e30_eric Posted February 10, 2015 Share Posted February 10, 2015 Hey guys, I have a 2.5l tbi 5 speed Comanche that is misfiring and running lean. The components I have already replaced, include, map sensor, plugs, wires, distributor cap, and rotor. I have tried two different ignition coils. I have checked the voltage differential in the tps sensor and it is within range, I tested the ohm differential across the Mass Air Temp sensor and it was 1000ohms which from what i have read is spec. Yet when i pull the plugs they are dry and nasty which leads me to believe that is running lean. I'm at a loss as to where to go from here, any help would be much appreciated. Link to comment Share on other sites More sharing options...
Strokermjcomanche Posted February 10, 2015 Share Posted February 10, 2015 Have you checked fuel pressure and compression ? You can rent those tools for free if you don't have them . Link to comment Share on other sites More sharing options...
Pete M Posted February 12, 2015 Share Posted February 12, 2015 yup, your fuel flow might be struggling to keep up with demand. what year is your truck? Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 it is a 86 i believe, it was titled as a 88 but from what i can tell from the paperwork its really a 86. I did do a fuel pressure test and it tested at 15psi which from what i have read that is perfect, i also did a egr delete in case it was stuck open and that did not fix the problem either. I'm wondering if it is the coolant temp sensor, from what i have read that tells the ecu to adjust ignition timing according to temperature Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 this is her Link to comment Share on other sites More sharing options...
Strokermjcomanche Posted February 14, 2015 Share Posted February 14, 2015 That is an 87+ for atleast three reasons , it's a short bed and a sportruck and the emblems aren't italicized. Have you also checked to make sure it he intake manifold bolts are tight and not missing. Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 ok then 88 it is, i have not checked to see in the intake manifold bolts are tight but i know they are there. Do those bolts have a tendency to back themselves out? I ask because the truck has been running great for a few months now and just recently started this misfiring/backfiring problem. I will check the bolts tomorrow morning just in case. by the way thank you everyone for the input so far Link to comment Share on other sites More sharing options...
Strokermjcomanche Posted February 14, 2015 Share Posted February 14, 2015 The bolts have a tendency to either break clean off or back out . Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 ok i will check tomorrow in the daylight. if they do happen to be broken do you know what size and thread pitch they are so i can replace them? Link to comment Share on other sites More sharing options...
Strokermjcomanche Posted February 14, 2015 Share Posted February 14, 2015 3/8x16x1.25 These may not be your issue but it's worth checking instead of throwing parts at it. Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 Thank you, yeh i agree i would rather test everything possible before throwing parts at the problem. I would still like to know if the cts could be the culprit Link to comment Share on other sites More sharing options...
Jeep Driver Posted February 14, 2015 Share Posted February 14, 2015 O2 sensor. Link to comment Share on other sites More sharing options...
cruiser54 Posted February 14, 2015 Share Posted February 14, 2015 Remove the ground cable from the firewall. the one that goes to the back of the head. Scrape the contact area down to bare metal and reinstall. Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 Cruiser before i made this post i went through your tips and re did and cleaned all my ground connections with no improvement. Your tips are awesome though! Jeepdriver i thought about the O2 sensor, is there anyway to test it before i replace it? Link to comment Share on other sites More sharing options...
cruiser54 Posted February 14, 2015 Share Posted February 14, 2015 Cruiser before i made this post i went through your tips and re did and cleaned all my ground connections with no improvement. Your tips are awesome though! Jeepdriver i thought about the O2 sensor, is there anyway to test it before i replace it? Good to know. I don't know if those O2 sensors work the same as the 4.0s. There is a procedure for them. Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 yeh i just did some quick searching around the forum and found a older post from you outlining the test procedures for the 4.0 i will try it and see what i come up with Link to comment Share on other sites More sharing options...
cruiser54 Posted February 14, 2015 Share Posted February 14, 2015 For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor. One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor. The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground. At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth with the engine runing. If the O2 sensor readings are not right, say they read 4 volts or 1 volt steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 volts on the orange wire, and 5 volts on one of the two black wires), and ensure the ground wire (power off) reads less than 1 ohm to the battery negative post. A leaky exhaust system or leaky fuel injector(s), or bad compression, bad rings or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem. Link to comment Share on other sites More sharing options...
e30_eric Posted February 14, 2015 Author Share Posted February 14, 2015 ok so after a little looking, the manifold bolts are there and are tight, they look relatively new so I'm thinking the po replaced them at one point. The O2 sensor looks extremely old, maybe the original one that came on the truck, so i am going to order one of those and see where that takes me. I attempted to test it using the outline you provided for the 4.0 but the wires are different colors so i assume its not the same senor as for the 4.0's Link to comment Share on other sites More sharing options...
cruiser54 Posted February 14, 2015 Share Posted February 14, 2015 Not a bad plan at all. Link to comment Share on other sites More sharing options...
e30_eric Posted February 19, 2015 Author Share Posted February 19, 2015 ok so the O2 sensor came in today and i replaced the old one, she is still backfiring and misfiring and generally running terrible up until 180*F once she reaches 180* she smooths out and drives great, so the O2 sensor definitely was the fix at operating temperature. I'm thinking to fix the startup will be done by replacing the coolant temp sensor to help fix the cold enrichment then she should run like new. Thank you guys for all your help! Link to comment Share on other sites More sharing options...
Jeep Driver Posted February 19, 2015 Share Posted February 19, 2015 You've got an air temp sensor too, I would not overlook that. Link to comment Share on other sites More sharing options...
e30_eric Posted February 19, 2015 Author Share Posted February 19, 2015 Your talking about the manifold air temp sensor correct? Link to comment Share on other sites More sharing options...
e30_eric Posted February 19, 2015 Author Share Posted February 19, 2015 If so i checked that with a multimeter and it checked out Link to comment Share on other sites More sharing options...
e30_eric Posted February 25, 2015 Author Share Posted February 25, 2015 I replaced the coolant temp sensor today as well as changing the spark plugs, she is running at her best now. Thank you to everyone who helped with the diagnoses Link to comment Share on other sites More sharing options...
Jeep Driver Posted February 25, 2015 Share Posted February 25, 2015 Get rid of the C101 and I promise you it will run better yet. Link to comment Share on other sites More sharing options...
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