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Discussion Starter · #1 ·
New to this forum, please bear with me:

Wife's car is a 2004 SE built in May 03; very rough idle. Took it into the local AAA repair facility for work on transmission (fluid and filter change), brake bleed, new plugs laser platinum .044, new Hitachi OEM coils all around, and plenum gasket kit and new valve (cam, really) gaskets. Before service, no codes (six months ago, CEL was for misfire on 6; replaced plug and coil), with LTFTs of +18 bank 1 and ~+16 bank 2). When we got the car back -- for the cost of a down payment on a new rig -- still bad idle after running for a while. But improved LTFT +13.5 and +12.8. Took it back, raised some hell: this time a bad 'lower' manifold gasket 96454A and a hole discovered in the intake collector (140107Y040)? and replaced EGR valve gasket. Got it back, still rough idle -- but improving -- LTFT, both banks at +10.9 (STFT oscillating around 0.4 bank 1, and dead solid at 10.9 for bank 2). Runs like a scalded cat up to 5500 RPM, but idle sucks with slight vibration.

Before I pull the plastic cover off and try to blow it up with an unlit propane torch, any ideas?

I will try the search community, next (just saw that area). As a long term member of Jag forums, I know to check there first.
 

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Several things that could cause a rough idle:
  • What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
  • The cam position sensor may be marginal.
  • Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The reading at idle should be around 51 psi.
  • There may be a major intake system vacuum leak. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
  • Dirty fuel injector(s). Run some good injection cleaner, like Techron, Redline SL-1 or BG products 44K, through the system; give the cleaner about a week or two to do it's job.
  • Leaking fuel injector(s). A good way to test for leaking injectors is to Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail and turn the ignition switch to the run position without starting the engine; then turn the ignition switch to the off position after a few seconds. The fuel pressure will be at around 51 psi for quite a good period of time; if it falls off rapidly, that indicates a leaking injector(s).
 

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Discussion Starter · #3 ·
Thanks, all good stuff.

Plugs are new OEM (they paid a fortune from the dealer). CPS is new (four months old and no indication of failure in rear one); fuel pressure is spot on (runs like a scalded cat . . . ); given that it is running rich in both banks, leaning toward vacuum leak. They used a Wynn product to clean the injectors and valves (originally thought it may have been carbonization causing the rough idle, but not for that long), I had previously been using both BG 44K and SeaFoam, pretty sure injectors are not the problem, att;

Don't know where the plenum bellows is (not a term I am familiar with in BMWs, Jags and Land Rovers), but this is an area of likely leak since they had to remove almost all of the stuff above the engine to get to the rear plugs and coils. Supposedly they checked those on the second try, when they found the lower manifold gasket leak and hole . . .

Where can I put the vacuum gauge? As a very old mechanic, I understand vacuum gauges -- haven't used mine in 30 years, wonder if it still hold pressure?

MAF sensor is most common cause of enrichment from vacuum leaks on 4.0L Jags -- might check that sucker, particularly the pins -- some heavy handed techs can bend them.
 

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The intake plenum bellows is the plastic air intake tube that connects the throttle valve to the MAF; check for hidden cracks and make sure the clamps are tight. Nissan may call that piece a "collector hose".

You can connect the vacuum gauge hose to the vacuum port that's located at the back part of the intake manifold; just in front of the power valve. Don't confuse it with the power valve actuator vacuum hose; the two hoses are close together. The power valve actuator connection uses a modulated vacuum, so don't use it.

You probably should perform an ECU code readout with a portable scan tool to see if any fault codes are set. The tool can be purchased at most auto parts stores or online at Amazon.com. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction. If you have a copy of the FSM for your vehicle, the code readout procedure is described there along with a listing of codes. You can download a copy of the FSM from this web site: Owner's Manuals. The section EC.PDF is the one you need to read.
 

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Discussion Starter · #5 ·
Thanks. I will see if I can find the vacuum fitting. I am also thinking some of these vacuum hoses are really old and maybe leaking. I will check the air tube from the air filter to throttle once more, as well.

Some thoughts on forum process: Normally when someone provides you the long term fuel trims they are using a diagnostic tool reading from the OBD II port (mine is an ACTRON CP9575): no codes in six months, last was a P0306 which was fixed with a new coil.

Sorry for the snark: appreciate your staying engaged.
 

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Discussion Starter · #6 ·
Finally got around to checking for vacuum leaks. One rubber line was badly worn: cut off the end and reattached. Checked the air tube from the air filter -- no leaks but not as tight as I would have it. Cleaned the MAF sensor while I was in there. Then used the propane torch to search for any other possible vacuum leaks. Nothing! However LTFTs, both banks, now +9.4.

This site is corrupted with popups for Norton and other crap!
 

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Finally got around to checking for vacuum leaks. One rubber line was badly worn: cut off the end and reattached. Checked the air tube from the air filter -- no leaks but not as tight as I would have it. Cleaned the MAF sensor while I was in there. Then used the propane torch to search for any other possible vacuum leaks. Nothing! However LTFTs, both banks, now +9.4.

This site is corrupted with popups for Norton and other crap!
WHAT!!; used a propane torch to search for vacuum leaks; caveman style; a good way to set the engine compartment on fire!. The proper way to search for vacuum leaks is to use a vacuum gauge; the reading at idle should be 18 - 20 InHg. If reading is under 18 InHg, you've got a problem.
 

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Discussion Starter · #8 ·
Cute: but wrong. Slight vacuum tick at 19. But useless for finding the location of a leak. Propane gets drawn in to the combustion chamber at the point of the vacuum leak and slows/roughens the idle. Done everywhere to find elusive leaks.

But no leak found!! Still rougher idle and slightly high LTFTs (but equal both banks).

Next step: try some additional cleaning of the valves and injectors. Wife -- aka, "the Judge" -- thinks it is now perfect. Maybe, just lower the hood . . .
 
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