EVAP guru's wanted

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mpbannon

New Member
Joined
Dec 30, 2016
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10
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Claymont, DE, United States
Hey everyone, I've posted this on another forum, hopefully I'll have better luck here.
I have a 97 Tj 4.0 with a check engine light that has me stumped. The code is p1494 [Leak Detection Pump Switch or Mechanical Fault. Incorrect input state detected for the Leak Detection Pump (LDP) pressure switch]. I have been reading and searching for months to no avail. Hopefully someone here has an idea that i have yet to try.
So far, heres what i have tried/replaced:
-Gas cap x2
-leak detection pump x2
-Replaced ALL the lines in the evap system
-Charcoal canister
-cleaned up the ground for the PCM and added as secondary jumper to the battery
Im no expert in electrical so forgive me if I get some things wrong. Ive checked the OHMS on the sense side of the circuit and checked out at .02. The power side was showing 13 and some change while running.
Also, when I checked the vacuum on the manifold side, I got 14in HG.
I know its not the PCM because it continued after I swapped in an auto with PCM. I dont think it can be the gas tank itself due to the vacuum on it after trying to bypass the evap canister.
If anyone has any ideas I can try or retry I'm all ears. I've been riddled with this issue for months and inspection is coming up.
 
They've both been non-lockable. The first replacement had a regular round rubber gasket, the now one had sort of a flared rubber gasket.
I bought the second one when I bought the second LDP hoping it was just faulty solenoid and figured for 10 bucks it wouldn't hurt to give that a try too.
 
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Reactions: Jerry Bransford
Check the evap barbed fittings on the tank. A partially broken one can appear to be connected but still leaking. Blow into a hose in the engine compartment that leads back to the tank and see if you can pressure it up. Or have someone blow into it while you listen back at the tank for leaks.
 
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I initially dropped the tank and completly replaced all the hose, including a new Y from both fittings to the evap canister. I will check again though im gonna drop the tank after the holidays. Does anyone know the normal routing of all the lines? The way I have it going is from the 2 fittings on the tank, to the Y, all the way to the vapor canister, then from the other fitting on the canister to the top fitting on the purge solenoid. bottom fitting to the intake manifold. Does anyone know the operation and flow of the vapor canister? I replaced the one that was on there with VC120(autozone) which another forum member said it was a readily available replacement from the parts store.
 
Thanks I do have the service manual, but it doesn't show too much on the evap system as far as trouble shooting. I just wanted to double check the lines just to be sure. A smoke test is my next step as I'm running out of options. Is this code definitely caused by a leak in the system if it's not the purge solenoid?
 
Been a while due to work and the weather here in northern delaware. I pulled and pressure tested the gas tank. Didn't use a gauge yet but i filled it with air and it held pressure for 12+ hours. Will be VAC testing the line from the tank to the EVAP canister tomorrow (brand new 1/4" rubber hose) I don't think it's the issue but gotta re check everything. The FSM doesn't show a diagram of the routing or any real diagnostics so that's no help.
 
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Reactions: Chris
Yeah, I've found that the FSM doesn't seem to have a diagram of the evap system for some reason, which would actually be really helpful.

With something like this though, you're right, you've got to check everything.
 
Just got a call, I’ve got codes showing up similar to the OP. Told them to hold off for now while I research. Will use this thread plus a few others I’ve found on other sites and start checking everything. Let the fun begin.


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From the research I've done for most people it's a disconnected hose or the metal tubing along the frame by the rear rusted out. If you could post your findings it would be appreciated. I'm still fighting this issue. Gas tank and hose up to the charcoal canister is air tight. My next step is trying a jumper on the sense wire to the LDP.
 
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Reactions: Chris
mpbannon said:
From the research I've done for most people it's a disconnected hose or the metal tubing along the frame by the rear rusted out.
Click to expand...

This is what I've found seems to be the issue for most people as well.
 
mpbannon my 97 is having similar symptoms, throwing a code 31 - EVAP circuit or solenoid. Check the voltage to the purge valve solenoid with engine idiling to confirm proper voltage, course that test alone will throw a 31 code.. I'm in the same boat, inspection due in 3 weeks
You might double check the emissions routing diagram, the label on the underside of the hood..

20180206_130224.jpg
 
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