My 2002 S-type V8 with 117,000 miles is doing something strange. First, when I am going 45mph on a flat road and take my foot off the gas, it keeps going 45 and maintains 1500 rpm. If I go up hill or down hill, the car will slow or go faster - as expected. When I slow down for a stop light, it feels as though the car wants to continue at speed, but when I am just about slowed down, and the rpm gets to 1000, it then bumps momentarily up to 1200 and finally when I stop it is at about 650. Not sure if this is a fuel injection issue or transmission. Would really appreciate inputs - want to get my S-type back to its wonderful "normal." Thanks in advance.

 

Look at fuel trims. Could well be an air leak.

 

Hard to say with very little information.
Have you checked for codes?
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Good point - no check engine light, but will hook up a OBD-II and see what might be lurking.

 

Thanks for the suggestion. Will hook up my OBD-II and see what it might show with respect to fuel trims. This is a new measurement for me. From what I read, short term fuel trims for both bank should be in the single digits. Let's see what I find. Will look forward to sharing with you and asking for your further thoughts.

 

LTFT matter more than STFT but do them all.

 

OBD-II shows no pending codes. short term trims for bank 1 sensor 1 at idle: -10.90 min, -0.6 mean, 8.6 max, for bank 2 sensor 1 at idle: -10.9 min, -0.4 mean, 10.9 max. Short term trims for bank 1 and bank 2 sensors 2 are both at 99.2. Not sure if this is a problem or if there are no sensor 2's.


O2 voltage for bank 1 sensor 1 is 0 min, 0.5 mean and 1 max, for bank 2 sensor 1 is 0.1 min, 0.4 mean and 1 max.


Does this information offer any insights? Again, thanks in advance for your help.

 

OBD-II shows no pending codes. short term trims for bank 1 sensor 1 at idle: -10.90 min, -0.6 mean, 8.6 max, for bank 2 sensor 1 at idle: -10.9 min, -0.4 mean, 10.9 max. Short term trims for bank 1 and bank 2 sensors 2 are both at 99.2. Not sure if this is a problem or if there are no sensor 2's.


O2 voltage for bank 1 sensor 1 is 0 min, 0.5 mean and 1 max, for bank 2 sensor 1 is 0.1 min, 0.4 mean and 1 max.


Does this information offer any insights? Again, thanks in advance for your help.

 

See post #6

 

Appreciate your request for more info. STFT Bank 1: -10.9 min, -0.6 mean, 8.6 max. LTFT Bank 1: 0.8 min, 4.8 mean, 7.8 max. SFTF Bank 2: -14.8 min, -0.3 mean, 7 max. LTFT Bank 2: 0.8 min, 3.4 mean, 6.2 max.

 

Even though you don't have any codes I don't like the -10 or -14 trims.

Can you monitor the fuel trims real time and then spray carb cleaner at your vacuum hoses and gaskets? The trims will go nuts if you have a leak and the cleaner gets pulled in the engine.

I think you do have a vacuum leak but might be so small the computer does not flag it?

Have you checked the plastic tubing on your intake system for holes? Several guys have looked and found holes worn in the bellows and other places.
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I do appreciate your continued suggestions. Will definitely do as you say this weekend and let you know what I find. Again - thanks!

 

Ideally you'd watch the trims as the engine speed up occurs.

It's most odd and appears unique.

Although problems like this are usually an air leak, it might be a sticking actuator (EGR?) or what tends to be called a "lazy" O2 sensor - meaning it's slow to respond and needs replacing.

I can't promise it's not the trans or injectors but I am pretty sure not.

Almost all the time, the PCM is reading sensors and relying on them to be almost accurate then controlling actuators (injectors, EGR, EVAP you name 'em!). It can compensate a bit for aged things but only so much. A diagram of the typical "modern" (last 30 or so years) engine www.onboarddiagnostics.com/page02.htm

The CL (closed loop) control is the PCM reading sensors and setting actuators many times a second.

Now, if we didn't mind more cost, weight, complexity etc there would be even more sensors so faults could be discovered even better than they are now - but of course such things as cost etc do matter. So, some issues are not flagged as faults and I think yours is one.

 

Thanks again for your input. Update: I used the carb cleaner spray on all the hoses and connectors that I could find and could not get any change to occur in engine idle speed. Then I took the Jag on a short drive and noted that the speedup when coming to a stop seems to be somewhat less pronounced. I have been running fuel injector cleaner in the gas tank for about 200 miles - not sure if this is having any effect.


When I returned, I took some scans of key measurement while the Jag was idling: STFT bank 1 jumping between -2.3 and +2.3, LTFT steady at 2.3; STFT bank 2 jumping between -3.5 and +3.5, LTFT steady at 2.3; Mass Flow steady at 0.6; O2 Bank 1 Sensor 1 jumping between 0.1 and 0.9, Bank 1 Sensor 2 coming down from 0.8 to 0.2, Bank 1 Sensor 1 jumping between 0.1 and 0.9 and Bank 2 Sensor 2 was jumping around before coming to idle but then steady at 0.75.


does this data suggesting anything unusual, perhaps O2 sensor on Bank 2 Sensor 2?


Your suggestions on monitoring scans during the speedup just before coming to a stop is good - will try to do it tomorrow.


Really appreciate your continued interest.

 

Just thought of something. Before this happened I replaced an ignition coil and cleaned the throttle body while I was in there. Could the condition I am experiencing be due to "idle air relearn" and I just have to wait a bit longer for the "learning" to take effect?

 

Cleaning the TB is risky. It's a high precision device with a special coating.

Engine idle changes with carb cleaner - OK for big leaks, not so good for small ones. You'd need to watch STFTs.

You have the older-tech O2s. The upstreams (sensor 1s) should keep cycling if they're hot enough. (The later cars have current-sensing ones of a newer tech and don't cycle.)

It usually relearns quickly but you can force another from-scratch relearn if you wish by a hard reset. Then let the engine idle as per the initial pages of the codes PDF (in JTIS). You don't have to follow the drive cycle, but it speeds things up if you wish to.

High-revving need not be an air leak (but commonly is). It might be a TB that's unhappy. Try not to make it worse if it is

 

Appreciate your continued comments. About the hard reset - is there a bespoke method? I've read some that have questionable steps - like jabbing the throttle 7 times in 20 seconds, then turning the ignition key on and off 4 times, etc. Waiting for someone to add "swing a chicken around your head 3 times." Just kidding. But your guidance is appreciated.

 

The numbers you posted look fine and I see the trims have dropped back down a good bit.

Lets see if you can get anything different when you get the engine speed up when coming to a stop.
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hard reset - often-posted (don't go for any weird posts)

 

Another way to check for leaks is spraying with a bottle of water to look for bubbles