1997 AWD 4.0L with 112k

Background:

Last year van suddenly overheated (almost to red). Turned on front and rear heat full blast and drove home - about 5 miles. Tested thermostat in boiling water and found that it would not open.

Fix:

Pressure-washed radiator fins. Flushed system with Prestone kit and cleaner. Replaced thermostat and temp sending sensors (gauge and ECU) with Autozone parts. Added 50/50 coolant and correct amount of Bar's leak. Temps returned to normal.

Problem:

As outside temps warm, the van now consistently runs between "A" and "L" in "NORMAL" on the gauge. AC on/off makes no difference nor does sitting or highway speeds.

Initial Fix:

Suspecting the non-OEM Autozone thermostat to be the cause, I replaced the thermostat with a 197 degree Motorcraft unit from the dealer. The van still runs at the same temp.

My Thoughts:

Fan Clutch - Unlikely. Resistance when turning by hand. No fluctuation between sitting and highway speeds implies airflow is adequate

Thermostat - Unlikely. New OEM unit.

Temp sending sensor for gauge - False high reading a possibility. May try to measure temp of radiator supply hose directly to see if it's accurate. May also try to measure sensor resistance to see if it's within specs.

Radiator - Likely? Older van, so scale may be causing blockage. Temps have increased with time. No fluctuation between sitting and highway speeds supports blockage. No issue when outside temps are colder also seem to support blockage theory.

Conclusion: Radiator is okay with winter temps, but reaches it's cooling capacity with summer temps.

Do you agree or disagree?

 

Welcome to the Aerostar forum on FTE!

Agree, radiator most likely at this point although other possibilities would include, a collapsed/kinked hose (may be internal collapse, not visible outright) or a water pump with erroded turbines (rare, but happens occasionally on neglected systems). Is the system losing coolant or staying at a constant level? The temperature you indicate is rather high for these systems, so a blockage of some type (radiator or otherwise) sounds likely.

 

Yeah, water pump is not likley, I support the blocked radiator theory on this one. Anyway, the wwater pump can easily be checked. First, with the system at operarateing temperature, squeeze the upper radiator hose to block off flow through it, and as you release, you should be able to feel the water pressure surge though it. Also, look ath the weep hole, and see if there is white scalely buildup coming from it. If there is, it is an almost sure sign that the pump has failed. If it passes both tests, then it is most likely not the cause. The radiator is the most likely cause.

 

I wouldn't rule out the Water pump. You can have partly eroded fins and that would not make it cool as fast as it should in hot weather.
A bad Radiator...Yes a good possaibilty.
But I also wouldn't rule out a leaking head gasket. If it's getting some exhast leak into the water system it will heat it up fast.

Jay

 

One thing that I have found in my past vehicle overheating problems is that you need to check for any leak, even if its a super tiny leak of some sort. The system has to stay at a certain pressure in order to operate correctly, and a small leak in the heater core, lines, or such will allow air to enter the system as it cools and will cause problems the next time you run it.

 

Thanks for the replies so far.

Van does not show any evidence of water/coolant in the exhaust, so I don't believe the head gasket or intake manifold gaskets are bad. I knew the water pump was also a possibilty, but I thought it was unlikely. Nevertheless, I will check it using the "hose" method that was described to rule it out.

I think I will also try running it without the thermostat in place to see if that affects the temp at all.

Does anyone know of a good way to check the coolant temp externally? I was going to tape a high-temp thermometer to the upper rad hose, but that may get interference from the exhaust manifold, etc. I think the remote infra-red type would work too, but I don't have way to get a hold of one.

 

Just a thought... Does the radiator boil over when it gets hot? If not, you might have a faulty temp sensor or gauge. Many years ago I replaced over 300.00 worth of stuff on a mustang, when I finaly found out that the only problem was a faulty temp sender...

 

Has anyone removed the radiator from an AWD 4.0L with rear air? The compressor lines are routed directly over the top of the radiator. I didn't check it out completely, but it looks like they will NOT move out of the way enough to get the radiator out. I would hate to have to disconnect them . . .

 

I have the same set up and have replaced my radiator recently. The easiest thing to do is to pull the four A/C compressor mounting bolts (10mm) and move the compressor and lines out of the way. Its easier to do than it sounds. Put a piece of carpet scrap/protective pad of choice between the grill and the front bumper and allow the compressor to rest there. Use a piece of wire to actually suspend the compressor from the hood latch so the compressor lines are not taking the strain. The most difficult part is trying to get the new radiator past the fan shroud without damaging the cooling fins. A helper is handy - one person to maneuver the radiator and the other to maneuver the shroud. Otherwise, it would be best to remove the fan clutch and shroud (as a unit) before trying to remove/install the radiator. AutoZone/Kragen/etc will loan you the special wrenches needed to remove the fan clutch. On the 4.0L engine the nut is right handed but is on very tight. Leave the fan belt on while trying to break the nut loose. Blood sacrifice is required for Aerostar repairs so wear gloves or else appease the gods as you see fit.

 

Update:

Checked hoses. Bottom was not collapsed and top was passing coolant. During warm-up I held my hand on the t-stat housing and the t-stat is definitely working. The temp changed and the upper hose pressurized very quickly once the t-stat opened.

I bought a new temp sending unit (TSU) from Napa (Echlin) and did some tests to see what resistance it is at different temps. This particular TSU works under the principle that as temp INCREASES, resistance DECREASES.

Knowing this, I heated up water to a boil in the microwave and placed a thermometer and the TSU in it. With the bottom portion of the TSU immersed, I measured the resistance at different temps as the water cooled.

At 198 F, it was ~35 ohms
At 85 F (ambient room temp), it was ~240 ohms
I have other values written down as well, but I don't have them handy.

Knowing this, I measured the resistance of the current in-place TSU as the van was warming up this morning.

At 85 F (room temp via same thermometer as above), it was ~215 ohms
Once between the "A" & "L" in the dashboard gauge, it was ~30 ohms

My next test will be to remove the in-place TSU and find out what temp 30 ohms corresponds to using the thermometer and water like above. Given the readings of the new TSU above and assuming the both TSUs are fairly close to each other, I'm expecting 30 ohms to be around 205 F - 210 F. Is this temp acceptable for this engine?

However, I also think it is important to note that the 85 F resistance readings of the in-place TSU vs the new TSU (215 vs. 240) suggests that the in-place TSU may be reading falsely LOWER resistance when compared to the new unit. If that's the case, installation of the new TSU should cause the dashboard temp gauge to read LOWER.

 

Problem SOLVED. It was the TSU. As I suggested in my previous post, the old TSU was providing falsely low resistance to the gauge. Who would have thought since it was replaced just last year. It must have been failing over the past couple of months.

With the the new TSU, the the gauge shows the van temp at just below the "R" in "NORMAL" with no A/C at idle and right at "R" with A/C on idle or cruise. What's more, I have confirmed that van is running 200 - 205 with the new TSU via resistance. I'm glad I decided to take some time and measure the resistance at different temps. A new radiator would have been waste and wouldn't have solved the problem.

 

Old TSU probably damaged by overheating and over pressure from thermostat failure and red zone operation last year.
An engine overheating incident often causes other failures; heater hoses, sediment movement in block and radiator with blockages, sensor failures, cracked and warped heads and blocks, blown water pump seals, blown heater cores, auto trans. stress and failure.

 

[QUOTE=96_4wdr]Old TSU probably damaged by overheating and over pressure from thermostat failure and red zone operation last year.
An engine overheating incident often causes other failures; heater hoses, sediment movement in block and radiator with blockages, sensor failures, cracked and warped heads and blocks, blown water pump seals, blown heater cores, auto trans. stress and failure.[/QUOTE-----
--------------------------
Absolutely 100% correct. I've seen cases too, over the years, where line flushing of the heater core, solves 99% of odd overheating problems, that are chased with cooling system parts replacement that do not solve the problem, where you know the thermostat is the correct temp. (going with a cooler 'stat on a modern vehicle causes more problems than good. 195 degree stats, with a 3.0 or 4.0 V6 running HOT (but not overheating) = an engine running clean, low emissions out the tailpipe, maximum MPG and power. Carb cars, trucks + vans from the 50s-60s-70s, you could "band-aid" a heating problem with a 160 stat (or remove it) instead of a 180 or 195 degree thermostat. By direct flushing of the heater core, I am referring to flushing the core, with a "T" splice directly at the heater core inlet hose, and out of the exit hose. Doing this, you'd just might be shocked at the sediment, muck, rust, and scale that comes out of a heater core, as the radiator petcock (or plastic drain plug) is flowing clear water. Here is an easy, simple way, to flush your heater core, for a LONG LONG service life. At your h. core inlet hose, install a flush "T" (3/4" common on Aerostars, 3.0v6. I'm not sure if the extended Aerostars with a 4.0 v6, has a different size of heater hose?) which is cheap and common, as you find in a Prestone kit. At the outlet hose, slice the heater hose with a razor blade, and install the correct size nylon plastic coupler. Try to locate these access points, about 6"-8" from where the hoses connect to the firewall. Then simply disconnect the outlet coupler so the sedimet, runs out of the heater core. At the inlet hose, connect your garden hose to the "Flushing T" I hope this make sense, my discription. Flush the core once every 2 years, and it's a 15 minute job that will not only add years of life to your heater core, in the autumn and winter, you'll have HOT, toasty heat, on cold snowy days + warm heat out of the defrosters. Lastly, try and make a habit to run your heater on occasion, even during the warm summer months. Likewise, run the AC, on occasion, during a freeway run during the winter.

 

Linked to Temp gauge reading cool...oops..

p.