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1990 Jeep YJ Wrangler 2.5L TBI Engine Has Intermittent High Speed Idle!


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Sinisterwillys1940...The engine needs two things to rev this high:  air and fuel.  Must be getting both.  I'd bet on the IAC (idle air control) system or an EGR valve that is not seating properly.  

When you place a load on the engine, like putting the transmission in 2nd gear, holding your foot firmly on the throttle and lightly releasing the clutch pedal just enough to load the engine, what happens to the rpm?  If it drops substantially, you have an air leak, either the IAC or the EGR valve, an air source and not fuel.  If the engine continues to feel strong, look for a leaking injector or idle control trouble.  Something sticking?

Make sure your EGR valve vacuum is from ported source and not manifold vacuum.  Ported is above the throttle valve and would have little or no pull when the engine is idling.  If hooked to manifold vacuum, the EGR would be open at idle.  The EGR, if sticky or defective, may not be seating properly.  Worth checking.

See if this leads to a solution...

Moses

 

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Sinisterwillys1940...The ground circuit path is always battery to engine block/head, to body, to frame—not necessarily in this order.  D.C. systems require the same amperage carrying capacity on the ground side as the hot (positive) leads.

Wicking of corrosives up wires and cables is always a possibility, and rust, scale or oxidation create subtle increases in voltage resistance or voltage drops...Use your intuition, look for oxidation and any kind of ground resistance issue.  The 2.5L engine to firewall/body ground can be troublesome.

Parts often get replaced needlessly.  Don't overlook the obvious and simpler items, like the battery cables and loose or corroded terminal junction points.  The power distribution box can be a source for oxidation issues.  Also check around the ECU, there may be loose connections, oxidation or poor grounding.

Moses

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The dipstick ground is a regular topic of discussion and trouble spot, Sinisterwillys1940. Your 2.5L YJ Wrangler's ECU should be mounted beneath the instrument panel between the steering column and heater A/C housing.  Attached by three screws to a bracket.  It should say, "Renix" on the ECU housing...

Moses

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I went through the dip stick ground and looked at the engine to fire ground. The dipstick ground I cleaned with some electrical cleaner and sand paper, as for the engine ground the last owner who installed the Napa rebuilt engine mounted the ground wire under the head bolt on the engine. I also looked at the Ecu and I can't figure out how to unplug the connector 

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Sinisterwillys1940...Be careful with the clip-lock connectors on plastic harness connectors.  A harness repair at this location would be a PITA, for sure!  In my view, rosin core soldering of wire splices, with either multiple layers or thicker heat shrink tubing for insulation, is mandatory with ECU voltage sensitivity...Other members with Jeep 2.5L TBI YJ Wrangler models may have insights into the safest way to un-clip this particular harness.

The factory manual does note the need to disconnect the negative battery cable before fiddling with the ECU connector.  Good idea, as a voltage surge could short the sensitive electronic board.  Also, in sequence, the ECU is detached from the mounting bracket before disconnecting the plug; see if there's a good reason for this step.

Moses

  

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  • 7 months later...

I didn't how to start my on troubles on a separate topic sorry for that. I have a 1989 jeep wrangler with 2.5 and I had to replace the head when I got it cause the other one was cracked. Well got it all back together and it started up and idled like a new one, and then drove it down the road and back, just letting it idle and then all of a sudden it just cut off by it self. The fuel pump is workin and I'm gettin fuel to the tbi, but I realize I'm not getting spark, so read some of the people on the Internet having similar problems. I've changed the crankshaft positioning sensor(twice) the ignition control module and coil, the distributor cap, rotor, spark plugs and wires I've heard the ecu could be bad, could someone help me out here, it will just crank and want start up at all, the distributor and rotor are turning when cranking 

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Hi, Jason...It's possible that the ECU has failed, that's one cause of no spark.  Before condemning the ECU, though, I would chase down your engine to body and frame grounds.  The only work you did was cylinder head related.  Among the causes for no spark are bad grounds, and there are leads from the engine/cylinder head to the body and frame.

See whether you left a ground loose or possibly paint is preventing a good ground to a fresh cylinder head.  Check the ground lead near the dipstick.  This is a common source for grounding problems on YJ Wranglers with the 2.5L TBI engine...

We can go from there.  The ignition failure can be tested in its flow chart sequence if you do not find a ground solution.  I can share the troubleshooting flow sequence.

Moses

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