You’ve got a vehicle in your bay whose engine isrunning hot, maybe even has overheated. Themotorist wants a diagnosis, but perhaps only tosee if he has to get the repair now or can safelypostpone it. In his mind, the approach of fallwith lower ambient temperatures may lead

him to believe his problems are over until next summer.However, some of the root causes for hot engine tempera-tures also can be responsible for lack of heat in the coldweather. In addition, some of the basic reasons for unsatis-factory passenger compartment a/c cooling in summer can P

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28 August 2002

DIAGNOSINGCOOLINGSYSTEM WOES:DO IT NOW

BY PAUL WEISSLER

A neglected cooling system doesn’tread the calendar, so there’s no‘right time of the year’ when itcomes to cooling system service.Warn your customers: Postponeservice now and you’ll regret itwhen the seasons change.

There’s a large buildup of rusty pow-der on the cap of this radiator on a4.3L V6 in a three-year-old GM van.Mileage was only about a third of the150,000-mile service limit, but thecoolant level was low. The radiator fillneck doesn’t look any better. Although

the rest of the system might liveafter a good backflushing, re-

placing both the radiator andf ill cap probably is the

safest approach.

29August 2002

lead to inadequate warmth in winter.So the problem you fix now also

can deliver benefits in the coldmonths to come. Anything that re-duces coolant flow in summer canraise coolant temperatures and possi-bly cause overheating. That’s straight-forward. In winter, low coolant flowcombines with some reduction incoolant temperature (from low ambi-ents and heat loss from the radiatorand engine block) to produce aheater problem, even when coolanttemperature itself isn’t low. Here’swhy:

You can’t just look at actual tem-peratures. A half-gallon of coolant at220°F contains a lot less heat energy(in BTUs) than two gallons of coolantat 160°F. If some problem keepscoolant temperature relatively highbut significantly reduces coolant flowthrough the heater, the heater can’ttransfer as much heat to the passen-ger cabin in the same amount oftime. Approximate numbers, if we as-sume a half-gallon of coolant perminute is flowing through the heaterat 220°F vs. two gallons at 160°F:

•850 BTUs every minute for thehalf-gallon of 220°F coolant.

•2500 BTUs every minute for thetwo gallons of 160°F coolant—aboutthree times as much heat energy

available to warm the cabin.Here are the service problems that

may have two or more causes:

Restricted Cooling SystemWhat can restrict a vehicle’s coolingsystem? There are many causes, butthe two we see most often are rustand corrosion and sealer in the sys-tem. Electrolysis is also a big contrib-utor to restricted coolant flow.

Rust and corrosion. In case youthought that everything is made ofaluminum and won’t rust, take noteof the fact that trucks and SUVs arehalf the market, and those enginesusually have cast-iron blocks atleast—even the Toyota V8. With poorcooling system maintenance, you canbelieve the reddish-brown stuffforms.

Sure, the extended-life coolantsusually carry a 5/150 or 5/100 changeinterval. However, poor maintenanceis not just failure to change thecoolant on a time-based schedule.Even worse is allowing the coolingsystem to run with a low coolant lev-el, which allows air into the system.That creates a great breeding groundfor rust, and there’s no more severeexample than GM’s 4.3L V6. Thephoto on pages 28-29 is not a one-in-a-million case. This is a frequent

problem, at a fraction of the 5-year/150,000-mile recommended ser-vice interval for DexCool.

The OAT (organic acid technology)coolants are in wide use and gainingin OE usage. However, I can’t say I’ma big fan of wholly-OAT antifreezessuch as DexCool, which seem to beoverly sensitive to low coolant level(among other things). However, inthe GM 4.3 V6, the root cause for lowcoolant level at low mileage, withouta leak, is a weighted vacuum valve inthe radiator pressure cap. A factoryfix to forestall the problem is a capwith a spring-loaded valve.

The longtime OE weighted valve isopen until closed by buildup of cool-ing system pressure. Before that oc-curs, some debris in the system canbe deposited on the upper side of thevalve. Over time, the buildup isenough to keep the valve from seal-ing closed when the pressure builds.Coolant flows out of the radiator intothe overflow reservoir, which itselfeventually overflows, causing loss ofcoolant.

The replacement cap deals with thatproblem, but cleaning out the rustpowder buildup in the system is some-thing else. There’s a GM bulletin onthe possible use of a “heavy-duty” cool-ing system cleaner, which is a specific

30 August 2002

DIAGNOSING COOLING SYSTEM WOES

Here’s a side-by-side comparison of the weighted valve-type radiator cap (left) vs. the spring-loaded vacuum valve.The spring-loaded vacuum valve is closed until drawn open by vacuum developed in the cooling system as hot coolantcools down. The weighted vacuum valve is open (allowing debris to accumulate) until pushed closed by coolant expand-ing under pressure. When enough debris accumulates, the valve does not seal even when pushed “closed,” so coolantflows out into reservoir, even overflowing the reservoir and resulting in a low coolant level.

Weight

VacuumValveSpring

Lower Sealing Gasket Lower Sealing GasketVacuum Valve Vacuum Valve

PressureValve Spring

PressureValve Spring

PressureValve

PressureValve

two-step product—oxalic acid and neu-tralizer. It’s not easy to use properly,and if you make a mistake, don’t say wedidn’t warn you.

Of course, you can’t just install theright cap on a plugged system and ex-

pect good results. One possible ap-proach is to backflush with a full-fea-ture flush & fill machine—first theheater core and then the engine block(thermostat removed because of thedebris issue). Replace the radiator, then

really fill the cooling system (a goodflush & fill machine also is key to suc-cess in this area) and warn the motoristto keep the system topped up and tohave the cooling system checked oncea year.

GM continues to endorse DexCoolfor this engine (and all others except itsnew Pontiac Vibe, which is Toyota-en-gineered). However, some shops pointout they never saw these problems withconventional American green/gold (sili-cated) antifreeze. So if you agree withthat line of thinking, you may be morecomfortable with installing a green/goldantifreeze with silicates in cast-ironblock engines. Green/gold may requirea shorter service interval, but the mo-torist who encounters the rusting prob-lem should be ready to shorten mainte-nance/inspection intervals anyway.

Just because a system won’t rust be-cause it’s almost all-aluminum (includ-ing the engine block and heater core),doesn’t mean it can’t suffer from corro-sion. Aluminum corrosion particles canbe carried to the radiator, where theymay be deposited and plug the tubes.

Also remember that not all radiatorsare aluminum. Many OE and aftermar-ket radiators in vehicles currently onthe road (particularly heavy-duty units)are copper/brass radiators assembledwith high-lead solder. This solder isprone to “solder bloom,” corrosion de-posits that block the radiator tubes.

Sealer in the system. Cooling sys-tem sealer has been shown to be a use-ful product. In fact, it was called the“great Band-Aid” for the Cadillac V8sof the 1980s, in which it stopped cold-engine coolant leaks. However, latelyit’s been used for preventive mainte-nance, a practice instituted at GMplants. GM reengineered its coolingsystems to prevent leaks, but kept thesealer in use as a “security blanket” in afew plants. Well, GM finally stopped allsealer usage on the assembly line overtwo years ago, and so should you.

Sealer should be used only to actual-ly stop leaks, and only when the cus-tomer can’t go for replacing the leakingpart. Don’t install more than a singledose of sealer. It can and will plug thenarrow passages of some radiators andin heater cores. Excessive use of sealer

32 August 2002

DIAGNOSING COOLING SYSTEM WOES

The end tank was removed to reveal the white solder corrosion that coversthe ends of the radiator tubes in this copper/brass radiator that was as-sembled with high-lead solder. The result is restricted coolant flow.

If you suspect the system is plugged with sealer or other debris, check forrestrictions in lines, hoses, heater and radiator with an infrared thermome-ter. If the readings are not reasonable for the coolant temperature and loca-tion, backflush the entire cooling system with a flush & fill machine.

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creates real problems, and en-gines that had a factory-in-stalled dose obviously wouldbe prone to this if more sealerwas added later.

Many coolant flow passagesin the engine are anything butwide highways. A buildup ofexcessive sealer can slow ordeflect the coolant flowthrough the engine. If thishappens in an area wherecombustion raises metal tem-peratures to levels where highcoolant flow is essential forproper cooling... Well, howabout engine knock, head gas-ket burn-through or evenmelting of metal?

If the customer is the of-fender, he may tell you howmuch sealer he’s put in, if youtake a careful vehicle history.If the source of the problemwas a series of shops just look-ing to get the vehicle out thedoor, you’ll have to do somemajor investigating.

Some sealer particles gen-erally accumulate in the reser-voir, particularly the pressur-ized type. Because GM usedsealer on the assembly line forso many years, it is particularlyaware of this problem. It re-ports gelatinous deposits of acolor that may resemble rust,but actually is sealer buildup,plus some debris from hosedegradation. The problem has beenmost common on the 60° V6 (both3.1L and 3.4L pushrod engines) andthe 3800 V6.

Electrolysis. MOTOR readers haveread our discussions of electrolysis as acause of corrosion and hose internaldegradation, producing debris that canrestrict a cooling system. We won’t re-peat them here, but you can get a re-fresher at www.motor.com (most re-cently with the Trouble Shooter col-umn in the September 2001 issue).

Checking for RestrictionsWhen sealer buildup or rust/corro-sion accumulations result in overheat-ing, you may find the restrictions lead

to uneven heat transfer. So if you taketemperature checks all over the cool-ing system with an infrared ther-mometer, you may get unexpectedreadings. We can’t tell you specificallywhat might turn up, but you’ll knowwhen readings don’t make sense,such as a hot engine, but only 110°coolant at the inlet heater hose, or abig temperature drop along a hose orcoolant metal line.

We can’t provide a rule-of-thumbspec for temperature drops across theheater hose necks because the num-bers vary so greatly according to vehi-cle model, ambient and coolant tem-peratures, antifreeze concentration,blower speed, etc. However, you’re

usually likely to see a drop ofup to 20° to 40°F, so if theinlet hose neck is close toengine coolant temperatureand the outlet neck is much,much lower—perhaps closeto ambient temperature—aplugged core is pretty likelyif heater output is poor. Tocheck things further, you candisconnect the heater hosesand attach auxiliary hosesand a water hose to the inlet.If there’s just a trickle fromthe outlet, that makes theproblem pretty apparent.

The radiator may not beplugged enough to resist agood backflush, but a heatercore just might be beyondcleaning. If you can’t get asolid column of water to flowthrough a heater core, re-place it.

Low Coolant Level,ConcentrationThe rusting problem withGM’s 4.3 V6 really involvedboth low coolant and a re-stricted system. Althoughother engines don’t end upwith the rust accumulation,they also may suffer fromoverheating in summer andpoor heater output in winter.One major cause is an im-proper fill or a system withsome of the space taken by

combustion gases leaking across thehead gasket. Not every system loadedwith air or gas bubbles is self-bleed-ing, and even if it is, the self-bleedingmay be far slower than the entry ofthe bubbles.

If you see bubbling when you openair bleeds, that pretty well confirmsthe issue. You can identify gas with acombustion leak detector in thecoolant fill neck (or cover the fill neckwith clear plastic wrap and pushthrough the probe of a gas analyzer,just into the neck opening, out of thecoolant). If you find combustion gasbubbles, you’ll have to identify thesource (such as head gasket or otherleak) and eliminate it, or you’ll en-

33August 2002

When checking for combustion gas bubbles in coolantwith a gas analyzer probe (engine idling, cap off), coverthe fill neck with plastic wrap and hold it in place. Makea slit in the plastic and push the probe through, just intothe open neck. To avoid damage to your gas analyzer,keep the probe tip out of the coolant.

counter the same problem down theroad.

Air and gas pockets are tough toeliminate unless you use a premiumdrain & fill machine or take the timeto do the job manually, opening allthe coolant air bleeds. Some techsthink there are just one or two airbleeds on an engine, but these bleedsalso may be built into a hose (includ-ing radiator and heater hoses). Not allair bleeds, particularly on an engine,are in plain sight, so check your CD-ROM information system.

As for low coolant concentration, a50-50 mix is what everyone recom-mends, and although there’s somemargin in that (45-55), when the an-tifreeze percentage gets down to the15% to 20% level, the system may bemore prone to corrosion than with100% water. So if the system hasbeen topped up with water periodi-cally, check the percentage with a re-fractometer or hydrometer. A test

strip is not as reliable an indicator,and even less so now with all the dif-ferent antifreeze dyes out there.

Weak Water PumpA deeply recessed water pump cantake half a day to replace, so a worn-out pump is not good news. Whywould a pump wear out so it doesn’tpump as much coolant as it should?The primary reason is called cavita-tion erosion/corrosion—the suddenformation and collapse of bubbles inthe coolant as the pump draws in andthen compresses the coolant. As thebubbles collapse, they “implode,”hammering the surface of the waterpump chamber, eroding the anticor-rosion protective layer of the an-tifreeze.

Some water pumps are more proneto cavitation than others. The silicate-type corrosion inhibitors in conven-tional green/gold antifreeze work veryquickly to reapply a protective layer

before corrosion can eat away thechamber and reduce pump perfor-mance. Silicates work much fasterthan OATs, so Chrysler’s new long-life antifreeze is a so-called hybrid.Based on a formula originally used byMercedes in both diesels and gas en-gine vehicles, the hybrid contains justenough silicate inhibitors to provideprimary protection and fast “repair”to the protective layer in water pumpchambers, and some organic acids(and other inhibitors) for extendedservice life.

The hybrid antifreeze is similar towhat Ford is phasing into its U.S.products, although it has other reasons,too. (An organic acid inhibitor used inmany wholly-OAT products seems toaffect gaskets in the Ford V8s.) So youcan look for a third type of antifreezeon the market. Valvoline, affiliatedwith the European company that al-ready sells a hybrid for use on the Con-tinent, has just started marketing this

34 August 2002

DIAGNOSING COOLING SYSTEM WOES

Air bleeds can be anywhere on the cooling system. In this vehicle, the red plug in the radiator hose is an air bleed.

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third type now. It’s called Zerex Ex-treme G-05, and a similar formula alsois available from the car dealers here ifyou want to go that route.

Tracing a problem to a weak waterpump isn’t easy, because a plugged

radiator may reduce coolant flow toproduce similar hot-run symptoms.The difference is that a restricted ra-diator is more likely to be responsiblefor higher coolant temperatures inhighway operation than a weak

pump. And you often can test or in-spect a radiator in less time than ittakes to pull and check a water pump.

Auxiliary electric water pumps arecoming into wider use (both as origi-nal equipment and as factory-fixretrofit devices), and if they fail, theymight be responsible for both over-heating in summer and poor heatoutput in winter. It depends on thecomputer strategy, because the basicobjective in all cases is to increasecoolant flow. If an electric pump istriggered by a cylinder head metaltemperature sensor, as on the LincolnLS and Jaguar V8s, a pump failurecan cause the engine to run too hot.If a pump is intended to improveheater output in low-speed cold-weather operation, as on 2000 GMminivans and 2001-on Dodge Duran-gos, it comes on in low ambient tem-peratures. Only if the electric pumpis controlled for both situations can itcause double trouble if it fails.

Air LeaksAn engine may not be running hot,but you still can be facing the sameroot cause for poor a/c cooling insummer and inadequate cabinwarmth in winter. The real issue maybe neither coolant flow nor the refrig-eration system; rather, it may be amatter of airflow. Okay, you knowabout the temperature control door orthe heater coolant control valve, andyou check those as a routine item.

However, outside air leaks justdon’t get respect as a source of theproblem. Yet they allow hot ambientair to leak into the cabin during thesummer and cold ambient air to enterduring the winter. Bad seals aroundthe door openings, between theHVAC case and the cowl and evenaround the fresh air intake may be re-sponsible. Now that you’ve got themin mind, get them in the crosshairs ofyour diagnostic sights.

36 August 2002

DIAGNOSING COOLING SYSTEM WOES

This water pump from a Dodge van with a 5.9L V8 running with a wholly-or-ganic-acid coolant suffered severe pockmarking of the chamber after a roadtest in the Colorado mountains. The result was cavitation erosion, followed bycorrosion. You can also see pockmarks near the edges of the impeller blades.This was a two-year test run over 100,000 miles.

This water pump is from another vehicle in the same Colorado test. It used ahybrid fluid (low-silicate antifreeze similar to Mercedes coolant) and obvious-ly fared much better. There’s no evidence of cavitation erosion or corrosion.

Visit www.motor.com todownload a free copy of this

article. Copies are also availableby sending $3 for each copy to:

Fulfillment Dept., MOTOR Magazine,5600 Crooks Rd., Troy, MI 48098.

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CavitationErosion, Corrosionof Pump Chamber