Hello, I recently picked up a 98' XJR with about 160k miles on it. The car was very fast for the first month I had it until one day the head gasket gave out and had to be replaced.

Now that the car is running again, when I stomp the pedal to the floor in standard mode the car just will not snap into acceleration like it used to, it seems to have lost a lot of power. I put it in sport mode and it seems to be much faster, like there is a much larger difference between the two modes than there was previously.. it's almost like in standard mode when you punch it, the car won't drop down into a lower gear and take off.. it just slowly ramps up in the current gear.

What can be done to put the car back the way it was, if anything, and is there any reason why it would be running slower now that it was before?

By the way the head gasket had worn out over time, it was rusted through in a couple of places and let coolant into the engine block and steam out the exhaust. I had to get all of the valves and headers resurfaced as well.

Thanks for any suggestions.

 

did they remove the throttle body or mess with it during the head gasket activities? It's possible...search on here how to "tighten" the throttle cable on the throttle body.

Over time it can get kinda loose. you can see if it's loose by using an OBDII reader and seeing, with the engine off but key to the accessory position, if the TPS reads 100%

if not you get to the throttle body, cut the zip tie, twist the square nut until the steel cable throttle on the throttle body is tight, and then reinstall a new zip tie

hope this helps

 

Thank you, I will take it down to the shop and plug into the OBDII reader soon. Am I understanding that you suggest putting the gas pedal to the floor with the engine off and read the TPS (throttle position sensor?) to ensure it also reads 100% with the pedal down? If not maybe I need a bit more of an explanation.

I appreciate the help very much. Is there anything else that could set the car to not downshift into a lower gear when the pedal is floored in standard touring mode?

Thank you,

 

This is correct, get it as close to 100% against the kickdown switch.

I'd check for codes, some O2 sensor faults reign in the power - what's the fuel pressure at? might have one of the pumps failing as a long shot. These don't post codes but leave the car with a lack of power that's hard to chase up.

 

Thanks for clarifying that Sean, and I will also check into the O2 sensors and fuel pressure when I can get the car over to an OBDII reader.

 

I wonder if your transmission needs to learn your driving style again. I have heard of a way to reset the Mercedes transmission. Search the forum, it's out there somewhere.

 

Thanks ATV, I will search for that too. I have read lines here and there about how this car 'adjusts' itself but I've never gotten any detailed information about that either and am very interested to know.

 

The trans from base to learnt isn't a factor, the difference is hardly noticeable what is, is a lack of power. Pull the codes.

 

Thanks Sean

 

On mine, the kickdown switch was intermittent and it really affected the behaviour of the car. It shifted around 4500 rpm in normal mode. You can disconnect the switch behind the dash and check it with a meter.

 

... and thank you again ATV.

 

OK so a few more things to report. I took the car out tonight and drove it gently until it was warmed up and then punched it once in sport mode and it seemed to be OK.

However, once I got on the freeway and hit 70 I tried to punch it again while still in sport mode and the car jerked back like it was ready to take off, but instead of doing that it popped once and then again, like it couldn't make up it's mind about taking off or not. There were actually two good jolts within the span of about 4 seconds, so I laid off the gas and just cruised.

About 10 miles down the road suddenly the amber light comes on at the bottom of the MPH gauge, it briefly reads 'reduced performance mode' and suddenly the temp gauge has dropped all the way to nothing, this is while the car is running and had been running for many miles.

I pulled over and turned off the car. After about a minute I tried starting it again and the amber light was off but there was an 'ER' reading on the thermostat for the AC system and the engine was sputtering when I gently drove it around the parking lot I had pulled into. The temp gauge was still at the bottom, so I turned the car off again.

This time I waited about 5 minutes then restarted the car and the temp gauge went to halfway and the car seemed ready to be driven home gently, which it was. Along the way the temp gauge was bouncing around between halfway and 1/4.

When I returned home I popped the hood and I could clearly hear the sound of boiling liquid coming from the engine block/intercoolers area. The sound also seemed to be coming from the cooling reservoir tank. Forwhatever reason the engine was extremely hot but the car's own cooling fans didn't engage. I pointed my own box fan on the engine and let it run for a couple of hours to cool it. I have a mechanic coming up tomorrow with his full OBDII kit to take some readings, but given these latest events does anyone have any more possibilities of what could be going wrong or more things we could look for with the OBDII kit in addition to what's already been suggested?

Thank you!

 

One more thing of note.. last night I topped off the coolant reservoir, literally filled it to the top. Tonight after the engine has mostly cooled down (it's still warm 4.5 hours later) and having the car parked in the same spot it was last night, the coolant level has dropped to maybe 1/4 tank full in the reservoir.

I didn't see any coolant leaking out so... I'm now worried that the new gasket might be leaking coolant back into the engine somehow.

 

this link is the same trans as you have - I suggest you disconnect your battery for about ten minutes prior, then after following these instructions, drive like a maniac for a half hour.

 

Thanks for that YouTube embed. Right now the car is running OK but it still may need the trannie to be dialed in.

Turns out the primary problem was the thermostat sensor being broken. The mechanic read the codes and that's what came up. He put a thermostat sensor from another Jaguar he had on his lot as a temporary fix until a new one arrives in his shop this week at which point we will swap it out.

Between that and correctly crossing the coolant hoses from the expansion tank, the car is now driving much better and doesn't seem to overheat as much or have to kick the fans on as much (or as high an RPM) on engine shutdown.

This fix was all done in my garage and using the mechanic's limited portable OBDII reader. He's got a rather large one in a big metal rack with a monitor built in back at the shop which we will use to read the rest of the diagnostics when I drive the car to his shop this week. I'll report back with what we find.

 

dc, I'm guessing the procedure is the same for the Jag as the Benz in that video?

 

I believe it is the same as Merc.
This W5a-580 trans is fitted to so many different vehicles and known by a multitude of different part numbers. This is the method that is used to program the same trans (known as NAG1) in the Chrysler C300 - I did it to my vehicles, - hard to tell with me, I drive like a maniac, I did have to write the sequence down, I got in the car and forgot what to do. Also your Trans electronic controller TCM is the same seimens as the Merc. Good luck