Hi All and Happy New Year,

Where to begin? So I have a 2003 jaguar x type with 177000 miles on it. I have been trying to solve this problem for a while. It is throwing the cursed p1071 and p0174 codes and has a rough idle (on start up and when coming to a stop). It is also doing the 3000 rpm limit thing.

Things I have replaced:
all spark plugs
all coils
fuel filter
both IMT o rings
brake booster vacuum hose
all 12 intake gaskets
MAF sensor
breather hose

Things I have checked:
Vacuum to fuel rail- no leaks
Listened for vacuum leaks and I am unable to detect any.

I have reset the codes multiple times and the codes always come back. Before they do it idles fine and the 3000 rpm thing does not happen.The CEL seems to come back with the rough idle when I brake. Could it be the brake booster? Can a bad brake booster cause the cel to come on? The brakes are not spongy but are stiff sometimes. Is there a way to tell if the booster is good or bad? What about the gas cap? Could that be it? Any help or suggestions would be greatly appreciated!

 

Loads of people with the 171 and 174 codes these days! I have one I'm trying to diagnose as well.
If the brake vacuum line from the inlet manifold to the master cylinder is broken/leaking you would have this problem so you should check that. Shrinkwrap or gasket sealant on the round middle section is a way of testing.
Does your code reader show fuel trims? If you could post your long term fuel trims at idle and at 2500 revs when the car is warm we can tell you if it's a vacuum leak.
Was there any oil on any of the plugs?

 

Stop clearing codes.

Watch fuel trims as so many posts say.

 

Thanks for the reply. I will check the break inlet again. Yea there was a bit of oil on some of the plugs? Would a small valve cover gasket leak make it throw the codes?

My obd II reader only can read and clear codes. It is just a cheapo HFT one.

 

elm327 is nearly free and good.

 

Thanks for the info on the elm 327. I bought one today and it should arrive soon. What should my fuel trims read at?

 

rag87: See this post and the one above it for an example of how a vacuum leak causes high fuel trims at idle that drop close to zero when the RPM's increase to highway speeds: https://www.jaguarforums.com/forum/x...9/#post1374641

+1 to JagV8's comment that you can buy an ELM327 for ~$15 and use an iPhone or Android app to see real-time fuel trim results. Well worth it.

 

Easiest way to check the brake booster vacuum is to pull the vac line from the booster after the car has sat for a few hours. When pulled it should suck in air rather aggressively(the booster), and will be very hard to pull out the rubber connection. If there is a leak it will pull out rather easy, the longer it sits the easier it will pull out if there is a leak there.

Sometimes with the leaks it will suck in moisture and can cause rust inside the booster eventually leading to a rust hole and air leak.

Another way to check it, with the car running pump the brakes, the pedal should feel the same each pump unless you have a leak it will get stiffer each pump as the vacuum pressure decreases from the leak. This is more of a judgement call method.

 

Ok so my elm 327 finally arrived, they took their sweet time in shipping it. I hooked it up and here are what the fuel trims are reading at.

700rpms
Bank1
Long term 25
Short term 19.53
Bank2
Long term 25
Short term 19.53

2500rpms
Bank1
Long term25
Short term 5.47
Bank2
Long term 25
Short term 4.69

Also brake booster seems to be ok because it "pops" when the line is pulled. Another thing I replaced is the pcv valve. I am going to try the carb fluid test right now.

 

rags: Please double check those fuel trim numbers. Typically a vacuum leak causes the LONG term fuel trims (not the SHORT term fuel trims) to be elevated at ~19-20% at 700rpm, and drop to ~4-5% at 2,500rpm.

 

Ok thanks, I thought it was weird the long term trims didn't seem to be moving. They were stuck at 25. I will play around with the program.

 

Ideally, the long term fuel trims are near zero, and the short term fuel trims bounce slightly above and below zero.

But a vacuum leak allows extra air into the intake manifold, causing the air / fuel ratio to become too lean. So the ECM compensates by injecting extra fuel, i.e., keeping the fuel injectors open longer. At idle (~700 rpm), the long term fuel trims can be ~10-20%, meaning the ECM is adding ~10-20% extra fuel to compensate for ~10-20% extra air from the vacuum leak.

At ~2,500 rpm, the engine 'breathes' ~3.5 times more air than at idle, but the air leak stays the same, so at high rpm the air leak is diluted and the ECM adds only 3-5% extra fuel to compensate.

So when the long term fuel trim drops from ~10-20% at idle to ~3-5% at ~2,500 rpm, suspect and search for a vacuum leak.

With the engine idling, spray carb cleaner or propane gas at the possible vacuum leaks (of course, taking care not to start a fire in the engine bay). When you find the leak, you'll hear the engine idle change and see the short term fuel trims bounce more erratically as the ECM attempts to compensate for extra fuel sucked in thru the vacuum leak.

Make sense?

 

Maybe was a cold engine? Need it hot or the O2s can't work and trims are meaningless.

 

Hi,

Sorry on the late reply. Here is the update. So after re-inspecting the the brake booster there was a slight hiss at the connection to the intake manifold. I used some gasket maker and sealed it up. Check engine light off. Then about 25 miles later, CEL back on and I get horrible idling, worse than before. I checked the codes and its reporting a multiple engine misfire and misfire codes on one side of the engine. The side where I noticed a valve cover leak when previously changing the coils and spark plugs. I then replaced all the valve cover gaskets and put in higher quality spark plugs. CEL off. Then again within 25 miles, poor idling returns with CEL back. This time its both lean codes again. :/ LTFT reading at 19.53 on both banks at idle and 3.26 on both banks at 2500rpms. I performed a smoke tests with no apparent leaks. Also used throttle body cleaner on the vacuum connections with the rpm not effected. I noticed that the throttle body makes a strange continuous before starting. Here is the video link below.


Could a malfunctioning throttle cause lean codes? Any suggestions? Thanks.

Also thanks dwclapp for explaining the fuel trim numbers!

 

Since it is so cheap and easy, if you have not yet, I would get some TBody cleaner spray and a soft bristle tooth brush and clean the flapper and it's chamber really well just in case it is sticking open when the car warms and stuff starts expanding.
Did you do your leak/smoke checks with a fully warm engine? Some leaks only show after the engine is at normal op temp.

 

Had all the same syptons, changed plugs, and all gaskets. Ran better but still bad with slow starting, rough idle and jerky stops. Cleaned throttle body and MAF sensor, better but still not good. I got carberator cleaner and sprayed around looking for a vacum leak. I found a large leak at the large black intake manifold runner vane control. The carb cleaner got sucked right in and engine speed changed immediately. A new gasket and everything has settled down. Error codes have not returned in 2 weeks and gas mileage is way up.

 

A leak is a leak. Spraying a burnable gas - at the right place - will immediately change the trims and often you hear the engine note change, too. If it's not happening it's because you have not sprayed in the right place - but that can be because it's not accessible. E.g. on some engines there are hoses under the manifold that you can't spray at. Or an evap/purge valve or a hose that's not in the engine bay. You'll need to figure what applies to any particular car.

 

Again I apologize for the late response, I got bogged down with other projects. So problem solved. As I suspected earlier, the throttle body and or tb position sensor must been malfunctioning; the whine coming from the throttle body motor in the video above. I got lucky and purchased a brand new one off ebay for 100 bucks! Now, no more lean codes! No more poor variable idle on start ups. She accelerates better than ever! Thanks to all that contributed to this thread!