Soaknfused

OK, just as it sounds.. I will be as specific as possible, I have searched many forum posts and found zero specific information about my particular trouble.

Here is what it does, when driving down the road at 55 mph and letting off the gas completely, as the car coasts from 55 to 50mph the engine RPM fluctuates from 1500-1700 RPM consistently (up, down, up, down rapidly). as soon as it hits 50mph it stabilizes at 1500 RPM. It's not the tach as I can feel the engine speed changing, it is rather annoying to say the least.

Has anyone had something similiar happen? I have replaced many many parts and this has been the same way for quite some time now. All I have left to replace is the TPS and MAF and to clean the IACV.. every other sensor is new, all injectors have been replaced etc.

Thanks in advance :-)

LvR

Auto or manual box?

Clean the IACV before you even start trying to solve this

Soaknfused

Auto :-).. so, IACV could be a culprit eh.. I wasn't sure what bearing it had on engine operation above 1000 rpm.. I really need to find about a 1500 page book about this IACV.. i get interested in learning about something and I can't help myself lol..

LvR

The IACV is in fact nothing more than a computer controlled throttle (just like the one you control with your foot) sitting there ready to make your life a misery soon as things go wrong.

I would guess that given the right circumstances (of things wrong or not as per design on the motor) the IACV can affect the engine's ingested volume of air/fuel to cause revs to climb as high as 2500+rpm (park or neutral on the box)

Not saying in your case it is the cause, but if you know its suspect start there.

With the auto showing that particular behavior its quite possible that you have a funky solenoid signals coming from the GCU - so check:

1. Alt and charging system OK - 13.5V above 1500rpm irrespective of electrical load?

2. All engine and box electrical connections OK? - especially ground connections? - go through them all and remove clean and install them all again

3. Box itself - fluid level and quality of it?

4. If you disable OD, do you still get the same response when slowing down? - to me it basically sounds like OD kicking in and out when you experience the problem................

5. Another possibility - the lockup converter feeling ill? - either by itself or as a result of funky control signals from the GCU

6. Does this happen only when the box is warm or does it also happen when its cold?

Nmax92

the same thing happened to me. it turns out it was my tranny(auto) that failed on me. It felt like the it couldn't find the right gear to get into. Get it checked out.

nismax1994

Mine has done the same thing for years now. In my case its the torque converter locking and unlocking. I first noticed this in '01, had the tranny rebuilt in '02 (for other problems). And it still behaves this way. I just avoid lifting the throttle between 50-55. I figure the TC operates on some criteria but gets indecisive at this point.

Cliff Clavin

iTrader: (11)

Manual or Auto tranny?

Also, what engine?

Did you check for codes?

Soaknfused

Once again, its a VG Auto (92 GXE with 140,000 miles). I have checked for codes, replaced pretty much every sensor on the vehicle (engine anyhow) the transmission works great, good solid shifts and has had the fluid changed every 30,000 miles since new. The fluctuating is actually in the engine RPM.. i can feel it surging slightly and it stays in gear and is not affected by turning off OD and it still does it when shifted into neutral. The change from the lock up of the torque converter happens at 55-60 and I have paid attention to that and it happens at a different RPM than this issue. I have replaced all injectors, O2 sensor, TPS, plugs, wire, cap and rotor, CTS, cleaned the manifold, TB, MAF and have checked everything for vacuum leaks. Only thing left to fix/service is the IACV. Other than the things listed above, the only thing I haven't replaced is the thermostat and knock sensor and at this time, i have no intentions of replacing either untill needed.

CapedCadaver

iTrader: (8)

i'd say whip out the fsm and a DMM and start testing all your harnesses and such. just because a sensor is replaced doesn't mean it works. take my O2 sensor for example. I replaced it, but it didn't help at all. Wasn't until then that i noticed my harness was the problem. I had broken the groundwire (well, the PO had it break when they had it. cuz mpg was always bad when i got it) aaaaanyhow... i regrounded it and now i get better mpg. (click any of that part to see pictures of my fix) So it's not always the sensor that causes the problems....

Soaknfused

I continued my quest to get my max as close to running like it was new as possible and took off the IACV and cleaned it all out. My RPM fluctuation between 50-55 mph is completely gone. After disassembling the IACV and cleaning it I really learned a lot about the operation of the device. It appears there is a thermostatically controlled aparatus inside that changes the position of the butterfly inside it to control idle speed during cold start ups, had no idea that was in there. It wasn't terribly dirty which I take as a good sign but definitely benifited from a good cleaning. When I completed the task and started it up it idled at almost 1500 RPM, so I followed the procedure in the FSM to set the idle speed correctly (diag mode on ECU, 2-3000 RPM then idle and adjust) I had to turn the idle screw in almost 6 complete turns to get the idle to 750 RPM. It now idles at 750 no matter what, in park, drive, AC on or off etc. All that is left to do now is move onto the mechanical things such as replaced worn control arm bushings and replace leaking flex section in my exhaust (planning on a WSP Y Pipe to kill that problem. Once I get everything up to snuff its time to move onto a 5-speed swap, suspension upgrades and then TURBO :-) I have to say, I am loving this site and donating to get search ability was a very very good investment. I plan on keeping my max forever, yep.. I said forever :-)

CapedCadaver

iTrader: (8)

me and you are twins!! both have wonky IACV, both have flex-section exhaust leak, both have crapped out LCA bushings, both want to do WSP to fix the leak, both want 5spd swaps, suspension upgrades, and turbo. In that order? It's cuhwayzee. I think i am going to really clean the mess out of my air cut valve (the coolant-controlled butterfly thing), because i think that may be part of my problem. Like it's not acutating at all.

Nmax92

Soaknfused - what did you use to clean the IACV?

CapedCadaver

iTrader: (8)

i used carb cleaner last week. it worked, idled low for a day. but didn't idle-up for gear or AC... and it's back to its old crap again after just 2 days and i just don't know what to do to make it work and stay working.

Nmax92

do you think you might need a new ?

Nmax92

i meant a new one

CapedCadaver

iTrader: (8)

not yet. i don't want to spend all that money until i get my TPS issue resolved also. Got a baddish harness on it, lots of green stuff on the connectors. So i'll do that first, and Aaron (NissanWORKS phenolic spacers) is sending me his test IACV and i'm going to put that on there.

Soaknfused

I used TB cleaner and a tooth brush to clean everything out. I took it all apart and cleaned everything, replaced the gasket too (made my own) Its working great now.

side note, I also replaced the TPS as well.