4.2 misfiring after overheating


Original poster
Aug 3, 2022
I’ll do my best to describe the details as I recall. Just bought this 2005 Enviy with the 4.2. Needed the frame welded for inspection but I got a deal. Drive it around for 10 minutes, went to the title place and signed the papers. Then drive it for about 25 minutes to my brother-in-law’s garage. Was overheating but I didn’t know it as the gauges don’t all function. Lost power and misfired pretty badly and wouldn’t start once I got it parked. Next day it starts and he fixes the frame. No idea how it was running. I went to pick it up and it’s misfiring badly. Filled it with coolant/water to get it the 20 minutes home and made it (barely). Still has fluid boiling up and out of the cracked overflow tank when I got there so at least I know it wasn’t dry.
Checked for codes with my cheap tool. Got a P0301 cylinder 1 misfire. Changed only that plug. P0301 code. Swapped coil 1 and 2. P0300 code random misfire.
Swapped coil 2 to 3. Same. Swapped everything back to original spots (with the new plug in 1 and I’m still getting the P0300 misfire.

Biggest current concern: I cooked the engine and blew a head gasket. No evidence of coolant in oil. Plug 1 was dry, no coolant in that hole.

Any ideas of what to try next?
It’s acting very similar to my old 09 Traverse when the catalytic converter was clogged up. Anything that could have happened when it overheated to cause that? Worth a check?


Lifetime VIP Donor
Oct 22, 2015
Tampa Bay Area
Welcome to GMT Nation...

Caveat Emptor...

Paraphrasing from The Bible:

"It is easier for a Camel to Pass Through the Eye of a Needle... Than it is for a Superb Mechanic to Replace the Engine Head on a GM-GMC 4.2L LL8 Engine... While the Motor is STILL Mounted Inside of the SUV..."

I'm NOT Kidding here in the LEAST... It would be easier to perform an Engine Swap...

Test each Cylinder for Nominal and Near Balanced Compression 1-6 as follows:

Follow "Eric The Car Guy" on this Compression Test Journey:

Here is a Dude with a Post Compression Test Result that will Make a Grown Man CRY:

There are THREE Possible Causes for Overheating:

(1) Thermostat
(2) Failed Water Pump
(3) Clogged Radiator

At First Blush... Replacing the In-Operative Thermostat will probably be your initial thought. However, confirm that your Water Pump is not working FIRST because changing the Thermostat on this In-Line Six Cylinder Motor is a HUGE PITA... Just ask THIS Guy:


THESE are the correct Choices for the Replacement Thermostat. Insist on getting the ACDelco version if possible. It comes as a Complete Unit, Flange is Fitted with an In-Dwelling "O"Ring... DON'T FIDDLE AROUND WITH IT. Observe the Real Differences in how these are designed:


Believe it or not... Changing out the Water Pump is quite a bit easier to do. We have an FAQ on this procedure and take note of @Matt 's Brilliant use of a Chain Segment in securing the WP Flange while backing off "The Jesus Bolt" that holds the Fan Blade Assembly onto the Water Pump..

Here is a decent Water Pump R&R Video:

The ONLY Pump Worth Choosing is again... Made For GM by ACDelco:


In the Very Unlikely Event of this Last Problem... Replace the Radiator ONLY if it WILL NOT FLUSH or if it is Leaking...
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Master Blaster
Dec 4, 2011
Ottawa, ON
Sounds like it's blowing exhaust gasses into the coolant if it boils over that fast in the reservoir. There are exhaust gas coolant check kits available. A compression and a leakdown test will probably be the determining factor.

And if it is the head gasket, it's cheaper and easier to replace the engine than the gasket. After the cost of machining/rebuilding the head, extracting the inevitable broken head bolts (and it's ALWAYS the rear most inaccessible ones that break), parts, labour, etc, etc, better to get a good used whole engine.

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