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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 29438
Experience:  GM Grand Master Technician 2007. 14 years experience.
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2005 H2 180xxx miles. Has random misfire at ghway speeds.

Customer Question

2005 H2 180xxx miles. Has random misfire at highway speeds. Have changed plugs wires done the relearn and put a new ecm Still has misfire. The misfires r mainly on cylinder 3 and 1 very little on 5-7. Never on bank 2
Submitted: 1 year ago.
Category: GM
Expert:  GM Tech (Cam) replied 1 year ago.

Hi there, I am Cam. Can you tell me if you have any codes set?

Customer: replied 1 year ago.
The misfire code us staying in it now and I have a random evap code that comes sometimes
Expert:  GM Tech (Cam) replied 1 year ago.

OK, sO to have codes on bank 1 or the drivers side of the engine suggests that you might have an exhaust issue, a ground issue for the coils on that side of the engine. What I would do is test for plugged catalytic converter on that side. Do you plan on doing the repairs yourself or are you looking for ideas of what the issue is to get it taken care of?

Customer: replied 1 year ago.
Truck is at the second garage now. The owner used to be a GM tec. He was highly recommend. He's leaving state for vacation and I'm taking truck to another garage probably GM dealer. I don't wanna keep throwing parts at it with no fix. I also put a new o2 sensor on bank one can't remember if it was put on before or after misfiring started
Expert:  GM Tech (Cam) replied 1 year ago.

I don't think for a second it’s a bad sensor, rather I think the converter is starting to restrict flow and would cause this. Do you notice any lack of power?

Customer: replied 1 year ago.
When its running right its hard to tell. Maybe a little. Before engine light starts flashing I can feel loss of power. Once it stops flashing and clears up seems to run OK How do u test the cat??
Expert:  GM Tech (Cam) replied 1 year ago.

The light was flashing, that almost confirms it. We test only when we have an issue. I am now thinking very much about that converter. You may find many exhaust shops will check for free. I would avoid aftermarket converters as a replacement as they don't seem o perform the same as GM or AC Delco and will often set more codes.

Customer: replied 1 year ago.
OK. One more question. How does the cat make it misfire. I don't understand the connection
Expert:  GM Tech (Cam) replied 1 year ago.

The engine is a big air pump. Anything that prevents proper air flow in or out will change how it fires. A plugged cat causes back pressure preventing proper air flow through the engine.

Customer: replied 1 year ago.
OK. Thank u. Guess that's what I will have checked out next
Expert:  GM Tech (Cam) replied 1 year ago.

Thank you for allowing me to assist you. Please rate my answer OK to Excellent service and feel free to ask any follow up questions regarding this issue you may have.
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