1991 dodge ram 5.9 gas computer issue??

Posted By: donsace

1991 dodge ram 5.9 gas computer issue?? - 12/29/12 05:56 PM

What signs would I be looking for if my computer is bad on my 1991 Dodge ram with a 5.9 gas,auto. Will the truck still run if the computer is disconnected? Thanks a bunch!

Posted By: jerseybud

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/29/12 07:43 PM

1. There are many signs of ecm failure, like a no-start. What is your truck doing?

2. The engine will not run with the computer disconnected, nor should you try to do that.

Most people get intimidated by the tbi engines, but they can run quite well and get adequate mileage to boot with some methodical care. Common failures overlooked are fuel pump/filter issues leading to inadequate fuel pressure (should be 14-15 psi) and Hall effect failure. For example, since 1988, my 88 has had a 2 failed Hall effect sensors, 2 failed coolant temp sensors, 3 failed fuel pumps (2 of which were b/c of poor maintenance by dad and sis), one MAP sensor and one throttle body charge temperature switch.

The 92 magnums are known for ecm failure but not so much the tbi ones.

Posted By: donsace

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/29/12 10:29 PM

Thanks for the info. My truck starts and runs but has a bad engine miss and is putting out black smoke out the tailpipe,like its getting to much fuel. I unpluged the ECU and it stil runs the same. Here is what I replaced-fuel regulator,complete fuel pump assembly,rebuilt the TBI,new map sensor,plugs,wires cap and rotor all new.Then I replaced out the dist pickup.Still runs the same?? The only thing not replace is the 2 injectors and the computer because of costs.The truck is all original and has 145000 highway miles. I checked for vacume leaks and has none.????

Posted By: jerseybud

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/29/12 11:12 PM

Sounds like its dumping fuel so a couple things to check:

1. Start by looking at the fuel pattern with a timing light strobing. Should have a nice cone pattern-no dribbles or drops. A failed or missing lower o ring on the injectors can cause this. You may find a bad injector as well or a loose connection at those black harness that sits on top of the injectors.
2. Check your fuel pressure regardless of whats new
3. ohm out the coolant temp sensor and check the oxygen sensor.
4. very low chance of the computer causing this.

Posted By: donsace

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/30/12 01:00 AM

Will do thanks again!

Posted By: superwrench

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/30/12 02:14 AM

Does your truck have an EGR valve on the intake?
If so, try disconnecting the vacuum supply and see how it runs. Then try to give the EGR valve shaft a good flick with a screwdriver to help it seat. Also you could just remove it and check to see if the valve is seating.

I'd be very skeptical that it is anything to do with the computor....like has been mentioned it's more likely to be a sensor/injector/egr problem.
I've seen ton's of trucks come in with stuck EGR's or bad EGR transducers (the black vacuum pot on the EGR).
Please let us know what you do find.

Posted By: 383man

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/30/12 04:15 AM

You say you unplug the ECU and it still runs the same. What are you refering to as the ECU ? It sounds like a bad coolant temp sensor as I have seen that a few times. The coolant temp sensor is like the choke on a carb for a cold injected eng and it can cause to much fuel to be dumped by the wrong reading and not set a code. As long as its in the operating parameters it wont set a code on yours. When you start it cold it will be in open loop for a few minutes on your 91 and it will ignore the O2 until the oxygen senser heats up in a few minutes. The coolant sensor has most control over the fuel mixture on a very cold eng along with the MAP and RPM signal. Ron

Posted By: Supercuda

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/30/12 05:05 PM

Quote:

You say you unplug the ECU and it still runs the same. What are you refering to as the ECU ? It sounds like a bad coolant temp sensor as I have seen that a few times. The coolant temp sensor is like the choke on a carb for a cold injected eng and it can cause to much fuel to be dumped by the wrong reading and not set a code. As long as its in the operating parameters it wont set a code on yours. When you start it cold it will be in open loop for a few minutes on your 91 and it will ignore the O2 until the oxygen senser heats up in a few minutes. The coolant sensor has most control over the fuel mixture on a very cold eng along with the MAP and RPM signal. Ron

Don't overlook a stuck open thermostat or one too cold.

Posted By: superwrench

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/31/12 01:22 AM

Upon re-reading this post, I'd think it may be advantageous to have the system scanned.....or at least cycle the key for a trouble code. It's probably going to come up with a code 51 or 52 anyways ...O2 going rich or lean...but that could be the result of whatever's causing the system to puke black smoke....not the O2 itself.
383Man could be right....the temp sensor is important....otherwise there is something else telling the system it needs fuel...i.e. a lower than normal vacuum signal. That's why I suggested the EGR.A stuck thermostat will only cause the truck to get crappy mileage...not cause smoke.
A scanner will show a poor vacuum reading from the MAP.
I'd be checking it for sure.

Oh...and an OBD scanner won't do....you'll have to find someone with the proper 6 way plug-in cable for the older Mopars or go to the dealer and they'll use the DRB scanner.

Posted By: MoPar Jamie

Re: 1991 dodge ram 5.9 gas computer issue?? - 12/31/12 02:45 AM

I had a computer die in an '89 D-100 and it caused the injectors to stick open. Ran so rich I couldnt get it to run at idle and black smoke bad out the tailpipe driving.