Overheating when under load

I have an XJ-S 1988, V12 HE. I’ve recently done quite a bit of work on the car. When I got it, it was overheating in traffic.

I’ve replaced the radiator, thermostats, fan clutch and fan blades, and the car will now idle all day without overheating.

However, when driving, the gauge moved up off the N towards the H if I’m pushing it hard or going up a hill. As soon as I slow down or the hill levels off, it very quickly (within about 20-30 seconds) settles back down to the middle of the N.

I also notice the car still feels pretty slow - I was wondering before if this was a timing issue but I’ve had the timing checked and the garage said it was now ok.

I’ve previously checked the vacuum and centrifugal advance systems and they seemed to be working too.

One thing i’ve not worked out how to check is fuel pressure, I did wonder if pressure is dropping when the engine is under load and causing a lean burn?

I did wonder about the head gasket but there’s no sign of contamination to the coolant or oil, and I bought a gas ‘sniffer’ thing which is showing no evidence of combustion products in the coolant.

At no point has it got hot enough to throw coolant or steam out or anything like that.

Any tips on the next thing to check? I’m running out of things to replace!

Thanks all!

Oh, and by the way, this is a UK car, so no cat or O2 sensors!

You didn’t mention your electric auxiliary cooling fan. Does it work?


Probably way off, but when is the last time you changed your ATF? Overheating fluid can tax the radiator.

You’ve probably already done so…

But just in case you might not have…

I have to ask…

Did you properly bleed the system? Are you fairly confident that you don’t have any air pockets?


I also notice the car still feels pretty slow -[/quote]

Compared to what?

Was it faster before the cooling system work?


Auxillary fan works but shouldn’t be necessary at that speed!

ATF: Never - only had the car for a year and it’s “on the list”… doubt that would push the temperature of the engine that much higher so quickly though!?

It was not any quicker before, but it feels slow compared to my other cars which in theory have less power. Measured 0-60 is about 11-12 seconds which seems slower than it should be. Pushing the accelerator harder makes more noise but no more power!

Yeah, that’s pretty slow.

Air filter? Fuel filter? That’s an easy stepping-off point.

Clogged cat converters?

Is the transmission shifting way too soon? It’s common-ish issue that really blunts acceleration. I’ve driven some that make the 1-2 shift at like 40mph (rather than 55-60 mph) under wide open throttle.

On a more unpleasant note, my '88 suffered a severe loss of acceleration due to a torque converter failure.


Air filters and fair filters brand new (no difference). Also new plugs, leads, coil.

No cats - UK car!

I held it in 1st then 2nd in the test - im sure it’d be even slower if I’d let it do its own thing!

As for the bleeding, I had the car facing uphill when I filled it. I then ran it, filled it, ran it, filled it etc.

If I open the bleed screw on the radiator I get water with no bubbles.

I guess that doesn’t guarantee no air pockets elsewhere though…

If that is the case, presumably it will gradually bleed itself?..

  1. Based on everything you described, if it was me that had same situation I would verify the timing is correct instead of taking the garage’s word for it.
  2. I have had recent case where brand new fan clutch was inoperative.
  3. And since everyone else mentioned everything else, you mention it happens while exercising the old girl then check oil cooler, how’s your oil pressure?

Yeah, I don’t have the kit to do that but thinking it might be a wise investment!

The fan clutch definitely works - at idle you can hear it ramp up and down, and when driving in slow traffic you can hear the fan roar when it’s warm. Again, around town and idling, the car does not overheat at all, it only does it when it’s on the open road at 40+mph.

Oil pressure is healthy.

It does seem to get from ‘N’ to half way between N and H very quickly - in about 10-20 seconds, which seems implausible. It stays there while I’m climbing the hill or pushing it, then it settles back down to N in a similar period of time, no more than about 30 seconds.

That manner of fluctuation suggests to me that you still have air in the system


Won’t bleed itself, no.

In my own experience, FWIW, you want a strong, steady stream of bubble free coolant coming out …almost like a geyser. And let it geyser away for a few seconds to make sure it’s for real. It makes bleeding a messy job.

Even then I’ve sometimes had to go back and re-bleed.

'Tis the heater core and hoses that usually trap the air, it seems. Adding a tee in the heater hose makes bleeding easier and more effective. Much has been written about this. It’ll be in the archives and Kirby’s book


Thanks, will check it again tomorrow!

Does anyone have any tips on how to measure the fuel pressure on an HE? I can’t see anything obvious on the fuel rail that I could connect a gauge to… and I am suspect of both the pump and the regulator since they’re 2 things I’ve no way of testing!

Being a UK spec car it always runs in open loop (no lambda sensors) so I do wonder if the mixture is off as a result of wonky fuel pressure or something… maybe that would explain the lack of power? I’m running out of things to test!

The method for proper bleeding is jack up front end, with left side (where bleeder is) higher than right side.

Also, do you trust your barrel gauge? I installed real water temp gauges so I know whats really going on.

I think your timing could be too retarded. That will lower power and make it run hotter. And when you test timing at 3000rpm with vacuum advance off, that will tell you if centrifugal advance is working. If not, then it’s running way too retarded at high rpm, generating heat.

Do I trust the gauge?.. Not really… I’ve been using an infra-red gun to measure temperatures too but that’s tricky to do at 40+ mph! :stuck_out_tongue:

Having said that, when just pottering around, the temperature is pretty darn stable, in the middle of the N to a bit higher but still touching the N (I have 88 degree stats). Wheras before I replaced the radiator and stats it was all over the place.

Looks like I’m going to have to learn how to measure the timing!

Also, a rich burn causes more heat.

Definitely worth knowing your timing. Also, make sure it was correctly timed for your region. USA models are 18° BTDC @3000Rpm with vac advance disconnected. I think non-USA V12s should be way more advanced than that because they are higher compression.

As an aside, my 88 HE was slow too.

  • I changed vacuum modulator so it didn’t shift into third so early. That helped.
  • I then removed big mufflers (under back seat), that made a HUGE difference.
  • Icing on the cake, I advanced the timing 4° and switched to higher octane supreme petrol.

Car now is quick.

The aux fan has no idea how fast you are going, it gets activated when the coolant temperature is too hot.
We experienced more than a few instances in our 1990 XJ-S convertible (5.3L w/ Marelli) when the aux fan did not come on when it should have and coolant temperature rose like you are describing. One time it was a bad relay, another time it was a bad temperature switch, and the other times I am not certain why. But each time the aux cooling fan did not come on the coolant temperature rose well above the “N”.
I modified the instrument panel to illuminate the unused green “Caravan” advisory light whenever the aux cooling fan is powered on. It is interesting to drive and watch that light go on and off regularly while driving even on mild days and at speed.
If I were you, the next time that you are driving and the coolant temperature rises noticeably above the N pull over where it is safe and see if your aux cooling fan is working or not. You can shut the car off and listen for the roar of the fan, or better yet you can get out of the car and open the hood for visual confirmation.
This test will only take a moment to do, and it will confirm whether or not your auxiliary cooling fan is working when it should be.
My observations of our “Caravan” light show that in our car the aux cooling fan comes on just below the top of the “N” and will turn off about the middle of the “N” on a fully warmed up engine.