88XJSport Posted March 7, 2009 Share Posted March 7, 2009 I don't know what else there is to do. JF is useless for these kinds of threads, and Ive started many threads on NAXJA about this, and I'm at my wits end. My 88 4.0 Renix has a loping/choppy idle problem. Anytime I am in gear, on the brake, at a light, my idle is about 600, and is really choppy, like its struggling for air or something. Then, a few seconds go by, and it jumps to about 800rpm, and the idle is smooth, then a few seconds later, its back to 600 and choppy. Ive changed Fuel injectors Fuel Pump Fuel Filter Plugs Wires Cap Rotor CPS IAC Adjusted year old TPS Bench Cleaned Throttle Body Plugged EGR. I honestly don't know what else there is to check/replace. I'm usually the one helping with renix issues, and I'm kinda embarassed. Thanks for help. Link to comment Share on other sites More sharing options...
Wildman Posted March 7, 2009 Share Posted March 7, 2009 Man......you changed about everything there is to change :eek: Couple thoughts :hmm: ........ Coolant Temperature Sensor??? CCV lines??? Causing a 'odd' vacuum leak / clog. I know Taz will jump in here........Bad grounds??? Might be a good point to change, check all your battery wires. I've heard that the ground strap from the fire wall to the engine will cause some strange running problem :dunno: Oh......and you know, we're not like the 'other' places :roll: So don't be embarrassed to ask ;) Link to comment Share on other sites More sharing options...
88XJSport Posted March 7, 2009 Author Share Posted March 7, 2009 I have changed the CCV system last year, actually. Also I changed my braided ground strap to a 2ga battery cable about a month ago. I also cleaned the grounds under the coolant bottle, and the 2 near the washer bottle and engine block. Coolant temp sensor is about 3 years old. Link to comment Share on other sites More sharing options...
Geonovast Posted March 7, 2009 Share Posted March 7, 2009 ECU? Link to comment Share on other sites More sharing options...
88XJSport Posted March 7, 2009 Author Share Posted March 7, 2009 Just though of this...would the knock sensor cause my symptoms? I want to exhaust all possibilities before turning to the ECU. Link to comment Share on other sites More sharing options...
Wildman Posted March 8, 2009 Share Posted March 8, 2009 ECU? Yea, after reading all the other things you replaced........same thought here. would the knock sensor cause my symptoms? Not really, at idle.......the knock sensor trims the timing more under a load. But then again........it feeds to the ECU :hmm: I want to exhaust all possibilities before turning to the ECU. I think you have.......thats the only thing you haven't changed yet. How many miles you have on this MJ??? And I read it that you have the AW4?? If you do, your getting feed into the ECU from the tranny and like you say, 'in gear, stopped', your getting the erratic idle. Link to comment Share on other sites More sharing options...
88XJSport Posted March 8, 2009 Author Share Posted March 8, 2009 Its got 192k. I don't know if it has anything to do with the AW4, but the rpms are lower when youre in gear on the brake anyways, the only difference is, is that my idle is choppy... Link to comment Share on other sites More sharing options...
BLHTAZ Posted March 8, 2009 Share Posted March 8, 2009 Anytime I am in gear, on the brake, at a light, my idle is about 600, and is really choppy, .This part of your statement is really making me think that it is a vacuum issue...possibly related to and internal issue with the brake booster. Is there ever a difference in the "feel" of the brake pedal pressure, position or holding power when this happens? Other than that...the ECU is about the only other thing I can think of too. As Wildman alluded too, my idle issues were caused by a bad ground at the main grounding point on the block where the negative battery cable attaches as well as 3-4 other ground wires that ground the entire electrical system of the truck...including the ECU. Once I took all of them off, replaced my battery cables, cleaned everything and put it back together...it has been good. Link to comment Share on other sites More sharing options...
88XJSport Posted March 8, 2009 Author Share Posted March 8, 2009 It sometimes idles like crap in park off the brake too, but always when in gear. My negative cable from the battery to the block is hard and cracked insulation. I am planning on changing this in the spring anyway, just didnt think it would cause idle problems. The connections at the block and battery post are clean though. So could a hard negative cable cause idle problems even though the connections are good? Link to comment Share on other sites More sharing options...
BLHTAZ Posted March 8, 2009 Share Posted March 8, 2009 So could a hard negative cable cause idle problems even though the connections are good? It is possible because the wire inside breaks down with grime & corrosion and can not trasnfer the proper ground. I would get that replaced and see what happens. I can't say that I expect it to fix your issue...but I battled with mine for many weeks until I found the bad battery cable & connection...so you never know :dunno: Link to comment Share on other sites More sharing options...
Wildman Posted March 8, 2009 Share Posted March 8, 2009 So could a hard negative cable cause idle problems even though the connections are good? Yes. Your talking about 20 year old cables, with 192K on them.......do an x-ray of the wire and see what it looks like inside the insulation. Of course that can't be done, but the point is.....the cables could be corroded inside the insulation, and that's where the problem lies. I brought up the AW4, because, beside the different TPS, manual or auto, there is also a tranny controller, and a feed into the ECU from the tranny, unlike the manual. IE: when you step on the brakes, it sends a signal to the ECU to drop the rpm's and start slowing down the engine for a stop, so that your not fighting the brakes against forward movement. Hum......now that I'm thinking of this.......could there be a problem with your brake switch??? Or......would that go back to the ECU being on the brink of going bad??? Face it......with 192K on the clock.....just maybe the ECU has seen enough use and is starting to get Alzheimer's :dunno: What you could do........is ask one of us to barrow a ECU for your specs and swap it out to see if that just might be the over all problem. Man......you changed out all the other items that make the thing run........there's only one thing left. But, I would move the changing of the cable up in your time schedule first, and see what that brings you, plus, it's not a big ticket item......what like $30 for cables?? Link to comment Share on other sites More sharing options...
88XJSport Posted March 8, 2009 Author Share Posted March 8, 2009 Do I have to make my cables? Or can I grab them from the parts house already fitted with the correct connectors? What would be the benefit of bigger diameter cables? Like 2ga? Would that help or no? I swapped my braided cable with a 2ga battery cable. Didnt make any difference. Link to comment Share on other sites More sharing options...
BLHTAZ Posted March 8, 2009 Share Posted March 8, 2009 I got my cables from the dealership. It came as a complete harness with positive & negative cables all together. I have seen others post of aftermarket solutions in pre-made cables too, but I don't really know about them as I prefer to get OE stuff as often as possible. Link to comment Share on other sites More sharing options...
88XJSport Posted March 8, 2009 Author Share Posted March 8, 2009 Ill grab new cables once it warms up (yes...I live in Michigan) :fs1: and Ill update if the situation improves or not. Thanks everyone. Link to comment Share on other sites More sharing options...
james750 Posted March 8, 2009 Share Posted March 8, 2009 Just something to check, it is the fist thing I check when I have idle issues. check your oil level. If it is over filled it will cause a rough idle. Just a thought as I have seen it MANY times before. Link to comment Share on other sites More sharing options...
88XJSport Posted March 8, 2009 Author Share Posted March 8, 2009 6 qts even. Thanks tho. Link to comment Share on other sites More sharing options...
SchneiderFishing Posted March 25, 2009 Share Posted March 25, 2009 Did you figure this out yet? Link to comment Share on other sites More sharing options...
Tomahawked Posted March 25, 2009 Share Posted March 25, 2009 I just finished piecing an 88 together from 90 motor and donor parts. It does exactly as you described. At idle in gear with brakes on it will drop down to around 600 or less and run rough. Everything loose in cab starts shaking and rattling. I put it in neutral and idles up just enough to stop running rough. But after a couple seconds it will idle back down to around the 600 and be rough again. If it sits long enough the idle rises just a little and smooth out for a few seconds then drops again. Its not a surge or wandering. I have other vehicles do similar without the real low idle that results in a rough condition. Link to comment Share on other sites More sharing options...
jtdesigns Posted March 25, 2009 Share Posted March 25, 2009 sounds like the IAC port needs cleaning,, and the whole TB Link to comment Share on other sites More sharing options...
88XJSport Posted March 27, 2009 Author Share Posted March 27, 2009 Still havent figured it out. I'm going to guess its a vacuum issue, because when I have my defrost on, the problem goes away. When I switch the HVAC to anything BUT defrost, its back to choppy idle. Link to comment Share on other sites More sharing options...
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