Hello, my 1987 xj-s 5.3 was running a bit warm on hot summer days last year and I feel it could use a good flush. Would I be foolish to try to flush it with vinegar? If that would be bad perhaps Mercedes citric acid flush would be safer.

 

If you have never done so, the best thing would be to remove the radiator and clean out the fins, as well as blowing out the fins on the condenser and oil cooler, which, once the rad is out, you can get to quite easily. You will be amazed how blocked up with crud the radiator fins will be.
Next turn the rad over and run a hose into the rad bottom and see if the water flows though freely, if not, get it recored. Then a couple of new thermostats and you will be good to go.

 

The first thing to check is the functioning of the mechanically driven fan. The clutches get weak as they get old, and you get chronic overheating at lower speeds.

The crude check is, with the engine warmed up, and the engine off, spin the fan. It shouldn't turn more than half a turn.

The much better test is to get a laser tachometer ($10 on Ebay) and check the RPM of the fan at idle.

Is it OK once you're cruising on the highway?

 

Just make sure you actually get whatever it is all flushed out.

I've put a flush product in a car once and left it in too long, 2-3 days later I was replacing a water pump.

 

Again, the first question is when it is overheating. If at low speeds, but not at high, the problem is not the radiator.

 

The thermostats are new, replaced them last year when I replaced all the coolant hoses. It runs just a bit above the N on the gauge when it's fully warmed up when it is over 80f outside. I was under the impression that is to hot. It does not seem to be dependent on road speed but possibly a bit worse in heavy traffic the electric fans (original and two pushers a p.o. installed car originally spent most of it's life in Texas) run and I thought the fan clutch seemed fine but I know the feel is subjective. It does not have the rubber flaps fitted to the shroud I didn't think it was airflow related so I didn't remember they were missing until now.

 

It is also still equipped with the yellow fan blade but I've looked at it closely and don't see any cracks in it. Perhaps it hasn't cracked yet because it is mostly freewheeling due to a failed clutch?

 

The problem is that gauge readings are all over the place. My current V12 runs at 1/4 on the gauge, while the last one ran at just over the N. But the true engine temp was the same.

 

Yes, that's quite possible. But I think a seized clutch would be worst for the fan.... imagine that fan trying to spin at 8,000 rpm..

 

Mine was running hot at low speeds and at idle, it was fan clutch.

Later I checked rad fins by having a friend move a flashlight around on the fan side of rad while I looked thru the front and had no problem seeing THE LIGHT, just to make sure I loosened ac condenser and took a flyswatter handle and moved it back and forth between, got some trash fall out but very very little, not enough to warrant what I did do.

 

LiquiMoly makes a good rad flush, I've tried just about every one. Would also strongly recommend running a full gallon of metal rescue (rust remover) in with coolant for a week. Shop-vaccing the intake openings at the top of the rad is a good idea to. I've since moved on to full-flow spin-on coolant filters installed on the exiting heater core pipe.

 

Had my Jag cooling system flushed last week, took 3-4 hours to do it, before I had clean water coming from the bottom hose. First I ran a rad flush for an hour with full heating into the cabin, then drained it completely and flushed with more water until clean.
Then filled with water and ran the car again until coolant at ~90C, drain, flush and repeat 5-6 times until I finally got clean water from the bottom rad hose after car ran ok for 15 minutes with full heating on. Must have used a half a ton of water in the process!
The only way to do it properly is to repeatedly flush it with water, otherwise you will still have rusty crap in the system.
Attached is a pic of what was coming out of the system until ~5th flush.
By the way, the system when warm didn't even get that much pressure so I didn't even have to tighten the worm clip on the bottom hose, just pull and push it and it wasn't leaking at all when full and warm so the whole job was easy, just time consuming.

 

To speed up the process you can put the flushing hose directly into the engine and the radiator, ideally backwards to the normal direction of flow. For the radiator this means into the bottom hose. Seal around it with a cloth or something. You'll be amazed what comes out.

 

Thanks to all that replied, I think I will order a replacement fan clutch and flush the system for good measure. If that doesn't seem to cure it in the spring I will probably just buy a new radiator.

 

Don't overlook the tstats. 90% of my cooling system maintenance has been replacing stats.

 

Can confirm. Replaced entire cooling system after running multiple flushes, rust-removing liquids etc still had non-standard engine temps until I remembered I forgot the thermostats sitting on a shelf still in the box.