TLDR Version
Multiple parts broken after shop performed coolant flush. Overheating caused by air infiltration via broken reservoir cap, broken radiator cap, then defective NEW OEM cap. All leaky caps replaced, system flushed/burped. Still seeing signs of leak near radiator cap--appears to be crack/pinhole at neck of upper tank...

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Full Description of Problem and Diagnosis
I have had an intermittent overheating issue off and on for a while now and am hoping some of you guys can weigh in on what I should do next. I have a meter plugged into the ECU all the time, so the temperatures listed below are readings taken from the sensor, not estimates based on dash gauge bar count. Apologies for the long post, but I try to detail everything that has happened up until now to help with the diagnosis.

Below is the sequence of events:
* I never had a cooling issue before, but took car car in for a coolant flush at a local shop, per the recommendation of the service schedule. Everything seemed fine on the drive home.
* 3-4 months pass with no issues
* I first noticed the problem (coolant suddenly read 235 F) while driving on the freeway. I pulled over and crept home slowly with the heater on (temp dropped to 205 F)
* Checked the coolant level in the reservoir and it was low and also discovered the reservoir cap had snapped apart (presumably from being overtightened at some point), so I added enough to bring it up to the max level.
* Several days later the overheating began again so I pulled over and let the car cool, then crept home slowly with the heater on (temp dropped to 197 F).
* Checked the radiator cap and discovered it was damaged (picture below) with parts missing, so I ordered new radiator and coolant reservoir caps.
* Overheating issue did not reoccur while waiting for parts arrived...
* Parts arrived so I replaced both caps and verified the reservoir level is good
* Several weeks later, the temperature spikes to around 230 F while driving on the freeway, so I ran the heater and limped home again.
* I stop to pick up a Lisle spill free radiator funnel on the way home, let the car cool, raise the front of the car, fill and burp the radiator
* 2-3 months pass with no issues
* Today, the temperature spikes to 235 F while driving surface streets home, so I run the heater and limp home with the temp at 197 F

New (left) and damaged radiator caps:


My questions are:
* Could I have just done a poor job burping my cooling system?
* Could the missing piece of the old radiator cap be circulating in my cooling system and causing an intermittent blockage?
* Could the missing piece of the old radiator cap have made it all the way through my radiator?
* What is the recommended course of action now?

Thanks so much in advance to anyone who's read this far and can offer some insight as to what might be going on. I am really hoping to sort this out before the scorching summer months roll around.

Recent Events and Solution
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* pulled thermostat and pot tested per the recommendations of forum members - thermostat ok
* refilled/burped coolant once again
* continued to observe random fluctuations in coolant temps (195F cruising / 227F idling in traffic)
* observed coolant drips near radiator cap (was I just sloppy when I last refilled?)
* cleaned coolant drips
* conducted test drive the following day and came home to find more coolant drips near radiator cap
* purchased/installed new autozone radiator cap
* burped coolant again and cleaned all coolant drips in engine bay
* conducted test drive the following day and came home to find NO coolant drips near radiator cap
* had coolant flush performed by dealership (to replace the off brand coolant I used while diagnosing)
* temps are now normal once again (187-195F cruising / 205F max, idling in traffic)
* conducted test drive and came home to find more coolant drips near radiator cap -- possible hairline crack/pinhole near neck of upper radiator tank
* <parts ordered - planning to replace radiator with Mishi unit>

 

Does your heater blow out hot? Might have a sticking thermostat. If the thermostat sticks it might not open all the way. do the cooling fans work?

Was the car on while you were burping it?

 

Yessir! The heater feels like it's blowing scorching hot, consistently.

Yes, the car was on during the burping procedure. I followed Billman's procedure, on a sloped incline (nose up) as prescribed.

 

After you replaced the radiator cap and overheated subsequently, did the coolant level in the radiator drop? You indicated you purchased the Lisle system, so it appears you had to add coolant? If so you either have a leak (do a pressure test) or a headgasket issue (perform a leakdown).

When you burped the system, did any air pockets come out?

Also, the missing piece that broke away from your cap is probably sitting at the upper plastic end-cap of your radiator. If you drained your coolant and tipped the radiator (or car) enough it should show itself thru the radiator cap opening. Unless the piece(s) are smaller than the radiator core, it's not circulating anywhere - so not to worry there.

A thermostat swap is probably the easiest (and cheapest) first attempt. Although I must say Honda thermostats are quality parts. How many miles on the car and how old is it?

 

After the first two overheating incidents, there was a drop in the reservoir coolant level due to bubbling (loss via damaged reservoir cap), but inspection of the coolant level in the radiator (once it was cool enough to uncap) showed the coolant level right up to the top.


I did not hear any hissing when I opened the bleeder valve (coolant just dribbled out) and did not notice any bubbles surfacing from the Lisle funnel.

Whew, good to know!

I spoke to a shop that I trust (with my wife's car..haha), this morning and he suggested starting in the same place (thermostat check/replacement), as well as draining and performing a pressure test on the cooling system. My S is a 2005 with 98K miles on it currently.

 

thermostat first, re-bleed system, if not possibly a bad coolant temp sensor?

 

Thanks, Matts2k87. I think this is the route that I will take. Hopefully it's something I can get knocked out sooner rather than later. I'll post an update as soon as any work is done.

 

Well, this issue is still occurring. I pulled the thermostat for a pot test and verified that it's working as expected. The fan switch appears to be ok as well, and the fan turns on at ~205F.

Eventually, I found that the brand new Honda radiator cap was faulty, leaking coolant, and likely to be the reason air was introduced into the system. This was confirmed by first cleaning the area thoroughly, then driving around to bring the car up to operating temp and finally checking again to find more evidence of leakage. I double checked the new cap after discovering this problem to make sure I hadn't installed it improperly, and also checked the radiator tank opening for possible debris from the old cap.

I am currently running a CST radiator cap from Autozone and found no more evidence of leaking from that area. Tomorrow, I am just going to take it to the shop and have them burp and change the coolant.

 

I'm currently having literally the same issue. Was driving about 30 minutes when I noticed my ap1 raised one car on the temp. Turned heater on went home with heat at normal range with heater on. Saw that my reservoir tank cap had ruptured. Replaced that and started noticing coolant levels in the reservoir tank fluctuating. Replaced coolant reservoir. Kept having same issue of reservoir fluctuating and even spitting out coolant sometimes. Since I lost alot of coolant I refilled the radiator and burped the system per billmans updated thread. (Someone asked if the car was on while burping, I let the car get hot, turn it off, and then loosen the bleeder with the car off as per billmans thread.) Had same findings as you, no hissing or anything when bleeding, just some coolant slightly oozing out upon squeezing and holding the upper rad hose while tightening the bleeder 3 or 4 times in a row. Finally realized my radiator cap looked exactly like yours, like it popped the bottom off. So bought a new radiator at oreily and everything was fine for 2 days. Then today I get home and theres fluid leaking out of the top of the radiator itself like it had burst at the seams near the top below the neck of the radiator, but on the actual radiator itself.
I'm baffled what this could be. Fans turn on when they should so thermostat is likely fine. It seems like there is too much pressure ? In the system perhaps? I blurted it several times when I added new coolant and the heat got nice and hot as the car warmed up.
VERY interested in your findings. Looking like I'm gonna need a radiator, however I would really love to know what the reasons are behind our issue.

 

Shoot, sorry to hear about your problems. It's a bit of a nightmare to chase down this issue and was extremely stressful since it occurred during the summer months during a project with long commutes. Hopefully you can get it sorted out quickly.

Here is a summary of the things I did (listed in the order they were done) to address the issue (copied from an older post):

• cracked coolant reservoir cap which prevented the reservoir from holding pressure - replaced cap with OEM part
• broken OEM radiator cap (inner parts and spring broke off and fell into radiator upper tank) - replaced cap with OEM cap
• leaky replacement OEM radiator cap (bad luck?) which allowed air to enter the cooling system - replaced cap with Autozone part
• pinhole leak at radiator fill neck which allowed coolant to dribble out at high pressures, and air to enter the system - replaced radiator with Mishimoto unit, and also replaced thermostat while system was drained
• failing radiator fan which allowed temperatures to climb significantly over time, when idling in traffic - OEM radiator fan motor ordered

Good luck!