Disturbed Posted September 5, 2012 Share Posted September 5, 2012 Something new has developed with my MJ. Truck is a 1988 MJ with a 4.0 and Renix injection, 5 speed transmission. If I accelerate like I am driving Miss Daisey I am fine but if I tried to accelerate any fast than that the MJ falls on its face, same thing when trying to give it some fuel heading up bigger hills. If I floor it it just dies like it is running out of fuel and if I keep the pedal down it never seems to recover, if I back off it will pick back up. Been having issues with the IAC but don't think that would cause this problem. Just for some back ground, fuel tank, fuel pump, fuel filter, and all rubber lines have been replaced in the last three years so I shouldn't have any issues there. Only think I did differently recently was dump some fuel into it out of a can in my garage. Fuel was from earlier this spring but was treated with Stabil and SeaFoam and has been being used in my lawn mower all summer without issue. Wasn't that much either, maybe a gallon, then topped it off with about 8 gallons of fresh mid grade fuel. Any ideas what the issue is? Link to comment Share on other sites More sharing options...
cruiser54 Posted September 5, 2012 Share Posted September 5, 2012 I might do a fuel pressure test just for the heck of it. May have a faulty fuel pump, especially if it was an Airtex. Link to comment Share on other sites More sharing options...
Disturbed Posted September 5, 2012 Author Share Posted September 5, 2012 I might do a fuel pressure test just for the heck of it. May have a faulty fuel pump, especially if it was an Airtex. That will be my first step when I have a chance but I didn't know if the resistor for the fuel pump would cause something like that, I am not very familiar with that setup and and do not know if a faulty resistor could cause issues like that. Link to comment Share on other sites More sharing options...
TylerJY Posted September 5, 2012 Share Posted September 5, 2012 Your issue sounds just like mine when my fuel pump was dying. Link to comment Share on other sites More sharing options...
Disturbed Posted September 12, 2012 Author Share Posted September 12, 2012 OK finally got around to messing with the MJ. Put a fuel pressure gauge on the rail. 30PSI at an idle, 39PSI with the vacuum unhook from the pressure regulator, Jumps from 30psi to 39psi when I try to rev it up but it still falls flat on it face, so it does not seem like a fuel issue. The truck had sat for a few days without being driven before this started to I pulled the air filter to see if some critter has built a nest in the air box, nothing the air box and filter are clean. Like I said the IAC was giving me issues so I am going to replace that but do not think that would cause this issue. If the fuel filter was clogged I would think I would see that in the fuel pressure. Anyone have any other ideas. It getting expensive driving my V8 ZJ and I want me truck back up and running.. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 12, 2012 Share Posted September 12, 2012 It would be nice to read the fuel pressure while driving. If your fuel filter were filled with crap, you could have fuel pressure at idle but not enough volume of fuel for rapid acceleration or pulling hills. Link to comment Share on other sites More sharing options...
Disturbed Posted September 12, 2012 Author Share Posted September 12, 2012 It would be nice to read the fuel pressure while driving. If your fuel filter were filled with crap, you could have fuel pressure at idle but not enough volume of fuel for rapid acceleration or pulling hills. I agree but I can get it to die just revving it up sitting in the driveway, even when it dies the fuel pressure is still reading between 30-39psi depending how hard I hit the throttle. I would think if the fuel filter was plugged up I would see a drop in the fuel pressure when it dies off and I don't, it seems to have plenty of fuel, if the pump wasn't keeping up, whether it is the fuel pump or a plugged filter I would see the fuel pressure drop off when I rev it up but I don't see that. Fuel pressure never drops below 30PSI and increases when the engine is revved up, That is what has be so confused. Thinking maybe I will try bypassing the fuel pump resistor to see if that makes a difference. Link to comment Share on other sites More sharing options...
HOrnbrod Posted September 13, 2012 Share Posted September 13, 2012 Had the same problem losing power going up hills. It turned out to be a clogged catalytic converter. Does the temp rise when you go up a long grade? Link to comment Share on other sites More sharing options...
Disturbed Posted September 16, 2012 Author Share Posted September 16, 2012 Had the same problem losing power going up hills. It turned out to be a clogged catalytic converter. Does the temp rise when you go up a long grade? No cat and the exhaust is all new from the manifold back in the last year. Link to comment Share on other sites More sharing options...
Disturbed Posted September 16, 2012 Author Share Posted September 16, 2012 OK worked on it more today. Replaced the IAC, not that I thought that was a problem, but I knew it was problematic so i replaced it. That fixed the idle problem but not the stubbling issue. Also replaced the MAP sensor, call it a shot in the dark, no difference. Tried bypassing the fuel pump ballast resistor and that made not difference. Tested the EGR solenoid and it functions properly. Hooked a vacuum pump to the egr valve and you can see that cycle so it seems as if that is working although bypassing that permanently might be a good idea. It acts the same way whether the truck is warm or cold, when I did my test drives today I started it up and ran 1/4 mile down the road and back, same issue so it was not warm enough to pull the EGR valve in. Still seems like a fuel issue but I hate to replace the fuel pump just to check it, especially since the fuel pressure tests OK and I can get it to do this in the driveway and see that the fuel pressure is not dropping. Any other ideas what may be causing this? Anybody think the TPS could be causing this? Since it does it warm and cold I do not think it could be the CTS, besides at WOT the computer should be running in open loop mode and ignoring the CTS. It does this in both open loop and close loop mode. If the engine runs fine an idle I think that rules out the CPS and the sensor in the distributor, if there was an issue there I would have an issue at idle and when cursing but the truck runs fine at an idle and when curing under light load. Load should have no affect on these sensors if there was an issue with them, am I wrong about that? Considering replacing the fuel filter. and just because it is cheap and easy. I am really going to kick myself if it does turn out to be the fuel pump. Anyone have a good plce to buy a fuel pump at a good price, the pump in my truck is a Bosch pump. Any idea's someone may have would be greatly appreciated, I really want my truck back on the road, I miss driving it. Link to comment Share on other sites More sharing options...
rgrmrk Posted September 17, 2012 Share Posted September 17, 2012 Any luck Disturbed? My 87 Comanche 4.0 is loosing power on hills after we replaced the head gasket. I'm looking for the vacuum diagram to make sure that is correct. I will also check the fuel pressure. filter and pump are both new. I will also check the manifolds, and do a compression test. Anything else that might give a problem? Link to comment Share on other sites More sharing options...
ParadiseMJ Posted September 18, 2012 Share Posted September 18, 2012 I'm having a similar issue, except at the lower end of RPMs' Cruiser54, Can you post up your TPS tech Link to comment Share on other sites More sharing options...
Disturbed Posted September 24, 2012 Author Share Posted September 24, 2012 OK I was able to fix this problem, it was in the TPS. I am thinking in the plug. I pull the connector off of the TPS to test the wiring, everything tested fine. Plugged to TPS connector back in an back probed the TPS, all of the readings were good there also. Fired the MJ up to see if it made any difference with it running, no everything tested fine but I also noticed that when I revved it up it was no falling on its face. Took it out for a drive, problem was gone. Drove it about 30 miles between yesterday and this morning to work and the problem seems to be completely gone. I am thinking that one of the pins in the TPS connector was not making a good connection anymore, from unplugged and plugging it back in and messing around with the pins while doing the testing that connection issue has been corrected and now the ECU is getting the reading it needs. Either that or is was just a coincidence that the problem went away after messing with the TPS. In either case my MJ is back on the road. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now