Dash temp gauge drops on acceleration

Any ideas as to why the temp gauge would drop when I get on the gas? It goes all the way down to 100 when I get up to 4k rpm's.

I've been battling an overheating engine since I picked up the XJ a few months ago. I solved that issue by replacing the PO's 180* thermostat with a 195* and fixing the piss poor coolant mix that was in there. It was almost straight coolant so I went ahead and used a cleaner and flushed it and now it's all good.
I was hoping this would have solved my dash gauge issue too, but it didn't. I suspect bad wiring but this may just be an old jeep thing. Not leaking any coolant, no air in the system, though I think my water or steering pump may be going bad as there is a whining like a bearing going bad on startup. I suspect this may cause some cavitation in the coolant and introduce air.

Sounds like you need to burp an air bubble from your cooling system.

Cavitation can not introduce air into your cooling system.

What year model is your XJ?

Whoops. I forgot I can't do a signature here so I don't always have my details posted. It's a 91 with an open system. Upper hose is firm and hot when running and expansion tank returns to its high mark once cooled back down but there is a slow drip from the heater control valve shaft I noticed this morning on startup so maybe air is getting in there?

Anytime you pull the thermostat, you will end up with air in the system and will need to burp it.

Maybe I'm burping it wrong. Warm it up, give the upper hose a few good squeezes and the air should make it's way to the cap side of the radiator and exit through the expansion tank, right? This gauge thing has been happening since I got it but I can't guarantee there wasn't air in the system then.

check the connection at the temp sensor at the rear of the head. Sounds like it may be loose.

dan1977p said:

Maybe I'm burping it wrong. Warm it up, give the upper hose a few good squeezes and the air should make it's way to the cap side of the radiator and exit through the expansion tank, right? This gauge thing has been happening since I got it but I can't guarantee there wasn't air in the system then.

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It helps to park it on an incline or jack up the front end too. I couldn't get rid of the air in my system after a thermostat and water pump swap until I did this.

In the past when I saw a coolant drop related to acceleration it was a weak thermostat.

Buy a good radiator cap, either dealer or Stant, and it will bleed itself properly. A good thermostat will regulate properly.

Why would you need to burp an open system? Shouldn't it burp itself?

bigalpha said:

Why would you need to burp an open system? Shouldn't it burp itself?

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Air hides in the back of the cylinder head. Usually not an issue, but every once in a while it happens. Service on an incline or pull the sender in the back of the head to let it out.

Its also only open when the cap relieves pressure, closed until then. Bad cap = nothing works

RedHeep said:

Air hides in the back of the cylinder head. Usually not an issue, but every once in a while it happens. Service on an incline or pull the sender in the back of the head to let it out.

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I see - I thought that was RENIX only. I just did my cooling system, so I'll pull that sensor and burp it