High Erratic Idle

[Jdm914]

My 1998 Honda Passport has very high erratic idle. It is a V6 4WD Automatic. It runs excellent except for the idle. Many times when decelerating the idle will jump up to 1800 rpm and down again. If I put it into neutral whilr driving at 30 mph, the engine rpm it will race to 3200 rpm. This is an intermittent problem which occurs about 65% of the time. I own an OTC Genisys and recovered the real time data while this was happening. All sensors check out fine (in range); but the timing advance is unstable causing the idle to increase as the timing advances. Normally the advance at idle is 16 to 18 degrees. For some reason the timing will advance at idle up to 36 degrees. I do not know why this happening??? The check engine light is not on.
Have anyone seen this problem before? If so, what is the cure? If not, do you have any ideas or suggestions? Here is a list of all the work that I have done to the car.

Tune-up: plugs, air, fuel filter.
Replace timing belt, water pump and all rollers.
Replace both upper and both lower intake gaskets.
Cleaned EGR valve and pipe.
Cleaned MAF sensor
Cleaned throttle body and adjusted butterfly. (Previous owner had messed with it)
Disconnected P/S pressure switch (can cause high idle)
Replaced MAP sensor
Cleaned transmission selector switch

Engine light was on when I purchase the Passport.
Codes were:
TPS (replaced)
Cam Sensor (replaced)

[amigo-2k]

Most likely it is the intake manifold gasket.

see the FAQ's

[Jdm914]

The upper and lower intake gaskets have been replaced. The intake vacuum is well within spec. and it also passed a propane gas check.

[amigo-2k]

what torque specs did they torque the gasket to? The are pretty easy to bust and Isuzu has changed the spec 3 times. My last gasket on my truck lasted 20k (its been replaced twice under warranty).

[Jdm914]

The intake is torqued at 14-16 ft/lbs. If it was the intake gasket the problem would be consistent. Sometimes it will run fine for several days. The passport passed the propane leak test. Thanks for your input!!

[Gizmo42]

Try cleaning or replacing the Idle Air Control. That has caused some idle problems in the past.

Btw, latest torque spec for IMG gaskets is 13 ft lbs.

[Jdm914]

The IAC is clean and working properly. Thanks for the suggestion.

[amigo-2k]

maybe this???

http://forum.planetisuzoo.com/viewtopic.php?t=14604

[Gizmo42]

The '98s werent drive by wire though so that wouldnt help. Not sure what else would cause it.

[trooperbc]

the thing that jumps out at me is:

"" Cleaned throttle body and adjusted butterfly. (Previous owner had messed with it)

there's a clue in there somewhere. first, since you have invested in that genisys, i assume you have likewise invested in the full oem factory manuals www.helminc.com
if not, i suggest it.

so did you use the manual to adjust the throttle valve?
what about the rest of the throttle body components? are you following the manual there, too?
and the thing i don't see mentioned is the Throttle Body Valve or air bypass valve. (but maybe i missed it, that is one crassyarselong list). this is my best guess from here. if its been treated badly, it can be ruined. it, with the computer, learns positions for particular conditions, and then resets from there. before going into this more, tell us what you have done with this, if anything, and if you have used the factory manual to diagnose this.

//bc

[Jdm914]

I do have both volumns of the factory manuals. The previous owner must have had the same problem that I am having. That is why I guess they messed with the factory sealed throttle body adjustment. I adjusted it back to what it looked like the factory setting. At this point the TPS was giving a .5 volt signal. This is within spec at idle. The range on the TPS is 0-5 volts. I have also switched the IAC with a spare with no change. What I think I will try next is a complete good used throttle body assembly and see if this makes a difference. I am not one to be a parts switcher but in this case I will try anything. Keep the suggestions coming!!! Thanks

[trooperbc]

okay, yeah, what i was trying to talk about there was the IAC idle air control valve. i agree to not being one to be a parts switcher, but then there's not much left, is there.

i just have the feeling that if you go back to the basics, maybe it'll lead you in the right direction. everytime i think of something, i check your list, and you've done it. and i get led back to the initial throttle position, and the IAC, especially the IAC.

btw, is your sense that the timing advance is a result of the rpm gain, or the rpm gain is a result of the timing advance?

//bc

[Jdm914]

My first thought is the erratic idle is caused by the fluctuating timing advance. The three sensors that have the most input for the timing are TPS, MAP sensor and the Mass air flow meter, but all three are in factory spec verified OTC Genisys. I have checked all ground points and they are OK. A bad ground would also show up on Genisys. The alternator is putting out the correct voltage (13.8v). I have doubled checked the cam timing and all is fine. So to answer your question my gut feeling the erratic timing advance is the cause but at this point I would not bet on it.

[Jdm914]

I tried a complete used throttle body with IAC and TPS. I am still having the same problem.
Has anyone out there seen a bad transmission range switch (4L30-E) cause any problem like this?
Bad or internally corroded engine wiring harness?

OPEN FOR MORE SUGGESTIONS!!

[amigo-2k]

have you tried tugging on the drivers side wire harness while the truck is running? maybe there is a break in a wire?