I've got a slight vibration in my XJR at idle and I'm trying to figure out how much is normal. I imagine that it's not going to be perfectly smooth even if everything is correct, but I'd like to know what others have experienced with cars they know are sorted.

When idling and warmed up completely, I can feel a slight vibration in the car, both sitting and if I put hands on the body. If I open the trunk then it's slightly visible in the trunk lid vibrating. I've done a good bit to the car (primary and secondary timing, and all the small bits along with that) though there's a few things I haven't been able to do yet (check spark plugs, trace possible vacuum leaks). Motor mounts look good, they'll get inspected again when I piece together my front end suspension/subframe bits and tools.

The fuel trims seemed good last time I checked briefly (think in the +7/+8 total range in both banks at idle?) though I did hear a whistling which sounded like a leak coming from the V8-plate to intercoolers area. Couldn't trace that down any more. Also been getting lower than expected MPG (about 16 driving conservatively on highway for a half hour) which makes me think of vacuum leaks again.

 

Sounds like your instincts are good, or at least as good as mine. If the spark plugs are an unknown then I would go there first. They could cause rough idle and poor MPG. Vacuum leaks as well, but if your fuel trims are pretty good I would think any vacuum leaks you have are not very significant. If you only checked fuel trims at idle then vacuum leaks move higher on the list as a possible culprit.

From there, wow. Bad gas, dirty fuel filter, failing fuel pumps/pressure regulator, dirty/bad MAF, restricted or malfunctioning fuel injectors, failing coil or coils etc. etc. etc.

But like you already said, plugs and vacuum leaks would be my first check.

 

I'd watch the trims. They should be near 0 (under 5), and vary little. Switch a/c off so they've less excuse to vary.

The noise may or may not relate to an air leak.

 

I only get 16mpg .... mine are very smooth .. no vibration.

 

On level interstate highway going 77 MPH with cruise control on and air conditioning running I get between 18 and 21 MPG in my XJR. If I go to pass or reach an incline in the highway that can drop to 13 MPG and when going down hill that can rise to 45+ MPG. On an all highway drive my average MPG is usually between 17.5 and 19 MPG. In city driving I am lucky to average 13 or 14 MPG.

All of this is based on the cars computer, presuming it's accurate. It usually lines up very close to my Ultragauge, but since the data is coming from the same place I guess that makes sense.

 

I know it's one of those things, could be a million causes, like when a car rolls into the shop and it's "making a noise"...

I should probably add, I've cleaned PLB, new air filter, new primary fuel pump, new fuel filter, MAF cleaned thoroughly. Heads were redone at around 75k maybe? I don't know much about this, except the seller's word, and the fact that the sides of the heads looked sparkly clean when I was doing timing, so they were obviously pulled and cleaned at some point somewhat recently.

Fuel pressure checking is definitely on the list as well, but I need a gauge for that and don't want to use my el cheapo vacuum/fuel tester for fuel, rather keep it to air-only.

Another thing I wanted to do was test vacuum at idle, I've been learning a bit about expected vacuum behavior with a good/bad system, but that's on NA engines. With the SC cars, if I understand right, the bypass valve is open at idle so it "behaves" like an NA engine there, so I could use the same methods for checking vacuum there? Has anyone done this, and know if I should expect the same values as other engines, and I guess a good place to tap vacuum lines for testing?

I'm not sure how accurate the computer is for fuel consumption, but I've tested distance and that reads correctly within the usual couple percent, so I guess if I really wanted I could run it down to a certain tick in the fuel gauge, add x gallons, run it down there again, then math out the actual consumption.

EDIT: pulled one of the plugs just to take a look, seems like it's at the end of its life, although I'm not great at judging spark plug condition. It's an NGK BKR6E and measured out to a .036" gap, far from the .049-.052 recommended for the AJ26 XJR. Maybe it'll be easy this time and I'll just order up a set of plugs.

 

Try OBD - easy and cheap plus tells you what the PCM (car's computer) is being told.

 

Looks like you are running rich. I would switch to iridium plugs with the correct gap.