Geonovast Posted October 26, 2008 Share Posted October 26, 2008 My 2.5 is acting up. Just got the AX-5 and 231 in there so I got my 4x4 back (207 locked up on me), and aside from my muffler falling off last night it's been fine... Except when it gets warmed up, for about 5 minutes, every couple of seconds it hesitates, feels like it's missing. If I keep giving it throttle, it starts backfiring. If I let off the gas, then give it again it's fine. After about 5 minutes it stops doing it completely. It did it once or twice a few months ago then stopped, but now it's every time I drive it. I wanna put my 4.0 down for surgery so I'd like this fixed before it gets toooo cold. I put on new cap, rotor, wires. I'm thinking either TPS, CPS, or ECU. I have a spare TB and ECU sitting around that I'm gonna throw in, but unfortunatly the CPS from the truck those came from had newer plug(rectangle plug instead of round one) Any other suggestions? Link to comment Share on other sites More sharing options...
IDXJ Posted October 27, 2008 Share Posted October 27, 2008 Has the O2 sensor ever been replaced? I just replaced the ones on my '88 MJ and my '92 XJ and it cleared up stumbling issues on both vehicles. Link to comment Share on other sites More sharing options...
Pete M Posted October 27, 2008 Share Posted October 27, 2008 EGR? Link to comment Share on other sites More sharing options...
FxRacing282 Posted October 27, 2008 Share Posted October 27, 2008 EGR? afaik backfire would indicate that the egr is letting air into the exhaust. :nuts: my truck did this to me and it was the timing. i guess after nonstop 4x4 the distributor spins a wee bit :mad: on second thought it could be egr. disconnect the vacuum lines to it and plug them see if it goes away. and check to see that the diaphragm moves easily Link to comment Share on other sites More sharing options...
JeepcoMJ Posted October 27, 2008 Share Posted October 27, 2008 1991 4 cylinders don't have EGR. they are high output correy, I thought I gave you a spare CPS for your 4 banger to use on andre's xj? even so, a 4.0 HO cps is the same as a mpfi 4 cylinder cps. I have spares of that, likewise you should too. Link to comment Share on other sites More sharing options...
Geonovast Posted October 27, 2008 Author Share Posted October 27, 2008 I did replace the O2 sensor with a known good one off a 92 4.0 when this all started in the first place and no change. I would try the two other CPS's I have sitting around, but they're both from 93/94 and have the rectangular plugs. Both my MJs have the circular ones. I could try the CPS from my 4.0, but pulling that thing off the trans and not having the bolts fall into the bellhousing is tricky. It's enough fun on a 2.5. I was thinking ECU, since I know the Renix era ones use a different set of preset values while it's cold, I couldn't remember if the chrysler computers did that too. It would make sense if it did, then the few minutes it was running weird would be the computer re-learning the motor. Could I use one of the rectangle plug CPS? I'd have to splice some wires I know, but they should be the same, right? On the left is what the trucks have (pic taken from 4.0 but plug is the same on the 2.5) and on the right is what the spare CPS is. Link to comment Share on other sites More sharing options...
pingpong Posted October 27, 2008 Share Posted October 27, 2008 Check your fuel pressure. I know on the 4.o's when it does this.. it is ussually a symptom of the fuel pump going bad. Ths is especially true if it takes a while for it to start. Link to comment Share on other sites More sharing options...
Geonovast Posted October 27, 2008 Author Share Posted October 27, 2008 Check your fuel pressure. I know on the 4.o's when it does this.. it is ussually a symptom of the fuel pump going bad. Ths is especially true if it takes a while for it to start. I will, but that better not be since I replaced the pump last fall. Starts right up. Link to comment Share on other sites More sharing options...
Lemmon714 Posted October 27, 2008 Share Posted October 27, 2008 I was just having a similar issue in my 96 Dakota. It wound up being clogged fuel screens on the fuel injectors. I would have hesitation, lean misfires, random cylinder misfires, and a check engine light (OBDII). I changed all electricals- Cap, Rotor, Plugs, Wires, Coil- no help. I changed the fuel pump since the regulator was bad- no help. When I pulled the first injector out, the screen was Black and 95% plugged. I went to http://www.mrinjector.us. I bought the 8 cylinder kit for $20.00. 1.5 hours later- engine runs like new. Link to comment Share on other sites More sharing options...
4x4 H.O.R.E. Posted October 31, 2008 Share Posted October 31, 2008 I don't know if you have one but theres a 1/2"x4"long white porcelin piece with two wires attached to it ,and is on the drivers side I had same issue then it would fire up back fire and die. it got worse I replaced this and never had the problem a used one works fine it seems to control fuel pump and variese presser and then none at all if you don't have this piece dissregard Link to comment Share on other sites More sharing options...
Pete M Posted October 31, 2008 Share Posted October 31, 2008 That part is called the Fuel Pump Ballast Resistor. It's purpose is to lessen the voltage getting to the fuel pump and thus lower the fuel pump's speed and thus lessen the noise it puts out. In 87 Jeep had a bunch of complaints about the fuel pump noise and the resistor was their fix. Usually when the resistor goes bad, the truck will fire up, and then die in a couple seconds as the computer switches fuel pump power from a direct line over to the resistor. But now that you've said it, it sounds perfectly plausible that it could be the culprit in trucks with poor fuel pressure. Thanks! :thumbsup: Link to comment Share on other sites More sharing options...
Geonovast Posted October 31, 2008 Author Share Posted October 31, 2008 I doubt it's the resistor, since I replaced it less than a year ago when the pump died. I was out there today and replaced the ECU (had one sitting around, figured I'd eliminate that), and I noticed the vacuum elbow fitting on the pressure regulator was cracked...everywhere. I replaced that and I'm going to be taking it for a test drive pretty quick to see if that did it. After seeing that, I thought it could be the culprit, since when the fitting was cold it might seal, but after the engine bay warmed up and the fitting expanded, it would start to leak. Then when I let off the gas, the injectors wouldn't open as much and allow the pressure in the rail to build back up. If this isn't it I'll have to swap CPS, and if it's still acting up after that I'll go to the pump. Again. I don't want to grab the CPS off the 4.0, with it being so close to the firewall I'm sure the bolts will get away from me. I do however have two CPS's sitting around with the newer style plug (see pictures above) If I splice wires, can I use those? Link to comment Share on other sites More sharing options...
ejndssn Posted November 1, 2008 Share Posted November 1, 2008 ... If this isn't it I'll have to swap CPS, and if it's still acting up after that I'll go to the pump. Again. I don't want to grab the CPS off the 4.0, with it being so close to the firewall I'm sure the bolts will get away from me. I do however have two CPS's sitting around with the newer style plug (see pictures above) If I splice wires, can I use those? what i have use when taking these bolts off is those little magnets from computer harddrives they are super strong and i have not lost any bolts using this yet just a thought btw tape it on the extension or socket :brows: they are a pain in the @$$ to get the off the truck or other parts Link to comment Share on other sites More sharing options...
FxRacing282 Posted November 2, 2008 Share Posted November 2, 2008 ... If this isn't it I'll have to swap CPS, and if it's still acting up after that I'll go to the pump. Again. I don't want to grab the CPS off the 4.0, with it being so close to the firewall I'm sure the bolts will get away from me. I do however have two CPS's sitting around with the newer style plug (see pictures above) If I splice wires, can I use those? what i have use when taking these bolts off is those little magnets from computer harddrives they are super strong and i have not lost any bolts using this yet just a thought btw tape it on the extension or socket :brows: they are a pain in the @$$ to get the off the truck or other parts magnets off 10'' sub woofers are pretty strong too :smart: Link to comment Share on other sites More sharing options...
Geonovast Posted November 4, 2008 Author Share Posted November 4, 2008 Changed CPS, that wasn't it. Tested fuel pressure, stays about 30 psi when idling and revving up a bit. That seems a little low to me. What's it supposed to be at? I'm grabbin new plugs and a fuel filter on my way to work today. Link to comment Share on other sites More sharing options...
ejndssn Posted November 4, 2008 Share Posted November 4, 2008 Changed CPS, that wasn't it. Tested fuel pressure, stays about 30 psi when idling and revving up a bit. That seems a little low to me. What's it supposed to be at? I'm grabbin new plugs and a fuel filter on my way to work today. may want to check the hose on either end of the fuel filter & fuel pump to i have read somewhere recently on this site that a rotten hose line was the problem and had some pic as well just a thought :hmm: found it its in the renix woes post second page it is worth a shot Link to comment Share on other sites More sharing options...
BLHTAZ Posted November 4, 2008 Share Posted November 4, 2008 If you check my post in DIY here... viewtopic.php?f=9&t=12517 It is the FSM and even though it says Renix, it also covers the 2.5. Go to the fuel pump section and it gives testing & adjustment procedures. Link to comment Share on other sites More sharing options...
Geonovast Posted November 5, 2008 Author Share Posted November 5, 2008 Thanks for the manual, I'll keep it around but it doesn't do me any good. In 91, along with the 4.0 got upgraded to the Chrystler system and it went to MPFI, so it's completely different. Link to comment Share on other sites More sharing options...
Geonovast Posted November 5, 2008 Author Share Posted November 5, 2008 Thanks for the manual, I'll keep it around but it doesn't do me any good. In 91, along with the 4.0 got upgraded to the Chrystler system and it went to MPFI, so it's completely different. Link to comment Share on other sites More sharing options...
BLHTAZ Posted November 5, 2008 Share Posted November 5, 2008 Yeah...sorry, I noticed after I posted that you had a '91, but I was in a hurry and didn't want to bother changing or removing the post... :oops: Link to comment Share on other sites More sharing options...
BLHTAZ Posted November 5, 2008 Share Posted November 5, 2008 Yeah...sorry, I noticed after I posted that you had a '91, but I was in a hurry and didn't want to bother changing or removing the post... :oops: Link to comment Share on other sites More sharing options...
Geonovast Posted November 6, 2008 Author Share Posted November 6, 2008 Changed the plugs, #1 was full of crap, #2-#4 looked normal. Figured since the plugs were coming out anyway, I'd do a compression test. #2, #3, and #4 held fine at 130psi. #1, however, was bleeding off when I got to look at the gauge, but I do know it at least got to above 100 psi. So now it's looking like something's working its way into the cylinder and gumming up the spark plug. Link to comment Share on other sites More sharing options...
Geonovast Posted November 6, 2008 Author Share Posted November 6, 2008 Changed the plugs, #1 was full of crap, #2-#4 looked normal. Figured since the plugs were coming out anyway, I'd do a compression test. #2, #3, and #4 held fine at 130psi. #1, however, was bleeding off when I got to look at the gauge, but I do know it at least got to above 100 psi. So now it's looking like something's working its way into the cylinder and gumming up the spark plug. Link to comment Share on other sites More sharing options...
FxRacing282 Posted November 6, 2008 Share Posted November 6, 2008 MPFI fuel systems run around 40PSI in the fuel rail. if you think something is getting in and clogged perhaps check your PVC and air filter. after i would seafoam the intake system Link to comment Share on other sites More sharing options...
FxRacing282 Posted November 6, 2008 Share Posted November 6, 2008 MPFI fuel systems run around 40PSI in the fuel rail. if you think something is getting in and clogged perhaps check your PVC and air filter. after i would seafoam the intake system Link to comment Share on other sites More sharing options...
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