RX7 overheating.... I think....

Soo, the last 2 Chumpcar races I have attended we have not finished due to cooling system problems.

This started last month at Watkins Glen. Friday was a 4 hour race. 2 hours into the race the low coolant buzzer came on. The temp appeared to be in its normal range on teh stock gauge (1/3 up). We added some water to the overflow reservoir and continued. The coolant level sensor was on pretty much the entire last 2 hours (temp gague still normal).

We decided we probably had a small leak somewhere. We changed the waterpump and the radiator and filled the system back up. Same issues. We took the thermostat out... Same issue. We checked over every hose clamp.... Same issue. Through all of the testing, we did get the engine above 1/2 on the gauge at one point. We pack up and go home.

I decide that it is probably an internal engine issue. (We could not see the any external leaks in the cooling system). Therefore, I changed the engine to a known good engine.

Guess what..... SAME ISSUE at Summit point this past weekend. Same symptoms.

At this point I decided that we are going to test some of our assumptions:

1) The temperature is staying normal... Installed an aftermarket gauge. normal on stock gauge = 240F on aftermarket gauge!

2) we don't have any leaks in the system.... Installed coolant system pressure tester... Found a leak in teh new radiator and one at the thermostat neck. Fixed both leaks.

3) our radiator caps are good.... WE bought new ones and installed them. No difference.

4) Our thermostat is good.... removed it.... No difference.

Engine still runs at 240F. Engine still pushes water out.

This car has run in this exact configuration for several years. We have NEVER had any sort of temp problems.

I am pulling my hair out trying to figure out what is going on. The only two things I can think of are POSSIBLY a lighter weight oil was used and the car MAY be running leaner than before.

In other words, I found out (on my first dyno run ever) that the car is running 14:1 AFR at the upper RPM. I ASSUME it has always been like this because the system was not changed (It is running that lean because of a bad vac line going to the MAP sensor).

The oil I put in (don't remember if I changed it or not) was 5 w 30 instead of the normal 10 w 40 that I run.

1 last thing... I took out the heater core for this year. I initially looped the lines together. This weekend we had them looped initially, but we later capped them both off by hose clamping bolts inside the hoses.

There is an undertray on the car that is in the same position as the stock undertray.

This issue takes at least 30 minutes to 2 hours to develop.

Any suggestions?

If you're pressurizing the coolant system, it is likely you've got a bad coolant o-ring or a hairline crack in the housing. It's a rebuild, either way.

(edit) you may be able to pressurize the coolant system and let it sit for a bit (24 hours or so) to see if you get any leakage into the combustion or exhaust chambers.

ugh. What Brett said.

There are coolant seals between each pancake of the engine (front iron to housing, housing to center iron, center iron to housing, housing to rear iron) It could be leaking into the engine from one of those seals. Pressure test will tell you.

I had a coolant seal leak on my '7 for about a year, but only ever autocrossed it. The worst thing you can do if you want to "just" rebuild it (that is, just replace the various seals) is to let it sit. Your iron will rust on the rotor with the leak and will be unusable. Keep it running and it'll stay clean.

Also a compression test will tell you which rotor has the compression leak into the coolant system (or if one exists). If the front reads 130-130-130 and the back 90-90-90 (as was the case with mine) then you found your problem.

When you replace the coolant seals be sure the seam is on the "cold" side of the engine (top passenger corner near the intake port).

Good times. (and good luck)

Having done my first piston rebuild, I can tell you that a rotary is CAKE! Just mind the various small bits. I have the DIY-RE video set if you want to borrow it.

I once read that if you're going to track an RX-7 for any significant amount of time then its a good idea to underdrive the water pump or to drill a small hole in the plate of the thermostat to reduce cavitation in the coolant. I cant say how well or even if it works, but it might be worth trying. Completely removing the thermostat may not have had the same effect. I'm afraid I never made it out to the track with mine for any sort of testing.

I think the replies so far neglected to notice that the OP replaced the engine with a known good engine, yet the trouble persists. It sounds like a coolant seal issue, but how does this happen to a "known good engine" as well as the original?

Exactly.

I bought a coolant system pressure tester, coolant dye and UV light, and combustion gas analyzer (to see if combustion gasses present in coolant).

I better be able to track this down....

Is this the first season you have tracked the car? What are you running for coolant? I had run water and waterwetter for many years in my TT car and it always ran hot, 200-220. Two seasons ago I switched to antifreeze and replaced the radiator as well so between the fresh radiator and antifreeze it knocked the temps way down, 180 in almost any condition. I also run an under drive pulley. How sure are you the good engine is good. When I first got my car I was putting running junkyard/parts car engines and had two that had coolant seal issues. The seal problem is usually easy to figure out, the hoses will get rock hard fairly quickly and if you take the cap off you see bubbles in the fluid. I also removed the tstat and have a bypass set up to keep the coolant flowing to all the correct places. Good luck, tracking down coolant problems is a pain.

No bubbles in coolant at idle or revving. Raced the car in this configuration for at least 3 years.

Motor was known good because I know the team took it out to go lsx and I bought the motor the week after they completed a race I was in also.

Water is what we must run. Sometimes we add water wetter. We did not add any this time.

I think I would look at replacing the radiator especially if you have an older stock unit, maybe it could be partially blocked. Do you have spare you can try to see if there is any difference? I have a Koyo radiator that has an extra row and I attribute most of my drop in temps to mainly the radiator and a little to using antifreeze. I was told when I first got the car that I should only use OEM radiator caps but have a hard time believing a good cap is any better. What about ambient temperatures? Were the last events hotter than usual? I can see if you have plain water and higher ambient temps you could get to 240 quickly.

DO NOT RUN A ROTARY WITHOUT A THERMOSTAT!!!!!!!! If you feel you must, block off the bypass opening visible once the thermostat is removed.

If you are not losing coolant, put a pressure tester on the cooling system with the engine cold then start it and watch the gauge. It will jump quickly to around 4 psi or so, that's normal. It should slowly climb as the engine warms up, if so then it's not real likely the coolant seals are bad. However, if it immediately pressurizes to, say, 10 psi and keeps climbing then yep the seals are bad.

I don't trust the stock gauges, I'd put a mechanical gauge on it. The best place I've found to install it is in place of the stock cooling fan sensor (yes they had them), this is the black sensor just below the thermostat on the back of the housing. The green one is for the EGI so don't remove it. Retap that hole for 3/8 NPT and now a mechanical gauge is easily installed. You can also drill and tap the front of the housing but that puts the gauge tube very close to the belts etc.

Oh yeah, make sure the oil cooler 'pellet' has been disabled. If those things go bad the rotors don't get cooled well and this can show as elevated coolant temps.

That heater hose: you'll see the original setup went from the rear iron to the bottom radiator neck. Rather than block this off, run a heater hose from the rear iron to the neck. This helps pull hot coolant away from the rear rotor.

Don't forget airflow. Make sure every molecule coming in the front opening has no choice but to go through the radiator. That's critical at higher speeds when you are demanding more from the engine.

We did add an aftermarket gauge. We only ran without thermostat for 10 minutes. We had the heater core lines looped at first and then blocked off.

Both radiators are less than 1 year old and are all metal.

Airflow has not changed.

Ambient temp was highish but not any higher than other races we have done.

I will do the testing tonight.

Do you have the correct thermostat? It needs the 'blockoff valve' on the bottom, like this:

Notice the spring loaded disc on the bottom? That's the bypass blockoff. A 'regular' thermostat won't block off the bypass and make it overheat. Like this, notice there's no disc on the bottom:

Sounds you are convinced there are no leaks internal or external therefore focus on the oil cooling system. The rotary transfers a significant amount of heat into the oil. Not sure whether this is a 1st gen Rx7 (air to oil cooler or coolant to oil cooler mounted below the oil filter) or 2nd gen (air to oil cooler). The 2nd gen has a thermo pallet valve in the front of the eccentric shaft that prevents oil spraying inside the rotors when cold which if stuck closed would be bad. The 1st gen had the valve in the oil cooler that bypasses the oil when it is cold. If that is always bypassing it would also be bad.

Water conducts more heat than glycol solution so 100% water will transfer more heat but will require a higher pressure to prevent boiling. The engine will run cooler with 5W-30 than with 10W-40.

Bruceman: I am actually not sure I have a leak. I am going to re-pressure test tonight and add some dye to the water to see if I can see leaks.

I also bought a combustion gas analyzer thingy to see if the coolant has combustion gasses in it.

Curmudgeon: My thermostat looks like the top one.

Once I get these tests done, I will know more. I will report back my findings!

Thanks!

Rob R.

Can you tell me more about the oil thermo pellet valve (car is an S4)? I am wondering if this IS an oil issue.

Thanks!

Couple of new questions:

What is the max "normal" temp for the engine oil and where would you measure it?

What is the max temp for the coolant without doing permanent damage (measured at the thermostat neck).

How long should I do the coolant pressure test? What is an acceptable amount of leakage? (2 or 3 psi per 24 hours?)

What is an acceptabel amount of leakage when doing the pressure test on the engine only? (1 or 2 psi for 24 hours?)

Thanks!

Rob R.

wvumtnbkr wrote: Can you tell me more about the oil thermo pellet valve (car is an S4)? I am wondering if this IS an oil issue. Thanks!

This valve should open at 140F. It is positioned in the front of the ecentric shaft right behind the front pulley bolt. The valve should protrude 6mm when in a fluid at 140F (60C). Many remove this and replace it with something which allows the oil to always flow to the rotors. http://atkinsrotary.com/store/Eccentric-Shaft-Oiling-System/

You should be able to tell if the oil is flowing through the oil cooler by feeling the temperature of the cooler hoses and the cooler. The bypass valve in the cooler should protrude 5mm to prevent bypass when in fluid at 149F (65C).

When replacing the thermo pellet, something you need to know: it's easy to accidentally dislodge the eccentric shaft thrust bearing. Before removing the e-shaft bolt, put the nose of the car on jackstands and use something to force the e-shaft forward and hold it there. I typically push the clutch pedal as far as I can, then use a 2x4 or similar to hold the pedal down. Only then should the e-shaft bolt be removed. Pics etc of the procedure: http://www.rx7club.com/2nd-generation-specific-1986-1992-17/racing-beat-eccentric-shaft-thermal-bypass-pellet-install-997414/

Mazdatrix page showing the bearing and explaining what happens: https://www.mazdatrix.com/faq/pulley.htm

As long as you don't remove the pulley, you should not have a problem. You might find a way to hold the pulley firmly back toward the e-shaft if you are concerned.

By the way, the thermo pellet's reason for being is to lower oil flow to the rotors when the engine is cold. This makes them heat up quickly for lower emissions when cold. It does this by opening a bypass passage which allows oil to return quickly to the oil pan, rather than being sprayed on the inside of the rotors. The RB and Atkins pellets replace the stock one with an aluminum 'block' which shuts off the bypass opening permanently and oil sprays on the inner side of the rotors as soon as oil pressure builds.

Faulty coolant level sensor or corresponding wiring?..... Have you checked or swapped it at all?

Your gauge reads normal, you can't find a leak and nothing has really been changed other than what you've changed since the problem started

It doesn't really make sense that both your original engine and the swap over engine have a bypass pellet issue.

In reply to daeman:

Agreed! However, water really is disappearing from both engines. The coolant level sensor is working correctly.

The aftermarket gauge I installed did read 240 F.

UPDATE!!!!!

The old engine (that I took out after Watkins Glen) held pressure all night (lost about .25 psi in 16 hours).

Also, the coolant system in the car was checked with dye and uv light and glasses. Not one spot of dye was found to be leaking anywhere. I need to pull the exhaust and check IN the engine. I plan to pressurize the coolant system to 25psi to see if I can see any leaks with higher pressure.

FYI, I wouldn't go as high as 25psi. Parts of the system may not be able to take the pressure (like WP seals). I generally don't pressure test more than a few psi over the cap rating. I don't know rotaries, but I can't see them being immune to damage from over pressurization .

In reply to HappyAndy:

I agree with you. However, I gotta find out if I have a leak. Regular pressures are showing nothing. Maybe 13 psi would do it (I figure that is what it is running under normal conditions).