Rough idle [to beat a dead horse]...need some direction

[RunnerPDX]

Alright, I'm in need of some direction...

Have '99 4runner, second motor 5vz-fe [155k on the second [new seals, timing belt etc etc] ...

Started with a sluggish hot start, that progressed to engine vibration/rough idle at +/-600 rpm. Drove me nuts. Beyond that, my MPG sank to about 12mpg around town. (was 16 around town) Highway, +/- 17 on a good day so long as I keep my foot out of it.

No codes, ever [CEL works]. Feels like a slight misfire at idle, put put put...and feels less powerful in the throttle...exhaust smells rich [but no smoke] and it's annoying the hell out of me.

Hooked it up to a diagnostic, no misfires, short-term and long-term fuel trims are acceptable.

Bring on the troubleshooting...here's where I have been:

1) Replaced MAF - OEM
2) Replaced both O2 sensors - denso
3) Replaced fuel filter
4) Brand new coil packs, plug wires and plugs -- all denso, ngk etc..
5) 6 fuel injectors from GB [cleaned intake plenum, new gaskets etc]
6) Throttle body, cleaned like a hospital toilet
7) NEW IAC valve and gasket
8) Tested the cat, not plugged...just fine

After ALL of this... still getting a rough idle. Sticky valve maybe? SOO time for a compression test... Plugs all looked great btw, no oil, excessive carbon ...

Engine temp is good also, and fluids are all clean

Cylinder compression:

1) 165
2) 165
3) 155
4) 160
5) 170
6) 165

Ok, #3 had me wondering...a little oil in that cylinder and tested again... shot up to 210psi... rings are likely worn for sure, but still getting acceptable (albeit not great) compression.

Where next?

Can this cylinder be causing the slight vibration/rough idle? OR, could this be an ever so slight vacuum leak...haven't done a vac test...

I'm lost at this point...

 

[davidch]

Valve clearances? (You're lucky in that you can check the clearance on #3 with only minimal surgery.)

I know you did the oil-in-the-cylinder diagnosis and this raised compression. However, a large enough amount of oil would of course raise compression irrespective of the condition of the rings. I always thought that oil trick - and I know it's been around since Henry Ford got his learner's permit - would work best only on an engine with a perfectly vertical piston. Otherwise, how much oil to add to a given cylinder to make the rings seal better in the few rotations of a compression check? Did Henry ever try it on a boxer motor?

 

[JayRolla]

You have pretty much covered every basis I would have done. I just went through this with a buddy and he replaced the MAF which has fixed it. Have you actually pulled each plug wire connected to a spark plug and watched to see if they are all firing? I would bring it into a good known dealer and see what they tell you, but again I think they would be lost also. I know I am.

 

[shotgun_willie]

Not the exact same issue as yours but my truck was overall running like shit until I replaced my valve cover gaskets. My truck was missing a bit, idling rough, and throwing several CEL codes. The valve cover gaskets were leaking a bit but I was able to ignore it (lost about a quart between oil changes). I replaced them 2 weeks ago and it's like driving a new truck. All of my above issues disappeared.

I'm sure you've probably already considered it but thought I might throw in my $.02.

 

[RunnerPDX]

Valve clearances? (You're lucky in that you can check the clearance on #3 with only minimal surgery.)

I know you did the oil-in-the-cylinder diagnosis and this raised compression. However, a large enough amount of oil would of course raise compression irrespective of the condition of the rings. I always thought that oil trick - and I know it's been around since Henry Ford got his learner's permit - would work best only on an engine with a perfectly vertical piston. Otherwise, how much oil to add to a given cylinder to make the rings seal better in the few rotations of a compression check? Did Henry ever try it on a boxer motor?

Ok, checked valve clearances, all good. Valve cover gaskets are good also...

Finally took it in and had a smoke test done, and was told that there is a very small leak coming from what appears to be the lower intake manifold gasket... [not the plenum but below that...] ...$%^&.

Not enough to throw a CEL, but enough for me to notice something is amuck and reduce some power.

How tough is it to remove the lower intake manifold on the 5VZ-fe? Anyone have a solid write-up for that...looks like something I could unbolt after I remove the upper plenum, injectors, rails, drain the coolant etc...

Even though it sucks, at least I have a direction...any help on the tear down is appreciated!

 

[Troybylt]

Have you checked or replaced the Throttle Position Sensor?

I was having crap power and Rough idle with terrible MPG and no codes for a short stent, ended up being the TPS.

 

[RunnerPDX]

Closing the loop here, fixed the problem.

Replaced the lower intake manifold gaskets. When I had the engine put in originally [ would have done it myself, but lack of tools and time]...the shop reused the intake manifold gaskets and put RTV on the end coolant ports in an effort to make those work when they replaced the knock sensors. Awful...

2 new gaskets and a TPS [just in case, since it's a fairly cheap part]...She's running strong again!

My MPG's hopped up to almost 16 around town. Up from 12, I'll take it.

Even though this was a small vacuum leak [luckily not coolant], it made one hell of a difference.

 

[LittleCaesar]

Good to hear.

My rule of thumb is to replace a gasket when digging into an engine if it has more than ten years on it or if it has come into contact with RTV, etc.