Erratic Coolant Temps, Befuddled
 

Hello Group, bet you've never seen an issue like this one!!!!
Been fighting erratic ECU temps for quite a while. Replaced the ECU temp sender, dealer part over a year ago, replaced any hoses that looked questionable, new cap, thermostat, coolant re-route w/ remote thermostat (949), flushed system and heater core, nice flow and no garbage. Holds pressure. Stock 1.6, 12lbs Boost, 177550 mi on the clock. Checked the High Flo CAT, can see all the way through. Where head meets the block front and rear, hot side is damp/wet and have seen tiny bubbles at those two points once. Do frequently have quite a bit of moisture out the tail pipe. Plugs have good color. No water in oil or vice versa. Temp Gauge never moves past a little before the 12 o'clock position. Leak down test good, runs absolutely beautiful when ECU temps hold steady.

Last straw chosen was a coolant flush. So after the flush, refilled, burped real good until good flow seen in radiator neck. While still runnin, put cap on, complete cool down, then highway test drive @ ++75 mph for an hour, temps held @ 88 and 90c steady, beautiful. Got off the highway, temps slowly start going berserk. 88, 106, 97, 88, 109, in a scan (fast) mode. Car becomes totally UN driveable. Park the car, no percolating heard, but I figure there must be air in the system even though I hear no percolating.

Next day, car is always fine through warm up (temps steady). Go to junk yard looking for radiator necks, (cause I think the remote thermostat is too far from the block), temps hold steady, and while I'm scroungin she'll cool off, and I can burp her again before heading home. Cap off, idle 30 min, no big burp, but some overflow until the thermo opened. Erratic all the way home (highway). Really got nasty once off the highway.

I'm lost??

I'm sorry but I am having a hard time understanding what is exactly going on in your thread. But which coolant sensor did you replace? The one on the front or the rear of the motor?

Also, if you want to make sure you bleed the system properly, get a coolant fill bucket like this:
http://ww2.justanswer.com/uploads/Fi...6_fill_rad.jpg

You basically keep the bucket filled with coolant and run the car until the thermostat opens. It is the highest point on the car, and it's filled with coolant so air is immediately replaced.

Sorry, better than a couple years on and off messin with this issue. Problem is erratic ECU Temps. Which sensor replaced, rear ECU temp sensor. Also replaced little coolant plug at the rear of the head. I like that coolant bucket. I've been tryin to figure out how to do what it is designed to do. (controllin the over flow until the thermostat opens) So long story short, I suspected the head gasket, plugged cat, plugged heater core, on and on and on. Now I'm back to trapped air in the system. Not aware of, or ever heard of, or ever needed the burping bucket, so I'm off to get one. You definetly pegged what I was looking for!!!! Still kinda suspect the head gasket, but really want to eliminate all BS before taking that head off. Thanks.

We used those buckets in the performance car shop I worked at and they are amazing. They have their own replacement radiator caps which the bucket seals too, and you just sit and watch the air come out of the system.

Does the coolant go rusty really quickly and do you lose coolant? I had a breach between the cylinders and the coolant passages and it did this shit, but it would also push coolant into the overflow tank all the time.

dustinb, as soon as I saw the item you passed along, I imediately thought, that tool is cool. Went down to the local NAPA, the associate was not familiar with it. He found two in stock. He and I were examining it like we just found a gold coin buried in the sand. So got the tool, let the car cycle at least five times, got air every time. I was suprised how many times it cycled and produced air. The third and subsequent cycles produced much smaller amounts though. The tool is effective. After the fifth cycle, buttoned her up and took her down the highway. Temps held steady, at 88c goin north, and 90c commin back south. Got off the highway, temps floated around a bit, but not near as bad, pre bucket. I think there is still air in there. Gotta make quick run to the left coast, so was not able to give it another go. Huge improvement. Bucket works perfectly!!!
Thanks again for the suggestion!!!:bigtu:

saboteur, Old coolant had a serious rust color to it. I'll find how quick new coolant turns color once I get back from a quick trip to the left coast and get to run her. Does not push coolant into the overflow tank. I've been suspecting the head gasket, but leak-down test showed all good. Cylinders 1-4, 14%, 15%, 15%, 10%. Appreciate your interest.

do u have a coolant reroute ? if so, most does the burping wrong.

Does have a coolant re-routre. How do you mean?

Got a chance to do another bleed of the coolant system. All fresh coolant after the flush. Coolant's getting a lot of a rusty substance really quick. No matter how many cycles the thermostat goes through, still get air burps. Not pushing coolant into the reservoir though or, maybe such a small amount I can't tell. So after letting her cycle at least ten times, took her out for a run, temps held steady for the first 30 miles or so, then the temps started going all over the place. I think I have, what you had...

I had a similar problem with my car. Very rusty water, and I could not control the temp. Turned out that there was some rust scale clogging up the radiator. Once I cleaned the radiator thoroughly, the problem went away. Check into that.

Did a really good flush. Good clear flow all around, and Radiator's not that old. This (rusty residue) is new stuff.

Pictures of re-route piping would be very helpful to me in formulating a hunch. I've got a guess, but I'd like to see it before I send you down a rabbit hole.

It's the 949 re route... Had the issue before the re route, also, re route has tamed it somewhat.

It does not matter if the radiator is new, the scale would be coming from the block. It will take more than one flush to get all that scale out of the block. I must have flushed my car ten times, and it still had some rusty water. For a long time I did not have any antifreeze in the car, and after a while, it caused me some headaches.

My cooling system runs backwards, so water enters the radiator from the bottom. I simply unplugged the radiator drain, and forced water from the filler neck. All kinds of crap came out of the radiator drain. Since your radiator is fed from the top, it may not work as well. You may need to pull the radiator flip it upside down, and force water from the bottom radiator hose. You do not want to try to force particles down the entire length of the fins. Do this over a surface that you can see what comes out. It would probably not hurt to pull the thermostat, and force water through the head, block, and then out the water pump while you are at it.

It seems like a lot of work, but it beats the hell out of pulling the head, only to discover that the problem still persists.

Well if it happened before the reroute, then it is either a breach into the cooling system, or the system is clogged.

Bad ground terminal? Do you keep them all clean?

I'm thinking it's a very good chance that the temp sensor ground is going ape shit. After I did my reroute, my needle was going up and down and all over the place...very quickly. Turns out that the spacer used and the new gaskates were isolating the sensor from being grounded. Try taking a piece of wire and attaching it to whatever the sensors are screwed into and grounding them to the engine. I know you've mentioned it was there for awhile, but I'd still try grounding to either find or eliminate an issue.

I have the same reroute.
it is very difficult to get burping right. 2 ways of doing it right.

1. use a vacuum pump
2. lift the rear of the car up, run the engine, wait till you see a big bubble (radiator will go empty). add water. then you do the same with rear lowered, front raised.

it takes many many tries to do #2, I ended up with #1, 15 minutes and done.


op: do you have a heater ? if not, did you put in a restrictor ? did you drill a hole on the thermostat and oriented at 12

see here for other recommendations on reroute:
M Tuned Coolant ReRoute - MX-5 Miata Forum

Radiator is a couple years old. When I did the re route, looked inside rear coolant cavity, pretty clean, front thermostat housing also clean. I forced water through the heater core thinking it might be clogged, not. Forced water through entire system, then reversed, did'nt get nuch, pretty clean. Did top down and bottom up through radiator.

Yep I thought about the clogged scenario, thus the flush. Good clear clean flow every where. I'm definetly thinking breach, especially after discovering and using the coolant bucket dustinb suggested. Just eliminating all avenues before pullin that head.

Cleaned them all, more than once.

Will check the housing ground. I would think a bad ground would cause the behavior at all time. Only happens once engine reaches normal operating temps. From cold to 88C, temps are rock solid steady.