P0442 on 1998 Dodge Dakota

I've got a '98 Dakota and have been troubleshooting a P0442 (evap sys leak) code for over a month now. Here's what I've tried;
1) I replaced the gas cap - light back on in a couple days.
2) I had it smoke tested at a local shop - They found no leak but told me to take it to a dealer to test the LDP.
3) Took it to the dealer - They vacuum tested the both caps (passed) and the tank, found no leak, suggested a new LDP and to update the computer.
4) Installed a new LDP - light back on in a couple days.
5) Back to the dealer - vac test passed - they suggested a new solenoid check valve
6) Installed new solenoid valve - light back on in a couple days.
7) Back to the dealer - They suggested another LDP because it is often bad from the factory.
Installed 2nd LDP - light back on in a couple days
9) Back to the dealer - flashed the computer - ran the forced LDP test - passed, reset codes, light is back on 3 days later.
10) I tried to isolate and test just the LDP by disconnnecting the 5/8 inch hose running back to the tank from the LDP. I connected a short line to the LDP and plugged it, reset the codes, and now the light is back on a few days later.

The dealer is now suggesting a new PCM for over $500. I haven't seen anyone mention a new PCM to solve evep leak codes on this or any other forum. I have read about filler neck and fuel pump seal replacement, but if that were the problem it wouldn't pass the vac tests or the forced LDP test, and it would pass the isolated LDP test (#10 above) right?

Also could someone explain the "internal leaks" I've seen mentioned? I understand that they would not allow smoke out, but how does it allow the vacuum to drop, and wouldn't that cause it to fail the forced test or the vac test?

Thanks in advance for any suggestions.
Mark

I'd doubt the pcm! Evap is one of the easiest systems to trouble shoot! I don't buy that they're isn't a physical leak somewhere! The evap monitor that they can run with the scan tool can tell if there is a leak before the truck is returned to you! You need to try another dealer or better tech at that one! The only reasons they aren't diaging this properly is because they can't or because somebody doesn't want to pay them too!

Thanks for the quick response Jeff.

They did the vac test twice and the forced LDP test once at the dealership while I watched. It passed each time. The mechanic was a smart and experienced guy with 22 years at the shop. I asked him a million questions, but I also doubted the PCM. I think he's frustrated by this one too.

Do you think it could be a temperature dependent leak? Does the PCM tell it to do the test at startup after the other conditions (fuel level between 1/8 and 7/8ths full) are met? The light always seems to pop on about 1/4 mile out of the driveway a couple days after the code reset.

And wouldn't the test from #10 above have prevented a failure due to the line, tank, filler neck, and gas cap? The only thing left connected to the LDP was the canister and 1/4 inch vac line.

I must apologize. I just went out to work on the truck again after my last post and the error code has changed to P1494 on the most recent light. I still have the 5/8ths line from the LDP to the tank plugged. Would that cause this code? Or more likely that I've gotten 2 bad LDP's?

if you're using the truck with any hoses disconnectted, plugged etc in the evap system or disconnectted electrically then you can generate that code, that code incidentally points to a problem with the ldp so I'd doubt you've gotten 2 defective from Mopar! You may very well have a circuit issue in the wiring somewhere! I'd hardwire the ldp cirtcuit direct from the pcm to rule out an intermittent wiring fault before throwing another pump at it? Has the purge solenoid ever been replaced ? likely by now it has? Any tsbs or flashes available for it? which engine?

The purge solenoid was replaced after the 2nd trip to the dealer. I found a couple TSBs (#2500401 and #180100) about faulty evap error codes but I don't know what the fixes are. The PCM was flashed last week.

The only thing I disconnected at the pump was the line to the fuel tank and capped the short piece of hose to make sure it didn't see the tank (and any possible leaks there). I did leave the line from the tank open for two days while I waited for it to run the test. Maybe that would cause a problem, but I thought it was normally vented thru the canister anyway.

It's got the V6 in it. Wiring direct from the PCM will be fun. I'll give it a try if I can find the correct wires. But I would think it probably wouldn't give me the original p0442 code if the pump wasn't functioning.

Well then it has to have a leak! The ssytem works on a flow and vaccum principle because obviously it can't be seen! An electrical issue can generate a leak code! You need to hook everything back up properly and then head back to somebody at a dealer who can fix it! I'd be hard pressed to believe a pcm would cure it but there's a first time for everything! Maybe there smoke machine is out of calibration! It sounds like everything but the cannister and the filler neck has been replaced, a cannister likely wont give you code as much as cause repet failures of valves etc from it breaking down! Pinched lines anywhere can generate leak codes as well! So can valves in the fuel tank, fuel pump seals etc!

Thanks for your time. It's great to be able to bounce ideas off someone with experience in evap system repairs.

I reconnected the line from the tank to the LDP. Now it'll take a few days to see if the code changes back to p0442.

Also wanted to get your opinion on my next move...
If I get the p0442 code again, I was thinking to take it in for another smoke test, but this time plug the 5/8ths line coming from the tank to the LDP at the LDP end. I think this would isolate the tank, filler neck, and fuel pump gasket from everything else to help find a very small leak.

And I was hoping you could explain the "internal leak" I've seen mentioned on some of the forums. What valve or seal is internal that would allow the vacuum to fall off without letting smoke out?

Im confused as to why you're smoking 1 line at a time! If it's hooked in at the service port with the green cap and no leaks then remove the service port from the circuit and send the smoke both ways, up towards the purge solenoid and back towards the tank. The service port has a schrader valve in it so in theory it could leak off through it! Sometimes I even fill it through the vent line to check for any leaks! An internal leak in the system can be through any valve or switch that won't hold vaccum like the purge solenoid, ldp assy, rollover valve in the tank etc etc! If the system is put in hold with the scan tool and it's moked and no visible leak then the vaccum is leaking off somewhere and youneed to start pinching lines off etc to isolate

I'm not trying to smoke test it one line at a time. I am trying to figure out a way to see if it's a very small leak in the filler neck or fuel pump seal and avoid their recommended $500 PCM. It has already passed the vac test twice and the forced LDP test once at the dealership. It only seems to fail after it gets home.

If I plug or pinch the lines at the LDP can it be smoke tested through the fuel port? Or would a better strategy be to plug the line to the canister and have it smoked? Or is there some other way to find a very small/intermittent leak?

I guess what I really need is a strategy to find this leak instead of throwing parts and money at it. I can't afford the labor rate at the dealership any longer. This is already running about $450. I figured that I will drop the tank and replace the filler neck and fuel pump seal next.

I just discovered that what I thought was the charcoal cannister is actually just an air filter on the vent side of the LDP. That is the line I thought I could plug to force smoke out of any small leak back near the fuel tank.

Does the filler neck and fuel pump seal cost less then 450? I'd recommend against that plan because why changes parts that may not be req'd and risk creating a leak from trying to seal a fuel pump module and filler neck that may be leaking intermittently! I mean they don't leak intermittently!