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When cold won't rev issue 89 xj renix auto

R

revmeup

NAXJA Forum User
Location
Reno NV
So before everyone chimes in.....new parts are as follows:
Cps
Tps
O2
Fuel filter
Battery
Icm
Coil
Plugs

Here is the issue:
Starts up first thing in the am. Ambient temp about 25. Idles ok but if I give it any throttle it coughs and sputters. If I let it warm up it runs fine the rest of the day. It acts like an open/closed loop issue but as you can see I installed a new o2. 5-7v on the preheat. One other thing I noticed is the plugs are showing to be lean. Fuel psi at idle is 38 and with throttle is 34. Tps is calibrated to 83%. 4.58v/3.79v Removed and cleaned iac and tb.

Open to ideas.
 
The sensor ground circuit can have too much resistance. This affects your CTS, TPS, IAT and MAP.

Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector
of the TPS . The letters are embossed on the connector itself.
Touch the black lead of your meter to the negative battery post.
If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be
necessary. The harness repair must be performed before proceeding.
I can provide an instruction sheet for that if needed.
 
I checked it the other day and had 1.5. Installed a ground from the neg post to the firewall right at the start relay and now have .8. Should I follow the instructions you sent me in the pm to mod the sensor ground harness anyway.
 
I traced all the brown with white wires. They all go into the loom which goes directly into the firewall just behind the intake manifold. No c101 connector present.
 
89 and 90 had no C101. Instructions still work. Wiggle the harness next to the valve cover and near the MAP and see if the reading changes. I would do the mod regardless.
 
Sounds like it might be a bad CTS or IAT sensor, coolant or Air Intake temperature sensors. Temperature related issues I would start there, but also recheck the O2 sensor data. Also check the vacuum connection to the MAP sensor at the throttle body (loose) for a small cold temp vacuum leak.
 
So I located the wire union for the sensor ground repair. Before cutting any wires here is what I have. Ohms at union is 17+ with pos lead at union neg lead on neg battery post key off. Also checked ohms at union and on body ground instead of the battery and got .6.
 
revmeup said:
Cts and iat are in range. Installed new grounds and checked ohms for sensor ground mod.
Click to expand...

When I see posts like this, with no data other than "in Range", I always wonder if the sensor was tested hot, and cold and if the temperature was tested with the resistance, and both data points used to verify that the sensor is in fact working at, at least two extreme temperatures when compared to the OEM FSM data tables?

17 ohms on the ground is a definite problem for some of the sensors, like MAP, TPS and the O2 sensor.
 
I checked them hot and cold and compared with fsm scale to confirm. They are both in range. And the 17+ ohms is at the battery post. I am getting .6 on the body where I ran a ground from the neg post to the body. So I infer the brand new ground from the block to the neg post is the issue. Except I get .5 ohms through that cable. So the resistance is between the sensor and the block. I also replaced the intake ground to the firewall and the ground from the dipstick to the body. Removed inspected and cleaned the wire harness at the dipstick as well.
 
Did the sensor ground repair suggested by cruiser54.
Also added a ground from the new union of sensor grounds to a separate body ground.

Resistance from neg cable to neg post is .4 ohms. Cleaned all parts and reinstalled with no change. Also checked both sides of the trans tps 3 wire connector. 5.39 volts for both sides and double checked with a spare tps to confirm readings. Then rechecked volts at the 4 pin and still have 4.56v/3.79v. Also checked ground again on the tps at the battery and now down to 4.1 ohms. However when I check the ohms on the firewall or any other body ground I get .6 ohms.

Good news is it now seems to have cured the initial issue of not revving when cold.
 
Resistance should be tested with no voltage or current running throught the circuit; essentially just the wire. You won't get accurate measurements if you have any current/voltage on the circuit.
 
It's from the A pin on the 4 pin connector to the neg post on the battery. I was getting over 17. Then like I said I checked it from the same pin to the firewall and I am getting .6 ohms which is below the level you said I needed. So next would be to back probe test the harness ground at the dipstick to see if I can figure that one out.
 
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