1978 Pontiac Bonneville


Old 07-15-03, 06:17 AM
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1978 Pontiac Bonneville

A couple of questions regarding referenced vehicle….
301 CID - Engine Code "Y"
Recently purchased this vehicle from my mom - VERY CLEAN - only 72,000 miles. My biggest headaches, I'm sure, are results of the vehicle not being driven much, if at all, over the past 5 years.
Item #1: I have looked high and low for a vacuum line routing schematic - The one adhered to the fan shroud is illegible. I have called Pontiac, several dealers and looked at AllData, which does not list the vehicle and the library to no avail. If anyone has one, or knows how to gain acees to one, I would sincerely appreciate the help!
Item #2: In a no torque situation, i.e., coasting down a hill, at slow speeds, I hear a clunking sound coming from the drivetrain. I'm thinking u-joints or something gone bad in the rear end. The noise is not present under "power" conditions. Seeking suggestions ideas for diagnosis and repair. - Thanks
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Old 07-15-03, 07:55 AM
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Thumbs up

This is a stellar old car. It is slow as molasses but it will run forever on spit and glue. That of course, coming from a guy who owns a 1979, 1980 and 1980 Pontiac (Trans Am) all Poncho powered .

1) Get a factory manual.

a) Dan Bower at [email protected]
b) Dan Klein at [email protected]
c) "Dave's Books" on Ebay.
d) Any seller on Ebay.

All three are fair sellers. An old Chilton or Motor book will also have good information, check your library. www.helminc.com sells new GM manuals, but the 1978 one you need is a reprint and doesn't have the fold out wiring diagrams---get a used one.

2) Sounds like U-joints.

Give the car a thorough inspection. Change all fluids and filters. Throw out all the belts and hoses, replace them all along with the heater hoses. Go through the braking system soup to nuts for safety. Throw out any questionable parts.

If maintained right, this car will run forever. One of GM's finest vehicles. Well built Indian.

My parents had both a 1978 and 1980 version of this car. None finer.

Good luck,
Old 07-15-03, 08:33 AM
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I don't know how you could call ANY car with a 301 engine "one of GM's finest cars!" LOL

The 301 was among GM's worst V-8s. Right up there with the 260 Olds, the 262 Chevy, and the mid 1970s-mid 1980s 305 Chevy.

I agree that these cars were otherwise pretty good, and they're cheap and easy to maintain.

Problems I remember:

1. Sagging doors due to worn hinge pins & bushings. These can be repaired cheaply & easily

2. Rusted rear bumpers due to poor quality steel & exposure to road salt.

3. I'm not sure if the Bonneville was offered with the THM200 transmisison, but many other B-cars were in this era & most had trouble at one time or another. The easy fix is to replace the THM 200 with an earlier model THM 350. Pretty much a direct bolt in & much more reliable/durable.

4. Miles upon miles of vacuum hoses, all necessary for proper function of the emission control system.

Other than those problems (all of which can be overcome relatively cheaply & easily) these cars are reliable tanks that last a good long time.

I *may* have a vacuum diagram in my attic/library. I'll hobble up there this afternoon & have a look. I'll post the diagram if I find it.
Old 07-15-03, 08:38 AM
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Thanks, Joe for your post and great advice. With the exception of the tranny fluid and filter, all fluids, filters, belts, hoses, brakes including fluid and master cylinder, have been replaced.
I replaced the fuel sender (the filter was partially clogged and collapsed during acceleration starving the engine from fuel) and cleaned the gas tank. I had the "dual jet" carb re-built thinking that the fuel issue was carb related. Otherwise, so far, so good.

This is the 2nd '78 Bonneville that I have owned. Got rid of the 1st one 6 years ago after squeezing 275,000+ miles out of her.

Is there any way to inspect and "test" the u-joints?
Old 07-15-03, 09:16 AM
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Shame on you good buddy

My family has owned 301s since they came out in 1977. I still own a few of them. The 1980 301W my uncle had in his 80 LeSabre was STILL going after he sold the car in 1986 for more than he paid for it . I STILL see the car every once and a while. LOL.

A Pontiac 301 is simply a 400 or a 455 with a thin wall casting. Problems come in with overheating of these, Pontiac engines like to be cared for nicely. I have never seized or ruined ANY GM engine I've owned since I started driving in 1988 .

I've also owned many 307 V8's from Oldsmobile, which is the 260s bigger brother. My current '84 Delta 88 has given me nary a problem.

Yes, I believe the THM200 was used in this combination and IS a junker. If you go easy and drive normally, and do regular maintenance, there's no problem. Then again, the 301 vin Y is sporting a whopping 150 max horsepower, so you won't be ruining it anytime soon. LOL.

Half the "myth" of these cars is maintenance. Even if you abuse it, it will run, albeit poorly. Treat it to regular TLC and it will run forever .

Compare this 1978 Bonneville to a Ford or Chrysler of the same era and you'll see that the GM is FAR simpler in every way to the other offerings at the time.

"Door droop" occurs mostly on 2 door models because the doors weigh a ton. As noted, any GM dealer will have the needed pins and bushings, and parts stores have them too in their "HELP" Motormite line.

Original poster: Osborn Reproductions makes the replacement label for under your hood. I have used his stuff on my Trans Ams, very very good! The label will set you back 3 bucks. I believe the URL is www.osbornereproductions.com. If not, look it up on www.google.com

U-joint check: Jack it up and grab the yoke and drive shaft. Feel any movement/slop? If so, wasted U-joints. Any parts store has them in stock, only a couple of bucks.

Since you've owned one of these B body cars, you know how good they are. .
Old 07-15-03, 09:18 AM
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Thanks, Knuckles. I'll check back later today.
Old 07-15-03, 09:26 AM
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And thanks to again Joe...

The 307 is indeed a good engine. Had one in my '85 Toronado Calante sp?)- Just a few months ago, after getting almost 300k out of it, the #1 piston let loose - probably a pin. It absolutely destroyed the spark plug. The body and interior is still decent. Anyone know someone who would be interested?
Old 07-15-03, 09:43 AM
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You could easily rebuild that engine or get a rebuilt GM Goodwrench engine for it. If the body and mechanicals are good on the vehicle, it's worth a swap.

I have probably half of another one of those cars. LOL (1979 to 1985 Toronado parts). Some on Ebay now. LOL

You could also Ebay the whole car for parts or for a project. Put it up cheap with no reserve and it's likely to get some bids.
Old 07-16-03, 04:41 AM
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Being a pontiac man, I agree the 301 is lackluster, it does not share much with the 350-455 engines. Many more differences other than just thin walls. If well cared for the 301 may last as long as any other GM v8 of the same vintage. I have not had much luck with them--that is why I consider them boat anchors. Good thing is the availability of 350-400 engines which will swap easily. Do not try to put one in front of a 200 tranny!
Old 07-16-03, 06:26 AM
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It does share some parts, albeit external stuff. Pontiac is Pontiac.

There is no "big block" Pontiac as with Chevy or even Oldsmobile. The Pontiac V8 is externally "identical" from the 265 vin S to the 455 vin W.

I hate when I see someone say "I have a Pontiac Big block". NO such thing. LOL

Main thing on a 301: maintain it and drive it. It will last. Speed with it or push it, it will grenade. It was constructed to save weight for the impending CAFE requirements in the 1970's and early 1980's.

Keep your Poncho all Chief .
Old 07-17-03, 07:18 AM
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LOL, I hate when people say I got a 350 big block! Externally they are all bigger than a small block chevy, and a 301 is significantly lighter than a 400--ask me how I know! I meant 350-400 pontiac motors--so yes keep your pontiac all pontiac!
Old 07-17-03, 07:21 AM
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Sure . I'd rather a 301 over another motor of the same displacement/GM make .

Must be the Pontiac in me .
Old 07-17-03, 10:22 AM
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Knuckles- Any luck finding that vacuum line schematic?

I appreciate evryone's post to my original topic. Lots of great comments, advise and insight!
Old 07-17-03, 11:31 AM
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I don't have the diagram you need. I have Mitchell manuals covering 1967-75...they're full of vacuum diagrams. I have Chilton Professional Manuals covering '76-81. They have only basic emission control info, no diagrams.
Old 07-17-03, 11:46 AM
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Original poster:

As I stated, Osborne Reproductions out of Georgia has a reproduction emission label for most Pontiacs. Speak to David Osborne there, he is the late owner's son. He should have what you need. Doesn't get better than that.

The service manual picture tends to be a rip off of this sticker in many cases .

Look up the URL on google.com
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