4.0 GEMS. Need help. My idle rpm are too high. (1500-2000 In P or N and 1200 in D) My chinese scanner showed me wrong voltage on MAF. I ve changed MAF but the problem still exists. At 10 mph when the trottle is not pushed rpm jumps 1200-1000. Is it MAF or smth else?

 

Something else - the Idle Air Control Solenoid. Yours just pooped itself. It obviously controls the idle speed by allowing more air into the plenum to hit 650 - 700 rpm. Hows the 02 readings? They control the injector pulses - either enriching or leaning the fuel mixture by pulse width. The two are tied together to provide idle. When you do replace it you'll need to have the ECU reset, usually an independent LR garage can do so and will adjust the idle speed along with the reset.

 

Thanks. I still hope the idle control solenoid is fine. I ve heard an opinion that there might be a problem with a maf connector because the wires near the connector are completely worn out. I dont have o2 installed right now. I need to change the connector of it. I also didn't reset ecu after changing maf

 

Is the wiring on this rig jacked up?

 

Sorry, may be I misunderstood you, did you mean if somebody tried to modify or fix the wiring on his own? If so, the engine compartment wiring doesn't seem to bee touched scince 96) I have only found the different connectors for o2 sensors.dont know why

 

If you don't have the upstream o2 senors installed you are wasting your time with anything else. They send critical info to the ECU which manages the engine.

 

Sure, I understand that. And I am going to install them as soon as possible. But I ve got this car for about a year already and it ran fine without o2's(except fuel economy, of course). So I doubt that the main reason of my problem are o2's

 

Probably not your issue, but I once had rodents of some sort lodge acorns by the throttle linkage and a piece of broken shell was trapped out of sight under the linkage preventing it from retracting all the way resulting in a high idle. I checked a bunch of electrical issues too. Couldn't see the broken piece w/o really getting up on top the engine ...took a while to track it down.

Yeah, it's expensive to replace O2 sensors, but bite the bullet and your vehicle will run a lot better with them (especially new ones), and you should feel better performance and fuel economy. There is a very defined scientific reason determined a long, long time ago that 14.7:1 is the optimal Air/Fuel ratio to strive for in an engine (and boilers, etc), and O2 sensors are one of the key elements to achieve it.
https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio
On the downside, new O2 sensors past the cats might indicate your cats are worn ...and then you have an annoying light to stare at, even if the engine is running properly in 'closed loop'.

GM produced some really great (IMO) training manuals that covers theory and understanding on fuel injection, throttle body systems, OBD 1-2, and others. Here's an example. IMO they are worth reading just for their in-depth theory and the information is transferrable regardless of vehicle:
1986 General Motors GM Fuel Injection Tech Training Service Shop Manual Book | eBay

I think the other guys' suggestion of the idle stop is good. Isn't there someone selling replacement connectors? I thought I remembered seeing some, or did you get some that didn't fit? Yeah, these vehicles will make ya pull yer hair out sometimes!

 

Thanks for detailed answer! Yesterday I checked the maf connector and actually didn't find any problem with it. I'll check the thorttle linkage on weekends. But I also found new sound from air intake. It seems that it takes the air from somewhere else, but I could not find where. I detached the air intake, maf and everything connected to it and didn't find any holes or cuts. Refited it but the sound still remains. Ill try to buy new IACS these days
As for o2's, a ve already bought the propper connectors and I just need to replace them. I ll try to finish with electrics on Saturday

 

and by the way my scanner shows me errors with voltage on maf and on intake air temperature sensor. I think it's time to check the wires and connectors..