the RPM bouncing up and down about 300RPM until I open the throttle, even just a little, which causes the engine to smooth right out.

Also, at idle with the A/C off and at operating temperature, my idle speed fluctuates rhythmically from about 675 - 750 RPM, back and forth.

You can watch the two videos that I've attached. The first is of the RPMs surging when driving down a descent at idle. You can see that after I accelerate, then let off the gas, the RPM bounces around.


The second is of the idle and oxygen sensor feedback waveforms at idle, with the A/C off. Notice how perfect the pattern is...


-Nick

 

So this is what I've done:

-All preventive maintenance items are replaced (air and fuel filters)
-Throttle Body has been cleaned
-All Intake Plumbing has been cleaned and tightened (I did insert a short piece of copper pipe inside a collapsed nipple on the intake elbow)
-The MAF sensor has been cleaned
-TPS has been adjusted using PDU to exact specifications.
-Throttle plate gap has been set
-Idle Air Control Valve has been set using the PDU
-All 6 Fuel Injectors have been replaced
-Spark Plugs, Wires, Cap and Rotor have been replaced.

Now, it bears mentioning that I've had an issue with the adjustment of the idle speed control valve. Even with the valve manually adjusted closed completely I wasn't able to get the idle speed down, so I adjusted the throttle plate closed to get the idle speed correct.

I've looked for causes of excess air being introduced, but I have yet to find any cause.
What would some possible causes be?

Thank You,
Nick

 

Hi Nick,

It's late and I'm tired, so please forgive me if any of the following initial thoughts are just silly:

1. Have you removed the IACV assembly, disconnected the stepper motor from the valve body and cleaned all the carbon inside? I'm not sure if the valve on your '92 is the same as on our '93, but the principle should be the same:

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2. Have you checked your transmission fluid? Low fluid can cause stalling when decellerating or coming to a stop, which could exacerbate other issues related to idle. Jaguar doesn't explain why this can happen, but my assumption is that the torque converter does not release properly, bogging down the engine.

3. One common point of an air leak is the joint between the large plastic elbow and the corrugated hose that leads to the TB. If the hose clamp is overtightened, the elbow can partially collapse, creating a gap on the underside that can't be seen from above.

4. Other places to look for air leaks are in the vacuum hoses; crankcase breather hoses; the intake manifold gasket (the torque spec is fairly low for the nuts and hex-head screws that secure the manifold, and it's not uncommon for them to work a little loose), and, less common, the gaskets on the six elbows of the intake manifold can develop leaks; if one of your breather hoses attaches to the oil fill tube, the O-ring where the bottom of the tube connects to the block/sump is a potential air leak point; if your car has the coolant-heated restrictor on top of the intake manifold, you might check its gasket. Misting around those areas with starting fluid or a similarly-flammable spray, you may get lucky and cause the engine rpm to increase.

5. I don't think this would contribute to your symptoms, but when you cleaned the TB, did you also clean all the carbon out of the EGR port on the underside of the intake manifold and check the seal of its gasket? Have you checked your EGR valve for proper operation?

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6. Another possibility that comes to mind is the Fuel Pressure Regulator (FPR). When the diaphragm fails, raw, unmetered fuel is inhaled into the intake manifold through the vacuum line fitting on the FPR, causing rich running that the O2 sensor sees but the ECM can't correct. You can check for this by pulling the FPR vacuum line, starting the engine and watching for wet fuel at the vacuum line fitting.

7. Fuel pressure? If the FPR has lost some or all of its ability to hold pressure in the rail, and especially if the fuel check valve in the fuel pump canister has weakened or failed, the fuel pressure in the rail may vary more than it is supposed to as the pump works to maintain pressure without the resistance of the FPR and the back-flow prevention of the check valve.

8. You've got a new distributor cap and rotor, is there any chance there is play in the distributor shaft?

9. The rhythmic nature of your rpm rise and fall could possibly indicate an ECM function, perhaps in response to a sensor input that is prompting fueling or timing adjustments. CTS? CPS? Can you see those signals on our PDU?

10. It can't hurt to clean the grounds used by the ECM - the two on the intake manifold studs and the one on the firewall right behind the cylinder head.

11. A couple of tests that might be helpful are a fuel pressure test and pressure leakdown test after shutoff, and a compression test on the cylinders.

12. One other question that comes to mind. Are you using the original type Champion plugs or a more modern plug? Some idle issues have been resolved by switching back to the OE Champions (RC9YC I think?).

Sorry if these ideas are all harebrained. I can't wait to see what you figure out.

Cheers,

Don

 

I can't thank you enough for all your guidance Don, you've helped me through countless issues in the past, and I appreciate it so much

I'll try to address these one by one.

1. IACV. I have taken the IACV housing off and cleaned it all out, and I've actually replaced the stepper motor with a brand new one. (I believe it was for a Renault, but was identical). I even purchased a new gasket (between the IACV housing and the intake) from the dealer.

2. Transmission Fluid - I saw that TSB too, and the fluid is fresh and new and at the correct level.

3. Throttle Elbow - This one is good. I haven't checked the seam on the elbow and I usually crank the hose clamps down very tightly... I need to check and see if this is an issue.

4. Intake Gasket - I did have the intake off to remove and replace the coolant hose underneath it. The green foam gasket looked very good, so I didn't worry to much about any leaks. IIRC, I had the stumbling issue even before removing the intake. I didn't check the torque on the bolts when reinstalling it, so I really should replace the gasket with new and torque the bolts to spec.

5. EGR - I have a long history with bad EGR on this car. It started when the original EGR failed to open (torn diaphragm). I ordered a new valve and gasket from a Jag dealer online. It was faulty and was allowing EGR gasses to seep past the valve, causing a misfire at idle... I sent it back and ordered another EGR valve, and I don't *believe* I have improper EGR valve flow, but I haven't ever checked either.

Also, I have cleaned the throttle body and a little inside the intake, but I've never been able to remove the EGR crossover pipe, which limits my access to the intake. I now have 2 sets of crowfoot wrenches, so I might have better luck at getting the intake off for a thorough cleaning now.

6 & 7. Fuel Pressure - I have pulled the vacuum hose from the FPR, and haven't noticed anything, but I guess a more in-depth check is necessary. I have also not been able to check the fuel pressure as I don't have the correct adapter for the car.

8. Distributor - The distributor shaft is tight and without any play.

9. ECM Sensors - The PDU can directly see the CTS over the serial line, and that's correct. The CPS can be seen by conecting the VIA to the PDU, using a breakout-style cable that connects between the VIA and the ECM. I *think* I have the correct cable, but I need to check to be sure. That would allow me to see the waveform for each sensor.

10. I've cleaned the grounds at the bulkhead and near the fuel rail, but I'm sure there are many more and will look into cleaning them.

11. Fuel Pressure Test and Compression Tests - Agreed!

12. Spark plugs are OE Champions. Valve cover gasket and O-seals have been replaced due to oil in spark plug wells when I purchased the car. No more oil seepage now.

It should be noted that my great aunt who owned the car before me have to have some pretty extensive engine work done. There was apparently a burnt exhaust valve that was replaced at the dealership. I don't remember seeing any other repair work to fix the CAUSE of the burnt valve, as there must have been a cause... Not sure if this helps anything?

Thanks again,
Nick

 

So I quickly checked the service records:

April 1, 1999, 38,957 miles:
-Exhaust Valve Replacement
-Head Gasket and Bolt Replacement
-Engine Oil and Filter Replacement
-Spark Plugs Replacement
-Brake Fluid Flush

$2,648.05

I don't have the actual service invoice in my hands. I'll have to check and see what the customer complaint was (might have been rough idle if I remember correctly).

There's no history of any other work before that that would have led up to valve failure.

The Jaguar "B" Service (Engine Oil Change, ATF & Filter Change, Coolant Change, Air and Fuel Filter & Spark Plug Replacement) was the only service before that major one, done at 29,400 miles in November 1996.

The interesting thing is that the dealer mechanic who did the valve work on the car is my current Jag Indy. He left the dealer and opened up his own place. Maybe he could shed some light on things...

-Nick

 

Definitely worth inspection - the air intake clamps don't have to be too tight (neither do clamps on most coolant hoses - I overtightened them for years).


No rush to replace the gasket, but do check the torque on the nuts and screws.


It's certainly possible that EGR issues could be contributing to your symptoms.


If you have a vacuum pump with a gauge, you can check the FPR to at least see if it will hold a small vacuum.


Check the schematics for grounds associated with Engine Management, sensors, ignition, fueling, etc. And it wouldn't hurt to clean the engine ground strap since it is the path for the ECM grounds on the intake manifold, the spark plugs, etc.


I've never done either one on our car, but in your case maybe starting with compression tests would be a good idea given the history of the burned valve.


[QUOTE]It should be noted that my great aunt who owned the car before me have to have some pretty extensive engine work done. There was apparently a burnt exhaust valve that was replaced at the dealership. I don't remember seeing any other repair work to fix the CAUSE of the burnt valve, as there must have been a cause... Not sure if this helps anything?[QUOTE]

A couple of known causes of valve burn on the XJ40 are incorrect valve clearance or carbon deposits on the valve that prevent it from fully seating, allowing hot combustion gases to be forced past the valve, eroding the edge of the valve over time so that it can no longer seal. A burnt valve can definitely cause rough idle and misfires - I hope you're not dealing with that again.

You've definitely done lots of the right things, so you're bound to narrow this down sooner rather than later. Hopefully another member who has had similar issues and resolved them will chime in with the right ideas.

Cheers,

Don

 

As unlikely as it seems that he might recall your particular car, at the very least he might be able to give you some things to check related to your current symptoms. Please let us know. I love/hate a mystery and am always glad to see it solved.

Cheers,

Don