Rough idle when cold then smooth

[2006Tahoe2WD]

Hey y'all,
For about a year I've had a rough idle or misfire when the engine is cold then when the engine heats up it is smooth. The boron level is going up on my oil analysis (from 50 to 79 in last 3 measurements every 5k miles and now 137 in the last 5k miles). I think this could be a symptom of a coolant leak into a cylinder. I'm not noticing my coolant level is going down. If it is it is very slow.
What is a way to check this situation?
The oil analysis also has low sodium and no %glycol.
Thanks,
-Henry

 

[justirv]

Hey y'all,
For about a year I've had a rough idle or misfire when the engine is cold then when the engine heats up it is smooth. The boron level is going up on my oil analysis (from 50 to 79 in last 3 measurements every 5k miles and now 137 in the last 5k miles). I think this could be a symptom of a coolant leak into a cylinder. I'm not noticing my coolant level is going down. If it is it is very slow.
What is a way to check this situation?
The oil analysis also has low sodium and no %glycol.
Thanks,
-Henry

If you haven't changed out your intake manifold gaskets, I would look there. When the engine is cold, they shrink, causing vacuum leaks and un-metered air to enter. When engine warms, gasket seals and problem goes away. Try spraying carb spray or whatever you prefer around the base of the intake manifold to see if your idle changes.

 

[Marky Dissod]

If you haven't changed out your intake manifold gaskets, I would look there. When the engine is cold, they shrink, causing vacuum leaks and un-metered air to enter.
When engine warms, gasket seals and problem goes away.

Kinda the same with exhaust gaskets, they do need replacing about as often as spark plugs - but don't spray stuff to detect exhaust leaks.

Could swear there is a CHEMICAL way to check coolant for exhaust gas presence.
My cheap@$$ way is to remove coolant cap on a COLD engine, start engine COLD, run back around and check for consistent bubbling.
Put the cap back on before the coolant escapes.

Oh, if you haven't changed the coolant in 3 years, it does get a bit thicker as it ages and collects more and more krapp, weakening the protective additive package.

 

[Fless]

Could swear there is a CHEMICAL way to check coolant for exhaust gas presence.
My cheap@$$ way is to remove coolant cap on a COLD engine, start engine COLD, run back around and check for consistent bubbling.
Put the cap back on before the coolant escapes.

How To Test Coolant for Exhaust Gases - A Complete Guide

Test coolant for exhaust gases is important for maintaining your vehicle's engine health. Exhaust gases in your coolant may indicate engine damage.

bryansgarage.com

 

[2006Tahoe2WD]

Thanks all.
I'll work on checking the intake for leaks also exhaust chemicals in the coolant.
Re. intake gasket replacement. Not sure if I should DIY or have a shop do it. It seems to be a bit fiddley and also might need to consider "while you are in there". E.g. knock sensors.

 

[justirv]

Thanks all.
I'll work on checking the intake for leaks also exhaust chemicals in the coolant.
Re. intake gasket replacement. Not sure if I should DIY or have a shop do it. It seems to be a bit fiddley and also might need to consider "while you are in there". E.g. knock sensors.

The intake is not too hard if you're used to mild/medium wrenching. Blow it all out with compressed air before disassembly. Take your time, block off intake passages while cleaning, and yes, recommend doing knock sensors/harness while your there, and seal properly. Use only original parts from reputable sellers. Lots of low-quality knock-off parts out there, which only add to headaches down the road. As @Marky Dissod and @Fless mentioned, exhaust manifold gasket replacement may also be needed. Not as easy, but still DIY'able. Access issues and probable broken-bolt extraction(s) needed.

 

[2006Tahoe2WD]

The intake is not too hard if you're used to mild/medium wrenching. Blow it all out with compressed air before disassembly. Take your time, block off intake passages while cleaning, and yes, recommend doing knock sensors/harness while your there, and seal properly. Use only original parts from reputable sellers. Lots of low-quality knock-off parts out there, which only add to headaches down the road. As @Marky Dissod and @Fless mentioned, exhaust manifold gasket replacement may also be needed. Not as easy, but still DIY'able. Access issues and probable broken-bolt extraction(s) needed.

I often buy parts at the dealer or get the top from RockAuto. I'm debating if I should replace the intake manifold gasket sort on a preventative basis. The Fel-Pro MS 98016 T set looks like the way to go? The rough idle is not bad and runs good otherwise. No codes/etc. Probably should let sleeping dogs lie. I know I have exhaust manifold broken bolts. I've been using one of the brackets for the last 35k miles or so and no noises. I don't relish trying to remove one or more broken bolts out of the heads.
Any thoughts on replacing the valley pan gasket assuming I don't see that it is leaking?