Truck misfires and backfires through the tailpipe when cold. Seems to smooth up when the truck is warm and RPM is over about 1200. I can still feel an occasional misfire after it's warm but it's definately worse when cold.

I replaced the cap and rotor and Sea Foamed the top end via the brake booster vacum line. I also added a can of Sea Foam to the tank. I'm about half way through that tank now.

Truck ran fine for about a day after I replaced the cap/rotor, then the CEL came back on with the same codes and symptoms just not quite as bad.

The truck sits out at night and outside temp is in the teens at night here now. When the problem started, temps were probably in the 30-40s at night.

Thoughts?

 

I had similar happen on 95 dakota with 3.9L V6. Mine was caused by coolant temp sensor.

- Dan M

 

How are the plugs ????


Dodge P0300:

Basically this means that the the car's computer has detected that not all of the engine's cylinders are firing properly.
A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.
Symptoms

Symptoms may include:

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Causes

A code P0300 may mean that one or more of the following has happened:

• [*]

Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.
Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

 

Forgot to mention:

Vacuum is steady at 18 inches at idle. I ohmed the injectors and they were all between 12 and 13 ohms.

I checked plug 1. Was not burned or carboned up. Gap was .038. I'll check the others this weekend hopefully.

Any way to verify the coolant temp sensor? The problems seem to go away after the truck warms up but the CEL stays on.

Any suggestions on a good code reader?

Thanks!

 

Most parts stores will have a code reader, that might also be able to read the data stream. Trouble is, you need to get it to them while the problem is happening.... (engine cold)

 

Where's the EGR valve on this thing??

 

As far as the coolant temp sensor goes, you can check for obvious issues. On my old 95 dakota, it was obvious when i tried to remove the wiring connector to test voltage that the sensor was bad. It has a plastic piece that the wiring connector plugs into on top of the metal that screws in and it bent so easily as I removed the wiring connector.

check the voltage cold and monitor it as the engine warms up. voltage should drop as the engine warms up.

I can't recall what the voltage should be, can't find my haynes manual for my old dakota (don't own one yet for ram).

The only way I was alerted to check the coolant temp sensor was i just got lucky and saw my temp guage in dakota drop very quickly and return back.

- Dan M

 

97 shouldn't have one.

 

actually dan m the coolant temp sensor(thermistor) works off of resistance(ohm's).colder it is the more resistance(ohm's)it produces.just the opposite acures when hot (less resistance ohm's).theres a chart in the manual you can refer to.oh and the 5volt supply to the sensor should stay the same when cold or hot.

 

Whew, that's a relief! Thought I was going blind or crazy... I looked all over for that thing.