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Discussion Starter #1
I have a '94 Dodge Ram 1500 with a 5.2L Magnum that suffered an engine fire due to a fuel leak back in July. The damage that I have found so far consists of: melted air intake, all plug wires burned, most vaccuum hoses burned, convoluted tubing surrounding wiring harness burned, plastic cowl melted, distributor cap seems like it might be burned.

I have replaced the fuel rail assembly (the leak seems to have started at the tube connecting the two fuel rails; I replaced the whole thing with a used assembly from a salvage yard), all the vaccuum lines, the plug wires, and checked the wiring harness for damage. There does not seem to be any damage to the wires themselves, just the black convoluted tubing the harness was wrapped in. I have not yet replaced the distributor cap, but will be doing that this Friday.

So that leads me to the problem: When I first start it up, it runs perfectly (no misfire, hesitation, etc), but after the engine reaches operating temp, it develops a noticeable off-idle hesitation. The OBD I is not showing any trouble codes at this point. Fuel pressure is a steady 40 psi, so that is not an issue.

Other than the distributor cap, any ideas?
 

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Mine behaved like this when the MAP sensor went south...and later became undrivable.
It seems like an closed loop problem so check lamdas and their wires.
 

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Agree. After the engine warms, the O2 sensors come into play to control fuel mixture. If it runs better when cold, the PCM adjusts fuel mixture with an internal program.
EGR coming on too soon can also cause a hesitation. Temporarily disconnecting and plugging the vacuum hose can prove this out.
If you have an OBD I scan tool capable of watching warmed engine fuel adaptives or fuel trims, you may find which bank or component is at fault. Not everything will set a fault code and sometime it will take awhile to set fuel system rich or lean fault codes.
 

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Discussion Starter #4
I initially had the vaccuum supply to the EGR disconnected, and it still hesitated; I have since hooked it up and the stumble is still there. I do not have a scan tool, but I can double check all the sensors in the vicinity of the fire. I suspect that it is a lean stumble; when I stomp it hard and it hesitates, I can sometimes hear it backfire into the air intake.

It only does this when I give it moderate or heavy throttle application, and mostly only from idle. If I baby the throttle or hit it from cruise, there is no stumble. At least not that I can feel or hear.
 

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Discussion Starter #5
Well, I replaced the distributor cap and that did not solve the problem. I visually inspected the TPS, the MAP sensor, the IAC valve, the wires for these and the wires to the distributor on the back of the manifold. They all seem to be fine. The only fault code the truck is displaying is code 37, which is a problem with the part throttle unlock circuit, although the trans shifts normal and the lock up converter engages and disengages without issue. It has displayed this code since before the fire, anyway. So where do i go from here? How do I go about narrowing down the area of the problem or testing to find the deffective sensor?
 

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I would look for a factory 1994 3.9L/5.2L/5.9L Powertrain Diagnostic Procedures manual: http://www.ebay.com/itm/1994-DODGE-3-9L-5-2L-5-9L-EFI-POWERTRAIN-DIAGNOSTIC-SERVICE-MANUAL-SY796-/221006211499?pt=Motors_Manuals_Literature&vxp=mtr&hash=item3374ff29ab
Without a proper scan tool (DRB or equivalent) for a problem like this, it is like working blind. A real-time sensor display on a road test would likely show the problem. Can you borrow, rent or pay for time with a good scan tool and helper?
 

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Discussion Starter #7
Have not really made much more headway into tracking down the problem, but I discovered something interesting the other day.

My climate control unit will only blow through the defroster vents, regardless of what setting I put it on. I was under the impression that this was vaccuum controlled, hence the vaccuum line routed to the climate control unit. I replaced this vacuum line and double checked it for leaks; I can find none, but it still only blows through the defroster vents.

I am wondering two things: 1.) if there is vacuum to the climate control unit, why would it do this? 2.) could this somehow be related to the drivability issue I am having?
 

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Blend door could be stuck, or there could be a vacuum leak at the 'octopus' vacuum manifold on the back of the heater controls.
 

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Discussion Starter #9
So I have not sorted out the climate control issue, but the hesitation is pretty much gone. I don't know what it was; maybe dirty fuel injectors, or bad fuel, or something else, but at any rate it gradually diminished the more I drove it and it is not doing it at all now.

I am happy about this, but a little bothered becasue I never did figure out what the problem was.
 

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Considering the fire, check both the MAP sensor and TPS sensor. The wiring to and from should also be checked. As for the heat, whatever caused the fire could have jolted something loose under the dash. Make sure your vacuum connector is secured to the HVAC selector switch. Typically, the HVAC vacuum line is tapped at the brake power booster. Check the check valve there as well. Since you didn't mention brake issues, I'll assume the vacuum to the booster is fine.

Testing the TPS requires an analog volt meter (or lots of patients with a digital version). Tap the signal wire and slowly open and close the throttle. If there are any hick-ups with the signal, you have a bad wiper (bad sensor). Replace the TPS. If voltage sweeps smoothly, chances it is just fine. For the MAP, I've seen them bad where no codes were tripped. Using a scan tool the voltages were erratic. You could test the MAP with an analog volt meter and vacuum pump. Just slowly apply vacuum to the port and watch your AVOM needle.

Hope this helps.

Mike
 

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The PCM adaptives will adjust the mixture richer or leaner over time to compensate for mixture problems. I agree that it sounded lean with the intake pop-back. Colder temperatures will also enrichen the mixture for better cold starts and running. Like a choke did in the old days. Fuel economy may suffer.
If vacuum supply is lost to the dash HVAC control, it will default to defrost for safety. The reason for this is probably out front under the hood where a vacuum hose has fallen off or is damaged or pinched. There is a vacuum diagram label under the hood to help determine the routing and locations of the various vacuum circuits.
Not related to the fire, the Magnum intake manifolds on these commonly developed vacuum leaks at the bottom plate gasket. The intake manifold has to come off to service this. You would have strong crankcase vacuum even with the PCV out and the valve cover holes blocked at idle. Oil consumption may also be high and you can see oil pool on the bottom plate looking down the throttle body bores.
 
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