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Ron, ASE Certified Technician
Category: Dodge
Satisfied Customers: 35515
Experience:  ASE Certs
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Dakota: I have a 1990, 3.9L, V6, 4x4, reg cab Dodge Dakota.

Customer Question

I have a 1990, 3.9L, V6, 4x4, reg cab Dodge Dakota. It started running rough about a year ago and getting less miles per gallon. Over time I added fuel cleaner numerous times, tried different gases and octanes, I then changed; spark plugs & wires, dist cap & rotor, hall effect pickup/camshaft sensor but nothing helped. Then just recently it started running very rough and dying off. If you start the engine with no trottle/gas, most often it will start ok and seem to run fine, but as soon as you press the gas even the slightest amount it runs extremely rough and can die. If you rev the engine to much higher rpm it runs better and does not die but miles per gallon is way down now. I bought an identical (same year, just manufactured 3 months later) fine running truck (but burns oil real bad) so I then changed/swapped out (to no avail) the following parts from the 2 trucks; Idle Air Control (IAC) Valve (I was sure that was the problem as it seemed to move just prior to the engine running rough), the ignition coil, both fuel injectors, Manifold Pressure (MAP) Sensor, Throttle Position Sensor (TPS), Starter Relay and I think I also changed either the Fuel Injection Relay, or Fuel Pump Circuit Opening Relay, or Computer Control Relay or the choke relay (all 4 look identical but I only have 2 relays on the far back right engine compartment, one of which I think is the Fuel Injection Relay and the other one is one of the 4 look alike relays). Truck still runs rough/no change. I then removed the Engine Control Module (ECM Computer) and was ready to swap it out also, but I stopped as I heard that may cause a problem. I do not have a plug in scanner and do not know how to find the trouble shooting codes.
Submitted: 2 years ago.
Category: Dodge
Expert:  Ron replied 2 years ago.
Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.The problem you are describing sound like you may have a failing fuel pump and the place to start with everything you have already replaced is to install a fuel pressure gauge and monitor the actual fuel pressure while the problem is occurring. Normal fuel pressure on the 3.9 needs to be right at 13 to 15 PSI. Any thing less then 13 PSI would indicate a failing fuel pump. That being said, as for the ECM (computer) on your 1990 the ECM are plug and play and do not require any programming so if the host ruck is the same year , make and model as well as engine size the ECM should be the same as well.
Customer: replied 2 years ago.
Hi Ron,Thanks for your reply. Can you please tell me where I should install the psi gauge and will I be testing static/no flow pressure (i.e. the fuel line is opened and the gauge is installed on the pressure side "capping" the line, or do I install a tee on the fuel line and take a dynamic/flowing pressure read, with the engine running?ThanksTom
Expert:  Ron replied 2 years ago.
Tom, You checking the pressure by attaching a pressure gauge to the fuel rail service port. Take a look at the fuel rail going to the injectors, you should have a service port that looks like a valve stem, connect the gauge on it, you checking static pressure when the engine is running and the problem is occurring . If you do not have the port then tee into the line down stream of the fuel pressure regulator.
Customer: replied 2 years ago.
Thanks Ron, it is 4:52 am here (Hawaii), I will wait till dawn to try that and then get back to you.
Expert:  Ron replied 2 years ago.
Not a problem, Im here most of the day if you need me.
Customer: replied 2 years ago.
Hi Ron, finally spliced a tee and psi gauge on my inlet fuel line (my dakota truck does not have a fuel valve stem like my old ranger) to the "carburetor" and it reads a solid 15 psi with the engine running rough! Now what?
Customer: replied 2 years ago.
Hi Ron, this site does not show my last message to you. I tested the fuel pressure and it is a solid 15 psi when the engine is running rough. What next, should I change the fuel pressure regulator?Tom
Expert:  Ron replied 2 years ago.
Your fuel pressure is with specs so I would go back to basics and start with running a complete engine compression test to at least rule out a bad cylinder. Let me know the results and we can work from there.
Customer: replied 2 years ago.
I am leaving for town in about 1 hour to go buy a compression tester. In case the cylinders are ok, I was also going to get what ever device I needed to read the codes. Can you please tell me if I need to buy a scanner and if so what kind or can I do a "key on/key off/read the dash lights" to obtain the trucks error codes?
Expert:  Ron replied 2 years ago.
All auto part sell code readers but they will not work your 1990 OBD 1 system , on your your going to need a advanced scan tool and this is not something cheep to buy. You may want to consider stoping by a repair shop and have them pull the code and or codes for you.
Customer: replied 2 years ago.
I tried the key on off trick and got codes 1, 2, 33, 55. Does this mean anything?
Expert:  Ron replied 2 years ago.
Im not showing any listing for these codes which usually means there not valid codes. Most fault codes on the 1990 are going to be 3 digit codes.
Customer: replied 2 years ago.
Ron, I got the compression tester but have not had time to test the truck. Also have not had it analyzed yet. Truck still running rough. Will get back to you as soon as I have more news.Tom
Expert:  Ron replied 2 years ago.
Not a problem, I'm here when your ready.