I had the same problem on my 1998 XJ8. My dealer told me it was the Mass Airflow Meter, which needed replacing to the tune of around $500. After a second opinion at my local Jag mechanic, it turned out to be a hose that was broken on the lower part of the front of the engine. Check the vacuum hoses for breaks or loose fittings.
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Re: restricted performance under load
Some times, it only takes a good cleaning of the Mass Air Flow Meter to get it working again. Also clean out the full air intake and down the throttle body, especially the butterfly flap. There are special cleaners for both the Mass Air Flow Meter, and the Throttle Body. Be sure to use the correct ones as there are special coatings that can be damaged by the wrong cleaners.
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Your tires as they wear will affect milage. Maybe an alignment check is do, All it takes is a couple PSI difference in the tire to loose a few miles per gallon.
A faulty tire guage may be decieving you.
Maybe you have different tires.
Maybe your need a good fuel injection service. Partially clogged injectors can decrease milage because fuel need to spray and become vapor to burn.
A restricted fuel injector will spit instead of spray.
You can do a search on problems with clogged injectors , and learn a little on it.
If only under a load, it is possible that the fuel filter is mostly clogged. Also check air cleaner (you probably already did). The concept is that under a load, your throttle body is mostly open, requiring high volume of air and fuel flow. Restrictions in either of these will make it run bad.
I had the same problem. Restricted performance and running rough at 3000 RPM. I have an OBDII code reader and found that the problem was in the air mass flow sensor, which is just after the air filter. I pulled this out and found it was dirty and had butterfly wings in it. The last shop where I had the oil changed replaced the filter for me but put in the wrong size. Close but no cigar. Bought some *** Flow Sensor cleaner at auto store and fixed the problem. Get an OBDII reader with the ability to erase the codes. Comes in real handy.
If the vehicle runs normally otherwise, your fuel filter is likely not the problem. A plugged fuel filter would likely cause misfiring under load, such as when going uphill.
Make sure, by the way, that you do not touch the gas pedal when starting. That could make it hard to start.
Old fashioned cars used a choke to feed it more fuel for a cold start. Modern cars use a computer to feed the fuel injectors more fuel on a cold start, and rely on a sensor or two to know that it's cold.
Couple of possibilities here:
a) Has the check engine light been on? If you have a bad sensor, such as oxygen, throttle position, or coolant temp sensor, the computer may have an error code stored in it's memory about this.
b) How many miles on the vehicle? Some other problems could be leaky injectors, leaky injector 0-rings, or intake manifold air leaks, all making it difficult to start when cold.
A fuel pressure gauge on the fuel rail would tell you if a leaky injector is causing the fuel pressure to drop....then a cold start would be difficult if there is no pressure in the system.
If there is a leaky o-ring or intake manifold, the vehicle will typically misfire and/or light a check engine light.
The following are malfunctions that could cause a rich condition:
^ Fuel pressure too high. Perform fuel pressure test.
^ Leaking fuel injector(s). - Perform pressure leak down test.
^ Vacuum leak around the MAP sensor grommet.
^ Oxygen sensor contamination.
^ Restricted exhaust or air intake.
^ Fuel Vapor Canister saturated with fuel.
IMPORTANT: If the front oxygen reading voltage is above 600 mV and the rear HO2S is above 600 mV all the time, the O2 sensors are probably not
It's just going to take some time and simple cleaning. If there is an Auto Zone around your area, you can buy all the things needed to get the car running 100% again in no time at all for under $40.00. Here is a few things that I do when i get a new car with a few years under it's wheels. Buy a bottle of injector cleaner for the fuel injectors, like STP, a bottle of Marvel Mystery Oil to add to the engine oil, and two can's of Throttle body cleaner for the throttle body and follow the instructions on the can. Make sure it's throttle body cleaner. The Carburetor cleaner is to corrosive for the throttle body gasket. Once you have reached 3,000 miles, service the engine and have them replace the oil (5W-30) and filter and add another Marvel Mystery Oil. This will clean out all the carbon and sludge in the engine to free up the oil passage ways, replace the Air filter, and PVC valve every 30,000 miles or once a year. Cleaning out the throttle body and adding the fuel injector cleaner will also help the fuel system and have them replace the fuel filter every 30,000 miles or once a year to ensure there is no fuel restrictions in the fuel system. Once this has been done you will notice the engine runs like new and you may have to use the Marvel Mystery Oil and fuel injector cleaner every 6 months to keep the internal engine free from sludge and carbon. Good luck and keep me posted, be glad to help you keep the car running 100% for a long time.
Have exactly the same issue on 04 jag x type 2.5. So far have replaced fuel filter, air filter k&n, plugs ngk. Codes cleared, same issue. Seems to appear when air temp is cold and eng is warm. Am replacing upstream O2 sensors and cleaning MAF next. Will keep you posted if it clears up. Did you ever find the cause?
a partial plugged cataylic convertor would case this. and under a load like towing a boat should have thrown a code. a exhaust back pressure test would show this. but normal driving would not throw a code.