Hi
My '78 (L48 auto, stock) developed a problem during a weekend event and I'm looking for suggestions that hopefully back up my theory!
Having driven 70 miles or so to the event we parked the car for about 30 -40 minutes, It had performed faultlessly on the journey, on a sunny day, warm, but not hot (85 degrees maybe). We then left for our hotel, but 200 yards down the road we crossed a river bridge an d the car suddenly was struggling to climb the very small gradient. Once over, it seemed to run better, but every time I applied more throttle, for a gradient, or to speed up, it was really struggling, sounding like it was firing on about 2 cylinders! It barely made it up the steepish hill to the hotel car park! But, if I was coasting, or put the car into Neutral and revved the engine, it seemed fine, revved Ok and showed no signs of missing etc. I am therefore pretty sure that it is not a fuel related issue.
The following day we inspected the distributor and found the centrifugal advance was seized and not doing what it should do. We freed that and re-assembled and all looked good, but the car still struggled to take any load. It started instantly, idled OK and revved, but still seemed to be firing on 2 or 3 cylinders under load.
The following day we opted for a flat-bed to get it home. The car had stood for over a day without running, and when I started it to move into line with the truck, of course, it ran perfectly! It drove up onto the flat bed. At home it ran OK as I manoeuvred it from flat-bed to driveway and into the garage.
Any thoughts?
Having chatted to some Corvette friends here, I have opted to buy a new coil (the one in the cap) and an ignition module, both of which are the original item as far as I can tell an d have done 133k miles.
Please tell me I haven't mis-diagnosed!

 

It's hard to guess, but it could also be a fuel pickup issue. The sock on the fuel intake in the tank could be partially plugged. I guess it could be the coil, it's cheap and easy to replace. generally ignition modules work or they don't. I would replace the coil and flogg it up a hill on a hot day in order to test it.

 

I had a similar problem where my 77 would idle great and would cruise under light load without problems. The moment I gave it any throttle it would sputter and miss, and had no power at all.

After fiddling with plugs, cap and rotor, ignition module, carb settings, etc. etc. i finally discovered that the #8 plug wire was burned and arcing to the engine block. I never noticed it since I only watched it run during daylight.

The wire appeared fine until you looked closely and bent it back and forth... then you could see the crack right where the wire entered the boot at the plug. Very subtle.

When I installed a new wire on #8, it ran perfectly.

 

Change fuel filter, when you have the fuel line off, verify the pump is pumping fuel......pull S-hose off of fuel pump and verify good gravity flow with tank full of gas.....you are losing fuel flow under demand, then the flow comes back when idling or light load.....
I seriously doubt it is the coil or module......it would run like **** all of the time......and in 35 years, I have yet to change an HEI coil that failed and never heard of one that is intermittent....and I have literally worked on hundreds of them.....

Jebby

 

I changed an HEI Coil, and when it failed. It failed. Car died about a half block from home. Neighbour guys helped me push it home and into the driveway. Replaced coil. Car ran fine again.
then, years later. Car developed a miss under load. Coil was loose on the frame. I remember posting on here about it. Put in a different Coil. (The one that was loose on the frame was a tiawan unit). Problem solved. Then I put in DUI distributor. HUGE improvement.

 

Just a couple of notes here Stephen,

This is why it's imperative that a module receives a paste under the unit. Its called Heat Sink Compound. (not dielectric grease)
If you do end up swapping out the modge, the better units come with the compound. But some do not. Decades ago, Radio Shack sold it.
The reason for the heat barrier paste is that inside the modge are electrical circuits & contacts. Heat & age eventually loosen these contacts, usually at operating temps. The contacts disconnect, circuit is broken, engine stops. After cool-down, contacts are joined, it fires right up.

Although coils in caps are affected by heat & age also, they usually faulter in output voltage (misfire). Where as the modge is like an On / Off Switch, controlled by temps.

However, neither of these IGN components act the way your Vette did. So, as others have said, sounds like a fuel starvation issue.
The pre-molded, poor design, "S" hose near the pump is a suspect, as is possibility of some crap in the tank.

When our Vettes act like this, every Vette owner should have a Sparkplug test Light in the rear compartment. (Harbor Freight or Ebay $6)
Just plug the tester in any terminal on the dizzy cap. Instantly know if you have IGN or not.

 

How about measuring the fuel pressure and flow at carb?

 

I agree with @stevedinino that it could be a burned plug wire. I experienced this a few years back, but in my case it wouldn't go over 4500 RPM.

Run it during darkness to help see if this is the issue.

 

I had an issue that is somewhat similar:

https://www.corvetteforum.com/forums...connector.html

incomplete connection at the dizzy caused high resistance and eventual failure of the terminal connection. IGN module was fine.

 

Personally I think it’s a pickup issue. I would inspect the filter in the carb (assuming you have the OE car and the filter is still there) and also the fuel sock in the tank plus the filter.

 

If the car runs fine at first then has a miss under load its usually the module. It doesn't show up until the distributor housing heats up.

They usually fail because the rotor is bad and the coil can arc through hit5ing the module. Signs of this are rust like dust on the springs and weights under the rotor.

When Ive had puckup coils fail they've just failed open and the motor just dies.

Never had an ignition coil fail after running HEI ignition for 40 years. Not saying I can't happen but I think it's rare unless coils are not original gm so less quality.

 

Hi All

A somewhat delayed up-date here as I had a busy weekend at the Le Mans Classic - with a fully functioning 1978!

The problems did in fact turn out to be the coil. I'd discounted a fuel issue (I a fuel blockage many years ago and it wasn't the same symptoms at all), so bought a coil and ignition module. It's virtually impossible to buy direct from the US now (we're in France remember) but I ordered them from Corvette Avenue, the place I use in Alsace and it arrived the next day. I was able to swap out the coil in about 40 minutes. I had a friend staying for the LMC and we discussed swapping the module (which came with the grease sachet) as well, but the terminal covers were very fragile, so we opted to try just the coil first. Together with the correctly functioning centrifugal advance, the car is going better than it has for some time - a shocking indictment of my poor maintenance routine during the last year or so! We had an extremely enjoyable weekend at Le Mans where incidentally, there were literally hundreds of Corvettes around the circuit!