1997 b4000 wont hardly idle when warm

[paulsb4000]

So yesterday I got the lower intake manifold gasket and valve cover gaskets replaced on the 97 b4000 we have that my daughter drives on an almost daily basis, after finishing the mechanic said he wasnt happy with the idle that it was kinda high and the car had existing codes when i took it in one of which was for a lean condition which i hoped getting the intake done would repair, well one of the codes he said he was getting was for maf, to make a long story short my daughter needed the truck for work and the mechanic said it was driveable and we could bring it back at another time and try get it a little more sorted it, I thought the maf code was weird because it was not one of the existing codes when I took it in, and sure enough I look when i get home and the maf is litterally not plugged in, so I am happy and figure plugging it in will help, wrong, now the truck idles ok when cold but barely idles when warm, I am going to have my brother come clear the codes and drive it and see what comes back, but any ideas would be appreciated, also during the winter, the iac and fuel filter was replaced, i am really baffled because its a mechanic i have trusted for a long time and though the truck has always been a little rough around the edges, now its worse than at anytime we have owned it during the last three years and basically went from driveable to not being driveable.

 

[RonD]

MAF should be cleaned every few years on the 4.0l engines
Good read here on cleaning MAF
http://www.therangerstation.com/tech_library/CleanMAF.html


The computer runs in "open loop" when engine is cold, it only looks at or uses a few sensors during this time, MAF, TPS, IAC.
Once warmed up it uses the O2 sensors along with the MAF to set fuel air mix, this is called "closed loop"

The sensor that controls this is the ECT(engine coolant temp) sensor, it is, in essence, the "choke" for a fuel injected engine.
The ECT sensor is only used by the computer, it is a two wire sensor.
There is a one wire "sender" that looks similar, it is for the dash board temp gauge.

If the ECT sensor is failing then engine would be running too rich after it warms up.
Since your '97 is OBD II you can view live data using a scanner tool, i.e. see engine temp(from ECT) and lean or rich data.
The Bluetooth interface OBD II runs about $25 and you view data on a smart phone, never used one so don't know how detailed the data is.

The ECT is a simple resistance sensor, the computer sends it 5volts and resistance thru the sensor sends back a lesser voltage, the lower the return voltage the warmer the engine.
At 3volts returned(70degF) the engine is cold and computer runs engine rich with advanced timing and high idle, computer is in "open loop" based on this 3volts, so running a preset fuel/air mix.
As return voltage from ECT drops to about 1.5volts(160degF) the computer starts to reduce the idle and lean out the mixture, but still in "open loop".
When ECT return voltage is at or below 1volt the computer switches to "closed loop" and all sensors are used and idle would be "normal", about 750rpm.

ECT sensors rarely fail, the computer also has a timer of about 5-7minutes that will throw a code if ECT sensor voltage is still high, this code can come up if ECT sensor fails or if t-stat is stuck open so engine is not warming up fast enough.

ECT sensor is easy to test, you need a volt meter and a sewing pin.
Key on engine off
Set voltage meter to 20vDC
connect black probe to a good ground
use sewing needle to pierce one of the ECT's wires, test its voltage, above 4volts is the incoming voltage wire.
test other wire the same way, the wire with the lower voltage is the return signal wire

Now test the return voltage at different engine temperatures.

This is a good guide for the ECT and a good site for general sensors and what they do.
http://oldfuelinjection.com/?p=28