Misfires Cylinder 1 - not head gasket?

A month ago, I was getting cold start misfires on Cylinder 4. I did a manifold and EGR clean out 9 months ago but checked again and it’s not clogged. I changed the plugs (also did this 9 months ago) again with the good ones and still had misfires so I changed the coils. No problems for 2-3 weeks.

Now I’ve been getting very intermittent misfires on Cyl 1, but this time they happen when the motor is warmed up, but at low throttle. I guessed I may have gotten one bad plug and I even tested the coil by moving it to cylinder 2. The misfires stayed on 1, so I changed the plug. Still a few misfires on cylinder 1. So infrequent, it doesn’t really throw a code, it just gives a brief death rattle, and I look at the non-live data at the misfire counts to identify cylinder 1.
I checked my coolant and it is low….. uh oh, sounds like a head gasket. So I’ve been trying to decide between a JDM gen 3 or gen 4 motor swap and realized I need confirmation before I get so deep in a project. So I got the combustion gas tester and tested it 4 times, in a cold motor, in a hot motor turned off, in maintenance mode at idle, and revving the motor a bit then while in maintenance mode. The blue liquid doesn’t even really turn green! Even after 2 minutes of testing. It does turn a lighter blue.
1. Does compression testing even work? My Prius does consume oil, about a quart every third tank of gas, so won’t the rings be stuck and fail compression even if the HG is intact?
2. Should I borescope it and add compression the the cooling system? Or is the leak probably too small to catch this?
3. should I just top off the coolant and drive it till the misfires are consistent? Note, I don’t think I’ve ever changed the coolant. It has always been a clear pink color and I never lost a drop until I spilled some during the egr clean out. I should probably change it all out either way.
4. What other ways should I test it?

I realize HG failure is basically inevitable and I have a 2012 Prius four with 260k miles. I am the original owner. All OEM parts (denso/ngk coils/plugs). I am not a pro mechanic but not afraid of big projects, but I don’t want to spend $2k on a JDM 4th gen and do the swap out frankenprius labor until it for sure needs it. Oh by the way a month ago I did change out fuel injectors with Rock Auto remanufactured OEM injectors. Thanks for any help or suggestions.

 

260k miles. It's gonna have a head gasket eventually but I'd hate to swap out a motor that still has a gasket intact. It has never overheated.

 

How do I test that? Just move #1 to #2 like you do with coils? I saw a video of somebody rigging up a fuel line and some 12v wire leads and blowing the fuel through the fuel line. Looks like a great way to get gasoline in my mouth or on my face . . . hmm not interested tbh but I respect the badass daredevil. The injectors are easy to get to, and since I changed them a month ago I know how to get to them pretty fast. I'll try the switcharoo this evening and let you know if the misfires move with the injector. My expectations of RockAuto would drop with that result I suppose.

I really just wanted to throw in the word "FrankenPrius" for fun. Yeah it looks pretty straightforward with a few simple variations which is why I would give it a whirl. And Mendel's contributions have me convinced the Gen4 is the way to go when the HG is for sure done.

I suspect there is a tiny leak that isn't giving a very strong indication, but may only leak coolant at high temperatures. It may require the bore scope method but then again if its a small leak that only happens at high temperatures and high pressures, even the borescope with coolant compression pump may not show a leak for confirmation; unless there is a clean piston top in the culprit cylinder.

 

DC1F8A76-0A07-47C9-A57B-9FB1F46273C9 by chaddarack posted Sep 6, 2023 at 2:53 PM

3E42790C-C3FF-47C6-9C3D-A6977AE7EBBC by chaddarack posted Sep 6, 2023 at 2:53 PM

BBA0A9E6-7A17-4A59-A865-CDFD06C25429 by chaddarack posted Sep 6, 2023 at 2:53 PM

For reference to the above discussion.

 

Update:
Current status was to keep driving it until symptoms became more apparent, and do an injector swap when I get a chance after work, or even a borescope check of the cylinder for coolant leaks. I topped off the coolant yesterday.

This morning, I started hearing a little bit of liquid splashing noises behind the dash - as someone else indicated this is another head gasket symptom. So I decided to check again for exhaust fumes in the coolant with the blue liquid. After releasing pressure from the cap, when the cap came off, a smoky fog was in the overflow bottle. No I didn't jump at the opportunity to stick my nose down there lol. But I huffed some of that into the blue liquid block tester stuff mentioned above to see if it would turn green or yellow this time. It did not. Still just light blue. So I took the tester around back to the exhaust (while in maintenance mode), and huffed in the air from right behind the exhaust. Still no change. Just light blue! Not even green. So my the blue liquid in my kit was defective. Light blue is a positive. I put some fresh, dark blue liquid in and stepped away from the car and pumped fresh air through it. It doesn't lighten up with fresh air, only with exhaust fumes. So I have a positive confirmation. Moral of the story: test your tester before you trust it.

It's gonna be a Gen4 swap for me. JDM. Wish me luck on getting a good one.

Also, somehow mine does not misfire much on cold start lately. I think when the motor is hot, it pushes 160psi(guess) exhaust from the running cylinder into the cooling system. Then when the motor stops at a red light and releases its pressure, the pressurized cooling system spits coolant into the cylinder at the red light and that's why mine misfires after hot. I don't know why it doesn't do it on cold start. Maybe the gasket is holding when cold, or maybe the coolant gets time to evaporate. This probably is common sense for an experienced mechanic. But everybody points to the cold start misfires, and that's not my pattern exactly. Sharing for any other novices who doubt because it doesn't sound typical.

 

I did look at the plug during the process. It was actually black on the electrode - fouled. When I changed all plugs and coils to new a month prior, all plugs were whitish on the tips. I think I blew the head gasket driving in hot August traffic here in Georgia, after I changed out my plugs so I expect the failed cylinder will show this time when I do the comparison. I'm about to pull all the plugs to check them, and the compression test to see if coolant leaks into the cylinder this evening. I'll share the pictures later. If I can get a screen grab of the borescope on Cylinder 1, I'll share that too.

 

Screenshots of Cylinder 1. Video, if it uploads, in order of Cylinder 1, 2, 3, 4, 1, and 2. Cylinder 4 looks steam cleaned. 2 and 3 have patchy crust, cylinder 1 was super crusty with fluid at the bottom corners.

I put a mirror on the borescope and tried to connect up the coolant system pump to see if I could make it leak. But the coolant pressure kit from Oreilly’s auto parts didn’t have an adapter for the coolant bottle so no go. Oh, and fun story! The borescope mirror fell off into the cylinder. I tried fishing with an antenna magnet for a few minutes but not worth my time. It’s getting a new motor anyway. I could get it when I take the head off one day, to show my son how a motor works.