I was driving my 89 xj40 yesterday holding a steady throttle position and noticed that the revs were going up and down in a small, (100-200 rpm) surge. I know that there can be many causes for this problem but all cars/engines have unique problems. Is there any place/thing I should look at first that may be a common problem for this engine? Any suggestions would be appreciated. Thanks!

 

You may try filling up with non ethanol fuel, it has made a difference in the vehicles we drive..

 

I have thought of that but the only non ethanol gas I can buy is regular, not premium. I have thought of using the regular 100% gas and then adding an octane booster. Have you tried that? Any downside to doing that? Booster brand recommendation? The guy I bought the car from said that he added "Sea Foam" to the gas about every other fill up to keep the car running in top form. I have filled the car up 5x since I have owned it and added a similar product to "Sea Foam". Any experience doing that ? Thanks!

 

I have a station near our home who claims the non ethanol fuel is 90 octane, Maverick has 87. I have run the Sea Foam in my Model A, but I quit using it now that I use the non ethanol fuel. I did use the Marvell stuff as well, I have a old tug that will vapor lock, but for some reason if I add about 2 ounces to 5 gallons the thing will run well.
I haven't run octane booster, cant help you there.. I thought maybe you were sucking air some where. But I will tell you this, I did use a small amount of the Sea Foam when I did fill up with the corn gas at times.. I like the Techron additive as well.

 

Hi dskul1,

A few possibilities come to mind:

1. Is it possible you were traveling at a speed at which the transmission was undecided about the best gear and was shifting up and down?

2. Is your transmission fluid in good condition and right at the HOT mark on the dipstick when checked with the car parked on a level surface after at least a 15 or 20 minute drive, engine running, transmission in Park after moving the lever through all the gears and pausing for a few seconds in each gear? Low, high or old fluid can allow slippage and other transmission problems that can be associated with variations in engine rpm.

3. How long since your last tune up (new plugs, air filter, fuel filter, distributor cap and rotor)?

4. Two common failure points are the Fuel Pressure Regulator at the front end of the fuel rail, and the Fuel Check Valve, which I think on your '89 is in the fuel line just ahead of the filter. The FPR and check valve work together to maintain pressure at the injectors. As they age they no longer hold proper pressure, requiring the fuel pump to work even harder to keep the rail pressurized.

5. Have you checked the Vehicle Condition Monitor for any stored Diagnostic Trouble Codes that might indicate a problem such as an air leak allowing unmetered air to enter the intake somewhere between the Mass Air Flow sensor (MAF) and intake manifold gasket? To check for stored codes, turn the key to postion II (ON or RUN) but do not start the engine. Press and release the VCM button on the trip computer to the right of the steering wheel. Watch the small display below the Speedometer for any message and write down exactly what is displayed. You may see something in the format of FUEL FAIL 28 or FF89.

6. Have you tried driving with the cruise control engaged, and if so, is the surge reduced or enhanced?

Cheers,

Don

 

Thank you for you lengthy reply. I have only owned the car a short while and haven't driven a whole lot. I was told that the car sat for 12+ years, hence the low mileage; 20K. I was told by the previous owner that he changed all fluids, plugs, etc. that needed to be after the long sleep, but I really don't know. All fluids are normal levels and appear to be clean/new. I will have to check out rotor, etc. soon. The digital display on the right side of the dash is only partially operative, and I cannot make out anything that could be of use. Does the car use OBD I or II? The cruise control does not function and I have not determined what the fault is. Thanks again for your useful suggestions and I will be getting to them soon.

 

Your car has the Jaguar (proprietary) version of OBD1 but has a built-in code reader.

To access the fault codes (if there are any), follow point #5 in Don's post just above.

Check for loose or tired vacuum hoses where they connect to the manifold - often these rubber hose push-ons get slack and allow a small amount of unmetered air to enter the manifold, causing a bit of a surge. If you find a sloppy connection, just cut off the end and push on a "fresher" section.


Larry