Hello, new member here with a 1990 Dakota that has a few problems that I am not quite sure how to fix or If I should take to a Stealership. I have had a couple of Dakota( never had problems with them) previously and now have gotten back with one for fun and work. 4x4 3.9 Auto. One of the problems is the idle is way high, I have not put a tach on it yet but it is high enough you really have to put your foot on the brake to keep it from moving in auto. I did a preliminary vacuum leak check and admit not very thorough. If you adjust the stepper motor screw it just compensates for the adjustment so the computer is holding it up. I replaced the temp sensor as it would not move the needle. I think I need to go back on the connections as the needle did not move much but then was jumping all over the place. I resealed the gas pump assembly as the clamp had failed also. The transmission does not go into OD 98 percent of the time and do not know if it is related to the idle problem I am seeking advice where to start as I was good fixing these years ago and seem to have forgotten a lot. Oh by the way it has no codes.

 

Look over the FAQ page. Check the codes. IDK 90's but it sounds like one of the sensors that informs the pcm that the mixture is right is giving bad data.
I had high idle in my 94. It turns out the guy I bought it from turned it up because he didn't know better. I think he was compensating for the bad plenum.
EDIT- there is a method for checking teh codes that does not require a code reader in the FAQ section near the top.

 

I checked the code and there is none, that is why I am confused.

 

Get a Factory Service Manual. Originally published by Bishko you can get a CD on ebay. You'll need it for this 29 year old beauty. All that I can offer is to clean the IAC if the 90 has one, Check for a plenum leak. and when I bought my 94 about 5 years ago I replaced all of the sensors in the intake, the throttle body, O2 sensor, just because the truck is that old and they all looked original.

 

A few notes.

1) Check COMPLETELY for vacuum leaks.

2) The gauge is on one sending unit; the ECU uses another one. I'd make sure the ECT sensor is in good shape; if it's open, you'll get a high idle. That's the OTHER one, the one over by the driver's side (2 wire). Shucks, pulling that is part of the instructions to set the idle speed at about 2500 RPM (!!!) (Special case with the sensor pulled).

3) I second getting the factory service manual; Haynes and Chiltons tend to vary from "Excellent" for a few items to "Doesn't even make good toilet paper" for others (the latter especially for wiring problems.)

That's what I'd suggest at first.

Also, do a complete (and I mean COMPLETE!) tune up; plugs, wires, cap, rotor, and air, oil, and gas filters. Don't skimp on ANY of it. Also, due to age, I'd do the O2 sensor. Being a 1990, I'd bet it's got an IAT sensor bolted to the throttle body; I'd replace that too. And due to age, I'd consider the oil pressure sending unit/switch assembly on the back of the block.

RwP

 

So I went back and looked closer and I did find a wire off the temp gauge sensor on the passenger side of the engine buried under a lot of stuff. I did a lot of checking vacuum lines etc. Engine is running about 22 inches of vacuum. at a rpm of 2000. After I plugged the last sensor in it started and operated normal. Today I finished with putting the tach in and took it for a drive . At 6 miles out it was starting to get higher rpm. By the time I got back it was at 2000 rpm so it is back to the drawing board. I did not check the timing because I can't get it to idle down once it is hot. the transmission shifted normally only for about a mile on that trip so it never dropped down at 60mph. Maybe the stepper motor is the problem. I may take it in.

 

I will repeat again - the ECU uses ONE sending unit, the GAUGE another one.

You're looking for the two wire sending unit - on a 1990, that'll be over at the driver's front of the intake manifold, lined up with the EGR valve just about.

Check that sending unit; if it fails, idle will go high.

Also, grab the factory service manual. It includes instructions on how to set the idle on our trucks (the TBI ones), and it includes unplugging the ECT and setting for about 2500 RPM when warm ...

RwP

 

Welcome! I always loved the 1st Generation Dakotas!!!

 

I replaced that sending unit first and went back and checked the connector second.
I don't have a lot of time during the week and need it so I took it to the dodge dealership and will see what they find and let you know. I did not plan on the need for the pickup this fast. The manual will have to wait until I see what they come up with.

 

So the dealership said all my idle problems comes from the Idle air control- well I took it apart and the nylon gears are stripped. so I ordered one last night and see what happens when I get it on. The plugs looked great, gapped them. The cap and rotor need replacement. If I can get the correct Idle, I will check the timing after I get the stepper motor on and set. then if it runs good I will tackle the transmission issues.