240: Radiator? Or something else I'm not thinking of?

[Randomletters]

My 240 has been running hot for a while, and I've been lazy, but now about to do a cross country road trip in summer so it really needs to get dealt with now. Also, it is quite audibly detonating, even despite switching to a colder plug (NGK BPR7ES). I'm not sure if this is just heat related or they have a common other cause.

The car always ran a hair over the midway mark. So when my water pump started leaking, I also swapped the thermostat. And of course all the belts. After that (I first noticed it about a week or two later) it started climbing to just under the red, and holding out there. As bad as this sounds/is, I've been driving it like this for about 35k miles, and it has yet to actually go into the red, but it often hovers just under. Or not. Sometimes it sits just a hair over the halfway mark like it used to for hours. Strangely, it usually starts to climb back up after going down a hill or something that lets the engine cool off slightly.

But it detonates. Badly. I have to run premium, and sometimes it still does. I swapped plugs for colder ones and it helped a little. But it still does uphill. It is worth noting that it detonates long before the temp needle climbs up, but not when the car is cold. I had thought this was based on a similar cause but not just the heat because of this, but I suppose it is possible that because of the temp compensation board, it is actually running a bit over operating temperature and hot enough to detonate before it actually starts climbing. Does this sound possible?

I had wondered if the timing was off, because that causes detonation and running hot, but I tried re-timing it twice yesterday, so unless the timing marks are off, I don't think the previous owner's mechanic, the mechanic who did the water pump, and me are all that bad at timing the car.

There are no coolant leaks normally, it doesn't seem to lose any. The radiator really does seem to flow, it is moving through both hoses, and squeezing them does effortlessly put water through the radiator when the t-stat is closed, and it isn't overheating all the time, but I'm out of other ideas. The other day I checked, and suddenly a bunch of coolant was gone. Put more in, and it isn't leaking again. I think it may have boiled over while I was driving. It is starting to slip into the red a bit as of a week or two ago.

[vlvman]

Replace the thermostat again. Get rid of the temperature compensation board.

Find out why it is detonating and fix it. Fix all the vacuum leaks, then read those plugs. Check fuel flow and pressure. Consider having the injectors cleaned and flow matched. Clean the MAF with MAF cleaner. Check the little hot air door in the air cleaner, make sure it is closed.

What year 240?

[vlvman]

Also replace the expansion tank cap.

[Randomletters]

I'll replace the thermostat again tomorrow and grab a tank cap and see if that does anything. I suppose I could jump over the board. It is definitely running hot though, not just a misreading on the board. I'm definitely not finding any vacuum leaks. I replaced a couple of hoses, and it isn't idling high. No leaks out the pressure regulator, although I haven't pressure tested it. At least at idle, it is clicking back and forth at the 02 sensor like it should. I tried replacing the AMM because it ran lean a lot with the old one, to the point of misfire in the cold, but it seems good with this one. The hose from the manifold is missing, so it isn't getting hot air regardless of what the airbox thermostat is doing. The plugs I just pulled out had 30k miles on them (copper plugs) and they looked just like normal wear.

It is an '87.

[Randomletters]

Okay, so I got to try some more today.

I pulled the thermostat out entirely, and it ran about middle of the gauge, just under. Figuring for sure that was it, I put in a new (87 degree) thermostat, and nope, right back up. It did bring it down a bit, clearly under the red now, but still closer to red than midway.

Jumped over the temp compensation board, and that pulled it down even more. Now it is closer to midway.

I'm not even sure where these gauges are suppose to hold out without the comp board, so maybe it is right now, but I'm a little concerned, because it is still a needle width above the midway mark cruising, and when I park it, it still climbs to almost 2/3 the way up. And it is actually pretty cool here today.

[John2x240]

The temp gauge or it's sender may be off. Get some temperature tape and stick it on the upper hose near to the thermostat housing or shoot it with one of those infrared temperature thingies and you should get a reasonably accurate reading.

[TrickMick]

1) determine the accuracy of the GAUGE ITSELF
2) get a COMBUSTIBLE GAS TEST run on the COOLANT
3) MECHANICALLY timed correct (all marks on sprockets correct?)
3) accessory drive BUSHINGS OK (effects BELT TENSION)
4) auto trans??
5) IR thermometer sweep the rad core for flow
6) pull all plugs pressurize the cooling system to the cap rating and LEAVE it for an hour...
if the pressure remains *good* if not - step to the side and spin the engine watching for
a COOLANT SPURT from a cylinder
7) make SURE that the crank pulley isn't "separating" and SLIPPING (you can see it w/a timing light...
8) fan clutch "whoosh"??
check and test per what all the above have suggested (they are ALL *good things to know*)