Overheating and computer issues

Hello everyone,
My 1988 XJS has one main issue that threatens its existence. It overheats almost randomly. At first, I thought it was just the outdoor temperature, but today, I realized it isn’t. My initial thought was a bad radiator, but today, I drove the car and it stayed at N just fine than a couple hours later (when the weather cooled down) it was running between N and H. It seems to randomly want to overheat. My guesses are the radiator, fan clutch (fan runs at idle but no way of knowing once I am driving), or perhaps bad sensors/gauge (something electrical).

My second issue is a little more minor. My trip computer doesn’t work. It just shows 0s for my fuel, miles, gas mileage, and everything other than the time. My current fuel gauge sensor doesn’t work so that could be why, but the electronic trip meter doesn’t work either.

Ideas anyone? I’m limited on money, so if you know of ways to test things before I replace a bunch of different parts, that would be appreciated.
Thanks everyone,
Colin Tracy
P.S. I haven’t shared my story with anyone other than Greg, so I want to tell the rest of you. In 1992, my grandmother bought this car. In 2004, I was born (so if you know math, I’m 16). In 2010, she parked the car after one parts car, an overheated seized engine, an engine swap, and lots of $. In 2016, she passed away from cancer. In 2019, I pulled the car out and started to work on it. I just wanted to share with all of you, so you understand why my knowledge is much more limited than yours. Thank you!

Just for grins, you might want to check the lower radiator hose to see if it’s soft enough that it might get sucked flat. That malady is known for random overheating issues. Sometimes it sucks flat and the engine overheats, then sometimes pressure blows it back open and cooling is restored.

Recommend posting about one issue at a time.

Welcome aboard! I’ve got less than 10 years on you so I feel your pain about being a young, limited budget, 80s XJ-S owner! My XJ-S (which I bought 2 years ago now–how time flies) was not owned by a family member before me but my love of Jags comes from my grandfather who had many over the years. His 80s red Jag was nicknamed RedCat and I call mine RedCat2 (also my plate) in his honor! You will do your grandmother’s memory proud to get the car sorted!

Your symptoms could be many things. One quick and easy thing is to make sure it’s well bled (jack up the front left or at least park uphill on a moderately steep hill for best bleeding effect. Also worth putting a tee back near the heater valve to assist bleeding that natural high spot in the system. I was shocked how much air came out of my system the first time I really worked at bleeding it (it took a good hour of bleeding, closing, checking, topping, repeat).

I bled the cooling system before and now am doing a coolant flush. I will try elevating the front. That is the one thing I didn’t do last time

I haven’t check yet but I’m assuming the way to fix that would just be a new lower radiator hose? The one on it is probably worth replacing anyways

Your XJS v12 overheats? Randomly? No way. This is just not possible. It’s well designed and fit for purpose (mr moderator - please move the topic to the archive).

Actually there a 21 basic things to check - just use the search option. Starting from your budget estimation in the time of purchase, through viscotic clutch in front, ending at the sump tank filter in the boot…

I would start from removal and inspection or each spark plug to eliminate the worst options at the beginning.

21 basic things to check? Where can I find that? I’ve replace spark plugs, distributor, fuel filters.

Apologies - I should say “at least” 21 basic things, not 21 :wink:

My theory was related to incorrect combustion - not no combustion at all.
If your head gasket is gone or the fuel-air ratio is bad - usually you can notice that by the look of spark plug’s electrodes…
If the your engine suffers due to fuel starvation (fuel-air ratio), leaking injector - you won’t find it in the coolant system - which only shows the effect of what’s above…

Those things of course shall be done after checking everything else (at least 21…) I recommend calibrated IR thermometer to assess the hottest place along your engine…
Your “ecu” will take time to show the values (ride) with fuel gause sensor not working - you will get no reading.

i would verify if it’s really random or not. While constantly moving at 40mph or more, the engine should run cool.
During stop and go traffic and idling for 10 minutes or more, the engine should run a bit warmer.

If you notice it happens more when you’re not cruising, I would suspect the fan clutch. Also make sure your electric fan is working.

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One other random possibility- if the instrument cluster has a bad ground, the temperature gauge will read higher when the headlamps/dash illumination is on. Sometimes hard to notice a pattern; you said when cooler (evening)?
On my ‘88, it caused the gauge to slowly creep up about a needle width. Also, once up the needle is very slow to drop, so an air pocket is another possibility.
As far as the travel computer, fuel consumption is read off of the ECM injector pulses; there is an interface in the trunk near the ECM that are a known weak link. If not recording miles, there is another interface in the trunk on the left- mine is bad on my ‘88. I just use the fuel consumption figure and the odometer to calculate.
Download Kirby Palm’s excellent reference and read it cover to cover- all you need to know to start your journey.

Some lower radiator hoses have a “spring” inside to prevent collapse. That spring is typically steel which rusts away, filling the coolant system with rust particles and allowing the hose to collapse. It could be fixed by simply installing a new spring – but a new hose hopefully comes with a new spring.

I’ve been recommending for years that owners eschew the OEM molded hose and spring and instead install a “flex hose”, one with a 90 degree elbow at one end. The elbow goes at the radiator end, the rest of the flex hose loops down and under and back up to the water pump inlet. Flex hoses cannot collapse because they have a wire coil built into the casing of the hose itself. They also typically cost more than the OEM molded hose.

I have NEVER had a molded hose collapse on me. Of course, I replace my radiator hoses regardless every 60,000 miles or 5 years, whichever comes first.

I should add that a collapsing lower radiator hose might be an indication that the radiator itself is obstructed. The pump is trying to suck, the radiator isn’t letting much through, so the suction collapses the hose.

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Alright. I will replace the hose because I don’t believe I felt a spring in the hose when I flushed my cooling system last week. I didn’t notice the hose collapse today but I’m still going to replace it, seeing as it’s 10-42 years old

I’ve also thought about replacing the radiator or getting it repaired at a shop. I’m obviously only going to do that if I conclude the radiator is the problem. Are there any aftermarket/“sporty” radiator that are better than the OEM or should I just stick to the OEM? Also any ways of testing a radiator?

i know you’re short of money, but I would at least replace bottom hose, both top hoses, and put in two new Gates thermostats (I went with 180F, i didn’t like having the standard USA spec 195F, I think that was all about better gas mileage)…all that shouldn’t break the bank.

Before suspecting radiator, it’s worth replacing fan clutch (only about $40) and easy to do, and if you still have the original yellow fan, search the forum about that!

I was in same boat as you a year ago. Not running too hot, but warmer than I wanted. I replaced hoses, coolant, thermostats, fan clutch, yellow fan, and auxillary electric fan. Ever since then, it’s been running very cool, so I don’t suspect radiator at all, although one day I’ll pull it and have it re-cored. The original radiators are good, and not too pricey to re-core. I’ve read horror stories here trying to fit aftermarket radiators, and redoing the plumbing.

I was planning on replacing the radiator hoses anyways but just haven’t gotten around to the passenger side and bottom one. My driver side hose exploded on me after the first week of driving it. I have a lot of parts from the parts car my grandma purchased. Is there a reliable way of testing the fan clutch?

Been there, done that. Note: A blown upper radiator hose is another sign of an obstructed radiator.

No. If there’s any doubt, the thing to do is install a new one. Then, while you know it’s good, get to know exactly what good looks like. How it spins when you spin it by hand, how the breeze feels when the engine is running, etc. Try to remember all that a few years down the road when you develop doubts about whether your fan clutch is any good any more.

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I think I might get the radiator recored. Do you think this would be the right decision? Things I should check before making this decision?
Also do you know what brand/product number the fan clutch is that you guys use? If not than I can check tomorrow