I have a minor overtemp problem on my 94 F150 5.8L. I just did a block test to check for combustion gas leaking into the coolant. The instructions with the NAPA block tester say if the test fluid remains blue there's no leak, if it turns yellow, theres a leak. The fluid did neither -- it turned green -- what does that mean??

Symptoms: Running empty @70MPH for 20 minutes with A/C on temp is normal (normal is needle on letter “O”), but if follow that by a few minutes of stop and go city driving, temp gauge goes 1/2 inch above normal. Starting out cold and only doing city driving, temp gauge stays exactly on the "O" no matter how long the drive is.
- Not losing any coolant, coolant looks like new
- Oil filler cap has no scum or foam,
- Oil on dipstick looks normal
- No steam out exhaust
- No rough running on startup
- Fan blade-to-shaft drag feels same as new.
<!--[if !supportEmptyParas]--><!--[endif]-->
Maintenance history:
- water pump, thermostat, and 2 radiator hoses (all new Ford OEM parts), coolant have 2 years, 7K miles.
- fan clutch (AC Delco) has 1/2 year, 2K miles.
- radiator (installed by EFD) has 4 years 45K miles
- coolant always changed on schedule
<!--[if !supportLineBreakNewLine]-->

 

Yellow and blue makes green so I think it's saying it don't know!! JK!!
Tower hobbies has a Duratrax infra red thermometer for $24.99 I would get one
before tearing in and replacing components or opening the motor. The stock gauges are not known to be real accurate I mean the temp is on N how accurate is that?
You could unscrew and clean the threads on the temp sender it could be corroded
and giving screwy readings with just a bit more heat expansion. Mine was corroded so
bad it wasn't making contact, a little bit of dielectric grease or silver antisieze when you reinstall it will help. What ratio are you running the water to coolant at 70:30 cools better 70%water to 30% coolant. Can you hear the fan kick on when the temp goes up, Try this test get it up to temp, let it cool for a bit so you don't get burnt,
start the motor run for a couple of minutes shut it off, wrap some electrical tape around one fan blade about 2-3 times then tie that off to something solid(not rad
hose or trans tubes) start motor and check that it didn't come off already if fan is
staying in place go for a ride, don't over rev just drive normal. Watch your temp
gauge closely, when you hear the fan come on or hear the tape beating around stop and take note of the temp on the gauge and with the IR temp gun. IF it takes a long
time to get the thermo fan clutch you may need to swap it out to a Heavy duty one
and be sure to get a thermal fan clutch, The other ones slip at a constant rate(60%)
usually and the silicone in them can get worn out, some you can swap in new fluid. I
would get a thermal one (and not ac delco my god!!) Or if it is hot where you live get
an auxillary electric fan for those real hot days in stop and go traffic, I wouldn't use a
electric one as a primary but a lot of folks do. That Ir temp gun can be used to check
the temp on hubs, transmissions, rear end, bearings, radiator, tires. More useful then
getting a aftemarket temp gauge.

 

I have an infrared gun; it is overheating. Since I wrote the post, I took it to my normal general repair shop; they replaced the thermostat and temp sender, with no improvement. I don't think gauge is bad, as it reads exactly the same position it always (for past 13 years) reads when at normal temp except now it's moving up 1/2 inch when overheats (slow speed after long hi speed drive). They also ran a block test (twice) and say the fluid remained blue!

Fan clutch is not engaging when it overheats (about 3/4 of scale, still in "normal" band), so clutch engagement may be bad. But why would the clutch even need to engage in 70 degree weather, no trailer tow or bed load, no grades? Since new, the only time (1994) the temp never moved above the 1/2 of the total gauge sweep was when towing my 6000 lb trailer on a grade, or empty climbing a very long grade in 100 degree weather at low speeds -- clutch would engage at about 5/8th position.

Fan clutch drag is very stiff when disengaged -- won't spin by hand more than an inch when push and let go.

I do have an A/C delco thermostatic fan clutch on it that has a few thousand miles on it. You mention those are bad news? I had problems with Hayden engaging at too low of a temp (gauge at 1/3rd!) and driving me nuts. Tried 3 Haydens and called factory rep to confirm I had correct part number. Put on AC Delco and everything was fine. At that time, clutch was no longer available through Ford (discontinued)

More info I forgot to mention: I tested radiator core with infrared gun. Lower half is about 60 degrees F cooler than upper half. Shop claims this is ok.

Just talked to shop as I'm writing this, they are going to replace the clutch, and claim the OEM clutch is available(??)<o></o>

 

Important typos fix to my last message: never -> ever

"Since new, the only time (1994) the temp never moved above the 1/2 of the total gauge sweep"...

should have said:

"Since new (1994), the only time the temp ever moved above 1/2 of the total gauge sweep"...

 

The way I read the instructions:
http://www.blockchek.com/instructions.htm

It seems like they say the fluid needs to remain blue to indicate that no leak is present. They also indicate that in a diesel engine, it may turn yellow-green - maybe due to a slight difference in the combustion gas mix. But it seems like any color change means there is a leak. Green may mean a small leak or that one has occurred, but may not be leaking at the time of the test.

One thing I look for is to see if the radiator cap makes a 'pshhht' sound when opened after the engine has run and cooled back down. This means combustion gases are in the radiator. Normally, expansion and contraction of the fluid will force all the air out and pull in coolant from the overflow tank, the result will be that the radiator is 'solid' with coolant - no air at all. If you open the cap and hear the air escaping, or if the level isn't 100% up to the top of the neck, something isn't right.

 

When I slowly lift the pressure release lever on the cap, I hear what sounds like coolant running into (or out of??) the overflow tank for a couple of seconds. I don't see any bubbles looing into the overflow tank during this. Coolant level is right up the cap seal level.

An update since my previous post: Shop replaced the AC Delco fan clutch with a new OEM clutch. This has almost completely solved the problem to my surprise, since the OEM clutch freewheels much easier with the engine stopped than the very stiff AC Delco clutch did -- but maybe that classic test isn't reflective of fan speed when idling. Also this nearly new AC Delco clutch was not engaging when temps got way too high.

I now see the gauge move up about 1/8 inch above normal in my original driving scenario, rather than moving up 1/2 inch. I don't recall it mioving at all that previous to about a year ago. So I'm still concerned there's some problem, maybe the start of a very small combustion leak. Also I'm not towing heavy loads these days so I don't know what would happen in those conditions (I sold my trailer so can't try it now).

The shop also did the block test a second time at my request and got the same blue result. Yet both times I tested I got green. fluid I wonder if they are reusing the test fluid.

 

there are two type of fan clutch thermostatic and non thermo. Of course the thermo
ones engage when a certain temp is reached and spin at near 100% of pulley speed,
The non thermo units spin at around 60% of pulley speed at all times so most of the
time that is fine but get above a certain temp and it will never bring the temp down
on it's own you will need to stop and let it cool. Always go for the thermo actauated
fan clutches when you live in a southern state or you tow or drive hard!! The silicone
in the non thermo units wears out from constantly slipping all the time, think like a
torque convertor!! Some you can swap out the fluid and dial it in for a different slip
rate to suit your conditions but it isn't worth it.

 

Both the old (ACDelco) and new (OEM) clutches were thermostatic.