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Renix 4.0 only overheats at idle. Likliest cause?

T

tayman

NAXJA Forum User
Location
Niceville, FL
1990 Renix XJ. stock cooling system. (the closed one)

This issue has just popped up fairly recently. It cools fine while moving, and "okay" if I throttle up (I have a hand throttle, I can keep my idle raised).

What's the likeliest cause here? Water pump, or clogged system?

Water pump HAS been replaced in the last couple years. radiator was flushed recently.
 
E fan is the first place I would check. Then coolant level, check for leaks. I doubt it's water pump.
 
E fan works. Coolant is topped up, no leaks. System is properly burped.
 
Sounds like a weak fan clutch.
Or Exhaust leak at the donut blowing exhaust on the block.

Define properly burped? Check the top half of the radiator to see if it is cold or hot, if cold it is not full of coolant!!!! Renix is bad about trapping air in top half of the radiator.

What is the temperature at the T-stat and the radiator outlet when the E-fan turns on?
 
weak fan cluch wouldn't surprise me too much.
I don't have a way to check temp other than the temp gauge.

by burped, i mean there's no air bubble trapped up at the temp sensor.
 
When the renix coolant system is first filled by most people, they radiator is still half full of trapped air. Have you had the system open recently to flush, or change parts? If yes, the radiator may be only half full.

Other wise the E-Fan temp switch maybe coming on too late, which is either a clogging radiator or drifting temp switch (going bad, turning on too late).

Get a $20 IR temp tester at Harbur freight.
 
The 1990 IIRC has two temp sensors, one for the ECU and one for the gauge. If the ECU one isn't seeing proper temp, it may not be kicking on the fan. I would definitely put some effort into testing what the actual temp is before you spend a lot of time chasing ghosts, I have had engine temp sensors go bad in the past, and it's usually a very disorienting experience.

However my bet would probably be on fan clutch. It's fairly easy to test, somewhat simple to replace, and usually a cheap fix.
 
I don't have the renix system in my 97 but when I gotten it it had been running super hot as in of water hit the hood, it would steam.... Change the thermostat and see if it gets better, that's what I ended up doing to mine and now it runs great with no over heating
 
BulletMaker said:
The 1990 IIRC has two temp sensors, one for the ECU and one for the gauge. If the ECU one isn't seeing proper temp, it may not be kicking on the fan. I would definitely put some effort into testing what the actual temp is before you spend a lot of time chasing ghosts, I have had engine temp sensors go bad in the past, and it's usually a very disorienting experience.

However my bet would probably be on fan clutch. It's fairly easy to test, somewhat simple to replace, and usually a cheap fix.
Click to expand...

The 90 has 3 temp sensitive devices.

A temp sender, back of head for gauge or light.
A temp sensor, lower block for ECU information (coolant temperature sensor)
A temp switch, e-fan control (doesn't send info to ECU).

After 1990 there are only 2. Newer models only have 1.

I would agree on the fan clutch...not expensive ($40-50), even if you get the ZJ HD fan clutch (NAPA part number 272310). You don't have to get the ZJ clutch, but since they are just about the same price and the same job, then why not.
 
Ecomike said:
When the renix coolant system is first filled by most people, they radiator is still half full of trapped air. Have you had the system open recently to flush, or change parts? If yes, the radiator may be only half full.

Other wise the E-Fan temp switch maybe coming on too late, which is either a clogging radiator or drifting temp switch (going bad, turning on too late).

Get a $20 IR temp tester at Harbur freight.
Click to expand...

Looks like you called it again mike. I was NOT the last one to play with the coolant levels, though I don't know why the problem didn't surface until recently.

Looks like it was just low on coolant. I thought it was full because there was fluid in the overflow bottle. But I squeezed the upper radiator hose, and a bunch of air came up and the tank emptied. I was able to add over a gallon by doing this.

I didn't want to post up to quickly with a "Got it!" only to have it fail again...

I may still replace the fan clutch, because I'm still not 100% that this was the whole problem.
 
I had been fighting and dealing with the Renix cooling systems on my then 2 renix jeeps for about 5 years, when I got to checking what I thought was full system one day, and the top half of the radiator was cold, and the bottom half was 220-210. And it was a new radiator. I was sure it was full too, and the bottle was 1/2 full and stable. That was when I realized from professional plumbing experience that the Renix cooling layout has an obvious P-trap, air trap in the top of the radiator with no way for the air escape while filling or running from the coolant bottle. I discovered that opening up the heater hose from the T-stat, by passes the closed-cold T-stat, and gives the system a way for air to escape the block, but that you still need to fill the radiator form the large upper hose first. I guess you did yours while hot with the T-stat open? The 88-90 is even worse I suspect as it has the heater valve in line and needs the engine running to supply vacuum to operate it?

The HO is different, and just needs an air vent for the block while filling.
 
I really don't know what they did before me. It was in California and I was in NC. It's here with me now (months later?) Yeah, it seems like it was something like what you're talking about. I may still pull the upper hose and make SURE it's full...
 
You can tell if it is full with an IR gauge, just test the top row of tubes. They cool so quickly with no coolant flow they will be near room temp, while lower tubes will be say 200 F.
 
good call. Only problem is I have NO idea where the nearest HF is...
 
tayman said:
good call. Only problem is I have NO idea where the nearest HF is...
Click to expand...

They sell on line, ebay and amazon have them too.
 
Harbor Freight has a store in Fayetteville, NC. I'm not really that familiar with NC, but their map claims 12 miles from Fort Bragg.

Or go to www.amazon.com and type "ir thermometer" in the search. $15 on up.

An IR thermometer is an excellent tool for troubleshooting lots of things. Also if you tow, it can be used to check tires and brakes for trouble spots.
 
"Also if you tow, it can be used to check tires and brakes for trouble spots."

Nice idea!
 
Ecomike said:
Don't recall seeing them at wallmart yet.
Click to expand...


you would be correct sir.

But it looks like that really was the issue. I've spent a lot of time in stopped traffic in the last couple days. It's running a lot cooler than it was before, and the gauge never even tries to move while it's running. I've also intentionally let it sit and idle for a little while to see if the needle would start to creep up. Nothing.
So I think we're good. :cheers:
 
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