[CruisinGA]

I'm helping a buddy with his recently acquired '88 Chevy 350 TBI engine (K5 Blazer). The engine had some work done, and then wasn't started for almost a year. We've gotten it running smoothing and starting well, however, it begins to overheat as soon as it warms up. Even with as many F/2F/1FZ engines I've worked on, I'm domestic dumb.

The rad is new, the W/P is supposedly so, and we just changed the tstat.

Apparently this is a wierd year that has some kind of "reverse flow" waterpump that looks and bolts on the same as earlier models, but flows in the opposite direction. It is the year with one ribbed belt and two v-belts, if that means anything.

We are trying to determine if the waterpump on it could be incorrect. As the engine warms up, the lower radiator hose will warm up and that corner of the radiator (crossflow type). None of the rest of the radiator will get hot, even as the engine begins to overheat. This leads me to believe the waterpump is the wrong model and is not effectively moving the coolant like it should.

Just to check my thoughts- coolant should be "pulled" from the radiator at the bottom hose and enter the radiator through the top hose, correct? So wouldn't this mean that the top hose and radiator surface closest to it would be the warmest, right?

Anyone familiar with this oddball or have any suggestions?

[Gumby]

The water pump is not a reverse flow. A reverse flow engine, like the 1FZE, has the thermostat on the lower hose. This allows the coldest water to cool the head and then flow down over the block, heating the block for a more consistent heat throughout the engine.

The water pump on the serp belted TBI engine turns the other way because the pump turns off of the back side of the serp belt. It's reverse rotation, not reverse flow. If you did not have to modify the serp belt pulley to get it on the pump, then it is the right pump. They have different bolt patterns. If the pulley turns the same way as the main pulley, it's a regular pump. if it turns the opposite way it's a reverse rotation pump. It'll take a different fan as well.

On any engine, the coolant on the other side of the engine from the thermostat will get warm. On the 350, that would be the lower hose and that corner of the radiator. On a 1FZE the top of the radiator and upper hose will get warm with the engine. The other hose will not warm up until the thermostat opens.


Let the engine run without the cap on until the thermostat opens. When it does, let it burp the air out and refill until no more air comes out. Then put the cap on and see if it still overheats.

[CruisinGA]

You mentioned this before, but I want to check-

When we pull the thermostat and run the truck, the top hose is significantly warmer than the bottom, meaning that coolant flow is engine>thermostat>top hose>radiator>bottom hose.

Is this correct or incorrect?

More background- turns out this is not a serpentine setup, never was. It just has a ribbed belt on the alt and W/P, but everything turns the same direction as the crank. The only reason this would be a reverse rotation pump is if the PO messed up. We compared the waterpump for an 88 non serp and 90 serp 350 at the parts counter, and they are externally identical, but the 90 model w/p is reverse rotation.

[Gumby]

Your water flow conception is correct.


The bolt pattern on the pulley/fan is very slightly different. If you take the pulley you need to line up and match it up there should be a minor difference. It won't fit over the studs on the wrong one. this has been my experience and I have been unable to confirm or deny this as an absolute truth.

Putting a caliper on it show a difference of about 1/8

[grant5127]

The only way to know for sure on the water pump is to remove it and examine the impeller....its not uncommon to get a boxed wrong water pump on these applications.....

[CruisinGA]

Gumby- what I meant was, is that flow direction correct?

From what I've gathered from the coloradok5 board is that we in fact have the right pump. We've also tested both thermostats in a pot of water on the stove, and they opened between 195-200.

You may be right Gumby, we never put the pumps side by side.

[Gumby]

Yes, the flow of water should be from the engine, out the top hose, to the radiator and sucked back in the bottom hose.

[atl zj]

We are simply speechless that Bailey cannot figure this one out, being the F/2F/1FZ expert that he is. Note that he is not yet a 3FE master. And what is all this business about coolant flow? Everybody knows coolant's just a power steering additive.

But it's got to be a waterpump or t-stat.

[CruisinGA]

You know no one can master the 3FE or the bottle as well as Dearborn...

Update: We reinstalled a confirmed good thermostat and the engine is overheating again, according to the stock guage, however, the top hose never even gets warm.

We are beginning the think that the guage may be bad, since the engine never gets hot enough for the radiator cap to release any pressure.

I think the next step is to get an engine temp reading with something other than the stock guage to see if it really is getting to hot.

Any other suggestions/ideas?

[WristPin]

if there is dexcool in there, of if it was in there, the system needs to be flushed with high strength radiator flush. you can also remove the radiator and dilute some muratic acid in there. dont know if it will eat the hoses. so thats why i suggest you take it out like that

the dexcool is the worst coolant ever made.
it gums up and forms brown sludge that is like rubber. it will clog even large passageways.

it was desinged as a 5 year coolant, and therefore far superior to green coolant, but it turned out to be as disaterous as the Hindenburg

[Gumby]

And yet GM still uses it.

After millions in warranty repairs and thousands of dissatisfied customers.

Go figure.




I think you're on the money with the gauge being bad. Test your sending unit with an ohmmeter


Coolant sensor resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.

[CruisinGA]

It has green coolant now. We were planning on a coolant flush eventually anyway. I agree than the Dexcool sucks, but if it never warms up w/o a thermostat installed, then a clogged passageway couldn't be the culprit, right?

Gumby- My buddy's out of town until the 26th. We'll test the sending unit in a pot of water with another thermometer then.

Another question- We ran the truck for about 20 minutes after installing a known good thermostat, and while the guage was reading very hot, the top hose remained almost cold. Seems like regardless of the guage being correct, after running for ~20 minutes the engine should be warm enough to open the thermostat and let some coolant flow through the top hose?

[PGH854runner]

my friends jeep cherokee does the same thingthe gauge shoots up and says it is overheating(or damn near it)when the engine was checked with a WIR thermo and was running cooler than another friends cherokee that was reading 195 on the gauge...........damn domestics..............We're not in Japan anymore Toto....lol