jubalb Posted October 18, 2015 Share Posted October 18, 2015 OK here is the deal I have a 1990 jeep Comanche pioneer. I'm having some problems. It's hard starting and the idle is rough after a few minutes and then dies. I checked the fuel pressure and its at 24 psi. new plugs cap an rotor. ignition mod is good. but not a strong spark. compression is great. cat is clear. all grounds front and back are clean and tight. brake controller is fine also. If I put it in drive just bogs out and tries to stall. back fire comes out on top, not through exhaust. now here is another weird part of it. i checked all vacuum lines and there all fine. But when I checked the line on the brake booster to see if the valve was dirty or clog idle came right up. I also checked the egr that's working also. any suggestions before I drop the tank. Link to comment Share on other sites More sharing options...
gogmorgo Posted October 19, 2015 Share Posted October 19, 2015 Try bypassing the fuel pump ballast resistor first. I'm pretty confident 24 psi isn't enough but I don't know the numbers for a Renix system. You need to check both at idle and with the fuel regulator unplugged. Having the idle increase when you unplug the brake booster is normal, as you've just created a large vacuum leak, so more air is getting into the intake. It has the same effect as opening the throttle. Link to comment Share on other sites More sharing options...
jubalb Posted October 21, 2015 Author Share Posted October 21, 2015 Thanks for the info. Everything seems good on those quick checks. Good to know to rule some other things out. If you have any ideas hit me back. And I'm still trying to get the right psi on the fuel pressure. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 21, 2015 Share Posted October 21, 2015 Fuel pressure is way too low. Should be 31 with vacuum line attached to the fuel pressure regulator and 39 when you pull the vacuum line off. Could be as simple as the ballast resistor as mentioned, and/or a deteriorated rubber hose inside the tank, or a bad ground for the fuel pump. Ever refresh the ground behind the left taillight? Link to comment Share on other sites More sharing options...
Marine1Texas Posted October 21, 2015 Share Posted October 21, 2015 check with regulator unplugged should be around 38 to 39 psi. If pressure is good and then drops to much below 31, your regulator needs to be replaced. If not then check filter out, then move back to your pump. could be a small hole in the line gas line in the tank going up from the pump. if not then could be a weak pump. Every 10 years or so I have to change the small 4 inch line of hose in the gas tank above the pump. it splits and starts spitting gas back in to the tank not allowing pressure build up. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 21, 2015 Share Posted October 21, 2015 check with regulator unplugged should be around 38 to 39 psi. If pressure is good and then drops to much below 31, your regulator needs to be replaced. If not then check filter out, then move back to your pump. could be a small hole in the line gas line in the tank going up from the pump. if not then could be a weak pump. Every 10 years or so I have to change the small 4 inch line of hose in the gas tank above the pump. it splits and starts spitting gas back in to the tank not allowing pressure build up. He's at 24...... Link to comment Share on other sites More sharing options...
Marine1Texas Posted October 21, 2015 Share Posted October 21, 2015 missed that! Thanks Cruiser.... Check that wiring in ground behind tail light and the ballast resistor as mentioned above. Make sure you can hear that pump kicking in, mine is loud when first put key in to start for 2 secs or so. That hose seems to always be the problem for me in the tank, maybe a small hole in it. Make sure to check pressure with the regulator vacuum unplugged to see if the psi is higher. that is the easy way to check the regulator. Link to comment Share on other sites More sharing options...
jubalb Posted October 24, 2015 Author Share Posted October 24, 2015 Thanks again for the tips. I will be testing the regulator tomorrow. This site is great. It really is a jeep thing. Link to comment Share on other sites More sharing options...
Dom U Posted October 24, 2015 Share Posted October 24, 2015 Seems it's been narrowed down to your low fuel pressure, however I know you stated checking all the vaccum lines but I'd definitely give the line from throttle body to map sensor a double look. My 88 pioneer would act exactly as you stated when putting it into drive or giving it gas. Ended up being that line even though I had checked it numerous times and thought it was good there was the slightest LITTLE crack in the hard plastic that was barley noticeable unless you slightly flexed it the right angle, would even seal and you could plug one end and blow into and it would seem air tight. Replaced it and my truck has ran amazing ever since. I'm by no means as knowledge as most on here but that's my experience with the exact same way your truck is acting. I'm sure the low fuel pressure is the biggest issue. Link to comment Share on other sites More sharing options...
jubalb Posted October 25, 2015 Author Share Posted October 25, 2015 Thanks I will double check it to make sure. Link to comment Share on other sites More sharing options...
jubalb Posted October 31, 2015 Author Share Posted October 31, 2015 Ok I replaced that line on the map sensor and there was no change, I rechecked the fuel pressure and I'm still around 24 psi. But when I take off the vacuum line off the regulator pressure doesn't go up. Could this mean bad regulator or still just a weak pump? Link to comment Share on other sites More sharing options...
cruiser54 Posted October 31, 2015 Share Posted October 31, 2015 More than likely weak pump or the internal hose. Link to comment Share on other sites More sharing options...
jubalb Posted October 31, 2015 Author Share Posted October 31, 2015 Right on. So I guess its time to drop the tank. If anybody has anything to look out for while dropping the tank suggestions are welcome. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 31, 2015 Share Posted October 31, 2015 Right on. So I guess its time to drop the tank. If anybody has anything to look out for while dropping the tank suggestions are welcome. Look out for falling dirt and rust. wear safety glasses. Link to comment Share on other sites More sharing options...
jubalb Posted August 23, 2017 Author Share Posted August 23, 2017 So I'm Back it's been awhile. Just wanted to say thank you for the replies. It turn out to be a disconnected return line on the fuel pump inside the tank. It ran great for almost two years until a blown head. Almost done with that. JEEP ON Link to comment Share on other sites More sharing options...
jubalb Posted August 23, 2017 Author Share Posted August 23, 2017 check with regulator unplugged should be around 38 to 39 psi. If pressure is good and then drops to much below 31, your regulator needs to be replaced. If not then check filter out, then move back to your pump. could be a small hole in the line gas line in the tank going up from the pump. if not then could be a weak pump. Every 10 years or so I have to change the small 4 inch line of hose in the gas tank above the pump. it splits and starts spitting gas back in to the tank not allowing pressure build up. Hey I missed most of this one a year or so ago. Thanks, you were right. Link to comment Share on other sites More sharing options...
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