TheDude Posted February 23, 2020 Share Posted February 23, 2020 I've been working on my good buddy's 86' 2.5 4 speed 4wd he recently purchased. He put a fuel pump in and it runs consistently but it's very rough and very rich. I'm not as well versed on the TBI setup so the stepper motor and the crazy intake setup threw me for a loop. Also no port for checking fuel pressure is making diagnostics difficult. Anyone have some good ideas on likely suspects? I have an MT2500 BTW and MAP seems to be reading correctly and engine vaccum seems good as well. Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
eaglescout526 Posted February 23, 2020 Share Posted February 23, 2020 Right where I circled in red is the test port for fuel pressure. It is plugged by an allen head plug. I have yet to find a pressure tester to fit that port but I also haven't really looked lol. How does the O2 look? Perhaps the PO jumped the pump wiring just so he could get it to run. TPS is a good one to check to. Actually checking all the sensors is a good thing to do too. Link to comment Share on other sites More sharing options...
TheDude Posted February 23, 2020 Author Share Posted February 23, 2020 Thanks for the pressure port info.My friend out the pump in himself so that shouldn't be the issue but I would really like to know for sure.The O2 is old and reading high but that would be correct if the Jeep is running overly rich.Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
Jeep Driver Posted February 23, 2020 Share Posted February 23, 2020 The air temp and coolant temp sensors are just as critical as the O2 and the MAP. Replace your line to MAP and clean your TBI ports. The stepper motor is.....well........ You can unplug it and adjust the plunger to manually set the idle. You will find it listed for Cadillac, BWD...........and a POS and rather meaningless anyway. There are actually two gauge port plugs, careful, plugs are brass or some kind of pot metal and will strip very easily. Threads are 1/4 NPT. Also, if all else fails.....may be a bad injector also.......see https://www.motormanfuelinjection.com/New-Fuel-Injectors.html you may have to call. Pressure should be 15-17lbs. Link to comment Share on other sites More sharing options...
cruiser54 Posted February 23, 2020 Share Posted February 23, 2020 Make sure the intake manifold bolts are snug. Inspect these hoses closely. Link to comment Share on other sites More sharing options...
TheDude Posted March 7, 2020 Author Share Posted March 7, 2020 So we replaced the coolant temp sensor and now the coolant and intake temp sensors are reading negative values on the scanner.MAP reading 30hg as well.Unlikely both those sensors are bad, so is there a ground both of these share?I also replaced battery to block neg cable and added ground to rad support. Block to firewall ground has already been replaced.Front most manifold bolt is broken, rest are tight.Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
Warren Mohler Posted March 7, 2020 Share Posted March 7, 2020 Neg. values are a problem if old sensors did not read the same, I would swap the old back in to start.I have had injector o-rings leak and it's hard to see running,sometimes aiming a timing light at inj. spray may show trouble-large droplets, stepper motor MUST be working as ECU needs to see closed throttle or it will do all kind of bad stuff Link to comment Share on other sites More sharing options...
cruiser54 Posted March 8, 2020 Share Posted March 8, 2020 54 minutes ago, Warren Mohler said: Neg. values are a problem if old sensors did not read the same, I would swap the old back in to start.I have had injector o-rings leak and it's hard to see running,sometimes aiming a timing light at inj. spray may show trouble-large droplets, stepper motor MUST be working as ECU needs to see closed throttle or it will do all kind of bad stuff Yes!! Intake air temp and coolant temp share the same ground. And both go through the c101. Link to comment Share on other sites More sharing options...
Jeep Driver Posted March 8, 2020 Share Posted March 8, 2020 58 minutes ago, Warren Mohler said: stepper motor MUST be working as ECU needs to see closed throttle or it will do all kind of bad stuff I drove with it unplugged for 5 years. It does nothing. Link to comment Share on other sites More sharing options...
TheDude Posted March 8, 2020 Author Share Posted March 8, 2020 Yes!! Intake air temp and coolant temp share the same ground. And both go through the c101. This is an 86' I thought the first year didn't have the c101 but maybe I missed it.Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
cruiser54 Posted March 8, 2020 Share Posted March 8, 2020 I got that from the 1986 2.5 wiring diagram..... Link to comment Share on other sites More sharing options...
TheDude Posted March 10, 2020 Author Share Posted March 10, 2020 I got that from the 1986 2.5 wiring diagram..... This 86' doesn't have a c101 so I'm having a hard time finding info for testing.The IAT shows 1.3v in and has ground.I for the life of me cannot find info on what sensor should get what voltage and I have an 86' Comanche manual I've been looking through.Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
cruiser54 Posted March 10, 2020 Share Posted March 10, 2020 IAT should only show ground. Ohms. No power. Link to comment Share on other sites More sharing options...
TheDude Posted March 11, 2020 Author Share Posted March 11, 2020 That makes sense now.So I have a spare ECM of the same year and setup and when I hook it up all the temp sensors read correct. A good 60 degrees more than with the original ECM.But this ECM isn't known good and I lose spark when it's plugged in so now I'm thinking they're both bad just in different ways.Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
Jeep Driver Posted March 11, 2020 Share Posted March 11, 2020 I'm getting as far away from Renix/Chryco as I can............can't run fast enough or far enough. Link to comment Share on other sites More sharing options...
TheDude Posted March 25, 2020 Author Share Posted March 25, 2020 So here's an update: Found a matched ECM local by chance, plugged it in and both ECT and IAT read perfect. So I started it up and it was the best it's ever ran for me. I figured cool that solves that problem and moved on to replacing all the brakes, lines, calipers etc.. I go to run it again today it's running rough, BUT it's not running super rich like it was before, O2 switches, Temps read good, everything I see seems good except the map seems a tad low. Checked the fuel pressure at the filter and showed 19psi. Tried lightly clamping the hose to bring it down and it didn't run any better. Sprayed the intake with brake clean to check for intake leaks but no dice. I sound like their may be a exhaust manifold leak? The exhaust is trashed so it's really loud regardless any thoughts? Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
Ωhm Posted March 25, 2020 Share Posted March 25, 2020 Both BARO ("Hg) and MAP ("Hg) display a absolute scale for their measurement. MAN VAC ("Hg) is displayed on a gauge scale. BARO-MAP=MAN VAC. Your vacuum readings seem to be normal for a running engine. I noticed engine running in CLOSED LOOP mode. This is good. Short Term Fuel Trim (STFT) is low, meaning its cutting back on INJ PW time, but well within its range of authority. Cause is O2S is seeing RICH (engine blowby, poor spray pattern on injector, high fuel line pressure (19psi ??) as some possible causes. Link to comment Share on other sites More sharing options...
TheDude Posted March 25, 2020 Author Share Posted March 25, 2020 Both BARO ("Hg) and MAP ("Hg) display a absolute scale for their measurement. MAN VAC ("Hg) is displayed on a gauge scale. BARO-MAP=MAN VAC. Your vacuum readings seem to be normal for a running engine. I noticed engine running in CLOSED LOOP mode. This is good. Short Term Fuel Trim (STFT) is low, meaning its cutting back on INJ PW time, but well within its range of authority. Cause is O2S is seeing RICH (engine blowby, poor spray pattern on injector, high fuel line pressure (19psi ??) as some possible causes. Well I did try partially clamping the fuel line out of the tank to lower the pressure to see if ran better and no difference at all.The fuel injector sprays the throttle plate evenly, but it's hard to tell how fine the spray is.For the fuel pressure is 4 psi too high bad enough to cause a rough idle?I've also read the return lines sometimes get plugged causing raised pump pressure, do I just take the return off the TB and blow it out?Founding Member of the Comanche Preservation Society Link to comment Share on other sites More sharing options...
Ωhm Posted March 25, 2020 Share Posted March 25, 2020 14 hours ago, TheDude said: Well I did try partially clamping the fuel line out of the tank to lower the pressure to see if ran better and no difference at all. Not sure if that would lower pressure since Fuel Pressure Regulator controls that. Definitely would lower flow rate. 14 hours ago, TheDude said: The fuel injector sprays the throttle plate evenly, but it's hard to tell how fine the spray is. That a tough call. 14 hours ago, TheDude said: For the fuel pressure is 4 psi too high bad enough to cause a rough idle? Don't know, but STFT and CLOSED LOOP seems to be handling it OK. 14 hours ago, TheDude said: I've also read the return lines sometimes get plugged causing raised pump pressure, do I just take the return off the TB and blow it out? True. Don't know best practice for this procedure. Link to comment Share on other sites More sharing options...
KawaJeep Posted May 11, 2021 Share Posted May 11, 2021 Hey everyone, I’ve got the same problem here. I recently acquired an 86 and a few years ago I got an 88. Both have the 2.5 TBI. The 86 is a longbed, 5-speed 4WD. 88 is a shortbed 5-speed 2WD. My 86 ran great but it lacked power, so a few weeks ago I was under it with a sawzall and the cat and muffler fell on me while I was just laying there!🤣🙄🤣 It then ran better than ever and had tons of power and averaged 26MPG. Last week it started losing power and running extremely rich and hesitated accelerating. I cleaned the injector, replaced the ECU from my 88 (which I know to be good), put the 88 fuel pump in it, and it made no difference except that it doesn’t hesitate to accelerate anymore. It starts up and runs great cold, but once it warms, it runs rich enough that it’s only getting 5MPG!🤬 MAP sensor is good (bought it new for the 88 last year before it became a parts-truck for mechanical failures), temp sensors check good, I know it has sufficient fuel pressure, and the injector is fine. How to I test the O2 sensor, or bypass the O2 sensor with a resistor? Link to comment Share on other sites More sharing options...
cruiser54 Posted May 12, 2021 Share Posted May 12, 2021 Have you cleaned the cable at the firewall? Link to comment Share on other sites More sharing options...
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