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88 2.5 Running Really, Really Rough After 10 Minutes


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88, 2.5, 4 speed 2wd.  Picked it up last December, and it's been a perfect angel until now.

 

First time it acted up,it was after an hour of driving on the highway, once I started getting into town, it had a massive loss of power over 2000 rpm.  Acts pretty much normal any slower than that.  Feels like it's missing, or not getting enough fuel.

 

A week before it happened, I made an identical trip into Branson, and it was fine.

 

Now, it's getting worse.  It doesn't take even 10 minutes before it starts acting up.

 

I did cruiser's ground refreshing, except the C101, as I don't have any cleaner.  The only ground that even looked iffy was the taillight.  New plugs, new-ish wires I stole off my now parked XJ.  Cap and rotor look near new.  It was suggested I look at the IAT, and resistance values aren't exact, but pretty close to the chart Hornbrod put up in this post: http://comancheclub.com/topic/41105-86-25-periodically-runs-poor/page-2

 

Checked the TPS, and I was getting 0.455 volts with a 4.96 volt input at idle (key on, not running).  Adjustment procedure tells me it's supposed to be 0.843 volts (17%).  After my ground refreshing, I adjusted it as far as it would go, to about 0.740 volts.  Took it for a test drive, and other than a slightly higher idle at first, it acted the same way it had.  After it got almost undrivable, I pulled over, and checked the IAT and TPS again with it hot and running.  IAT was normal, but my idle TPS output was about 0.340!  Is this a dying TPS, heat soaking problem, or is it expected to change with temp?

 

02 sensor on the thing looks brand new.

 

 

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Change out the TPS.  If that doesn't do it, check that your fuel pump is putting out the required 14-15 psi.  May be a clogged fuel filter or dying fuel pump.  Your symptoms sound eerily similar to the problem I had with my 2.5L.  Changing the TPS was my first step but it did nothing.  Ended up being fuel pump stalling out.  New one solved my problem.

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Oh yea... about that... I completely forgot about that on the 2.5.  :doh: There is no schrader valve as there is no fuel rail.  The way to check fuel pressure is to tie in a T-fitting in the fuel line after the fuel filter and measure it that way.  A PITA, I know...

A little insight to what I did to double verify my pump was in fact really the culprit was to listen to it as my truck started to die.  I could hear the pump clicking on and off till it finally just stopped (and with it the engine choked and died).

 

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I ordered a new TPS.  I'll listen to the fuel pump after the next drive, but she idles fine and revvs ok when not under load.  Not ruling it out, but given the major voltage change in the TPS, I'm really leaning towards that being the problem.  Shouldn't be too hard to add in a permanent schrader valve at the throttle body with all those fittings in there, gonna do that regardless.

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I wonder if your catalytic converter is messed up. 

 

A vacuum test would verify that.

 

CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION
 
Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction. 
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Wires were replaced at the start, not new, but from my XJ, which were newish (within the last 3 years), and it was running just fine.

Looks like the TPS did it.  Took it down to Branson and back today without issue, although it was cooler and rainy.  Won't really call it until I can get a good drive when it's up near 90 degrees again, which is where it was when this started.

 

Am still going to do the suggested vac test and get a schrader valve in there.

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