As I have written before, I have a rough idle at the normal idle speed of around the 735-745 rpm. There is no real fluctuation in idle except what one would expect and it stays in that range (I wrote Gus and asked if he knew where it should be and he checked his out and it is in the same range) This gets more noticeable when in gear and less so in neutral or park and when the compressor engages it is most noticeable.

If I accelerate to 800 rpm- it is gone. It is more like a slight tremor and you can see a very small (very small) shake in the engine on the manifold side.

Having had a crimped wire before that was broken, I know what it is like to have RP with a dropped out cylinder and I semi-live in fear of something of that nature happening again.

For the record:

1. I have ZERO codes- just a 1111.
2. In the last year have new coils, plugs all the way around, new pcv, new Coolant Temp sensor, new O2 sensors both sides up and down, new cats both sides. Air filter replaced every 3 months whether it needs it or not (and yes done the correct way) Maf stays cleaned, new fuel filter, use Shell prem.- 90% of the time.
3.The IMT o-rings were replaced twice by Jag under warranty.
4. No indication on LT or ST fuel trims of any problem, be it being driven, or at idle or at 2500 rpm. All the LT trims sit within 1.5-2 points on either side
(- or +) of "0" and most of the time at about 0.08 minus or plus.
5. No indication of mileage being down or any power or noise problems. If you were not sitting in the car or looking at the engine very closely (where all you would see is slight vibration of the engine on manifold side (but you would have to look carefully) or sitting in the car and feel the tremor, you would hear a very quiet engine.
6. No indication of any motor mount problems.
7. Throttle body is clean- took Gus's suggestion and blew some cleaner through the car at 2500 rpm- (no change)
7. Checked last weekend for vacuum leaks with an ether based product- checking all the vacuum lines that Rick so nicely photographed and colored for us. Checked around all O-rings, joints/connections nothing with one minor exception.

I blew product all around air filter housing- nothing.

The only indication of any affect where at idle the ST fuel trims went from around +1 to a quick -8 to -9 (negative side) was when I blew product on the pipe area between the MAF and the air filter housing. (but not every time) There is a seam there, but nothing under magnifying glass or heavy amount of light indicated any cracks of any sort or fractures and when I sprayed directly on the MAF and it did the same negative bounce. My MAF O-ring looks fine and when removed you can tell it has a tacky/stuck feel with where it is attached and the 2 screws are quite tight.

So I am at a loss on the tremor/rough idle except for the area around the MAF and that seam before it goes to the air filter housing.
Would that be enough to cause a tremor with showing any ST or LT trim problems and since the seam area is before the MAF -would that make a difference. Someone was saying that even a crack in the air filter housing or before the MAF would make no difference, but I do not think that is right, although I am not sure of what the affect would be.

For info, I got a refurbished Denso MAF like the one I have and put it on for 2 days, disconnected the battery and reset and no change at all in Fuel trims or tremor. Took it back, put the old one back on and there I am. The scanner showed the exact same stats with the refurbed one as with mine.

All suggestions are welcomed

Thanks

Tom in Dallas/Plano
2005 S-Type 3.0 85K

 

Unless you got the spray into the actual air intake, doing what you did should not affect the trims. So, you may have a tiny leak there.

However, I'm a bit doubtful it would cause a rough idle if it's before the MAF.

I take it that it's not caused by the a/c switching on and off? And does it even if the a/c is off?

Does it do it in N as well as P?

 

Are you sure it's not normal? You have done good trouble shooting and you have no codes. Maybe everything is OK?

I find V-6's don't idle as smooth as a V-8.
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Just sharing because I had similar issue; there is vacuum line under the intake flex line. It's hard plastic line pushed into the flexible intake between air filter and throttle body. I missed this and it caused uneven idle.