Regarding my '98 Disco I;
I ran a smoke test through the PCV line on passenger side of truck and found a very minor vacuum leak on the back of the intake mani. I will take care of this problem. but should the idle be so high (2500 RPM) just from a very tiny vacuum leak? Sometimes when I start it up the idle is normal too. Also, it has been very hot here (80-90), and the truck seems to go above normal operating temp (3/4 of the way up) if im using the AC. if I cut the AC off, temp goes down within a minute. no problem sitting in traffic or cruising, unless the AC is on. Radiator Fan works fantastically. I am getting code 1178 with the vacuum leak, but nothing else. could the computer's fuel compensation for the vacuum leak be causing the overheating issue? or maybe unburnt fuel? Im also using slightly more fuel than usual (as expected with vac leak) I guess Ill find out if they are related once I fix the leak, but Im just curious if anyone else knows anything more before I start on it.

 

Could it be the AC fans? When are they supposed to cut on? full time with the AC on, or just when it gets warm?

 

Yikes! 3/4 guage is way too hot. This a'int Detroit iron, and if you use a scanner that can read coolant temp you will find that 150 - 215 is the bottom 1/3, don't move much. 1/2 is about as far as most members consider OK. Above 1/2 and you are really cooking.

That said, here are some ideas, in order of difficulty and cost:

1. If the extra heat from AC makes the system that unstable, your system is on the brink anyway. Coolant needs to be full.

2. Air flow - look for trash behind the grille, between the condenser (AC radiator) and main radiator (I had a compost pile there was so much in there), and mud/crud in radiator fins (hold a light behind it and look all the way through). Air flow cools things off, of course, but hot air is also needed to bring the face of the fan clutch to above 170 degrees to start bringing the fan back on line at increased power.

3. Since you said problem happens with AC on, we assume low speed, because at higher road speed the fans are of no issue. AC fans should come on when AC is engaged, and they should blow toward the engine block, not out the front of the radiator (been there, got the t-shirt). Main fan clutch should cause fan to "roar" at cold start up, because it moves more air. In a minute or so the clutch has warmed up, and the flow decreases as does the noise. Mine moves enough air cold to spin the AC fans even when they are not on. When cold the fan should not spin more than a small fraction of a revolution. When warm, it should not spin more than 1 revolution and certainly not free wheel, if it does, the internal fluid is low and it won't come back on at full power when hot. When engine very hot, fan should be stiffer to turn and maybe roar when running. Forum has good write ups on replacing with $50 fan clutch from 2000 Chevy Express 4.3 liter w/o AC.

4. Thermostat could be stuck open. If closed it would over heat right away. Stuck open it does not leave water in the radiator long enough for it to cool off. You can't tell this on the "low resolution" temp guage, but a scanner that reads data will show that the coolant emp is all over the place - 160 - 205, and more. Also, if the thermostat slips down during installation, it will leave an crescent shaped opening at the top. The gasket will seal up around the out-of-alignment bottom of the stat, and the gap at top will let that brand new stat you had shipped in perform just like one that is stuck open. Stats are cheap, and in the old days people used a winter and a summer stat. I have used a 180 "high flow" for a while, but have gone back to a 195 to see if gas mpg improves (14 mpg). A 160 is too low, the automatic tranny needs to see 150 degrees before 4th gear enables.

5. Radiator could be ready for service. If you have not changed coolant, or if you have not used distilled water, there could be a build up of calcium in the radiator which acts like extra insulation, and reduces flow. You probably have the D1 standard brass and copper radiator, which can be flushed out, or at a commercial shop "boiled out" (hot acid dip) and rodded out (they un-solder the side tank and run little stiff wires or rods through the tubing). Way cheaper than new radiator. Was able to get it done for $65 near me, but that is farm country, you might need to shop around. I am told one test for a radiator is no more than 10 degrees change across it. Some people have used white vinegar to flush out radiators, and there is commercial stuff at any auto parts store.

Note - it is possible for the coolant temp sensor to be defective, you have two, just to the driver's side of the upper radiator hose. The unit with one wire drives the guage (clean the connection), the one with a two wire plug tells the ECU how warm things are.

6. Now for the pricey things: water pump on the edge; head gasket (using any coolant?) combustion gas in coolant - test kit from auto parts store); AC high side way out of spec, unusual, because there is a 450 PSI or so safety switch; valley gasket (usually shows up as coolant loss). Hopefuly no coolant in oil or oil in coolant.

On you high idle, I would start with the IACV (idle air control valve) - a new one is cheap and shared by Jeeps, etc. Good to continue on the vac lines as well. The IACV is just a computer controlled vac leak.

Other may know more about the mixture, but I don't think that the ECU would do something to run the coolant temp to 3/4 range.

Dang - you are so lucky to live in a town with online junk yards that email you when vehicles of interest arrive (U pull apart).

 

what did you use for the smoke test ?

 

Savanna, great write up.

I'm doing my cooling system this weekend, W/P, upper and lower, t-stat and rad flush, will check flow and see if it needs rodded. I'm getting intermittent rising heating issues when idle at a red light when outside temp is over 105 and A/C is on. Oh and all was purchased from rock auto for under 100 bucks so woohoo.

 

On a D1 at least, the AC fans come on for three reasons (1) AC is on and interior fan is on any position other than off; (2) ECU believes coolant is at 212 F and engine is running; (3) ECU belives coolant is 110 Centigrade AND fuel temp is 70 centigrade and engine is off, fans will run for 10 minutes. This comes from section K5 of the Electrical Troubleshooting Guide.

The D1s and early Rangies have a brass and copper radiator, can take flushing, acid treatment, rod out, etc., and still bounce back. Some people have removed radiator, laid it flat, and filled with brick washing acid (muratic acid). Some have used white vinegar. The plus is that the treatment is only the radiator and not the engine. However, if you shop the indy radiator shops, you may find it fairly cheap to take the radiator to them for "boil out" and rodding. I paid $65, the guy was on the edge of farm country, and someone that does heavy road equipment, bus, dump truck, and tractors does not need to make his boat payment from the next SUV that comes in. I think the boiling term is actually more like a hot citric acid that is pumped through for a period of time to flush and dissolve as much calcium as possible. When you remove the plastic fill plug on top of the radiator (you will replace with a brass 1/2 NPT plug for $2) you can see down inside on some of the ends of the tubes and see calcium. I also tried a radiator from the salvage yard, only to find that the oil cooler section was 95% restricted, cranked the truck and the oil light stayed on, while Mr. Dummy (that be me) was busy checking for leaks as it warmed up! So the old radiator had "killed" the donor Disco by oil starvation. Got my original radiator fixed. Now that is not to say that salvage yard parts are not a great deal, I just failled to make basic tests on the unit before I carted it off. And failed to think of priorities when starting a truck to test - the oil light going off is a lot more important than some coolant on the floor... Some have suggested that a D1 does not need the oil cooler lines in a pinch and that could be closed off, or run to a remote oil cooler. But keep in mind you have tranny, oil, and water radiator in the same box, and no mixing allowed.

And note to those replacing a D1 radiator, the driver's side top bracket also supports the power steering fluid bottle, and this needs to be level, not twisted to one side....

 

There is a third water temp sensor which is actually a switch in the cooling system. The two sensors on top of the t-stat housing are the ECM and cluster gague sensors. The third one is located on the outlet elbow where the outlet hose is clamped. That two wire switch is directly responsible for turning on the A/C condensor fans without the operation of the A/C compressor (overheat situation).

If both condensor fans are not turning when the vehicle is on and the a/c button is depressed and the a/c blower is called for then that will certainly cause a slight rise in engine temp. Make sure all three fans (both a/c and engine fan clutch) are in good working order. Also check in between the Radiator and Condensor to see if there is a bunch of leaves and gunk in there.

Could just be as simple as your rad being old and needing to be cleaned out or replaced as stated above.

And YES, a vac leak anywhere in the system post MAF will cause your idle to rise. if the leak is more than the IACV can compensate for then it will be high and sometimes will hunt around too.

 

I found out my AC fans are not working, they cut on for a second when car first starts and then cut back off. if i try to spin it, it will come back for a second but go off again, so i need 2 new fans. I cleaned my intake mani and put a new gasket in, 100 miles later CEL goes off, and no codes are read, and idle is normal. My temp gauge has always (for years) stayed at just a hair under 1/2 for normal operating temp. should it really be lower?

 

You need fans, and your AC will not work very well at idle or slow traffic on warm days. Need those fans to hold down AC compressor head pressure. When you replace them, make sure blades turn the right direction and air is blown toward the engine block.

Heres what my D1 runs - see photos. It helps to have a scanner that can read live data.

 

No. My gauge runs right there as well. Just use that location as the "normal" mark. If it goes higher, figure out why.