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My 1978 GMC motorhome

Mike_H

Off-Camber is scary
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Figured I'd start something over here, as there seems to be some interest. In short, I am the current caretaker of a classic piece of americana. The unmistakable GMC motorhome.

They were made from 1973 to 1978, with approximately 13000 built over the five year run. Of those numbers, it is estimated that 8500 or so are still on the road, doing their thing. That's pretty amazing, in my opinion, as American auto makers weren't really focused on quality in the late 70's.

Anyway, mine is technically a Transmode, which means the vehicle is GMC, assembled in Pontiac, Michigan. It was shipped bare to Indiana where it was upfitted to an RV by Jimmy, a division of coachman.

These pictures are from the day we picked her up.
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Mike_H

Mike_H

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Well, the day finally arrived for us to drive our new beauty home. My wife and I were both pretty excited. I was a little nervous too. I had noticed that the tires were pretty old (like 2008 old) and it was 2019 when we bought this thing. But, we figured we'd take it easy, it was only 75'ish miles and I'd just monitor them as we went along.

We got 6 miles down the road...and my first warning light came on! Water temp. It flickered on, then went out...that was odd, I thought. I kept driving and after a couple miles, the light was on solid. Gauge only read half, or maybe just over. But, being cautious, I kicked the heater on, opened the windows and nursed it another 4 miles to the next gas station. I needed fuel anyway. I filled the MASSIVE gas tank (this pig holds 50 gallons) and I started going over the engine bay. Right away, I noticed the overflow was empty. So, I pulled around to the side lot and we hung out waiting for the engine to cool off. After about a half hour, I popped the cap and noticed the radiator was low on coolant too. So, I bought a couple gallons of water and topped it off. We made it home fine after that, thank goodness.

Once home, I made an appointment with a local shop for some new tires and a pressure check of the cooling system. As I feared, it wouldn't hold pressure. So...here we go. I made a few calls and decided to have the radiator re-cored. As I started tearing the engine down, I kept finding shit that wasn't right.

Like a distributor that wouldn't advance

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And exhaust manifolds that had stripped bolts, eroded gasket surfaces, and cracks...

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I also found a ridiculously loose timing chain

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My compression numbers were a bit wonky too...so, I decided that I was gonna tear the top end of the engine down and do a little refresh. I had the heads sent out for a valve cleanup. I decked the exhaust mounting surfaces, bought new manifolds, replaced the whole cooling system, put a new, custom mapped distributor in, new double roller timing chain, the whole nine. I even had to buy a new intake manifold. This emissions era BOP (Buick, Olds, Pontiac) engine has an exhaust cross-over in the intake to help warm the fuel. Warmer fuel atomizes better or something. Well, in the motorhome application, they run HOT...so hot that they have a tendency to crack the intakes. The central crack between the secondary bores isn't a big deal...The crack to the outside certainly is.

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This took me a long time. Seems that every time I touched something, I needed to take it apart further and buy more parts. It got pretty expensive too. An edelbrock intake for a SBC is about 180 dollars. My Olds 403, which is nearly the same physical size, required an intake that was 450 bucks!

Here we are, all pretty'ed up and ready to purr.

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And, in case anyone wants to hear what 403 cubic inches of American V8 sounds like after twin flowmasters and about 20 feet of 3" exhaust...


She's been running like a top ever since.

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Mike_H

Mike_H

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Our first trip in the ol' girl was a 500 mile, one way trip to visit my in-laws. Yeah, I'm a little crazy! I had driven it around a little bit before that though. 10 miles here, 5 miles there. I packed a pretty good tool-kits, said my prayers the night before and we took off!

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65 mph was about all I wanted to drive. The steering is really vague and it was gusting like 40 mph that day. NOT FUN!

I had my co-driver to help though...


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One of the nice things about traveling in a motorhome is you don't have to stop for food at a fast food place. My wife made sandwiches while we were driving, and we stopped at a nice roadside park to enjoy a peaceful lunch

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And approaching the Mighty Mac (mackinac bridge). For those who don't know, the Mackinac Bridge is the longest suspension bridge in the western hemisphere. Its about 5 miles, approach to approach. Center Span is about 220 feet over the Lakes. It connects the lower peninsula to the upper peninsula. It can be a bit nerve-wracking to drive over it, especially in 40 mph gusts in a vehicle with zero on center feel. Speed limit is only 25 though, so that helps tremendously.

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Another stop in the UP. Beautiful roadside park

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In all, it took us about 11 hours to travel the 500 miles and we got about 9.5 MPG fighting the wind. I'm actually not upset with that. We stayed for a week and a half over labor day. September in the UP can be really nice, or it can be really cold and dreary. We got cold and dreary this year. That leads me to my first repair while on the road. Tried to use the furnace...that didn't work out (to be discussed in a later post). Its a good thing this Luxury Motorcoach came with an electric heat pump. Its the only heat we had.

We also woke up one morning to the smell of rotten eggs (or dirty socks). I suspected the LP tank. I turned it off, and sure enough, the smell went away after a while. I got on the phone with the local propane company. I love the UP. Everyone is friendly and willing to help out, anytime. Its like mayberry. Anyway, I hauled the tank out of the coach and got is leak checked. Thankfully, it was still sound, but the valve packing had deteriorated. They replaced the valve and filled the tank for about 65 bucks!

While it was out, I couldn't help myself. I had to buy some supplies and paint it.

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I also pulled the door and resealed the LP tank compartment.


Finally, with that project done and some nice weather on the way, we decided a trip to the lake was in order. We packed up and drove to Twin Lakes, MI where my wife has a lifelong friend. We stayed with them for a couple days and went to Lake of the Clouds in the Porcupine Mountains.

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Finally we headed home, exhausted but happy. Had one issue on the way with a loose ground causing a no start in the gas station. That one issue FINALLY made my wife realize that my tool addiction was worth it. I figured it out and got it tightened back up in about 10 minutes and we were on our way!
 
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Mike_H

Mike_H

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I mentioned in the last post that the heater didn't work well. It would turn on, but then turn off after a full cycle. So, its another thing to fix. when we got home from our trip, I pulled the heater out and started to dig into it.

I found the reason for the shutoff...mud-daubers :mad::mad::mad:

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this is the air intake side. This nest caused the fan to run out of balance and shake the whole coach.

I also found the burner completely plugged up with dirt. That was causing the improper combustion, which was tripping the shut-down.

I cleaned the grooves out with a thin blade.

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Since it was all apart, I ordered all new seals and gaskets, painted the housings and tested the safety switches. everything checks out, so back into the motorhome it goes.

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jgaz

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That is awesome!

I worked at a Chevy/Olds dealer from 1976-85. We did the power train warranty on those units because they used a Olds Toronado drive trains.

I loved driving them! They drove and handled so nice compared to many motor homes of that era.
 
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Rick Flair

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That was the odd ball front wheel drive right? I knew a guy about 20 years back that bought one to restore. He loved the thing.
 
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Mike_H

Mike_H

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That was the odd ball front wheel drive right? I knew a guy about 20 years back that bought one to restore. He loved the thing.
yup. Front wheel drive. Used the drivetrain cribbed from the Olds Toronado and Caddy Eldorado. There are a few guys who have swapped in the Caddy 500 from the eldo too...I guess its not a terrible job to do. Mine only has the 403...but so far, I haven't really needed more power. It would be nice, but as long as you remember your driving a damn bus, you get used to it.
 

Rick Flair

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yup. Front wheel drive. Used the drivetrain cribbed from the Olds Toronado and Caddy Eldorado. There are a few guys who have swapped in the Caddy 500 from the eldo too...I guess its not a terrible job to do. Mine only has the 403...but so far, I haven't really needed more power. It would be nice, but as long as you remember your driving a damn bus, you get used to it.


Like most things big and heavy, it’s how fast you stop and not get up that mountain!



I dig it man, looking forward to this. 👍
 
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AMS417

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Always thought these were cool since the movie Stripes. Don't they have some weird beam style rear air suspension? My father in law use to work at a garage and the owner had a minty restored green GMC RV. I loved that thing, its was like new, and 100% period correct down to the cushions.

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Mike_H

Mike_H

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Always thought these were cool since the movie Stripes. Don't they have some weird beam style rear air suspension? My father in law use to work at a garage and the owner had a minty restored green GMC RV. I loved that thing, its was like new, and 100% period correct down to the cushions.

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It is a weird rear suspension. It has a longitudinally mounted air bag and the wheels pivot independently into the bag. No transvere beam which is how they got the ride height so low.

This isn't the greatest pic of the rear suspension, but it shows the whole Chassis layout. The two tanks in the middle are fuel. My Gray and Black water tanks are behind the fuel tanks.

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In addition to the interior mods, I plan on rebuilding my rear suspension this winter. Wheel bearings, parking brake (currently non-functional) check alignment, and Install a new "quad-bag" suspension. I already have the disc conversion.

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Squatch

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I always thought these were rather elegant, @Mike_H, and yours is a beaut! Very cool, indeed.

So, are you at all familiar with the Clark Cortez? Also a front wheel drive, but all steel panels (rust!) and initially with a Chrysler slant 6 and a manual transmission. I think they eventually ended up with the Oldsmobile 455 and an automatic in them. Someone told me there were a few years that had a small block Ford for power, but I don't recall what he said it had for a transmission. Anyways, nowhere near as elegant as your ride. Hope you're enjoying it!

Clark Cortez
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Mike_H

Mike_H

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I always thought these were rather elegant, @Mike_H, and yours is a beaut! Very cool, indeed.

So, are you at all familiar with the Clark Cortez? Also a front wheel drive, but all steel panels (rust!) and initially with a Chrysler slant 6 and a manual transmission. I think they eventually ended up with the Oldsmobile 455 and an automatic in them. Someone told me there were a few years that had a small block Ford for power, but I don't recall what he said it had for a transmission. Anyways, nowhere near as elegant as your ride. Hope you're enjoying it!

Clark Cortez
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Not super familiar with it, other than knowing it existed. The other "cousin" to the GMC is a Revcon. They made them for a while, sharing the same drivetrain layout, but when GM discontinued the 455/Hydromatic 425, they switched in a Chevy 454 and "regular" transmission and utilized some sort of transfer case to get power back to the front wheels.

 

SkylinesSuck

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I thought those all came with Olds 455's, not 403's. Interesting. Well, I know somebody giving away a couple of 455's if you ever feel the urge to rebuild a big block and have it sitting on a stand ready to swap in just in case ☺️
 
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Mike_H

Mike_H

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I thought those all came with Olds 455's, not 403's. Interesting. Well, I know somebody giving away a couple of 455's if you ever feel the urge to rebuild a big block and have it sitting on a stand ready to swap in just in case ☺️
Mid 77 they switched, due to emissions pressure. I'd be interested in a "free" 455!