Engine stumbling,very odd

[Yukon2000xl]

I’ve been having this pain of a problem with my 5.3l Yukon xl 2000 lately.She is my 3rd driven car so she sits maybe 4-5 days in a row ,usually used on weekends to haul quads up north or in winter up here in Canada where the half ass plow the roads.
Lately including this morning (truck was sitting 6days) she kinda bucks when idling,like a fraction of a second engine miss.Only when she gets warmed up on highway speeds does it stop.It also seems to not come back even after cool down that same day? No check engines, oil psi on start up 60 when warm 40-45.All fluid levels are on the mark,heat and amp gauges are indicating fine?
I’ve already changed the fuel pump and filter plus flushed tank maybe 1000k ago (used oem gen 3 Delco pump).Plugs were replaced (not wires).Water pump ,oil pump and pickup tube replaced.Battery is new glassmat battery for our extreme weather up here.Exhaust from manifold to rear replaced with new cats/O2 sensors all Bosh new,rear exhaust 3” dual Gibson stainless no back pressure for sure.
I don’t want to take changes with this truck cuz I take her places most jeeps don’t wanna go hauling atvs up north .The last thing I need is catastrophic failure at minus 45c, 4 hours in the bush.Any clues where I should start


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[swathdiver]

Wires, start with the wires.

 

[rockola1971]

YUP. If those wires are original it is well past the time for them to be replaced. Start the tahoe up in the dark with hood open (with engine cold) and look down around engine (both sides) and you will see a lightning show of flashed. Thats the insulation on the wires crack and broken down and high voltage escaping seeking chassis ground (This is what is suppose to happen only engine ground is the tip of your spark plug). You have whats called a partial short to ground but since there is no fuse from the ignition coil secondaries through your wires to the spark plug it will just intermittently keep doing it. Sometimes it can take out the ignition coil(s) but rarely.

 

[OR VietVet]

You didn't mention whether you have been seeing a CEL. With the misfire, if it is plug wires, you should have set a misfire code. If the wires are very old they should be changed anyway and may take care of the problem but I am wondering why no CEL, unless there is a CEL and you didn't share that info.

 

[Chubbs]

Different vehicle but had 1 bad wire & no 'CEL'

I found it when I was pulling the wires off of the spark plug end and a boot seperated from the lead...with minimal force. It was just degraded.

At the time, I found that it missed under high load, usually at the top of 3rd gear

 

[rockola1971]

You didn't mention whether you have been seeing a CEL. With the misfire, if it is plug wires, you should have set a misfire code. If the wires are very old they should be changed anyway and may take care of the problem but I am wondering why no CEL, unless there is a CEL and you didn't share that info.

GM purposely programs paramaters that must be met BEFORE a code is thrown. For instace the misfire code might be 10 or more instances within 5 seconds or whatever. His may not be bad enough yet for the code paramater to be met to even throw the code. But soon it will.

 

[OR VietVet]

Different vehicle but had 1 bad wire & no 'CEL'

I found it when I was pulling the wires off of the spark plug end and a boot seperated from the lead...with minimal force. It was just degraded.

At the time, I found that it missed under high load, usually at the top of 3rd gear

In this case the OP makes it sound like it has been going on for a while. I would check the plug wires as well but if has been going on I would "expect" a CEL or at least the OP telling us whether it has or has not flashed on. I like the "search and destroy" method of diagnostics instead of guessing at it and hoping for a positive out come. I get it, it does sound like plug wires but I also want answers to obvious questions.

 

[1_8TTony]

How old is the engine coolant temperature sensor? Have you cleaned out the throttle body? (The shop I work at) we repair this similar problem (LS series engines) once every few months sometimes by replacing engine coolant temperature (ECT) sensors or by cleaning throttle bodies (TB).

In our situations the ECT sensor became active once it got up to a certain temperature, but was inactive while below a certain temperature (cold). The PCM requires ECT info to make proper adjustments and fuel trims.

You can use a scanner and look at engine coolant temperature and ambient air temperature sensor readings. Both data should indicate similar numbers.

With scanner still connected, look at both data, start the engine, the ECT number should start climbing higher than the ambient temperature number (probably takes approximately 20 seconds before the coolant begins to warm up).

ECT sensor is located behind and below the alternator (incase you want to back-probe the electrical connector with a multi meter to get some wire harness voltage readings............you may have to move the battery cable bracket out of the way. This bracket is secured to the side of the cylinder head with two 10mm bolts. Good to check that the wires are in good condition.

And as for this situation being remedied after cleaning dirty throttle bodies.......we learned that when the A/C system is activated, it disrupts the intake air passing thru the dirty idle air control valve causing the engine to stumble.....................

But wait...........

You live in cold climate and this is happening with the air conditioner turned off.

Ok, so living in cold climate, you turn on the defroster to keep the front windshield clear, right? Well, the a/c compressor still comes on even though the vents are blowing hot air. Basically, when the defroster is on, it activates the a/c compressor, the cold air in the cab of the truck is sucked into the HVAC system where it is chilled even more making the air more dense then that cold air passes thru the heater core coming out nice, hot and DRIED............with situations like this, the problem will eventually get so bad that the engine will actually stall every time the a/c compressor cycles on and/or every time the brake pedal is pushed with engine idling.

Fuel injector cleaner wont fix this problem. Aerosol throttle body cleaner wont fix this problem......the throttle body has to removed and the idle air control valve needs to be removed from it........the cavity where the idle air control valve plunger inserts into the throttle body needs to be scrubbed out with a small plastic brush and scrubbed out with proper cleaning chemical......the plunger needs to be cleaned as well before it goes back on the throttle body.....don't lost the O ring seal when removing the idle air control valve.

 

[retiredsparky]

Coolant temp sensors on older engines should be replaced since they can start to fail by losing their accuracy slowly over time. This can cause the engine to run poorly. A $30 part that has a significant effect on the engine. I have repaired multiple engines of different brands over the years both gas and diesel by installing a new temp sensor. Of course this has no connection to the sensor connected to the dash gauge for engine temp.

 

[1_8TTony]

You know your stuff!!!!
The temperature guage in the dash is not accurate in relation to "real time" engine coolant temperatures.

Whenever working on coolant systems, I always use a monitor to see exactly what the PCM is seeing. Volkswagens are the worse. Normal operating VW temperatures the guage cluster shows 190 degrees.......I can force a coolant temperature fluctuation between 185 and 225 and the dash needle stays rock solid at 190.