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Stumped - Help!

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spiritofdriving

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My dad and I have a problem we can't seem to solve. He may have more ideas, being that he's much more mechanically intelligent than i am so far, but i certainly don't. Anyway, the car is a TVR 2500M but with a Ford Cologne 2.8 V6 with a "fast road" cam, Holley 390 carb, and (until last night) electronic ignition. The problem is that the car refuses to be driven on the highway. This may sound strange, but it could be driven in city style driving for hours on end, but once up to highway speeds it will (after no more than 3 miles) begin to misfire and inevitably die. After pulling over and restarting, it will do the exact same thing, negating any highway travel. After checking many other things, we checked the coil and found that there was no current being sent to the coil from the electronic ignition computer. So we switched back to points and took it back to the highway but it did the exact same thing. Also, we changed the fuel filter and we know that it's getting fuel. Any ideas?

p.s. I know this is more of a general tech question than an "other british car" question, but i didn't see any technical section to the forums.
 
Check your that all your vaccum lines are connected and in good shape. There are alot of them and a major vaccuum leak will prevent the spark advance from working correctly
 
Small leak in the fuel line - causing the fuel supply to not keep up with demand at 70+ MPH? Clogged pickup tube? Vacuum advance not working good - leak in vacuum line somewhere? Bad springs on the advance weights? Coil going out when hot?
 
Sounds to me like the coil is getting weak, and can't keep up with the demand at highway speeds and becomes overloaded.
Any chance you could swap it out and try a different one?
 
yeah, actually, that was our next guess, and this weekend we'll swap coils. We bought a new one along with the electronic ignition, so luckily we still have the old one. thanks again everyone
 
In addition to everything mentioned already...

In my experience, problems like his are caused by:

1)Ignition
2)Fuel pump on the way out

Re #1, check the plugs wires, change them if you have another set. Check the dwell, especially well above idle speed - in additional to being at or near spec, it shouldn't bounce around much. (If it does, see if the distributor shaft wobbles). Take a look at the plugs - is the heat range reasonable for the car? Examine the distributor cap carefully. Any tracking or cracks?

When/how did this start? Was the car running fine one minute and bad the next? What changed? Did you do any work on it? Did it sit a long time?
 
This is also a fiberglass car... How are the electrical ground connections? Could be a loose one letting go of the current at high speed road vibrations.
 
I had a similar experience with an old Dodge. I was on the highway when all of a sudden the engine started missing and power went way down. The car wouldn’t run over 30 Mph. Since it was a Sunday and nothing was open I limped home like that for 5 hours. Turned out to be the fuel pump. The diaphragm developed a tear and it would not draw fuel at the higher RPM. Put in a new one the next day and everything was back to normal.
 
Plumb a fuel pressure gauge into the fuel line (carefully!!!!!), and tape it to the windshield.Drive the car at speed until it dies, and see if fuel is being delivered. How is the tank vent piping done in your car? If I remember right, there was once a charcoal canister. The filter on the bottom could be clogged completely preventing large amounts of fuel to be pulled from the tank. If the canister is gone, how were the vent lines handled? Try popping the filler cap open the next time the car dies, and see if that makes a difference. Finally, does the Holley have the small filters inside the fuel inlet? You would have to remove the fuel line, and unscrew the housing that the line fits into to be sure. Hope this helps
 
I suppose this can't be it, but my first VW Golf (in England in 1970) had a design fault in that they had not put a breather in the fuel filler cap, and there was a pressure release return pipe from the output of the pump back to the tank. After a bit of driving, particularly at high speed, sufficient vacuum could develop in the tank that the pump simply returned all the fuel to the tank and none went to the carburetor! The engine would stop, usually at the most inconvenient or dangerous places, and would not start. If you waited ten minutes, it would start immediately, and then of course die again ten or twenty miles further on. Once I had discovered what was happening, I knew to release the filler cap, with a loud whoosh. Eventually there was a service recall to install a new cap, but I was moving to the US so I sold the car before that was done.

It's interesting that my old Rover's filler cap, 4 inches in diameter on the running board, has a tiny hole in the middle.

Ken G, 1925 Rover 16/50 (San Francisco)
 
The fuel pump is recent, so i hope it isn't that. A friend of dad's with a Triumph suggested the charcoal canister and we checked that also. I'm convinced it's the coil. I forgot to mention that this problem just came out of nowhere, i.e. it was running well for a couple weeks and then poorly the next.
 
My vote is for vacume buildup in the fuel tank... The coil should actually run cooler on the highway due to air flow. Is the coil externally resisted? I've seen those cause problems when they get hot, and a new ballast resistor is dirt cheap.

It's the sustained RPM cut out that leads me to fuel flow, and the venting in the tank has caused this problem for more people that you might guess!

Be sure to let us know!!
 
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