What doomed GM's 400 cid small bloc V/8 engine in '78?

[mustangsallysdad]

Warped cylinder heads and blown head gaskets...if these engines were tasked a little too much (towing, hauling or in a larger, heavy vehicle) a modest rise in coolant temperature was all it took to set the wheels in motion for a warped head and a blown out gasket.

 

[machmeter62]

How many of these were on tractors?

Wedge

I towed several tractors many miles the last 20 years with one.

 

[Car Doc]

I worked in Chevy dealerships back then and the lightweight cyl heads that they came out with about 1976ish trying to save weight for better cafe ratings caused a lot of early head failures in the entire SB engine line and they got horrible mpg and were generally harder to cool.

I think the MPG and power is what really did them in considering any old 454 could pull a barn down the highway w/o breaking a sweat and get about the same bad gas mileage they sold a lot more of them.


Steve

 

[machmeter62]

I really appreciate all the information and thoughts. I plan on towing a 6000# tractor about 600+ miles one way distance this summer, and I am concerned about the possibility of a major break-down somewhere along the route.

I put a tachometer on the vehicle, after the new purchase 32 yrs ago. The engine idles very smooth, but increasing the RPM in a static position,... off idle, and slowly increasing the RPM it starts to vibrate between 1200 and 1500 RPM....before the vibration ends....this is viewed with the engine cover removed. The engine oil shows no mayonnaise coloring from anti-freeze. I also checked the wiring harness with a probe check of each cylinder with a visual indicator of juice getting to the plugs. I haven't done a compression test yet, but that will be next.

The vehicle only has 16,600 total miles since new!

Another thought, with those tightly cowled engine covers, would that constant heat there cause a breakdown on the wiring harness, when at operating temp, that would create this vibration? Maybe it needs a performance grade wiring harness?

 

[Car Doc]

vibrate between 1200 and 1500 RPM....before the vibration ends....

It sounds like the EGR is all which would be normal as its supposed to operate in that range of throttle (just off idle) to keep engine from pinging under accel you can pull and plug the vac hose going to it to verify this.

The EGR is a silvery/goldish colored round device about 3" in dia at the right side of the carb twords the rear mounted on the manifold NOT the carb with one vac hose thats what you pull and plug for test purposes, if thats it causing the miss you can try running it w/o it and if it does not ping you can run it w/o the EGR plugged in and will notice a better running vehicle EGR's are known for causing stunble and surging.


Steve

 

[ME83]

I put a tachometer on the vehicle, after the new purchase 32 yrs ago. The engine idles very smooth, but increasing the RPM in a static position,... off idle, and slowly increasing the RPM it starts to vibrate between 1200 and 1500 RPM....before the vibration ends....this is viewed with the engine cover removed. The engine oil shows no mayonnaise coloring from anti-freeze. I also checked the wiring harness with a probe check of each cylinder with a visual indicator of juice getting to the plugs. I haven't done a compression test yet, but that will be next.

The vehicle only has 16,600 total miles since new!

Another thought, with those tightly cowled engine covers, would that constant heat there cause a breakdown on the wiring harness, when at operating temp, that would create this vibration? Maybe it needs a performance grade wiring harness?

You are probably looking at the carb running a little rich or the ignition timing isn't working quite right. I would make sure the vacuum line to the distributor isn't cracked and the weights in the distributor are able to freely move. If it's the carb and is running rich enough to stumble you should be able to smell it through the exhaust..

 

[carpenter383]

Sounds to me like it's a little lean off idle. Confirm by restricting the air inlet a small ammount while it's stumbling. If it gets better the mixture is lean.
Assuming it dosen't need tuned up:
Check for vaccume leaks.
Check the egr valve by unhooking as sugested. Make sure the egr valve plunger is fully seated.
Quadrajets have adjustable idle bleeds, you have to pull the factory plugs out of the lower front of the carb to access the screws. Back the screws out a tad to richen up the idle circut. I know it's not stumbling at idle, but the idle circut has some affect off idle too.

 

[machmeter62]

Sounds to me like it's a little lean off idle. Confirm by restricting the air inlet a small ammount while it's stumbling. If it gets better the mixture is lean.
Assuming it dosen't need tuned up:
Check for vaccume leaks.
Check the egr valve by unhooking as sugested. Make sure the egr valve plunger is fully seated.
Quadrajets have adjustable idle bleeds, you have to pull the factory plugs out of the lower front of the carb to access the screws. Back the screws out a tad to richen up the idle circut. I know it's not stumbling at idle, but the idle circut has some affect off idle too.

I dis-connected the EGR at the distributor, and the engine runs very smooth at all RPM now. The anti-pollution hoses on this engine is equal to the wiring, I think? Now that I know the problem, I assume I should leave it connected, huh?

I also want to thank all you guys for the history of that small-bloc, and all the information of its storied life too. And thanx again for solving that vibration problem....there is a wealth of talent here!

 

[carpenter383]

I dis-connected the EGR at the distributor, and the engine runs very smooth at all RPM now. The anti-pollution hoses on this engine is equal to the wiring, I think? Now that I know the problem, I assume I should leave it connected, huh?

I also want to thank all you guys for the history of that small-bloc, and all the information of its storied life too. And thanx again for solving that vibration problem....there is a wealth of talent here!

The egr valve dosen't go to the distributor.
The vaccume advance on the distributor has a hose that goes to the intake. Is that what you unhooked? Did u plug the vaccume leak when you did the test?
Strange, cause most engines like a decent ammount of advance. With good vaccume the total advance will be around 40 ish degrees BTDC When you unhook the hose at the dist. the ignition is at base timing (12 degrees ish) until the rmps overcome the advance weights/springs.
More details please

 

[brokenot]

I had a couple of 400s back in the day, not a bad engine at all. I know it's not what's going on with your engine, (since you're the original owner and are aware of its history), but in addition to the other 400 differences mentioned so far like the siamesed cylinders and stuff, there's another very important difference between the 400 and the other small blocks.

It's externally balanced.

The small block has always been an extremely popular engine for enthusiasts and hobbyists due to the swap-ability of its bits and pieces, but a 400 needs to have the correct harmonic balancer and flexplate or flywheel. I'm sure there have been quite a few engines assembled with parts from here and there by folks that aren't aware and have just bolted on a balancer and/or flexplate and thought it was okay just 'cause it fits....