A few years ago I had to get a new engine put into my 1995 Vanden plas. The engine had to be put in because of scarring on the engine walls. The engine I had put in came from a x300 that had 23K miles. Ever since I got the new engine the car idled very high (up to 1400 RPM) and would surge up and down. I took it back to the mechanic that did the work but it never really got resolved as I ended up moving in that time (this was about a year ago). Just a few weeks ago I thought I would try to tackle figuring out why it was idling so high. I took off the TB and noticed that the idle control valve was completely covered in gunk. I wiped the senor clean and reinstalled. The car was idle smoothly at about 700 RPMs. I drove the car for 2 weeks and it was driving great.

Last night I went to start the car after a week of it sitting. The car started but hesitated and died. It started right back up but was idling around 600 RPM. I drove the car about a mile and as I went to take a very sharp turn the car died. This is not an easy car to turn without power steering in a sharp turn but was able to keep it out of oncoming traffic. The car started right back up but the transmission failure light was on and it was clear the car was in limp home mode. I got the car home turned the car off and back on and the transmission light is off but I am really nervous about taking it out. Any one have suggestions on where to start in diagnosing the issue/common causes? Cannot afford to put much money into the car so I would like to try to work on it myself if possible but does anyone have a recommendation of a decent mechanic in the Portland Or. area? Thank you!

 

I had a similar issue, with my car dying on me at stop signs, red lights and slow sped turns. If the car dies while rolling it will throw a transmission-related code, because the transmission computer gets totally confused when the engine is not running but the car is moving. These codes may sound scary, but chances are there is nothing wrong with your transmission. The 4HP24 is a very robust transmission, and if maintained well it will last a long time.

You need to figure out why your car stalls. 650-700 RPM is normal idle speed for these cars. In my case the cause of the stalling turned out to be the igniton coils. The ECUs are notoriously bad at detecting ignition coil problems in these cars. My Jag went through 2 sets of cheap coils before the proper coils were installed that finally cured the idle issue. The coils are not the only possible reason for stalling at idle. It could be due to Idle Air Control Valve, bad throttle potentiometer (throttle position sensor) and EGR valve gunked up/sticking.

 

Thanks for the reply Juha_teuvonnen. I figured that was likely the case that the transmission light was caused from the stall. I just do not know where to begin on this. I usually let the check engine light guide me to the problem but with out a CEL Im lost where to start and I dont want to go replacing things without some direction. I guess I will start with cleaning the EGR valve then test the coils and go from there.

 

Chavid,
You will start with the spark plugs. Replace them. Use the basic Champion RC12YC plugs or equivalent. Your symptoms are EXACTLY what happens when the plugs are worn out.

That will also give you a chance to inspect the coils and boots, plug wells, and coil wires for trouble.

It is also a very good idea to clean the throttle body as you have done (including the IACV) and the EGR valve as you plan to.

I concur with Juha that the transmission limp-light is due to the stall, and most likely not a fault with the transmission. This is a known common occurrence with stalls.

I noticed recently that the owner's manual service interval for spark plugs is 20,000 miles. They weren't kidding around!

Lugging/stalling happened to me again around Jan/Feb this year. A new set of plugs cured it. Turns out the old ones had about 18,500 miles on them. I also faced the same problem in 2013, and new plugs cured it.

Then your Jag will not try to kill you, or anyone else for that matter. I believe the risk would be greater for "anyone else" because the XJ6 is built like a tank!

 

Got a set of spark plugs and a fuel filter and plan to install this week. I started the car today so the battery wouldnt die and I noticed the the exhaust sound had a deeper pitch to it than regular (out of both pipes) and a bit of a rattle. If I remember correctly, a vacuum leak can cause a deeper sound in the exhaust. Am I right to think this might be a vacuum issue? How would I go about diagnosing a vacuum leak? Any advice is appreciated!

 

Many years ago, when I was taught by a certain German car manufacturer how to diagnose their cars, I was told to eliminate ignition issues first, then look at fuel/vacuum. I would recommend doing the plugs first.

An easy way to diagnose a vacuum leak is to spray WD-40 in the vicinity of the suspected leak. If the engine RPMs go higher - you have found your vacuum leak. Keep a fire extinguisher handy when you do it, don't ask me how I know this

 

I finally got around to changing out the plugs and the ones that came out did not look too bad. I put my ear right by the throttle body and I can hear a hissing sounds. Makes me think it is likely a leak. I do not think it is coming from the throttle body (I used a small amount of gasket maker on when installing). Does anyone have a vacuum hose map? Any ideas on how to figure out which hose might be causing the issue? How much do the hoses run and how hard are they to replace (are they all pretty accessible)? Should I just play it safe and replace all of the hoses (if they are cheap and easy to replace that is)? Any help is appreciated.

 

Ditto SlickJag. I had this problem for a couple of years. New plugs, Japthug coils and resealing the cam cover to stop oil from the plug wells. After that it never happened again. And the trans light ....ignore it.