Not a Chevelle but a 84 Chevy Blazer, son in laws, built a mild 355 for it last summer, flat tops, 76 cc heads, mild RV cam, stock except for cam. He had his step dad install it last fall, it has been overheating since installed, Timing was slow so set it at 12 initial, changed stat, was a cheapo Murray, put in a super stat. still overheated, flushed everything out, radiator is clean, fan clutch was a little loose, put on a regular 6 blade, air moving good thru radiator. rebuilt water pump was installed when motor done and I ran it on my test stand to break it in for over 1 hour with no issues. I looked at it Sat. for a bit and idled it for over an hour, ran at about 200 according to his gauge soon as you put it in gear and put a load on it she shoots right up. the heater hoses seem to surge after the stat opens and sounds like there is air in the system, this one has me a little stumped, I normally install the motors I build my self but I had shoulder surgery in Aug. and that slowed me up till last month, still not able to extend to well, Any thoughts would be appreciated.

Sold Chevelle but new project in the works

How accurate is the temperature gauge?

That is my idea also, Im going to put a mech gauge next to get a good idea, it does get to the boiling point tho.

idled it for over an hour, ran at about 200 according to his gauge soon as you put it in gear and put a load on it she shoots right up.
Assuming the gauge is accurate, I'd say that thing has SERIOUS problems.

The cooling system can't pull enough BTUs out of the coolant at IDLE, never mind under load.

1. Corroded radiator air fins? Doesn't matter if the coolant tubes are clean if the air fins have lost contact with them. Happens a lot in areas that use salt on the roads in winter.

2. Reverse-rotation water pump? Or normal-rotation pump used with a serpentine belt system?

Other stuff that would be worthwhile checking:

Vacuum advance toast? What's the timing curve?
Lean fuel/air mix?

Put the fan shroud back on it.
Since you didn't list it ,I'm guessing it's not there.

I've seen some people put the intake gasket on backwards. They put the closed end in front. I also agree with checking the water pump rotation. I had a really dumb guy put freeze plugs right in the water pump holes one time. HEE HEEE!!! Still gives me a chuckle!!

Wrong rotation water pump is a really good guess.

Jerry,
How hot does it actually get ? My 85 Blazer used to run right at 210 or so from the day it was new. At about 140K I replaced the 305 with a 355 similar to yours and never had an issue with it running any hotter than that. I took everything off the 305 and bolted it on the 350.

84 should be carbureted so as long as you got the correct gasket set, you really cannot put the intake gaskets on backwards ... that only applies to the TBI gaskets.

I would also be suspect of the wrong water pump like Shurkey said. They look identical externally, but the pump for the serpentine belt has a different impeller because it is designed to turn the opposite direction. I have seen them boxed wrong a couple of times. Only way to tell is to pull the back cover off and look.

Sorry for not replying had to run to Wyo. this AM, Shroud is in place, fan sits centered in shroud, gaskets are on right done a lot of SBs, Im also leaning towards the water pump, I was thinking when I ran on the test stand I used an electric pump to break it in, the pump on it now is a Oreilly rebuild and Im getting a couple of reports on some of their pumps being bad, the engine will get to 260 when we shut it down and get it cooled off. vaccum advance checked OK and the card is just a touch rich if anything, We are going to pull the water pump this weekend and put a New unit on. Thanks for all the replys and suggestions, I will report on what we find. Thanks Again guys

I've seen some people put the intake gasket on backwards. They put the closed end in front. I also agree with checking the water pump rotation. I had a really dumb guy put freeze plugs right in the water pump holes one time. HEE HEEE!!! Still gives me a chuckle!!

got a chuckle out of that myself. :)

are you sure the block's good?

Update, replaced water pump with a new one and it cools like a champ, took the new (rebuilt) apart and the impeller from wear and being blasted so much was worn down to the point that it would cavitate. no more rebuilts, you get what you pay for, it never used to be that way, quality has hit the pot. Thanks for all the replys Guys.

Thanks for posting the fix, might have saved someone else a lot of work and hair pulling.

Jerry,you said base timing was set too low and then set it to 12 deg base.

Even though you found thre major cooling issue the timing is still not right with the cam your running which can also lead to slightly higher emgine temps & poor fuel mileage/throttle responce/lost power etc.

The 12 deg base timing is also too low/retarded with your mild aftermaqrket RV cam,the 12 deg base timing is still retarded enough to cause higher engine temps esp at idle in traffic & or when at lower engine speeds.

That mild aftermarket rv cam needs at least 16-18 deg base timing so the 12 deg base its currently at is approx 4-6 deg retarded & being up to 6 deg retarded can inc motor temps with most setups esp when you load the motor .

Try this,you need to setup approx 20 deg mech in the dist all in by approx 2800-3k rpms so then 16-18 deg base timing + 20 deg mech in dist = 36-38 deg total which is a decent place to be & start for dialing in a street perf ign timing curve.

You should also try runing the vac adv from full intake vacuum all the time becasue in general most street motors with mild cams work well with full int vacuum esp at idle & lower engine speeds .

Advancing the base timing to where it belongs along with vac adv hooked to fullint vac will result in cooler engine temps/better throttle responce/better fuel mileage/more power esp in lower to mid range rpms before the total timing is fully in .

I have seen mult times over the yrs when all else is decnt maning water pump/t-stat/fan pulling thru rad/etc and motor run hotter that when retarded base timing like 12 deg is advanced to 18 degt base when running even a mild rv cam the temps go down a few deg.

Use 93 fuel for testing this ign timing curve out to ensure no detonation/ping,if no pig then try next lower octane fuel to see in the motor will tollerate it.

Also,if the fuel calibration way lean that can also add to in creased temp issues.

Scott

Thanks Scott for the info. Ill be getting deeper into it when the weather breaks, I found a couple other items that the installer did that is not correct so there are a few adjustments it needs. Thanks Again Jerry

NP Jerry,if you could get by all my dang typos/LOL!!!!!!

Man,i cant believe i typed PIG for when i stated to watch out for ping.(LOL!!!!)

Scott