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1983 G30 motorhome overheating issues

Old August 23rd, 2012, 11:13 AM
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Default 1983 G30 motorhome overheating issues

I bought a 1983 motor home after Hurricane Ike washed my beach cabin out into the Gulf, planning on parking it there and letting it rust until the new cabin was finished. Unfortunately, my wife fell in love with the thing, and now wants to galavant all over the country with it. No problem, right? It has low miles, about 45k, and is in good shape. I have no idea what mods, if any, the previous owner installed.

The problem is, it overheats when you slow down from driving. When sitting and idling, it's fine. When driving down the road, it's fine. When you have to stop, though, the temp gauge shoots right up into the red, quickly, and it boils water out of the overflow. I've flushed the block, changed the water pump, radiator, thermostat, and all the hoses - it makes no difference. One hard run, and she's hot.

When I checked the timing, with the vac line removed from the distributor and plugged, it was set at about 16 degrees BTDC. I set it back to the 4 degree setting on the emissions sticker, reconnected the advance vac line, and the engine had no power, and wouldn't rev well even with no load. For now, I've set the timing back to about 8-9 degrees BTDC (vac line plugged), and it runs well enough with no pinging, but I'm confused.

I found the thread on vacuum lines, so I've done all that. I don't know when that valve on the passenger side exhaust manifold should open, but it doesn't seem to. It moves freely, but never moves on its own.

Any help or advice you guys could give would be greatly appreciated....
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Old August 23rd, 2012, 1:08 PM
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Given that those things sit for long periods of time without being run, it's possible that there are contaminates settled into the block and possibly the radiator.

I'd start off with a another REAL good clean and flush, take the hoses off of it, and reverse flush everything (block/radiator/heaters) after the cleaning.

Now (usually) the reason for a spike in the temp when stopping, is because the thermostat is slow to respond to the increasing temp. You might try finding another brand of Thermostat, possibly one that "advertises" increased flow when open. There also may be some information on the flow capacities of some thermostats, and you could select one which is more adapt. You should also check the depth of the gasket as compared to the depth of the cap neck of the radiator. It's remotely possible that the neck is a little deep, and not letting enough pressure set on the gasket.

Understand, that all vehicles spike temp, and it's not noticeable if everything is working ok. It's also possible that your Radiator Cap is not holding the necessary 15-16 lbs even though it's new. You could have it checked, just to make sure.

One other thing you might do is adjust the % of water/coolant mix.
This is not a good idea if you're running in high ambient temperatures, but, when you increase the % of Coolant to Water, you increase the overall boiling point. Normal is a 50%w/50%c, so you might try 35%w/65%c. This by the way drops the freeze protection down to about a -60 and is popularly used in Alaska and other very cold and high altitude areas.

Beyond the above, you might search out a multi-tube (thicker) radiator, but, concentrate on the "flow" issue, with enough, the boil won't happen.

Last edited by SWHouston; August 23rd, 2012 at 1:45 PM.
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Old August 24th, 2012, 4:45 PM
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Thanks....the block was flushed (and backflushed) several times before we got around to swapping the water pump and radiator. Two new radiator caps have been tried, along with three different thermostats. I didn't get into replacing more expensive parts quickly.....but now I have. I am concerned that just putting a stock radiator in it might not be sufficient - the things I've done have made the problem better, but it's still not right. Fortunately now, the temps have started to drop, and we're no longer dealing with 100 degree weather (I'm in central Texas). We also found the radiator shroud wasn't seated properly, and may have been allowing airflow around the sides of the shroud. It's all back together again, and we're going to drive it some this weekend to see what it does.

I was really wondering if anyone had seen timing issues like this one appears to have. It sure doesn't run right on the factory setup, but I don't know if the previous owner recurved the distributor or not, or even has it in the right position - is it possible to get it one tooth off on a 350?
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Old August 24th, 2012, 6:54 PM
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Obviously, checking the cooling system first, must be done. And, making sure you have an appropriate thermostat would be crucial. Don't buy into any personal recommendations or the marketing for 165-180 degree thermostats. These engines run HOT, and need a 195-205 degree thermostat. Otherwise, the thermostat remains open all the time, and defeats its entire purpose.

But... It sounds like it could definitely be a timing issue, to me.

I know that might sound a little far-fetched, but the smog-era distributors on these engines have as much as 24 degrees mechanical advance and up to 34 degrees of vacuum advance. That's the reason for the (low) 4 deg BTDC static setting (some 350s from the 80s went so far as 2 degrees ATDC!).

The whole idea is to delay firing as long as possible, under certain conditions, in order to get the exhaust gas temperatures as high as possible for post-combustion with the air injection system. But, to compensate for the delayed ignition, and get some power back when its needed, they added massive amounts of mechanical and vacuum advance.

My engine, for example, was completely rebuilt recently. With a normally-appropriate static timing setting of 16 degrees BTDC for start-up, I was topping out at 71 degrees advanced () when both advance methods reached their stops (22 mechanical, 33 vacuum). The pre-rebuild version of the engine was running the specified 4 degrees BTDC, but that still allowed 59 degrees advanced timing. That, of course, results in an engine that runs hotter than hell when vacuum kicks in. (Such as idling, decelerating, or going down long hills with little to no throttle.)

As parts wear and the mechanical advance springs get weak, the advance can come in much more quickly that it should. So, rather than having the intended 16-38 degrees of advance at idle, you may actually have significantly more because of the (weak/worn) mechanical advance coming in, too early.

My data shows that your distributor should be somewhere between 14-18 degrees mechanical and 12-14 degrees vacuum. But... there's no guarantee that it's the original distributor. If it came from a smog-era small block or was rebuilt, there's no telling what it might be. (From 1978-1986, there were 62 different variants of HEI distributors for small blocks. Add big block versions, and the number nearly doubles.)

Another problem comes from the fact that these engines were set up so the vacuum advance runs on ported vacuum. Again, it was for emissions reasons, but it means performance really sucks if you don't make other adjustments.
One thing you can try, is taking the vac advance can off of ported vacuum and hooking it up to manifold vacuum.

You're probably hooked up to a thermal switch/port on the front of the intake manifold right now. I would disconnect the hose at the distributor and plug it. Then, take the vacuum line that normally runs to the air cleaner (from manifold vac at the back of the intake) and hook it up to the vac advance. That will bring in the vacuum advance at much more appropriate times, and allow you to take some of the static timing back out without losing power (possibly back to the specified 4 deg BTDC). Taking vacuum from the air cleaner won't matter. It's a worthless system that never worked right from the factory.

If that doesn't help and you're not restricted by emissions requirements, ditch that distributor and go to one of the standard HEI replacements (even the cheap Summit HEI distributor is fine). You'll have to advance it farther than the smog sticker calls for, but it won't allow as much mechanical or vacuum advance, either. So, it shouldn't run as hot.

It is quite possible to install the distributor in a small block off by a tooth, or more. Right now, the entire distributor in my '86 G30 motorhome is actually clocked one tooth advanced (25.7 degrees), because I ran out of adjustment with a typical installation (vac advance hitting the tranny check/fill tube - I could only get 8 deg advanced).

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Old August 24th, 2012, 7:26 PM
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Sorry about the long post.
I figured this was one situation where explaining some of GM's stupid design theory might help shed more light on the subject, rather than just suggesting some possible causes.

And, a long-winded caveat:
I'm not a professional mechanic. I learned this stuff, because I had to. My G30 motorhome lives at 4,400 ft asl, and only goes up from there. Its normal operational altitude is 6,600 ft (over a 7,500 ft pass) to 11,000 ft. And... I live in a county that has emissions requirements 4 times more strict that at the time the vehicle was built (they cut allowances for '79-'93 vehicles by 50% in 1991, and another 50% in 2003). And, we're required to have the equipment installed and fully operational at all times (subject to inspection during any traffic stop, as well).

The engine ran plenty clean (even before the rebuild), but I had to find a way to keep the emissions crap from robbing power, forcing the engine to run hot, and unnecessarily leaning the air/fuel mixture without anything appearing to be tampered with. So, I dug into diagrams, schematics, design theories, operational theories, and all kinds of crap I never really wanted to see.

I learned way too much about GM's half-baked (or over-cooked?) approach to emissions requirements, smog-era distributors, and vacuum systems....
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Old August 25th, 2012, 2:37 PM
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Just sitting is really bad for an engine.
Rubber does deteriorate, tires belts, hoses.
Fluids get old.

Sounds like your Rad. fan might be bad, and not enguaging when it should.
I would certainly changes belts, and all hoses don't forget heater hoses as they get old too.
Check the age of the tires to see how old they are.
4 to 5 years, rellace them.

I went through all this before with a M.H. thinking the tires were good because of the looks.
Only the boorn on date can tell you how old those tires are.
Your speedo will read wrong while you speed down the Hwy.
Then BANG, there goes a lot of things like gas lines are torn out.
I now look ay the born on date.

Change all fluids in the engine compartment, and flush the Rad.
You should be ok for a trip.
Hope this helps from what we went through.
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