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

[mjarrels]

I had one in a 1970 Grand Prix... Would float the valves at 130 MPH. A steady 12 MPG with driver only, 12 MPG with driver and 3 pax pulling a boat loaded with camping gear. Always 12 MPG. Ate fuel pumps like crazy and starter bindix, but cheap to change.

mark

 

[machmeter62]

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

That "silverish" dish shaped thing appears factory mounted on the right side of the base of the distributor, with a vacuum hose running forward to the firewall, and just under the hood is a high-impact plastic canister, (4"x3") mounted there, that has two hoses connected to it. One is the 3/8" O.D. one from the distributor, and the other one... smaller hose, goes to the left inside a large harness full of wires, that head over to the drivers side. So far, I have been unable to find where it ends... but I think it looks like it connects into the firewall again?? That area is a junction maze!! I disconnected the one from the distributor to smooth the engine at all RPM.

 

[machmeter62]

That "silverish" dish shaped thing appears factory mounted on the right side of the base of the distributor, with a vacuum hose running forward to the firewall, and just under the hood is a high-impact plastic canister, (4"x3") mounted there, that has two hoses connected to it. One is the 3/8" O.D. one from the distributor, and the other one... smaller hose, goes to the left inside a large harness full of wires, that head over to the drivers side. So far, I have been unable to find where it ends... but I think it looks like it connects into the firewall again?? That area is a junction maze!! I disconnected the one from the distributor to smooth the engine at all RPM.

On the test, I didn't plug any hoses, and only felt slight vacuum with my finger capping the end.

 

[carpenter383]

On the test, I didn't plug any hoses, and only felt slight vacuum with my finger capping the end.

Yes the silver canister is the vaccume advance. Not sure about the maze of hoses you describe, but the one dissapearing into the firewall is prolly for the heater controls.
So since you didn't plug the disconnected hose the engine runs better with a vaccume leak:confused2:
Are you sure the choke is opening all the way when the engine is warm?
Did you check the egr valve?
Shine a flashlight into the carb (breather off) while the engine is idling. You DON'T want to see fuel dribbling out of the primary or secondary nozzles. If you do then some carb tuning would likley fix your prob.
Try pulling the pcv valve out of the valvecover with the engine idling, this creates a large vaccume leak. The engine should run rough and/or die. If it revs up some, but still runs good/ smooth, then the idle circut is too rich.
The reason I think you still need to investigate this matter is cause the vaccume advance makes a big difference on fuel mileage and overall running condition of the engine. I wouldn't just leave it unhooked.
Let us know what you find out.

 

[machmeter62]

Yes the silver canister is the vaccume advance. Not sure about the maze of hoses you describe, but the one dissapearing into the firewall is prolly for the heater controls.
So since you didn't plug the disconnected hose the engine runs better with a vaccume leak:confused2:
Are you sure the choke is opening all the way when the engine is warm?
Did you check the egr valve?
Shine a flashlight into the carb (breather off) while the engine is idling. You DON'T want to see fuel dribbling out of the primary or secondary nozzles. If you do then some carb tuning would likley fix your prob.
Try pulling the pcv valve out of the valvecover with the engine idling, this creates a large vaccume leak. The engine should run rough and/or die. If it revs up some, but still runs good/ smooth, then the idle circut is too rich.
The reason I think you still need to investigate this matter is cause the vaccume advance makes a big difference on fuel mileage and overall running condition of the engine. I wouldn't just leave it unhooked.
Let us know what you find out.

I did all the prior testing when the engine was semi-warmed up without driving it, since it's still unlicensed for this year, but will require a smog test for the license renewal soon. If it fails, they will give the reason on the report. When time permits, I will check it again with all your suggestions, before the smog test. Thanx again! There is no leaks or seeping anywhere on the engine while running it. And it seems to run smoother, with only a slight momentary vibration off idle, much better than before.

This is a California vehicle that was involved early in the state's pollution problems, because of the serious SMOG in the LA basin, and growing in the 60's. I have no idea what half of the phantom hoses do, but I don't plan on any permanent removing of them either. I have made notes of all your suggestions, and will report back, only if it fails within the next month. Once again, you guys gave me a great awareness of many thing that effect that small bloc.
I have confidence now, the engine is still healty with only minor issues.

 

[miker2702]

i have a 30 over 400 in my nova. flat tops 68 cc heads with stainless
valves and msd super hei ignition. current cam is 520 lunati. pistons are expensive to say the least. have built a few 383s they make good engines.
400s do run 20 degrees hotter than a 350. gm 400 straight shift flywheels
are hard to come by.

 

[machmeter62]

i have a 30 over 400 in my nova. flat tops 68 cc heads with stainless
valves and msd super hei ignition. current cam is 520 lunati. pistons are expensive to say the least. have built a few 383s they make good engines.
400s do run 20 degrees hotter than a 350. gm 400 straight shift flywheels
are hard to come by.

I am also curious of the stock HP/RPM rating of the '78 400 cu.in. small bloc. I have researched it, without any luck on this factory engine in my van. They seem to advertise in the cars, but nothing for a one ton Chev. van in '78? Most of the miles have a trailer behind it. Maybe it's just another stock car engine in that era? I have all the original brochures, but only states the engine options without the HP?

 

[lostcause]

How many of these were on tractors?

can't say i've ever seen any chevy v-8's in tractors personally, but i know for sure that there was a farm tractor in upstate new york fitted with a 4 cylinder saab engine back in the early 80's. i think it was a turbocharged one even. it prolly predates any video, but i'm sure there are some still photos floating around somewhere.

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.

since this is more a question of vehicle reliability and not a matter of why the model was discontinued, and you are considering using to do a long tow, i'd not be worried as long as i was comfortable with the cooling system. as was mentioned earlier, this block is known for its siamese bores - i cant remember for sure, but i thought it was the middle cylinders that were together: 3 & 5, 4 & 6. this made it harder to cool because the water jacket didn't go all the way around these cylinders. other manufacturers have used siamese bores too, but for some reason the 400 seems to be the one with the notoriety of overheating. make sure your coolant is in good condition - maybe flush the system out if it hasn't been done in years. also inspect the exterior of the radiator. if it's an old one, it may be deteriorating. fins will corrode & rot off. also check for airflow through the radiator. make sure it isn't plugged with foreign debris, such as mud. with no maintenance the 400 will usually be fine, but often people make performance modifications or put it under a heavy load (towing) with no consideration to the extra heat generated.