I finally got around to replacing the electric throttle body on my '05. The car has never had a problem, with the exception of the pots in the throttle that have slowly been tripping a code. (All the dash lights go on and the cars goes into high idle "limp home" mode...fixed by turning the ignition on with engine off - and slowly cycling the accellerator pedal 3 times). When it started tripping almost weekly for the throttle TPS, I figured it was time!!

During the replacement, I noted some oil puddled in the intake manifold. At the time I figured it must be normal. At first start up after the replacement, lots of blue oil smoke for about ten seconds, then it stopped.

On first drive, the downstream right O2 sensor tripped a code. Replaced that.

Next drive, the code P0171 tripped for lean right bank. I beat my head for 3 days, but there are no air leaks, the Mass flow sensor looks clean, and the air filter is new.

So, anyone know if all this is related? If I screwed up the throttle body, MAF was bad, or i have an air leak...it should trip lean on both banks. The car runs fine, but has a severe hesitation when floored. Once it goes, it goes well.

Thanks,

John

 

Well, went through the entire archives of P171/P174. It seems everyone else has found vacuum leaks in brittle hoses.

1) I pulled every hose off the car and pressurized them with ar, but no leaks in any of them.

2) Finally got the car to generate the P0174 code with the P171. I assume that means the leak is just barely tripping the ECM. It should also eliminate the possibility that the O2 sensor in the right bank that I replaced is introducing an air leak.

3) Tried disconnecting the crankcase vent valve from the suction hose and plugged the hose. Still generated the P171 and reduced perf. I had hopes for this valve, as it could account for oil in the intake and the excess lean. In normal operation this vacuum hose makes a loud hissing noise...but other posts indicated that is normal...and I confirmed the hose is intact. I assume the ribbing causes the hiss with the volume of air going through it.

4) I reseated the Throttle body, using a sealant to ensure no air leak.

5) I checked the entire intake duct from air filter to throttle body...completely intact.

I am at a loss. I downloaded the workshop manual, but it is surprisingly void of basic diagrams like a vacuum or EGR, or many other systems. I am confused about the hoses into the lower throttle body, as they appear to be coolant hoses to warm the TB, but they have no coolant inside. What's that all about??

Anyway, would appreciate any guidance. I will try to get access to a real data OBD tester to see what is happening with fuel pressure an the like...

Thanks,

John

 

Went thru the same insanity and it turned out to be the downstream O2 sensor after all. Try swaping them and see if the code changes to the other side.

 

New guy to Jags, but check the noise supressor attached to the air intake hose. It runs from the air cleaner the TB. There is an attached noise supression device that clips on to a bracket in front of the thermostat housing. Mine was cracked all around the junction of this supressor and the air intake tube. I added gorilla glue, and let it set for a few hours, and my XJ was good to go. If you cracked this during removal it will allow more air to enter the TB than the MAF sensor is registering. This in theory will cause the engine to run lean. Too much air, not enoug fuel. Hope it helps!

 

Thanks guys.

I think I have it fixed...got my fingers crossed anyway.

In trouble shooting I pulled all the hoses off 3 separate times to check them and the connections/o-rings/etc. I think I fixed it the 3rd time by using sealant on the throttle body gasket and re-installing the new throttle body...but then I toaded it up by missing the small breather hose that connects to the plastic intake duct. I feel dumb!?! Serves me right for working at night without good lighting. Once I re-connected that hose, all seems well. No codes and the acceleration is far less delayed after 50 miles of driving.

So, for posterity, I think there was a very small air leak on the bottom edge of the throttle body. Although I sprayed cleaner all around the gasket with the engine running (to look for surging which would indicate a leak), the location of the leak could not be accessed with the spray. The leak had to be very small...which shows how intolerant this 4.2 engine is to vacuum leaks!

Anyway, thanks for the support...I'll post again if I haven't fixed it.

John

 

Oh, forgot to mention...there was no more oil in the intake manifold when I removed the throttle body. So that is still a mystery. Unless there is a crack in the bottom of the plastice manifold, it could ony come from the PCV/crankcase breather valve on the right valve cover. During the trouble shooting I did remove that valve and clean it thoroughly.

So who knows??

John

 

I think you're approach was perfect, and your tenacity to locate the problem, admirable.

Though frustrating, it does feel good to claim being smarter then the car, at least for the time being. Now, giver her a good detailing, before she tries something else (oh, don't use pressure washer under the bonnet..in fact, no uncontrolled hosing whatsoever...just to be safe. damp cloth, spray bottle of simple green, wipe down and enjoy)

 

Well...it went about 120 miles and then generated the P171/174 codes again. Bummer. But, I have now found the problem...it was the mass flow sensor (MAF)! Here is what I went through to find it, just for anyone with similar symptoms in the future:

My wife drove the car for a week, without any codes, but it was all around town, nothing over 50 mph. I got in the car and took it on the freeway and bam , "restricted performance" with the MIL. Hooked it up and it was the same old P0171 and P0174.

I could not believe it was a fuel pressure issue (pump or filter), since it ran so well under full throttle, and it was not likely an injector issue, as it was affecting both banks at once...possible, but not probable.

At this point, I turned to the PCV, or crankcase vent valve, as it is sometimes referred to in the manual. Since I had seen oil in the intake plenum AND I have a lean condition, this is the likely item to cause both. The valve is made with a diaphram and a spring. At low vacuum the diaphram relaxes so it allows the spring to open the valve to full flow into the intake. At high vacuum, like coasting or cruising on the freeway, the diaphram closes the valve, so only a small amount of airflow is allowed into the intake. The downside...it is sealed plastic, so no way to check it without destroying it, of course! I replaced the valve ($52 at the dealer), cleared the code and took off, this time leavng the code checker plugged in.

As soon as I hit the freeway, cruising easy at about 70mph, bam..."restricted perf" and P171/174 again. At least now I narrowed it down to when it tripped. It was always easy cruise at 60-70mph. It was fine at all other speeds and throttle positions. This finally explained why sometimes it would trip right away, and other times it would go a week or two. It was waiting for us to hit the freeway...and it is a "2 trip MIL", so it has to go on the freeway twice before the dash light triggers. Sneaky!!

Now I hit Autozone. Picked up the MAF ($79 w exchange), a new air filter ($15), and a bottle of injector cleaner ($4). Installed them all in the parking lot. This is the interesting part...no more codes, but better still, the car drove better than it ever has.

From the minute we bought the car in '05, it always had a jerk as you accellerated. Annoying...you push the accelorator pedal like a normal car, and it would pause and then jerk as the throttle "hit". You had to try very hard to barely push the throttle until it started, and then push it normally to prevent the "hit". I took it to the Jag dealer 3 times under warranty. They checked the programming each time and said all is normal. I just assumed it is the way that these "electric motor throttles" operated.

Well, that "hit" is gone! At this point I have to assume the MAF has been bad since the car was new. It was under reading the airflow, so the car was always trimming to compensate, but it was trimming just below the 19% limit to trigger the lean codes. It ran fine, but the lean condition resulted in a surge anytime you increased the throttle, since it took the ECM a half second to retrim the fuel, and during that half second there was a lean "sag" in response. This accounts for the delay in acceleration...both at idle and when accelarating from one speed to another. The new throttle body must have altered the flow just enough to barely trigger the trim codes P0171/0174 during one particular speed and throttle...cruise at 70 mph.

So, problem finally fixed...although I still cannot account for the oil in the intake that one time!?! Hopefully the new crankcase vent valve will prevent anymore of that in the future...

Cheers,

John

 

Way to go Sherlock!

 

Elementary my dear Watson!

 

The MAF you talk about, is that the mass airflow sensor? I have this exact condition as well the 'hit' you talk about.

 

John,

I have been experience the basically the exact same signs after tripping the P 0171 and P 0174 trouble codes and following the same logical process. Codes appear simultaneously.

My first thought was it must be a vacuum leak on top of the engine, since the codes indicated both banks lean. After tracing down the vacuum hoses and finding nothing obvious, I also focused on the loud sucking sound coming from the big mushroom shaped PCV valve on the passenger side valve cover. After removing it and the hose, I could find nothing indicating a leak. However, I did notice clearly visible OIL accumulating inside the hose connecting it to the top of the plenum. Then I cracked the brittle hose that connects the PCV valve to the plenum in two. I assumed there was a slight crack I could not see in the congregated tube.

My quick solution was to replace the broken hose with rubber hose I had on hand. I carefully removed the brittle hose from the barbed connectors on each end of the hose. I cut the rubber hose to fit and re-installed the PVC valve and plenum connection.


I removed the hose connections between the air box and the throttle body and checked for cracks or anything that could cause a vacuum leak. Found nothing cleaned everything and re-installed.

Next I took carb cleaner and systematically sprayed the plenum to intake manifold area and all vacuum hose and plenum connections which did not show any signs of a vacuum leak. After re-installing everything and finding nothing obvious, I cleared the codes and that was it for about two weeks.

During that time the car was only used for in CITY driving. Then two days ago, it was driven on the expressway and "BAM" up comes the check engine light. The P 0174 & P 0177 codes were stored again. Basically the same scenario described above by John. Everything is normal until driven at expressway speeds of 60-70 MPH.

I am next going to replace the MAF sensor which appears to be the solution for this condition in John's case. Off to Autozone. Still don't know if oil visible in the PVC valve to plenum connection is normal or if I need to replace that PVC valve.

 

I realize this is a year and a half later...but I might as well add a post script. I had more issues shortly after the last post. I wiggled and worked and tried to figure out what was happening. I bought a decent OBD code reader that would show which emissions checks were pending and when they were complete. I would try a fix and then run the drive cycle to check the results. It's been a while, but the specific drive cycle involved something like a 10 minute idle and then some 3rd gear accelerations and decelerations. I must have run a dozen cycles, all of which tripped the P0173/0174 codes as soon as the drive cycle was complete. The car ran during this time, but it never "felt" right. It would delay acceleration and sometimes give a buck or two randomly.

In the end, the problems went away all on their own. In hindsight, I think the problem was a bad tank of gas. What finally fixed it seemed to be re-filling the gas tank with good gas, and after a while it started running great and has not tripped a code since. I have not used the gas station that I think sold me the bad gas...and I have had no drive ability problems with any of my cars for a year and a half.

So...although I have no proof...I think I had water in the fuel tank the whole time I was trying to troubleshoot.

 

With the only change being installing the new MAF sensor and clearing the codes again, the difference in performance is remarkable. (Cleaning the old MAF sensor had no effect.) The throttle is so much more responsive and the acceleration is improved significantly. You step on the pedal and the car takes off like it's supercharged. Drove around town and at expressway speeds and no codes so far.

Obviously the old MAF was deteriorating slowly and the engine compensated for that until it triggered a code. What I experienced with my 1985 Corvette is that when you have multiple degrading sensors or components the ECM compensates for everything. Fixing one may push something else over the limit or triggered another code. Just have to wait, but just installing this new MAF sensor made a significant change in performance. After the ECM learns everything, we will see what happens.

Thanks again John.