Overheats when towing after engine swap

[dwmitchell61]

1973 Chevrolet C20 Crew Cab heavy 3/4 ton. Replaced engine & transmission with engine & transmission from 1988 GMC TBI 5.0L and TH400 transmission that shifts good.

Runs great, but...

It overheats when towing a two horse trailer empty going up inclines. Temp gage reads about halfway when it overheats, may not be accurate as 1973 temp gage is connected to 1988 sending unit that was tied in to an electronic (digital) instrument panel.

I checked the OBD-I codes and it only has code 12 stored. Yes, the BATT+ on the TBI system is connected to the truck BATT +, so it should be storing the codes if there wee any.

So I am assuming it is a cooling system issue.

It has a clutched 5 blade fan with single core stock radiator from '70's era. Original radiator in truck developed leaks and was dual core. Original fan was a non-clutched 6 blade flex fan. The radiator cap is 15lb cap. Reutilized water pump from the 1973 5.7L.

New thermostat for 185 degrees was installed with engine swap.
Upper radiator hose replaced with a Gates universal.
New fan belts installed.

Any other suggestions?

1. New radiator cap.
2. Try the flex fan.
3. New fan clutch if flex fan does the trick?
4. New radiator.

[1985-GMC]

Put the correct sending unit in it for the year of the gauge and put the direct drive fan back on it.

I always have problems with 454s overheating in the Texas 100+ heat so I always put a solid fan on them and never have another issue. I can pull whatever I want with the AC on or let it idle and I will never overheat no matter how hot it is, I couldn't do that before even with a brand new fan clutch.

[Dead Parrot]

Install an aftermarket temp gauge and sender and I suspect you will find you don't have an overheat problem. If BB heads are symmetrical like SB heads, install the sender in the opposite side head hole from where the sender normally goes. If the engine temps show normal, you can then find a matched set of OEM gauge and sender which isn't always easy to do.

I did this when chasing what I thought was a series of bad thermostats. Turned out to be a sender that would sometimes decide to read full hot.

[dwmitchell61]

Definitely overheating.... Full boil.... Percolating into the overflow tank.

I am putting the flex fan back on. The clutch had no resistance when I spun it immediately after shutoff.

If that doesn't work I may just get a new radiator from Rock Auto. Someone said having an old radiator re-cored costs a fortune.

[dwmitchell61]

Originally Posted by dwmitchell61

1973 Chevrolet C20 Crew Cab heavy 3/4 ton. Replaced engine & transmission with engine & transmission from 1988 GMC TBI 5.0L and TH400 transmission that shifts good.

Runs great, but...

It overheats when towing a two horse trailer empty going up inclines. Temp gage reads about halfway when it overheats, may not be accurate as 1973 temp gage is connected to 1988 sending unit that was tied in to an electronic (digital) instrument panel.

I checked the OBD-I codes and it only has code 12 stored. Yes, the BATT+ on the TBI system is connected to the truck BATT +, so it should be storing the codes if there wee any.

So I am assuming it is a cooling system issue.

It has a clutched 5 blade fan with 2 row core radiator from '70's era. Original radiator in truck developed leaks and was 3 row core. Original fan was a non-clutched 6 blade flex fan. The radiator cap is 15lb cap. Reutilized water pump from the 1973 5.7L.

New thermostat for 185 degrees was installed with engine swap.
Upper radiator hose replaced with a Gates universal.
New fan belts installed.

Any other suggestions?

1. New radiator cap.
2. Try the flex fan.
3. New fan clutch if flex fan does the trick?
4. New radiator.

Thermostat is definitely opening. Temperature of the upper radiator hose rises dramatically when it does.

[patdaly]

Originally Posted by dwmitchell61

1973 Chevrolet C20 Crew Cab heavy 3/4 ton. Replaced engine & transmission with engine & transmission from 1988 GMC TBI 5.0L and TH400 transmission that shifts good.

Runs great, but...

It overheats when towing a two horse trailer empty going up inclines. Temp gage reads about halfway when it overheats, may not be accurate as 1973 temp gage is connected to 1988 sending unit that was tied in to an electronic (digital) instrument panel.

I checked the OBD-I codes and it only has code 12 stored. Yes, the BATT+ on the TBI system is connected to the truck BATT +, so it should be storing the codes if there wee any.

So I am assuming it is a cooling system issue.

It has a clutched 5 blade fan with single core stock radiator from '70's era. Original radiator in truck developed leaks and was dual core. Original fan was a non-clutched 6 blade flex fan. The radiator cap is 15lb cap. Reutilized water pump from the 1973 5.7L.

New thermostat for 185 degrees was installed with engine swap.
Upper radiator hose replaced with a Gates universal.
New fan belts installed.

Any other suggestions?

1. New radiator cap.
2. Try the flex fan.
3. New fan clutch if flex fan does the trick?
4. New radiator.

Dumb question, did you keep the belt system from the 73 and the water pump from the 88?

If so, your water pump is turning backwards.

[rich weyand]

The correct temperature sensor for your 1973 temp gauge is GM 1513321. These have been obsoleted for many years. *All* the cross-reference parts are not the same resistance curves and will give bad readings. BUT...

The Corvette restoration boys got really annoyed with that situation and someone finally built a reproduction unit that is exactly correct, down to the connector style, and they also made the correct connector. This is what I put on my '78 a couple years back, and the gauge finally reads correctly.

http://www.americanautowire.com/shop...ding-unit-5028
http://www.americanautowire.com/shop...ure-sender-con

[rich weyand]

Originally Posted by patdaly

Dumb question, did you keep the belt system from the 73 and the water pump from the 88?

If so, your water pump is turning backwards.

Ooo. Good one.

[andrewmp6]

Yeah a water pump turning the wrong way is bad,Or you got a big air bubble trapped in the engine.

[dwmitchell61]

No.

We used everything from the front of the 350...

I am assuming the crankshaft rotation did not change in the GM small block from 1973 to 1988!

The '88 had serpentine belt, the '73 components use v-belt, so no... wouldn't have worked that way.

We used the front items (alternator, water pump, power steering pump and mounting hardware) from the 1973 350 because the 1988 305 was missing the alternator and some mounting hardware.

Had me worried for a moment...

I will try the flex fan without clutch tomorrow.

What about an intake leak? I replaced the throttle body to manifold gasket and air cleaner to throttle body gaskets during the install. So it would have to be the intake manifold to head gaskets if I have a vacuum leak. I would think it wouldn't run as nice as it does if it had a leak, though. I may try spraying some starting fluid around and see if the RPM changes to check for a leak, that sound right?

[1985-GMC]

I bought one of these all aluminum champion radiators a while back, really nice looking and you can't beat the price. http://pages.ebay.com/motors/link/?n...185895&alt=web

The local radiator shop wanted 400 bucks to re-core the stock copper/ brass radiator so I thought that was ridiculous.

[bilfman]

Could be timing. Tbi are set at zero degrees with the brown wire disconnected.

[dwmitchell61]

Thank you Tony!

When funding allows, I will buy a Champion MC716 for it, 4 row. Original was a 3 row brass radiator that started leaking. The two row may be a bit anemic and may have some clogging from scale. This truck is dedicated for towing her horse trailer with horses. Weather is cool now but summers get hot in the desert here in Eastern Washington.

Thanks Bilfman!

I will check the timing tomorrow. Yes, I recall the connector for the timing set when I was getting the TBI harness ready for the engine swap. I had to extend the harness to put the ECM under the front seat. Older dashboards just weren't as deep as the newer ones. I will go to the local parts store and get a loaner timing light tomorrow.

[geezer#99]

Check the bottom rad hose.
Could be collapsing.

[dwmitchell61]

Thanks Geezer, I will get a new hose for it, also. It is my girlfriend's truck... I have so many hours of labor in it now... She is on a tight budget with the project (she has horses... laughing).

[dwmitchell61]

Originally Posted by andrewmp6

Yeah a water pump turning the wrong way is bad,Or you got a big air bubble trapped in the engine.

I may just remove the thermostat housing, drill a 1/8" hole in the thermostat flange to ensure this is not the case.

Thanks Andrew!

[geezer#99]

Originally Posted by dwmitchell61

Thanks Geezer, I will get a new hose for it, also. It is my girlfriend's truck... I have so many hours of labor in it now... She is on a tight budget with the project (she has horses... laughing).

A new hose might collapse too.
Put a coil in the one on it.

[1985-GMC]

No problem! Even one of the 3 cores should do you just fine with a small block.

[dwmitchell61]

Originally Posted by bilfman

Could be timing. Tbi are set at zero degrees with the brown wire disconnected.

I disconnected the timing jumper and put the timing light on it.

It was 2 degrees advanced. So I loosened the distributor and set it to 0 degrees. Then I tightened the distributor and verified it was at 0 degrees one more time before reconnecting the timing jumper.

I will call the radiator shop tomorrow and see what they charge to "boil" a radiator out. I will take both the three row and the two row radiators in to be cleaned and see if the three row can be soldered where it is leaking.

The block has a "Remanufactured" tag on it just behind the power steering pump. I noticed that when I was under the truck marking the timing mark with whiteout.