1994 Dodge Ram



September, 18, 2013 AT 10:21 AM

Problem: A month ago I pulled my 3/4 ton Dodge into the drive and it stalled. Since then it will not idle or drive correctly.
It sounds terrible, typically I need to pump the gas in order to keep it from stalling once it starts. If I keep my foot on the pedal, it will stall and if I lay off the pedal it stalls, so I push down, let off and repeat and that combo seems to work.
It will not drive, it will stall when I let off the gas for a light/ stop sign or even if I slow down for a turn, when I accelerate there is big lag and at this point I can barely attain 20 mph on a straightaway.
I can hear it is not firing right either.
What I have done: For another recent problem I: Replaced battery, crankshaft position sensor, camshaft position sensor, distributor cap and rotor, plug wires, ignition coil and added another ground to the battery,
For this problem I have replaced: Precat o2 sensor, alternator, throttle position sensor, idle air control valve,
I have 3/4 full gas tank and I have put plenty of high quality injector cleaner in the gas and tried to run it long enough to let it work in there. I've also checked and repaired the micro sized vacuum lines associated with the EGR valve.
I'm able to read the codes off the computer and I am getting nothing that helps-- Code 12: battery cable disconnected recently and 55: end of all codes, occasionally code 51 (after replacing o2 sensor) comes after some rough idling.
My remaining thoughts would be to start with replacing the EGR valve.
Possibly look into a new PCM (computer)
One x one the fuel system- fuel pump, filter and pressure regulator are all together in the gas tank and I wonder. Fuel rail and injectors are a lot more accessible. I just need an intelligent and experienced mechanic to guide me to a good working truck. I need this truck to work for my business.


4 Answers



September, 18, 2013 AT 11:50 AM

This is a case of where it would be a lot less expensive to have gotten a mechanic involved. You're looking at parts that won't cause the symptoms you've described and you're wasting too much time and money.

The clue here, if I read this right, is the engine will stall unless you keep moving the accelerator pedal. That is a common way the MAP sensors used to fail, and it was the secret to how we could drive the vehicles into the shop after the owners had to have them towed there.

You were on the right track to read the diagnostic fault codes but the reason nothing showed up also has to do with how the codes are set and how the sensors fail. The MAP sensor is fed with 5.0 volts and ground. It puts out a signal voltage that decreases as intake manifold vacuum increases. The typical voltages range from 1.2 to about 4.2 volts. The acceptable range is from approximately 0.5 to 4.5 volts. Anything outside that range will set a fault code.

Chrysler is the only manufacturer that has been able to make an engine run right without the mass air flow sensor all other manufacturers use. On their engines the MAP sensor has the biggest say in the fuel metering calculations. The running problem comes from the sensor reporting the wrong signal voltage, but that voltage stays within that 0.5 to 4.5 volts so no code is set.

If I am right, the engine may run better if you disconnect the three-wire connector from the MAP sensor. An internal "pull-up" resistor in the Engine Computer will put 5.0 volts on the signal wire to force a code to set. At that point the computer will know it can't rely on that voltage so it will disregard it and it will inject approximate values to run on based on calculations from the other sensors and operating conditions.

The only thing that concerns me about this story is those sensors usually went from working properly to what you described to total failure within a day or two. They never lasted a month, but there's a twist. The original sensor was developed by GM. It had a piezoelectric crystal, similar to a phonograph cartridge, that developed a voltage based on changes in pressure. A rubber diaphragm tugged on that crystal due to manifold vacuum. (The sensors are actually sensitive enough to be used to measure engine speed because they are so sensitive they can detect when each piston pulls a gulp of air). GM had a huge pile of problems with the design, so they sold it to Chrysler. Part of the problem was caused by over-filling the gas tank. You were okay if you stuffed it full, then drove far enough to use up a few gallons, or if you stopped filling when the pump shut off. Problems occurred after repeatedly stuffing more and more fuel into the tank after the pump initially shut off, then parking it on a warm day. The fuel would expand and the fumes would work through the vapor recovery system and up to the sensor. There the fumes would deteriorate the protective jelly, then start to work on the sensing element.

The later design used a strain gauge instead of that crystal. That was a very long, thin wire wrapped around a plastic core. As the diaphragm pulled on it, that caused the resistance of the wire to change. The sensor amplified that and used that to generate the signal voltage. Those have been pretty reliable.



September, 18, 2013 AT 6:41 PM

Update: Today I went back to the shop and went ahead and replaced
The EGR valve, MAP Sensor and PCV valve.

Idle now.
It idles a little smoother now. A little more consistent.

If I try to drive it, it still lags and hesitates pretty bad when I put gas to it, but it responds better than before although it's obviously not fit for the road.

Key Issue alongside this.

Brakes: Idling In Park, if I hold down my foot on the brake for 10 seconds or so, it will start to stall unless I let off the brake- at which time the rpms will shoot right back to normal.

Drive and Reverse: If I press the brake it will start to stall quicker than in Park

Neutral is the least likely to stall with the brake, it virtually is unaffected and so I have found I can avoid stalling by slipping the truck in Neutral when I need to slow down and back in gear to accelerate. This is all for testing purposes.

Just came home and read the reply.
Try again.



September, 18, 2013 AT 7:50 PM

Decided it was worth a try.
I disconnected the Map sensor and started the engine, it ran for 3 seconds and died out. Tried again, same result.
I reconnected the Map sensor and it idled.
So I shut the engine off.
As said before, problem has now shifted specifically to hesitation, lagging and pinging during acceleration and stalling is isolated to braking in park, reverse and drive.



September, 18, 2013 AT 11:30 PM

Okay, a lot of what you're describing could be related. The MAP sensor, (manifold absolute pressure sensor), very accurately measures intake manifold vacuum. If you have a vacuum leak, that can affect its reading, it will affect engine idle speed, and it can result in stalling.

When shifting into gear affects the symptoms, look for a vacuum hose or connection that is opening up when the engine shifts position. Electrical connectors can do that too but for engine stuff that should be detected by the Engine Computer which will set a fault code. Things like that will be aggravated by accelerating because the engine will rock more.

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