2001 Infiniti QX4 rough idle - leaking fuel damper vacuum port

Question

I have a bit of an involved problem, so I’ll break it down a bit and try to provide as much detail as possible.

My main problem:

I have a rough idle on my 2001 Infiniti QX4. It’s not extremely rough, but quite noticeable. It doesn’t happen all the time, though it happens most often after going up a hill, getting off the freeway, or after heavy acceleration.The next stop after a few minutes of slow easy driving usually yields a smooth idle again. Acceleration and all other driving is super smooth with no hiccups or loss of power.

The only other problem is warm startups are weak, as if engine was starved of air. When cold the engine starts up with no problem or hesitation.

The state of things:

New parts (all replaced within the last year):

• injectors
• fuel pump
• fuel filter
• fuel damper
• fuel pressure regulator
• fuel hoses
• vacuum lines
• coil packs
• spark plugs
• idle control valve

Other notes:

• The air filter is a K&N cone that’s part of a CAI and is mostly clean (can still see the color and metal mesh).
• The hose attached to the fuel damper reeks of fuel, but doesn’t leak fuel out of the port when I pull the hose and run the engine for a few minutes.
• The fuel pressure regulator’s vacuum port doesn’t smell.
• I don’t hear any vacuum leaks, and the CEL is not on.
• Fuel pressure and manifold vacuum are perfectly within range per the FSM and are rock steady.
• I cleaned the throttle body recently and removed all carbon buildup.
• I only use 92 grade (per manufacturer’s recommendation) from Chevron.

What I’ve tried (beyond the new parts):

• I’ve swapped in and returned five other fuel dampers from various retailers including the dealership, and all immediately exhibit the same symptom.
• I’ve replaced the vacuum line each time, and the smell is present after driving a few miles.
• The ends of all of the dampers have the same part numbers and stampings despite being sold under different brands, so I believe they’re actually all the same manufacturer: SMP. I’ve contacted SMP and the dealership and both shrugged their shoulders and refuse to do anything citing that they’ve never received complaints.
• I bought a scan tool that shows sensor information and drove around watching the fuel trim. I watched the long-term because short-term was very erratic and hard to get a good picture from. For the most part, it didn't go any lower than -5%, except for once or twice when it jumped down to -7%. The 1 and 2 values usually differed by 2%, with 2 being the closer to 0. Idle when not moving was always 0%. Going up a hill at 3k, the two numbers both settled at -2.3%. With no throttle applied and moving at about 30mph, the numbers were between -1 and -3%.

My questions:

1. Any ideas on what could be causing the rough idle?
2. Should the fuel damper be leaking fuel at all? This seems like the obvious smoking gun but the manufacturer’s adamance that it’s not a problem and it not weeping fuel sans vacuum hose introduces some doubt.
3. Any ideas on other damper manufacturers to try or if I should? I haven't tried Borg Warner or Airtex/Wells, but BWD are hard to come by and are a subsidiary of SMP so are likely the same part anyway. Airtex/Wells have a horrible reputation from what I’ve seen and I’m afraid I won’t be able to return them as they’re sold online by a place I haven’t done business with. The fuel pressure regulator is also SMP, for what it’s worth.

Answer 1

Thanks for the great detail. Considering this is a bit of an odd issue, I'll try thinking outside the box a little. But first I would use a multimeter to test the coolant temp sensor, then o2 sensors. Perhaps even a quick check of the upper radiator hose to make sure the thermostat isn't stuck open. I say to check these first because it sounds like you're running a little rich even after warm-up. If these all check out, then on to my real suspicions:

A possible problem with the camshaft or a malfunctioning crankshaft position sensor, which the latter can be tested with a meter if you haven't gotten a check engine light for codes. The ECU uses this sensor (among others) to adjust timing and, since that could be directly affecting the sequential injection, it could be causing that fuel smell right at the damper, as well as the performance issues you describe. Extremely small leak could just be evaporating or the damper could be stressed from working harder due to fluctuations in the injection sequences. Just a thought, not necessarily intended as an answer but it wouldn't let me comment as opposed to answer so sorry I had to write this here. Maybe I don't have enough credibility yet for that function since I'm new here.

One other possibility is clogged exhaust somewhere.

Anyway, it's worth a test! Good luck.

Mustangguy

Answer 2

I recently troubleshooted a very similar issue on a friend's Outlander. Different car but physics is physics.

He was complaining of a similar rough idle and occasional stalling when the car would come to a stop. Turning the A/C on helped smooth out the idle. The dealership was tried cleaning the throttle body, which reduced the severity of the symptoms but didn't solve it.

In that case it turned out to be a cracked air filter box (accident damage) that allowed air to be drawn in from an alternative route. Note that this was upstream of the MAF, so I wouldn't expect to see abnormal long-term fuel trims (which we were sadly unable to verify). Plugging the crack immediately smoothed out the engine's idling. The fix was to replace the air filter housing.

I would suggest assessing if this situation applies to your car. Focus on the section of the air intake that precedes the MAF.

This question may help:

• How can I detect vacuum leaks without access to a smoke machine?

Happy troubleshooting!