Had been getting the "too lean" codes P0171, P0174 recently on my 2003 4.2L. The car idles roughly at start up and then smooths out as it warms up to operating temp and all stays well. When I looked at the live data on the OBD reader at startup, the short-term trims showed the max +25%, but the long term were normal at less than 10%. True to form, as the engine warmed up, the short-term trims dropped back to a normal plus/minus 5%. I have reviewed the lean-code writeups, but have not found any answer(?). I replaced the MAF a year ago and the upper O2 sensors three years back to solve previous lean code issues. I think I have applied all the common air leak fixes. A sticking component or an unknown air gap that closes on warmup? Any ideas greatly appreciated.

 

smoke machine or fire the parts cannon for every part known to cause a vac leaks

 

I don’t know the plumbing equivalent on the 4.2, but on my ‘02 4.0 XKR I had fluctuating trim numbers that baffled me for a while. It turned out that the part load breather connection at the valve cover was suffering from disintegrating O-ring seals. There are two O-rings at that location and both of them had gaps and were falling apart. I replaced those with the highest quality viton O-rings I could source. Then did a hard reset to clear any substandard info that might stored.

Since then I’ve fuel trim numbers of fluctuations ranging from -1 to +1 . The best readings I’ve had since buying the car. 4 years ago (and 54.000 miles under my ownership , so far.

Z

 

Could be air leaks at the two VVT solenoid O-ring seals. They bake in the engine bay, get hard and brittle after just a few years, and lose their effectiveness. Do you see any signs of oil seepage below yours? My wife's 2006 XK8 that turned 123,000 miles yesterday is currently on its third or fourth pair....

 

I can't accept the idea that a air leak would be massive at cold start, then closeup as temp increases. Could the engine temp sensor be faulty?

 

After a cold start the trims are in effect meaningless until the car can go CL (closed loop).

 

That's my point. When it starts in open loop the mixture is richer, factory set. I get the impression his car is not getting that added fuel, therefor running lean until it warms up . Could the sensor be not providing the information that the engine is cold?

 

I just replaced my air intake duct and the codes did not come back after resetting. The duct had become brittle and had holes everywhere. Good luck

 

Just read each sensor: ECT (mainly), IAT

May as well check any others you grasp for plausibility.

It's mainly going to trust the MAF & ECT I reckon.

It also has to trust things like injectors work but they can't all be bad and besides then non-cold running would be off.

 

Thanks to all for your ideas. Some additional info may be helpful (but not to belabor it - only real problem is "Restricted Performance" every so often). The high readings occur only after the car has sat overnight and is absolutely cold, and only at idle. Accelerating to 30mph drops readings to 12 - 15% before they drop down to normal plus/minus 2% at operating temperature. An engine coolant temperature is read at all times and rises uniformly to 190 degrees. The intake air temperature rises slightly from 70 to 73 (that's why we pay ridiculous prices to live in southern California). The mass air flow sensor reads zero to two pounds at all times, and the other readings appear to be normal (at least the usual ones that I can understand). Welcome any additional thoughts.

 

I like this!!! Thanks!

 

you might have noticed I made no mention of the part load breather connection at the throttle body.
thats only because getting to that connector looks like more work.

I’ve no reason to think that the o-ring at that end of the hose is in any better condition, I just can’t get to it without some disassembly. so replacement gets postponed into the mythical future. Or at least until the fuel trim numbers go haywire again.


Z

 

You're reading my mind...!

 

Hello everyone, I am a new member and have a nearly perfect 2003 S type base 4.2 at 111K miles with the same problems described above. On cold start, it idles rough, chugs sometimes, and sometimes sets a CEL with "limited performance" but not always.

I get codes for lean fuel mixture right and left when the CEL comes on P0171, 0174. When I clear the codes it runs like new again. When the engine is warm it runs perfectly. I let it idle for a few minutes upon cold starts and it will sometimes line itself out without setting a code or CEL. Other times not. Brakes are only affected on the first use when cold.

I have had the intake/vac lines checked for leaks with smoke and find no leaks. I have also tried carb cleaner all around the engine compartment and found no leaks, or at least no drop in RPM.

The spark plugs and coils were replaced about a year ago, and has only had about 5000 miles on it since then. Notably, it was not having this issue before or after they were changed. The problem started about 4 months ago and we have driven the car very little since then.

It has been to a highly recommended shop that supposedly specializes in Jaguars (they replaced the throttle body saying it was cracked along with the heater valves) and after $2600, no change. They found no other issues with the car and I had them go thru it pretty thoroughly. Compression is good, cooling system holds pressure, etc.

So resorted to throwing parts at it. The throttle body was replaced, the intake gaskets were replaced, the engine temp sensor was replaced, the MAF sensor was replaced, fuel injectors were replaced, and no change.

I'm kind of at a dead end.

Anyone have any other ideas? Please help!

 

widebands and cats the parts cannon still needs ammo lol.

plug the brake booster as well they’re known for internal leaks

 

I am still getting short-term trims at the 25% max at start-up and then the decline to normal once the engine is fully warmed up. The live data on my OBD reader shows the EGR valve is open upon start-up, when it should be closed. This result makes me suspect the valve is clogged in the open position and letting enough additional air into the intake stream to generate the high ST trim (?). As others have tried, I have spent hours running the smoke testing and fixed all notable leaks. This effort has eliminated the "Restricted Performance" episodes, so I have lived with the somewhat rough idle during the first few minutes of run-time. My car, a 2003 4.2L XK8 coupe, now has 149k on it and runs fine otherwise - only minor electrical error codes. Would appreciate the thoughts of the more knowledgeable members on this EGR valve theory and whether cleaning/replacing it would help.

 

If it's not gone CL (closed loop) then the trims may be fake. (It's running rich on purpose, a bit like using a choke in the old days.)

 

sdbill,
Have you tried shooting spurts of starting fluid, carb cleaner, or what not flammable fluid around the engine, like Ricksjagg? You can hear the engine rev when a leak draws in the aerosol. It is handy having a cell phone running a bluetooth OBDII sensor app displaying rpms for higher sensitivity.

One issue that can cause a lean mix without a leak is poor Throttle Position Sensor adjustment. It may be responsible for a "restricted performance" display in Ricksjagg's case. I plan to post a TPS adjustment technique I've been playing with soon.

 

For what it’s worth, another variation of this technique is to use plain water in a spray bottle.

if there is a vacuum leak, the rpm will vary, just like it does when using a flammable spray.


But by using water instead of the flammable spray one does eliminate the possibility of catching the car on fire, which I’ve seen happens few times in various shops where I have worked .

 

To look for small leaks you need to watch the STFTs.

You should be squirting tiny amounts so should be no risk of a fire!!