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Welcome to the 3500HD.com forum created October 2009

Author Topic: ECM going haywire?  (Read 2133 times)

Andrew

  • Single Axle Driver
  • *
  • Posts: 3
ECM going haywire?
« on: October 20, 2015, 10:10:58 PM »
I'm not sure if GM calls it an ECM or a PCM. Feel free to school me!

My 99 with the 7.2 (454) has been misfiring at an idle and at low speeds. Upon first start, the truck runs nice and smooth. Rev the engine up, then let it idle down, and it starts missing.

After the engine would be warm, and had been off for 10 minutes or so - it would crank for a while before starting. So I'm sure the stock injectors needed replaced, as I'm sure they were leaking.

Truck has about 148,000 miles.

 - I replaced plugs, wires, distributor cap, and rotor. (these needed replaced regardless)

 - I replaced the injectors (with the Bosch) and replaced the fuel pressure regulator.

And the problem still exists.


We connected a scanner to the truck tonight.:

 1) Upon start up - engine runs smooth and revs smooth. Sounds and runs like a new truck. Scanner reports no cylinders mis-firing.

 2) Let the engine idle down, and engine starts running rough, scanner said cylinders 3, 4, and 6 were mis-firing.

 3) Scanner says (1) engine code. But no service engine light.

 4) click on read check engine codes and the scanner says no engine codes exist

 5) service engine light does NOT come on when turning ignition switch to on, nor when truck is running. But the scanner can turn the service engine light on, which tells us the bulb is working.

 6) scanner is saying that fuel system is putting out 25% more fuel. (which is why it's idling rough - its flooding, in other words it's running like a lawn mower when the choke is on).

To us it appears like the ECM has gone hay-wire and either needs re-programed or replaced. It's appearing as if the ECM is confusing the fuel system.


Anyone have any thoughts and experiences?

Thanks!












fabman72

  • One Ton Driver
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  • Posts: 79
  • Age: 47
  • Location: Blanchester ohio
Re: ECM going haywire?
« Reply #1 on: October 21, 2015, 03:02:29 AM »
Try replacing the distributor my uncle was having the same type of miss you are describing with his and it fixed it.

240g

  • Single Axle Driver
  • *
  • Posts: 19
Re: ECM going haywire?
« Reply #2 on: October 21, 2015, 12:12:14 PM »
x2 on the distributor. I had the same issue with my 96, that was the problem.

ED-NY

  • Administrator
  • 3500HD Hero
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  • Posts: 1081
  • Age: 68
  • Location: Champlain, NY
Re: ECM going haywire?
« Reply #3 on: October 21, 2015, 09:43:03 PM »
Since it runs good cold (closed loop) the ecm is not monitoring the O2 sensors, when the engine warms up (closed loop) the ecm relies on the O2 sensors for data.

I would use the scanner data stream for the up-stream O2 sensors to make sure they are cross counting properly.

 


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