[SOLVED for $0] -- 2010 misfire, stumbling, stuttering, coolant loss, head gasket, etc. symptoms

I've been on track for Gen4 into Gen3 engine swap or minimally a head gasket replacement in my 2010 Prius V. It's been stuttering and stumbling BAD at nearly every stop sign lately.

As far as I understand it, this is the same issue/set of symptoms that Toyota is replacing intake manifolds, EGR's and people cleaning their PCV through EGR system (replacing PCV's, cleaning pipe, cleaning EGR cooler, unsticking EGR valve, even installing Oil Catch Cans / OCC's). I've done all that and more (replaced plugs, coils, cleaned injectors, etc.). Hell, my avatar is of the first dump of the contents of my own oil catch can -- which did NOT solve the terrible misfire/stumbling issue.

So how crazy is it that removing the o-ring from the coolant cap solved all symptoms instantly?

I mean instantly. No more stumbling. No more coolant loss. What the heck.

One thing I can add to the mystery is that I did notice that even after 24+ hours of sitting, or sitting overnight in the cold - when I would unscrew there coolant cap there was always considerable or noticeable amount of pressure there "PSSSSSSSSSSSSSSSSSH!" each time I would open it to add a little more coolant.

I discovered this "fix" on youtube from this guy...

 

So here's where I was.... I was ready to keep adding coolant ("cheap" ZEREX ASIAN red at this point -- because like a 1/2 a pint a day was going somewhere) until the thing totally shook itself silly and broke down. Then I was going to do the 2016 Gen4 engine into Gen3 swap. I figured... I have a short 10 minute commute, I'll get the most I can out of the existing engine before spending $900-1200 or whatever on Gen4. The longer I go, maybe the cheaper that 2016 engine gets.

So without being a master mechanic, I assume it was getting more pressure than it should have in there from a small leak (let's say cyclinder 1) pumping gas/pressure back into the coolant system. But apparently not enough pressure to throw the relief valve??

OR maybe the coolant cap relief valve is/was just broken?

Consequences? I have no idea. You would think that all the coolant will boil and vapor off since the system ins't sealed. But drove for an hour and did not experience that at all. For the first time in a long time -- zero stumbles and not a drop of coolant loss (that I can see).

So, back to you then...

Why do you think it helped? What do you think the other consequences will be?

 

Could it be... these mysterious stumbling issues that Toyota has been chasing down. Could some of these failures be caused by faulty coolant cap pressure relief valves on the coolant reservoir caps??? How stupid easy would that be to fix.

Huh... or could it be that Toyota overshot the PSI spec for the cooling system cap?? If a bunch of cars shook themselves to early death as a result, that might be recall worthy if so. Again, not a mechanic. Just speculating.

For what it's worth, my cap looks original and says 108kPa.

 

Have you ever been in one of these Gen3 Prius's while they were shaking and misfiring? Not sure how it could be much worse.

 

Good point. I'll get one of those OBD bluetooth dongle things for Torque app and measure all the temp points I can with and without the coolant pressure.

 

Good catch (pun). I photoshopped the face from the scream in there, because I felt the same.

 

The only problem with this is that while I do plan to replace the engine with a 2016 Gen4, I don't want to replace anything else outside (i.e. heater core, 3 way valve, etc.) that might get damaged by "stop leak". I'm having that problem right now on a 2006 prius -- had to flush out the heater core because stop leak was used for head gasket repair.

 

So in theory, if your theory were to be occurring, then my coolant level should drop over time? Or are you suggesting "boil to vapor" in one area and distilled or whatever and recaptured in another? As mentioned in first post, my coolant level was dropping 1/2 pint or more a day. It is stable now.

 

So then I have to wonder... what was T-SB 0012-10 about?

(The TSB where Toyota was replacing intake manifolds and EGR pipe gaskets.)

But where does this problem actually START.

It's clear that it happens often enough to indicate a design flaw of some sort, but where does it originate?

I'm not entirely sure I have a head gasket leak, but I've done so much here without fixing the problem I don't know what else it could be.