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94 Jeep Wrangler 2.5 timing

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  • #661424
    Bill BrowningBill Browning
    Participant

      I have a backfire on a 94 Jeep Wrangler and it bogs down on acceleration. Starts and idles fine. Car came in with the alternator locked up and driver complaint of not running right. Replaced Alt and belt and then test drove it. That is when the backfire was noticed. The bushings in the distributer where whore out so I replace it. Had 10 degrees of slop in the timing, chain replaced that. Replaced exhaust / intake gasket as gasket was leaking. Rotor contact is centered at contact on the cap with the timing mark on the crank at TDC. There is no adjustment on this distributor and from what I have read timing is controlled by the ECM. My question, is there a way to test the ECM as I don’t want to shell out a couple hundred bucks for a used one and it not fix the back fire? By the way I tried a crank sensor but it didn’t help.

    Viewing 12 replies - 1 through 12 (of 12 total)
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    • #661434
      PatrickPatrick
      Participant

        TLDR: I have a similar problem, but no answers, sorry…

        Wow! What you’ve described is almost identical to what I have experience with my 97 wrangler (2.5 I4 150 MT US). I’m going to include the description (and evolution) of my issue here, in hopes that someone can help both of us.

        The Initial Problem:
        Engine has short misfire at 1750rpm under medium acceleration (whether in drive or stationary); under hard acceleration misfire becomes near-stall. At 3000rpm, no noticeable misfiring, but engine would ‘tremble’ under load: the acceleration would decrease and become inconsistent. At high loads (uphill) some intermittent misfiring and power loss occurs. The only other noticeable symptom is a long crank time to start. Usually, if the engine is cranked for at least 3 seconds, then stopped, then cranked a second time it will start up ‘right away’ on the second go. Depressing gas peddle to any depth has no noticeable effect. Priming fuel lines has no noticeable effect.

        Things I’ve Tried:

        • checked for vacuum leaks
        • verified spark with inline tester on each cylinder
        • swapped out upstream, downstream, and then both 02 sensors: no effect
        • swapped out throttle position sensor: increased idle speed on startup, but otherwise no effect
        • swapped out crankshaft position sensor: misfire has transitioned to weak backfire at 1750 rpm, otherwise no effect

        I was going to check the timing with a gun, but as you pointed out the Distributor appears nonadjustable. One other thing of note: I currently have a bad EVAP solenoid, and have until now considered that an unrelated issue. Does your vehicle have a bad EVAP as well? Mine almost never throws a DTC but I’ve confirmed with a multimeter that it is quite dead. I have not checked the timing chain on my engine yet.

        Finally, the local mechanic I used took a look at the car before the CKP was replaced. His testing found that the computer was indicating Lost Crank Reference (which when stored should be p1374) but the CKP was consistently putting out signal. Additionally, the lost reference was not occuring often enough to result in a stored DTC.

        Patrick

        #661458
        EricTheCarGuy 1EricTheCarGuy
        Keymaster

          As pointed out in the other post, it could be another issue that has nothing to do with the ignition. I would eliminate those other possibilities before you went for the ECU. Traditionally you go there last when looking for a problem like this. More info on solving problems like this here.

          http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

          Good luck and keep us posted.

          #661868
          Bill BrowningBill Browning
          Participant

            Ok here is what I have tried, timing light show 12 to 14 BTC steady at idle, increases at rev as it should. Compression is 125 to 135 on all 4 cylinders, vacuum at about 20 at idle and only has a slight bounce of the needle at idle goes to 0 with wide open throttle. Tried a new coil and crank sensor with no change. Have unbolted exhaust pipe at the manifold but that didn’t help either. There is a rattle in the cat but I thought that unbolting the exhaust would relieve any back pressure? Distributer is new, cap and rotor are good. Timing chain is new. Intake and exhaust gaskets new. I have cleaned the throttle body and did not find any vacuum leaks. Checked all the ground wires and all are clean and tight. Engine backfires through the intake and it bogs down when mashing the throttle. It does better if increasing throttle slowly but still has hesitation and some backfire. I have run out of ideas on what to do next and any help would be appreciated.
            Thanks Bill

            #661941
            Andrew PhillipsAndrew Phillips
            Participant

              Bill, I have seen 6 of these with this exact same problem. On 4 of the 6 (67%) it was damage to the vacuum line going to the MAP sensor (can be very tiny damage like a pinhole–test the hose, visual may not be enough), on 1 it was the MAP sensor itself, and on the other it was a bad upstream O2 sensor.

              #661942
              Andrew PhillipsAndrew Phillips
              Participant

                [quote=”Majesticpark” post=134230]TLDR: I have a similar problem, but no answers, sorry…

                Patrick[/quote]

                Patrick, do the same checks I just advised Bill regarding the MAP sensor and MAP vac line, and also check your grounds and neg battery cable/clamps. A bad ground can cause some of the symptoms you described. I might also suggest getting the alternator load tested as well. If electrical checks out, turn your attention to fuel pressure. You could have a faulty fuel pressure regulator or failing pump.

                #661946
                DavidDavid
                Participant

                  I would also check spark plug gaps.

                  #661954
                  PatrickPatrick
                  Participant

                    Cap – Thanks for the insight, I’ll try running some of these possibilities down today. What would you suggest for testing that tiny little MAP vacuum line? I’ll double check the grounds as well. I know that there is a splice in the negative cable near the battery from the previous owner(s). It sounds like I should consider examining that as well?

                    Wokko – Thank you as well. Spark plugs all looked good.

                    Patrick

                    #661971
                    Bill BrowningBill Browning
                    Participant

                      Thanks for the information I will give it a try in the AM, have a Cub Scout meeting tonight and a guy just has to have his priorities.
                      Thanks again,
                      Bill

                      #661980
                      Andrew PhillipsAndrew Phillips
                      Participant

                        [quote=”Majesticpark” post=134750]Cap – Thanks for the insight, I’ll try running some of these possibilities down today. What would you suggest for testing that tiny little MAP vacuum line? I’ll double check the grounds as well. I know that there is a splice in the negative cable near the battery from the previous owner(s). It sounds like I should consider examining that as well?[/quote]

                        Remove the hose, plug one end with your finger, apply suction from a vacuum pump (or your mouth) and see if it holds. Or, just replace the hose. Yes, check the grounds; do voltage drop tests if you can (measure from battery voltage to both sides of the ground terminal or splice, it should be the same or within 100 millivolts).

                        #664530
                        PatrickPatrick
                        Participant

                          Quick update: sorry this took so long, I got bogged down with work and finals. I was able to fiddle with the MAP and check its tiny hose. The hose looks good. I did not have a chance yet to check voltage drops, hopefully i can that this weekend. I’ve found that if I run the jeep in the driveway, it will run fine from idle through 3000 rpm. At or near 3200 rpm however, I get a regular and constant misfire or blurp. The mis happens, the rpm drops about 400 and then returns to 3200 where it repeats the cycle.

                          #664556
                          EricTheCarGuy 1EricTheCarGuy
                          Keymaster

                            Just for the heck of it, why not check the fuel pressure? If it’s slightly off it might show up at higher RPM’s as you point out.

                            Good luck and please keep us updated.

                            #664568
                            PatrickPatrick
                            Participant

                              i had a similar feeling; that’s on my to-do list as well

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