lostissues Posted October 26, 2009 Share Posted October 26, 2009 when I depress the accelerator to a certain point my truck will buck, stutter, backfire and bog down. I have to baby the accelerator and it is still pretty iffy. If I am still and rev the engine by pressing the accelerator to the floor and then letting up it will bog once it hits about 3 grand. happened after wheeling today, I checked air filter and distributor cap both are dry. any suggestion? Thanks, Jonathan EDIT: 1987 Comanche 4.0 Link to comment Share on other sites More sharing options...
mvusse Posted October 26, 2009 Share Posted October 26, 2009 Sounds like it's not getting enough fuel. Either a plugged filter, plugged sock on pump, or bad pump. Link to comment Share on other sites More sharing options...
Geonovast Posted October 26, 2009 Share Posted October 26, 2009 Sounds like either it's fuel-starved, either by a bad fuel pump, pressure regulator, or bad TPS adjustment. Can you get ahold of a fuel pressure tester? Link to comment Share on other sites More sharing options...
lostissues Posted October 26, 2009 Author Share Posted October 26, 2009 trying to get a hold of a fuel pressure tester. autozone didn't have one to rent out and I didn't feel like paying 50 bucks for one. my buddy has one but I cannot get it till Thursday. Link to comment Share on other sites More sharing options...
Geonovast Posted October 26, 2009 Share Posted October 26, 2009 I paid 30 for one at Sears. I think it was a good investment, cause I was able to prove my dad wrong about something that I diagnosed from 600 miles away. Well worth it. Besides, if you buy one, rent it out a couple times, and it'll pay for itself. :thumbsup: Link to comment Share on other sites More sharing options...
lostissues Posted October 27, 2009 Author Share Posted October 27, 2009 well I figured I should replace the fuel filter anyway because I don't know how old it is. so I replaced it and it still runs like poop. still waiting for Thursday to test the fuel pressure. Link to comment Share on other sites More sharing options...
buddy208 Posted October 28, 2009 Share Posted October 28, 2009 did you check the plugs, cause my brothers friend had the same problem and figured out that he blew up a plug Link to comment Share on other sites More sharing options...
larrymo Posted October 28, 2009 Share Posted October 28, 2009 All of the above...plus add new ignition wires and clean the throttle body with carb cleaner. Good luck, Larry Link to comment Share on other sites More sharing options...
lostissues Posted October 28, 2009 Author Share Posted October 28, 2009 I picked up new plugs, wires, cap and rotor. unfortunately it is raining so I cannot do anything. tomorrow I should be able to get a hold of a fuel pressure tester. I hope it isn't my fuel pump because I want to go wheeling this weekend and getting a pump to my door and installed is not something I can probably get done in one day. I may still go and just limp the whole run. it seams to still run OK at low speeds :shake: Link to comment Share on other sites More sharing options...
mjeff87 Posted October 29, 2009 Share Posted October 29, 2009 happened after wheeling today, I checked air filter and distributer cap both are dry. checked your exhaust....tailpipe, muffler, etc? Maybe you munched something. Jeff Link to comment Share on other sites More sharing options...
Mongo Posted October 29, 2009 Share Posted October 29, 2009 I have an 87 also, when mine did what you described it turned out to be a bad O ring on one of the injectors, which caused a vacuum like leak and made it run like poo Link to comment Share on other sites More sharing options...
Oizarod115 Posted October 30, 2009 Share Posted October 30, 2009 check fuel pressure yet? check your tps for adjustment, if its WAY out and can't be adjusted to spec you need one. with it idling you can spray ether or another combustible spray at the intake and check for vacuum leaks ANYWHERE there is rubber or a gasket with air in it. do you have a catalytic converter? possible that it's clogged up and causing the truck to have to much backpressure. Link to comment Share on other sites More sharing options...
lostissues Posted October 30, 2009 Author Share Posted October 30, 2009 Just check the fuel pressure, assuming I did it correctly: when the fuel pump primes it goes to 40 and when idling it is at 30. pretty sure it is low, what is the 4.0 supposed to run at? I'm pretty sure it is a fuel pump because it is LOUD. Cat is brand new just installed a few weeks ago, I will go and test the TPS now and report back. Link to comment Share on other sites More sharing options...
lostissues Posted October 30, 2009 Author Share Posted October 30, 2009 assuming I tested the TPS correctly (Haynes manual was little help) the TPS reads about .621 volts when throttle is closed and 4.48 volts when wide open. Link to comment Share on other sites More sharing options...
mvusse Posted October 31, 2009 Share Posted October 31, 2009 If the pump is LOUD, I would definitely pull it and see what the sock looks like. I went through 3 pumps in short order, and the 3rd pump would still idle fine, but barely keep me going 20 mph and getting worse. I was 350 miles from home, in a different country and 4 hours away from the nearest store that could honor the warranty on the pump. So I dropped the tank, drained it, wiped it clean, removed the pump and cleaned the sock. I didn't replace the sock because I would be changing in another pump and new sock 4 hours down the road. Put it all back together and ran great again. A lot quieter too. Never did replace that pump yet a year and 20,000 miles later. A plugged sock on the pump will make it loud, and will test enough pressure, but will not flow enough or keep up the pressure at the higher needed flow rates when you push the pedal down. A new sock is cheaper than a pump, and to keep the warranty on the pump you need to replace it any way when you put a new pump in. Most pumps do not come with a sock (although some do). But if the sock is the problem, I would drain the tank and wipe the inside clean (hope you have skinny arms), and to do that you do need to drop it. Link to comment Share on other sites More sharing options...
GATOR88 Posted October 31, 2009 Share Posted October 31, 2009 yeah my 88 mj is starting to do the same thing it will start up and bog out until i romp on it but i am rebuilding all of it bigger injecters, lower gears etc. i am going to need all the power and torque i can get when i put 44 under the ole mj Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted October 31, 2009 Share Posted October 31, 2009 Is the tank a stock tank? I ask because an after market tank will not have the internal support of the factory tank, this will cause your fuel pump to be quite loud all the time... Just like mine!! :fs1: :wall: :wall: :wall: CW Link to comment Share on other sites More sharing options...
lostissues Posted February 6, 2010 Author Share Posted February 6, 2010 well I dropped the tank a while ago and discovered that the sending unit kind a broke (tubes where it goes into the tank broke loose on the sender and were able to rotate) so we JB welded the heck out of them and replaced the fuel pump. this was before winter and I was able to drive it around and it seamed to be OK. I have driven it a couple times since and noticed it doing the same thing though not quite as harsh. when accelerating sometimes it will work OK and then bog down it sometimes bogs until I press the pedal more, sometimes pressing the pedal more will not do anything. it sounds like it wants to go but it hesitates. I have new plugs, wires, rotor, new pump, stock tank. other things I have not checked that people have suggested are injectors, exhaust leaks, TPS. what is more likely? easiest to test? cheapest to replace? Link to comment Share on other sites More sharing options...
jimoshel Posted February 6, 2010 Share Posted February 6, 2010 Fuel pump pressure should be at 40psi at all rpm's. If stomping on it from a idle it may drop a couple p's but will return to 40 in a second. Link to comment Share on other sites More sharing options...
maddzz1 Posted February 6, 2010 Share Posted February 6, 2010 I believe wildman came up with a fix for the cracked sending units and, if i remember, will even send you a repaired one if you send him your old one. Link to comment Share on other sites More sharing options...
lostissues Posted March 30, 2010 Author Share Posted March 30, 2010 First thing first is I @#$%ing hate the fact the the backspace button doubles as the page back and when you do that all of the $#!& you typed in this @#$%ing window disappears :headpop: :headpop: :headpop: :headpop: sorry, back to REtyping all of this again OK trying to figure this out still. same symptoms. new parts are: fuel pump, fuel filter, cap, plugs, rotor, wires, cat, adjusted TPS, checked voltage at fuel pump, orange wire is getting 12 volts and black is getting ground. http://www.jeepforum.com/forum/f11/87-4 ... ll-990797/ this is the thread on Jeepforum I made with all the diagnostics so far. over there we have it narrowed down to a possible bad fuel pressure regulator because removing the vacuum line (which does have suction) did not affect (effect) the fuel PSI at all. my question is if its a bad FPR it would be either stuck open or closed right? so if its stuck closed then the fuel pressure in the fuel rail would be high (its not) if it were stuck open then the fuel pressure would be low right? but I did pinch the return line with a pair of pliers and it had no affect (effect?) on the fuel PSI. does this sound like a reasonable assumption? I really want to drive my truck and unfortunately my inspection ran out this month :fs1: so I need to get this fixed and the get the truck inspected somehow. any help would be greatly appreciated. Link to comment Share on other sites More sharing options...
88pioneer Posted March 30, 2010 Share Posted March 30, 2010 Same thing happened to mine, bad TPS an bad ballast resistor, the fuel pump wasn't sending enough fuel to the fuel rail. I also put a new fuel pump, filter, regulator and spark plugs too, but the TPS and the resistor did the trick Link to comment Share on other sites More sharing options...
lostissues Posted March 30, 2010 Author Share Posted March 30, 2010 ok, if I go to the junk yard I will grab a tps and fuel filter just to check. Link to comment Share on other sites More sharing options...
fiatslug87 Posted March 30, 2010 Share Posted March 30, 2010 Have you checked the EGR valve? I had similar symptoms, turned out that was it. Here's how to check the EGR, from the '90 FSM Link to comment Share on other sites More sharing options...
kro10000 Posted March 30, 2010 Share Posted March 30, 2010 Id just unhook the EGR solinoid, and bypass the ballast resistor Link to comment Share on other sites More sharing options...
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