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Spits and spudders first 5 min.


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I have a 87 MJ with a 2.5 5 speed and I have been having this problem. When I first get in it in the morning warm or cold outside it doesnt seem to matter, the truck seems to spit and spudder around 1500 RPM. I have thought it could be something with the butterfly mechanism on the front of the airbox but I havent been able to troubleshoot that yet. I hope that someone in here has came across this problem and has some suggestions. After I have driven it for aprox 3-5 minutes it runs fine but when it sits at work for 9-10 hours its back then it goes away again. Thanks in advance for any help.

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When you first start up, the ECU is pretty much in control and the sensors are not used. When warming up, the engine runs in "open loop" mode, ignoring sensor input and using a pre-mapped fuel curve that's built into the ECU. Once the coolant temperature reaches a certain point (I think it's 165 degrees, not sure) the ECU switches over to "closed loop" mode, in which it reads input from the various sensors and alters the fuel/air delivery accordingly. On my '88 XJ, I can feel a distinct bump or surge when it switches over.

 

It sounds like there's something happening with yours that affects only the open loop mode warm-up cycle. The trick then will be to diagnose what. For lack of any intelligent suggestions, perhaps try running a double dose of injector cleaner through your gas tank and see if the injector might be gummed up and not responding properly in open loop mode.

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If that's not it, I have an 86 2.5 4 speed ECU sitting around that you can literally have if you meet me on my way down to MO. Closest I get to you is Cedar Rapids.

 

I don't know if it's good though. We picked up the truck for parts, was told it would run for a few minutes then overheat.

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  • 4 weeks later...

So far I replaced the air filter and that did not produce any results then I bypassed the butterfly assembly on the front of the air box this seemed to help some . I also used a meter and tested the fuel injector and that also seemed to be good. I have yet to check the fuel pressure. Is there a port to check the fuel pressure on a throttle body inj. ??

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So far I replaced the air filter and that did not produce any results then I bypassed the butterfly assembly on the front of the air box this seemed to help some . I also used a meter and tested the fuel injector and that also seemed to be good. I have yet to check the fuel pressure. Is there a port to check the fuel pressure on a throttle body inj. ??

 

Yes, there is a test port - it's on the side of the TBI facing the driver's side fender, upper right corner, above the throttle position sensor (TPS). Pressure should be 15psi. Haynes/Chiltons manuals say a range of 12-15psi is acceptable, but in my personal experience, anything less than 14 is a sign the fuel pump is starting to fail or the fuel lines are clogged.

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  • 4 weeks later...

Well still dealing with this problem, I have noticed something that may make a big difference. After starting and when it starts to spit and spudder if I apply more throttle it somewhat moreless comes out of it. If I ease up on the throttle or keep it whare its at not the case. Eagles post was very enlightening I wasnt aware how that worked thanks for the lesson Eagle jamminz.gif :cheers:

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Did you ever fix this? I have an 87 2.5 that runs the same way on start up. Based on my symptoms, Eagle sound dead on in his diagnosis. I am going to check the injector tonite. I will let you know how it goes.

 

Aside from having to really clean out my injector, a clogged CAT was the culprit for me. If the "spit and sputter" has no ryhme or reason and you can "power" (2.5 L :rotf: ) through it, I would take a look. I hollowed mine out and welded it back in. When it comes time to smog again I will have to replace it, but other than that the truck is running great and its even got a beafier sound after killing the cat.

 

-Skip

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  • 11 months later...

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