Jkey8195 Posted March 13, 2022 Share Posted March 13, 2022 Okay so I just replaced everything in my cooling system on my 1988 Comanche 4.0. That includes water pump, 195 thermostat, thermostat housing, pressure tank, heater control valve, and the fan clutch with the Napa grand Cherokee clutch I had seen being recommended. After flushing the whole cooling system, and then install all of those parts and using a vacuum fill I am having a weird fluctuation at my coolant gauge when it gets to operating temperature it fluctuates between these two marks while idling and while driving. Is this normal for a closed cooling system? Thanks for any info. Link to comment Share on other sites More sharing options...
ghetdjc320 Posted March 13, 2022 Share Posted March 13, 2022 How fast is is fluctuating? Is the gauge fluttering? Often times I would see the gauge drop down when accelerating after a a short time idling. Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 13, 2022 Author Share Posted March 13, 2022 16 minutes ago, ghetdjc320 said: How fast is is fluctuating? Is the gauge fluttering? Often times I would see the gauge drop down when accelerating after a a short time idling. It’s not fast it will creep up to the 210 mark then once it gets there is creeps down to the lower red mark I made in the picture. And it just constantly does that at idle and while driving. I guess I should have also stated it was about 30-40 degrees out today. Link to comment Share on other sites More sharing options...
Pete M Posted March 13, 2022 Share Posted March 13, 2022 I would still try bleeding out the temp sensor. not because I doubt the effectiveness of the vacuum fill system, but because renix 4.0s are notorious for trapping air at the back of the head where the sensor is. Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 13, 2022 Author Share Posted March 13, 2022 Just now, Pete M said: I would still try bleeding out the temp sensor. not because I doubt the effectiveness of the vacuum fill system, but because renix 4.0s are notorious for trapping air at the back of the head where the sensor is. I guess I can try that, it’s just hard for me to believe there is any air in it but I know I have seen a lot of people have issues getting these renix systems to bleed properly. I’ll use my lift at work and raise up the back of the truck as high as I can get it and let it get hot then crack the sensor on the back of the head and see what it does Link to comment Share on other sites More sharing options...
Pete M Posted March 13, 2022 Share Posted March 13, 2022 no need to let it get hot. get the rear up 6", crack the sensor, then start the engine. it's easy to go too far and have the sensor pop off which would then spew hot coolant all over. I usually suggest covering it with a rag when doing it. Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 13, 2022 Author Share Posted March 13, 2022 9 minutes ago, Pete M said: no need to let it get hot. get the rear up 6", crack the sensor, then start the engine. it's easy to go too far and have the sensor pop off which would then spew hot coolant all over. I usually suggest covering it with a rag when doing it. Okay will do I’ll try and update this Monday evening after seeing what happens. Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 13, 2022 Author Share Posted March 13, 2022 1 hour ago, saveevryjp1998 said: When it's fluctuating, is your heat or defrost on? It could be due to the bypass valve, inconsistent vacuum on it, or issue with the heater core. It might just be the sensor starting to go, poor wiring, weak connection there. The temp range popping around there on a cold day isn't soo bad. The fluctuating isn't normal. A tiny hole in the system like in the heater core or if it's a plastic vacuum bypass(notorious for cracks) may cause that when it's cold out but go the opposite when it's hot. If it's a bad aftermarket plastic bypass, it will eventually rupture when steady temps increase towards warmer months. I've never had one last and would put a 30 year old metal style in over a new plastic one any day of the week. I'd keep the heat/defrost in the off position in my first round of troubleshooting. See if it changes any. If it doesnt, next carefully check every inch of the system for pin leaks from bypass forward. If that's good I'd chase the temp guage wire making sure it has no issues anywhere with clean contact points. Each step or change you make, watch for gauge consistency. The heater core leaks can be very tough to catch. They love to leak up high in firewall leaking down in the floor deadening material first. Slow leaks are very tough to catch there because people often assume it's from the passengers feet. Even that would most likely mean no floor mat to spot it. I had my 1st xj do this to me in the core for 2 years before I figured it out the second summer unhooking the lines and blasting it with the garden hose with brand new to me junkyard oversized random floor mats. The water puddled since the mat was pushed all the way from the seat to up past core housing in order to make fit. It also could be due to some restrictions in head especially near the Tstat. Many t stats are junk. Some guys blast the rtv in the housing. That can make them stick. I myself like to put a super thin bead on the Tstat. Just enough to make it hold on its own to get the housing in. I've seen guys do the same but with a half a tube or put the Tstat in backwards lol. I test them through a dozen or more times in hot water. Many of them will suddenly start to stick after a half dozen cycles. Especially the autozone junk. A slight bit garbage in the coolant can cause them to stick. I've been hosed 1 too many times dealing with the old renix coolant closed system from bad t stats or cutting corners not cleaning it I hate admitting it. I spend the coin now on a quality brand if I'm keeping the closed system renix anything and the time to thoroughly clean it. Lol their definitely a love/hate relationship but it's what my first Jeep was, how I learned(without the web or a manual), and I'll always have a soft spot for them. Well I don’t think I have any leaks I pulled a vacuum on the system to fill it and let it sit for 15 minutes or so and had no signs of the system leaking down, I flushed every part of the cooling system each individually until I had clean water coming out of everything. I personally do not use any rtv on the thermostat housing I only use the paper gasket, and the thermostat is a murray brand and I have not had a issue with them in my other jeeps, not to say that this one isn’t bad out of the box, but I think I’m going to start by cracking the cooling temp sensor and making sure I don’t have a air pocket like suggested above. Also to note the gauge did not fluctuate before I redid the system, which I only did due to previous owner filling system with water only and it was all rusty looking crud in there and the coolant pressure tank had a crack in it, and I was having issues with starting to overheat when sitting at a light and it would be fine as long as I was driving. Link to comment Share on other sites More sharing options...
gogmorgo Posted March 13, 2022 Share Posted March 13, 2022 How much have you driven it/let it idle? I’ve noticed a few vehicles do this after draining and refilling the cooling system, but it usually settles down after 20 miles or so. Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 14, 2022 Author Share Posted March 14, 2022 8 hours ago, gogmorgo said: How much have you driven it/let it idle? I’ve noticed a few vehicles do this after draining and refilling the cooling system, but it usually settles down after 20 miles or so. I would say roughly 25-35 miles or so and sat and idled multiple times for more than 10 minutes letting the cab warm up before leaving Link to comment Share on other sites More sharing options...
Jkey8195 Posted March 15, 2022 Author Share Posted March 15, 2022 On 3/12/2022 at 9:57 PM, Pete M said: no need to let it get hot. get the rear up 6", crack the sensor, then start the engine. it's easy to go too far and have the sensor pop off which would then spew hot coolant all over. I usually suggest covering it with a rag when doing it. Well I bled it last night and now it only fluctuates when it’s fairly cold out like it is in the mornings and if I am moving but at idle with it 65 degrees out it’s sitting here and not moving Link to comment Share on other sites More sharing options...
omega_rugal Posted March 16, 2022 Share Posted March 16, 2022 by fluctution you mean the needle moves between those values non stop? you can see the needel moving? if so maybe the sender is bad Link to comment Share on other sites More sharing options...
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