1987 Jeep YJ Wrangler 2.5L Odd Scanner Readings

[tshasha]

Hello,

 I have a 1987 YJ 2.5L. It has a bad miss at idle once warmed up.

It has a new fuel pump, MAP, Coolant sensor and the mechanic put a used fuel pressure regulator in it. 

Today, I hooked up my old Snap-on scanner. It shows that it is staying in open loop, it has 0 hg. of manifold vacuum, and coolant is not going over 155 degrees. I changed the coolant sensor and it reads exactly the same.

Any thoughts would be greatly appreciated.

Tony

[Moses Ludel]

tshasha...Although your scanner may be accurate, I would run some manual tests.  With an infrared non-contact thermometer, see what the actual temp is at the thermostat housing and radiator.  If it's truly 155 degrees F after warm up, change the thermostat to a new 195-degree F unit.  See if that takes the system out of open loop and stabilizes the idle.

As for manifold vacuum, same thing.  Check the actual manifold vacuum with a simple vacuum gauge at a manifold port below the throttle body.  These mechanical tests will indicate more than a scan, as the scan tool depends solely on sensor and ECU data.

If the problem persists after these checks and repairs, test the pressure at the TBI pressure port.  It should be 14-15 PSI.  Verify that the fuel pump part number is correct for a 2.5L TBI application.  A later pump application for MPI will put out too much pressure.

Let's go from there...

Moses

[tshasha]

Hi Moses,

  Thanks for what you do here.

 I did check with an infrared and the temp at the thermostat and surrounding area was around 150 degrees so I will get another thermostat. 

But, I went a bit farther checking the temp sensor and circuit. The troubleshooter on the scan tool said that when unplugging the CTS with KOEO, the scanner reading should go to 247 degrees but it actually goes to -40 degrees.

I then checked the CTS circuit. The ground circuit is good but the circuit from the computer that should have 5 volts has just .62 volts with KOEO. I assumed I had a break in the wire somewhere. The wiring diagram I found shows that wire going straight back to pin 15 on the computer but when I traced the line it went straight to the diagnostic connector. 

What am I missing here? I have a feeling the wiring diagram doesn't show the complete picture.

Thanks in advance.

Tony S.

[Moses Ludel]

tshasha...You're trying to be thorough and using good troubleshooting methods.  In case you don't have an FSM for your model, below is a diagram from the FSM, covering the CTS wiring.  Zoom-in for better detail.

Do a continuity/Ohms resistance test from Pin 15 on the ECU to the CTS connector.   Check Ohms resistance for opens or high resistance.  Then test your CTS itself.  Forget the scan tool for the moment, simply disconnect the wiring from the CTS and check across the poles.  According to the wiring chart, at an ambient temp around 100-degrees F, you should get an Ohms reading of approximately 1365 ohms.  If you run this test with the engine or CTS at 220-degrees F, the reading should drop to 93.5 ohms.  Factor for temperature points between these thresholds.  These sensors usually work or they don't.

I would rule out the CTS and poor wiring connectors first.  The most likely culprits are 1) a poor connection, 2) corrosion/oxidation at connectors or 3) a defective CTS.  Don't replace the CTS unless it fails the Ohms test. 

Grounds are critical on the 2.5L TBI YJs.  Check your grounds, especially the firewall/dipstick tube ground.  Oxidation can build over time and increase resistance.  The ECU is always a possibility, though that would be a long shot.  If you clean pin connectors, use a soft nylon tool and electrical spray cleaner.  Do not gouge terminals with a metal scraper or screw driver.  You simply want to remove oxidation, which is usually black.

Let's see what you find...

Moses

 

 

[tshasha]

Hello,

Resistance between pin 15 to the connector is 00.1  Crossing the pins of the disconnected temp sensor, I read 1.4K (infrared said about 100 degrees at the temp sensor). I also checked continuity between pin 32 (ground) at the ECU and the ground wire at the connector at the CTS and it is good. 

Even though I am only getting .62 volts from the computer at the connector, it is reading 155 degrees on the scanner, which is about what the infrared is picking up at the thermostat housing. I read that the computer may be substituting air intake temperature if there is a problem with the CTS circuit.  

I think this is starting to point to the computer?

Thanks

[Moses Ludel]

tshasha...These ECUs deteriorate or fail at higher mileage.  That's always a possibility.  I would, however, install the new thermostat before getting your ECU rebuilt or exchanging it for a rebuilt unit.  The thermostat needs replacing regardless and could be the cure for your problem. 

Where did you get the information that Pin #15 should have 5 volts with Key On/Engine Off?  Based on how these sensors work, #15 pin and #14 pin should not be feeding 5 volts to the CTS or Manifold Air/Fuel Temp (MAT) sensor.  The CTS and MAT sensor each complete a ground.  I think of these sensors like rheostats.  The ground signal from Pin #32 is constant input to one pole of each sensor.  The ground "amount" or resisted ground flow to Pin #15 or #14 is governed by the sensors' resistance at a given temperature.

Temperature at the sensor varies the degree of Ohms resistance across the sensor poles.  As the CTS or MAT heats up, the Ohms reading across the CTS or MAT poles drop in resistance.  If the coolant temperature sensor could reach near zero Ohms, the #32 ground side Ohms would be nearly the same as the input (ground) Ohms reading to Pin #15.  (The 0.62V reading that you get at Pin #15 is likely residual feedback from the ECU circuits.  Is this negative or positive DC voltage on your meter?  It may be immaterial.)

The MAT will likely not read 155 degrees F or the same as the coolant temperature at the thermostat housing.  (Aim your non-contact thermometer beam at the base of the MAT when the coolant is at 155 degrees F.  Is the MAT reading lower?)  The MAT is the intake stream air/fuel mixture temperature and not the engine's cooling system/coolant temperature. 

#15 and #14 do share a common, full ground from Pin 32.  However, the two sensor circuits are independent of each other.  The ECU is apparently comparing a full ground (Pin 32) to the partial or resisted grounds feeding to Pins 14 and 15 from their sensors.  Again, at varying temperatures, the sensors act to reduce or increase the ground signal Ohms.  The ECU weighs the Ohms readings at Pin 14 or 15, which is a reflection of each sensor's temperature reading.  

The base ground signal (Pin 32) establishes a "constant" reference point.  The #14 or #15 feeds from the MAT or CTS to the ECU vary in Ohms.  Each sensor is affected by temperature.  The MAT reads incoming air temperature.  The CTS reads the coolant temperature.  Worth noting, the Ohms resistance calibration for each of these sensors is the same at a given temperature.  See the charts below:

Glad you tested the #32 ground feed.  The sensors do require an accurate ground signal from the ECU.  This signal is then fed through each sensor and altered according to the temperatures.  The sensors' output poles take the Ohms resistance to the ECU pins #14 and #15.

Footnote:  Continuity/resistance tests are not the same as a load test.  A good test for the integrity of ground circuits is a "lamp test".  This will indicate the load carrying capacity of the ground wire(s) and the circuit.

Scan tools are very useful, but there are some basic mechanical and electrical functions involved here.  Before you dwell on the scan tool findings, try changing the thermostat to a new 195-degrees F replacement unit.  Make sure the thermostat's bypass bleed hole is at 12 o'clock.  You may find the fix here...

Moses