I have a 98 Catera, and when it is sitting in the driveway idleing, no problem, temp gauge gets to the 1/2 mark then drops down 1/8in or so, i think thats due to the stat opening, but after I drive it for an hour, it gets very hot, not to the red mark but very high. Do you think my thermostat is half opening? I've read lots of the overheating threads on this forum. I just want to be pointed in the right direction.

I'm still checking this one out myself, already made a lot of progress. I originally had a failed heater aux water pump. Didn't find it until after I removed the oil cooler and added an aftermarket cooler. I noted it warmed up slower but still kept going until the second stage of fans kicked in.

It cools better at idle because on this car there are three pumps, one mechanical, and two electric, and most cars the mechanical can't keep up at idle and it gets hotter. If your car gets hotter at idle check that heater aux water pump (damn I say that a lot lately it seems to fix all kinds of problems!!!). It runs all the time and gets a lot of abuse and causes radio problems too.

I first (as emergency measure) removed the connector from the heater aux water pump and plugged it into the aux water pump and that helped until I got my replacement. It had an extension cable already installed. Looks as if that was intentional OEM for testing or emergencies. Good tip for someone who finds themself with a failed pump, make that other pump run in its place (don't move it just unplug it and plug the heater pump connector in its place). That actually made it work ad good as the original until temps get above 90F then it needs both pumps running.

But even after I got the new pumps I wasn't happy since I drive a lot and its damn hot out there, I don't think this car was set up for hot climates and the setup of the auxiliary fans and pumps are conservative (meant for temporary hot operation not extended high temperatures above 90F). But damn there are enough pumps and fans and I have external oil cooler so I studied the service manual and noted how my car performed and looked for way to improve the cycling.

My observations with operational water pumps and external oil cooler:
70 degrees and below - warms up to thermostat open and levels off (thermostat full open at 198 that is right below the half mark on gauge)
75-85 - may cycle up to first stage fan and then cycle back to half (that is 9/16 position - between half and the next mark)
90-95 - will ride in the first stage of fans (one fan half-speed) between 5/8 and 3/4 mark
95-105 - will cycle on the second stage of fans at idle (two fans full speed), will cool slightly at idle but will not lower at slow highway speed, stays at 3/4 and cycles on the third stage (three fans high speed plus the aux water pump).
It would cool some at idle below 100F outside but barely moved above 100F outside.
The third fan runs whenever the AC is on but if it is not is comes on at the third stage with the aux water pump. The car runs cooler with AC on all times unless its already at the 3/4 mark. If its that hot and continues rising more than 3/4, than shut it down, and pity the poor hot bastard looking for a ride.

HOWEVER - you can make ALL FANS cycle on the first or second stage and keep that cat cooler in summer. There's enough cooling capacity but it seems wired more as backup for a car designed for cooler markets. It can be changed very easy with a jumper on a test connector.
I remembered something I read about the coolant test connector, and did an experiment. I looked at wiring diagram and noted the relay test points for the primary (two switches) and secondary (one switch) temperature switch. Pin #1 is for the switch cycling between 95C-100C. Pin #5 is for switch cycling 100-105C, pin #4 for switch 105-110C. The trick is to short the connectors to change the cycling making the second or third stage relays pull in on the first stage switch, or make the third stage pull in on the second stage switch. For summertime use especially as hot as it is I decided I wanted all fans and pump to kick in earlier so I put a jumper between #5 and #4.

Now after completely warmed up - at idle in 100+ heat, it will actually move the temp gauge below the half mark, and at highway speeds it will cycle on and off the first stage of fans, because the jumper makes all fans and aux pump run at 212F (100C). If it hits the second stage it goes back down and cycles off.
I recommend it at least thru August, then you can snatch the jumpers off. From the notes I took before the jumper it seems it isn't needed if the temp outside is below 85F, if anyone having problems cooling below those temps I suspect a problem. I would likewise remove the jumper as soon as temps outside get below 85 as a high, so if I do see that gauge move to 3/4 I can look for a problem and not hide it until it fails.

You cannot use the first-stage switch because it is bypassed whenever the AC pressure switch is ON and this switch is powered on at all times.

FYI - I tore apart a floppy drive connector. The female pins are perfect size to fit the coolant test connector pins. The coolant test connector is under the hood fused box behind battery. From top right clockwise (leaning on driver fender looking down) 1,2,3,4,5,6.

Thanks, that looks like its going to be a lot of help. I will check it out when I get home from camping (yes I have a wireless card :thumbsup: ) I dont have radio problems. Do you still suggest its the heater aux water pump? Where is that located and how hard would you say it is to change? So your suggesting putting a jumper going from number 1 and number 4 ?? I will have to get home and take out my battery to see exactly how it works. Thanks for the help, very detailed and complete. I will have to give that a try.

actually jumper #5 and #4 cause jumpering #1 pulls everything in when engine is off. FYI I am still investigating. According to the service manual "description of operation" the aux water pump should come in on the first stage but the schematics seem to agree with my analysis. There is at least one disagreement with two sections showing the same relay K22 contacts. Could be the heat. But I know the #4 and #5 jumpers get everything at full speed at least 10 degrees cooler if everything else is working right. That simply makes the second stage turn on both second and third stage.

So put a jumper between #4 and #5? I hope this works, I'm already fed up with this car and would sell it for very cheap lol.

So put a jumper between #4 and #5? I hope this works, I'm already fed up with this car and would sell it for very cheap lol.

Don't give up on your Catera. It's a great car if you can fix it yourself, which you apparently can. Hang in there!

But you are VERY wise to stay on top of and ahead of any kind of overheating. Do not forget those aluminum heads. If she gets too hot the heads WILL warp. Then you will not have to worry about leaving your Cat behind; she will already have left you behind . . . . with a dead engine.

I have had the same problem with high temps. I read somewhere that it was a sensor problem being that the origional ones from germany were defective and read too hot. I bought one new from the dealership, $35, and will replace (there actually was a technical service bulliten on this part) the new part should read on it VDO. I will let you know if this makes any kind of a difference. Cross your fingers.

Ace are you talking about the coolant temp sending unit?? It's not that the gauge says its hot, you can tell its hot when you pop the hood. I love the inside luxury of this car, i feel like a million bucks....So your saying put a jumper between #4 and #5 lakeside??

bad you are right on both...the coolant temp sending unit corrects the gauge but doesn't affect the cooling, that is done by the thermostat and the primary / secondary temperature switch.

Jumper #4 and #5. That makes the second stage of fans kick on both the second stage and third stage. You will see that alternator needle take a dip but if its getting hot its gonna dip later anyway, the whole idea is to call in the calvary earlier while its still easier to push that temperature down.

I discovered a few more things from my experiments and study.
1) The aux water pump (behind radiator) will NEVER RUN when car is ON. It only runs when car is hot and off. the relay is N/C and wired direct to ignition ON power so it opens the circuit. I actually traced wires and measured relays. The wiring diagrams for the three-volume manual set disagree with each other, one section shows a N/O relay and the other shows N/C. Don't bother removing the relay and jumpering to make it run. That pump only cools the radiator and seems to have little effect on operating temps (I tested this with and without my 4-5 jumper and still saw temps rise up to 3/4 - according to my data logger it went to 230F)

2) It appears that the bypass diodes of two of the fan control relays work as a latch preventing any fans from dropping out until the first stage drops out when the 4-5 jumper is in place. When it reaches the second stage and fans come on they never all went OFF until temp gauge dropped below 1/2. I will check data logger tonight to see the setpoints and also look at schematics again. These bypass diodes are also why I don't jumper 1-4 or 1-5 since they are only on the relays pulled in at the first stage. Makes sense, if the car gets warmer than the primary cooling then it needs to run them all to cycle.

Gonna stay on this even though it appears I have solved my cooling problems, but too many people posting about heat issues...and its still hot out there.

hey guys,
I thought I'd revive an old thread that I think might be usefull for people now that it's summer

my cat was getting a lot hotter than it did in the winter so I thought I'd do something about it before something terrible happened. my heat was hover over 5/8 mark and just below 3/4 mark, which really started scaring me. mostly since my girlfriend drives the car 95% of the time, si I can't keep an eye on it.

I think the first thing you should try is water wetter. that stuff is pretty good. now I add it to all my cars.