My '01 XJR is acting up when cold. Starts normally, idles normally but sometimes stalls when put into gear, and runs rough/stumbles for a minute or so until warm - then runs fine. I've also gotten a Failsafe Mode on startup a couple of times - clears after shutting down and restarting. No codes. I'd suspect a bad O2 sensor heater but again no code, and these were replaced last year when I was chasing lean codes (dirty MAF sensor). Thoughts appreciated.

 

Assuming your MAF is still clean, I’d clean the butterfly valve and bore of the throttle body next. Whatever solvent you use (I used carb cleaner), spray it on a rag instead of on the part. If it still doesn’t want to come clean, you can keep the soaked rag between the butterfly and bore for a few minutes then it should come clean afterwards. If you have a K&N air filter installed, chances are the MAF is coated with oil and needs to get cleaned again. Put a standard filter in it. This is just a few things to look into. Yes, I know you don’t have any codes but, do you have the ability to look at a live data list while the engine is running?

 

Thanks for that. MAF sensor and throttle body were recently cleaned (within a few thousand miles), and we're running a stock air filter (clean).
I have an Innova 3160d scan tool - I can read and record live data according to the manual (didn't know that). The car is at operating temp at the moment - I'll do a cold start in the morning and record live data while it's in stumble mode, and report back. It looks like I can plug the scan tool into my PC (didn't know that either). The stored O2 sensor test readings are identical for both downstream sensors - low/high V for Sw .095/.895, and Min/Max V for test cycle 0.00/1.275 - not sure what that tells us:0) Thanks again for the help.

 

What are the RPM's at startup and when she settles down? Have you done any upkeep on the vacuum lines? These will dry up, crack and thus create weeping points. Many times all you have to do is trim the end off about a 1/4-5/16".

 

1000 or 1100 rpm on cold start and drops to 700 or so after 15 or 20 seconds. I haven't touched or even looked at the vacuum lines - will check, thanks. Wouldn't this be a constant (vs cold only) problem, or could they expand and seal with heat?

 

The live data you need to look at when you do a cold start is:
STFT (Short Term Fuel Trim)
LTFT (Long Term Fuel Trim)
For both banks. ^^^^
ECT for engine coolant temperature.
IAT for intake air temperature.

Both the STFT & LTFT when engine is up to operating temperature AT IDLE should be:

-10 to +10 range. << If this range is out when you do a cold start AT IDLE, post it here what it is.

When you rev up the engine in park or when driving, the STFT will immediately respond and can go outside those parameters above depending on the throttle angle. The LTFT will normally be much slower to respond.

Both of the temperature sensors should be within a few degrees (F) upon a cold start. The ECT should only reach 95* (C) for the most part. The IAT is depending where you’re at.

 

It's possible they could expand enough....but the draw of vacuum can bring the hose in to where it seals also, if its loose. Thus, when it idles down, it could move back.
Now that is just a slight possibility, but it should be a regular maintenance check. Plus, to eliminate a slight issue, makes it easier to dial in a problem one.

I'd follow Addicted's directions on this, get him that info he's looking for, ..but check the hoses.

 

Both STFTs ranged from -3.1 to -5.5 on cold start and stayed within that -10 to +10 range at operating temp. LTFTs were both 19.5 on cold start and never varied from that - ECT reached 194F and IAT 96F. I went to record some live data taking off from cold but of course the beast ran flawlessly this morning without a stumble. Not sure if this helps.
I now have P1000 and 1649 (Flash Memory FCCP Monitoring Malfunction) codes - no idea what the latter indicates.

Cleared DTCs and recorded live data over a 15mi run - 1649 did not return.

 

LTFT of 19.5 indicates a significant air leak,

 

I agree with RJ. A positive number on the LTFT means the ECM is adding more fuel over the long term to compensate for the vacuum leak to keep the air fuel ratio more in check.

With the engine running up to temp, watch your STFT’s closely. Spray brake cleaner around vacuum lines and maybe under the bellows of the air intake tube. Another area is the brake vacuum booster supply pipe that goes on the passenger side beneath the throttle body. Also the part load breather corrugated hose on the drivers side valve cover that goes to the supercharger elbow under the D/S of the throttle body. The full load breather on the P/S valve cover can leak also. The supercharger lid has 2 large circular seals that can leak air also along with the large square one on top of the charger. The STFT’s will spike way out when brake cleaner finds the vacuum leak and the engine rpm’s change drastically.

 

Thanks for the feedback - will let you know what I find.

 

I attacked everything I could access with brake cleaner with no luck. I suspect the culprit could be the vacuum line at the P/S base of the throttle body that feeds the brake booster - when assembling the beast I remember wondering if the o-ring in there is really the only thing holding/sealing that line. BTW is that o-ring replaceable (it's original as far as I know)? Unfortunately this motor is stuffed into a Mark 2 and the area under the TB is virtually inaccessible. I'll have to remove the bonnet, and since there's no way to maneuver an aerosol can down there I'll probably have to remove the line, plug the opening and look for a change in the LTFT. I assume repeating the 15 mile loop I recorded earlier will be enough time for any change to show up? Let me know if there's another way to do this, as I'm not sure if running 15 miles without vacuum assist on the brakes makes any sense.

 

There is a breather tube running under the intake and plugs into the front left cam cover on one end, then it
Tee's into the throttle body and continues past the throttle body. That particular tube is very susceptible to
cracking and could be the source of a leak. You might also look at the brake assist vacuum line plugs
on the right side of the throttle body. There are little “O” rings inside that fitting that have been known to
crack and allow a vacuum leak. It may be wise to procure a smoke machine at some point.

 

Since this is a start up issue, I wonder if your temperature sensor in the cross over pipe is stuck and sending false data to the MAF and on to the ECU?

 

I have been thinking the same thing, Jim.

 

This is the connector on the P/S under the TB that joins your brake booster pipe to the lower charger intake. Disregard the actual brass collar because you don’t need to replace it. The very specific size o ring and that plastic quick connect will likely need replaced. I would not drive the car without vacuum assist. I’m not saying this is your problem but, they are prone to vacuum leaks from this.

 

It is repairable, ...I don't know if its still available, but it is repairable.

 

That was the problem I had with my 06. I just sealed it with RTV because I didn't know how it came apart at that time.
But I had a high LTFT on A bank(15) while B was under 10.

 

I didn't think of that but seems quite possible.
How would one go about "unsticking" it? WD40?

 

Thanks all for the info - I'm always amazed by the folks here that take the time to help solve someone else's problems.
Wondering if there is a simple way to test the temp sensor - the part is cheap enough so I'll go ahead and replace.
Addicted and HH - looking at the pics of the adapter, I don't even recall the plastic locking device being present. It could have been in the TB but I'm thinking my tube may be retained solely by the o-ring on the brass collar. The part is widely available so I'll order one and start removing stuff to access the area.