I very recently purchased a 1998 XJ8 with 64k miles. Previous owner provided documentation for tensioner change out. The problem I have is a rough idle. Driving at speed is great but when sitting in the driveway or the stop sign the motor idles very rough. I travel frequently for work so I have not had time to do a-lot of troubleshooting but have read a lot.

I own a scanner and found no codes present. I should get codes for a misfire right?

I am thinking a vacuum leak. All the hoses I have checked seemed to be in great shape most of them appear to have been replaced shortly before my purchase.

The only other thing I have noticed is that I will get restricted performance with the cruise control system on.

I have read that on 98 model that cruise control is vacuum controlled, or at least partially (not exactly sure). From my understanding there is a hose that runs from near the brake booster to a valve on top of or near the throttle body?

If this is correct would this be a good suspect?

I am asking because as I said earlier I travel a lot for work and will be home in a few days but will only be home for two days. I want to cover as much ground as I can get it fixed this time home?

If the cams were not properly adjusted during the tensioner swap could this cause what I am experiencing?

Any help or reccomendations would be great. and very appreciated.

Thanks so much

 

A very good method to find vacuum leaks is to spray brake cleaner on the vacuum hoses, connections and gaskets, when the engine speeds up you've found the offending part.

Poorly adjusted cams can result in lack of power while it doesn't really account for a rough idle. It's something you'll experience under load.

Is there any difference when the engine is cold? If so I'd suspect the sparkplugs. Doesn't harm to check them anyway.

 

I had that vacuum hose loose after the garage forgot to connect it, after cooler hose replacement.
Got the cruise control message, but no rough idle.
You can see the hose and the fitting where it has to connect on the left side of the TB.
I managed to put it back without removing anything, but it is very narrow and needed a lot of patients.
You might check the sparkplugs, coils and maf sensor as a cause for your idle problem.

 

A vacuum leak will give you high short term fuel trims at idle. Of course you'll need a scanner that will read those and check it while driving a little. You could pick up some CRC MAF cleaner and clean your MAF - when cold (cooled off). No codes is puzzling. I seem to recall seeing something about restricted performance and cruise control. Maybe someone will chime in, or try a search of the archives.

 

I'm a fan of shotgunning vacuum hoses on older cars. Just buy a bunch of length of hose, and go to town. Maybe $30 invested and it can prevent oddball issues, and more importantly, they won't break when you accidentally lean on them or manipulate them.

 

I have a scanner that I used to look at this data but have no idea what I was supposed to see can you explain what I would see if I had a vacuum leak as far as these trims go much appreciated

 

Well, my scanner, an UltraGauge, has the option to show short and long term fuel trims on bank A and bank B. I keep it hooked up to the OBDII port most of the time. The short term fuel trims (STFT) will fluctuate but at idle they will indicate if the ECU is being told the mixture needs to be richened beyond the normal (mapped) amount if there is a vacuum leak. This will show as a positive fuel trim. This is a deep subject, but the first oxygen sensors report the oxygen content coming out of the cylinders to the ECU. So, if you see +10 or greater at idle for the STFT, you probably have a vacuum leak. I wouldn't count out the cam timing though. If the tensioners were just changed and it idles rough, I think it could be the cam timing. One of the first symptoms of a chain jumping a tooth is a rough idle. Just a thought, and that doesn't throw a code. But that's not fun to think about ...

 

cleaning the MAF and throttle body never hurts.
Check for oil on the outside of the spark plugs. The gaskets leak and cause a slight misfire. Engine computer reset sometimes helps as well.

 

I finally got home and have the chance to update the situation

On STFT. My car jumps back and forth between about -3.8 to around 4-5

I also looked at the o2 sensors it jumps between rich and lean on the slider.

I also see an average timing advance of about 10.5 Degrees.

I know how to get the data not exactly sure what to do with it.

Also no codes well other that P1111.

I understand that just tells me all testing requirements have been met.

I am now off to clean sensors look for leaks.

Any information would help especially if you can gather any details from the information I listed.

Thanks

 

Where does the other end of this hose go to i just found it disconnected and laying in there

 

Pretty sure it goes here which goes to to two wire switches marked denso and the green label around the other hoses that connect to it says. Engine management tested ok. Well it goes around the wiring that soes to those switches<br/>

 

That's the vacuum switch for the cruise control. It goes to the solenoids in the right fender well.
The second picture I can't identify, maybe back off a little and get more of the surroundings.

 

Hi Techdowe,

For what it's worth, had same symptoms.

I own a iCarsoft 930 OBDII scanner and I suddenly got a "Restricted Performance" message in display when driving at relatively high speeds in thundering rain. Message came and went, but my OBDII reader did not give any error codes, but a reference to MAF sensor in a module other than engine (security lock module). Cleaned entire air flow line, nothing solved.

To cut a long story short, I finally updated OBDII reader with new firmware, and suddenly it gave a cylinder 8 misfire. Removed coil and found water in plughole due to loose fit on coil cover, letting water in. Dried, cleaned and everything was ok.

Maybe not your problem, but worth checking out?

(I think Cruise control got nothing to do with general erratic idle..)

 

Ok time to update

As to the pictures I got them figured out the first was explained to be from cruise module when I traced it back I found picture 2 there is a tiny pvc tee that was broken and conects between two sitches and this line. Cruise control works great no restricted warning when using it now.

As far as the idle still no codes.
May find a different scanner tomorrow and check again.

Cleaned MAF sensor and throttle body, found "donut" looking gasket that attaches big black hose to throttle body squeezed in any throttle body kinda kinked if you will straightened that out and put back in correctly.

Also cleaned fitting in from of driver's side front of motor little valve stub looking thing attaches to vacuum hose. The orings here were broke replaced those.

Overall idle hhas gotten better but not where it is supposed to be.

This maybe a little strange but if I let it idle and put my hand on the exhaust it feels like there is a skip that is pretty consistent that does not happen on the opposite side. This makes me think codes or not I need to inspect coils and plugs.

Does this make sense?

 

Yupp, refer to post above...;-)

 

Silly question I guess.

I don't know where to find coils and plugs willing to check them if I came find them

 

Download the JTIS manual from the stickies. There are many threads on changing out the coils and a few YouTubes. Run a Google search.

I would bet you still have an air leak in the air flow system from the air filter box to the Throttle Body.

 

For coils / plugs, read this exellent post: (picture over "stolen" from that)

https://www.jaguarforums.com/forum/x...-03-xj8-72827/

For MAF / air flow / throttle cleaning, read the first top posts in the forum for guides, instructions and other. Use google to search, it fequently reverts back to relevant posts on this site.

 

So time for update.

I have done the following.

Cleaned MAF sensor
Cleaned load breather
Cleaned throttle body
Removed and checked spark plugs all looked good

Inspected coils.

Found one that looked bad. Had what appeared to be cakes on oil on the tip underneath the little spring.

I cleaned this. Looked good after no pitting or any thing.

Aafter reconnecting battery and starting felt great for the first few minutes.

Then back to rough idle.

This makes me think a sensor of some sort. As I understand after changing battery the ecu must relearn the sensors?

Any suggestions?

 

You could do a hard reset. Remove the negative cable from the battery and touch it to the positive post for a second. This discharges all the capacitors. It won't burn anything up as long as there is no connection with the negative post of the battery. All your fuel trims will go back to zero. It will cause the car to idle high when you first start it, but it will settle down.
This may not help, but it won't hurt.