71' C-10 cutting out intermittently

[Hennyhog]

First off...thanks for all the feedback! Ok...so I put on a new set of plug wires Thursday evening. Truck ran great to and from work Friday and even work and errands Saturday morning. Then as my wife and I were out for a spin in the afternoon it happened again. Yeah....it's a bummer but have to keep at it so what am I going to try?

1. Some have mentioned that fuel lines near exhaust and even leaning against radiator hose may be vaporizing due to heat. The truck has had this happen to it 3 miles from home after having only been running for 5-8 minutes or 40 minutes after running it. So I kinda want to rule that out.

2. I'm pretty sure my fuel cap has a pressure relief doohicky in the center (little blue plastic piece) and when I fill it up I've never had that "whoosh" of air escaping while undoing the cap.

3. Still haven't read too many posts about intermittent mechanical fuel pump issues that act the way my truck is suffering from.

So with the crappy weather up here in MA I'm going to wait till later in the week to pull line off fuel tank sending unit, inlet side of pump, blow it out, and put a new pump in. They are cheap and it'll be another thing to rule out.

I still can't wrap my head around how it can be the pump (my brain tells me it works like a light switch. it's either off/on or pumps fuel or doesn't. I surely will keep you posted!

Thanks,
Henny

[MagmaJct]

Originally Posted by Hennyhog

First off...thanks for all the feedback! Ok...so I put on a new set of plug wires Thursday evening. Truck ran great to and from work Friday and even work and errands Saturday morning. Then as my wife and I were out for a spin in the afternoon it happened again. Yeah....it's a bummer but have to keep at it so what am I going to try?

1. Some have mentioned that fuel lines near exhaust and even leaning against radiator hose may be vaporizing due to heat. The truck has had this happen to it 3 miles from home after having only been running for 5-8 minutes or 40 minutes after running it. So I kinda want to rule that out.

2. I'm pretty sure my fuel cap has a pressure relief doohicky in the center (little blue plastic piece) and when I fill it up I've never had that "whoosh" of air escaping while undoing the cap.

3. Still haven't read too many posts about intermittent mechanical fuel pump issues that act the way my truck is suffering from.

So with the crappy weather up here in MA I'm going to wait till later in the week to pull line off fuel tank sending unit, inlet side of pump, blow it out, and put a new pump in. They are cheap and it'll be another thing to rule out.

I still can't wrap my head around how it can be the pump (my brain tells me it works like a light switch. it's either off/on or pumps fuel or doesn't. I surely will keep you posted!

Thanks,
Henny

I told you my story. Perhaps my story doesn't sound like yours to you, but your story sounds like mine to me.

I too wouldn't have thought a mechanical fuel pump would be intermittent until it died completely forcing me to see that it was indeed bad. Once replaced, years later the truck has never died from lack of fuel.

That "whoosh" from the gas cap does have me a bit concerned. It is my understanding the cap is supposed to let air in to prevent a vacuum, but block vapors from escaping, instead allowing them to vent into the charcoal canister. If the tank developed a vacuum this can cause fuel vaporization ahead of the pump.

Regards,
Mike

[Hennyhog]

Ok.....so 99.9% sure I rectified this issue having driven it for a few weeks without skipping a beat (watch it happen to me this afternoon!!!)

So I took all your suggestions as well with people I seek out daily to tell them what was going on. I went back and forth...one day I was sure it was a fuel issue...just to jump a few days later to the other side of the coin thinking it was no doubt it was an electrical/ignition issue. Here is what I did since adding the new plug wires.

1. I hooked up my VOM to power going into HEI. At idle I'd get 11.9 volts and when driving it it'd go as low as 10.1 volts. So...having read up online forums I came up with a few GREAT leads on solving my problem. One was do not use power/wire going to old distributor to power up HEI and two you can't give HEI's enough voltage. So....I came off my "unfused power" terminal on my fuse block, installed a 20 amp inline fuse, and ran it right to power in to HEI (next to lead coming off Tach). Started it up and had 12.9-13.2 volts constant at idle and under load. I thought FOR SURE this was it and drove it confidently for almost 3 days before it happened again and then after much disapointment I was back to go over to the "fuel" side of the coin.

2. I purchased a carb rebuild kit for my Holley 650 dbl pump & a new mechanical fuel pump. Because of limited time over the 4th of July weekend (kids and wife) I rebuilt the carb on Saturday afternoon which was LOADED with crap...and then installed fuel pump Sunday morning.

I like to do one thing at a time so as to know what the culprit was but not having the time to rip around with it after just doing the carb rebuild....I have been driving it since without the issue occuring.

So...that's it fellas. We can debate whether it was all the junk in the carb or possibly a fuel pump that was slowly getting ready to croak....I just know she's been running strong since having done everything on the ignition side of the coin....and now on the fuel side.

Hope this helps any and all.

Thanks,
Henny

[Mike_The_Grad]

@hennyhog,
I've read your thread and I hope you have finally resolved this frustrating issue. I myself had a similar problem. Two things ended up compounding together to keep my mind boggled for a while. #1, I also had a clear inline fuel filter. Mine was located after fuel pump before carb,level with valve covers. After a TON of research I realized the design of the fuel system for this era of trucks. When these vehicles rolled off the assembly line, they did not have inline fuel filters. They only came with two main ways of filtering what went into the engine as far as fuel is concerned, first was bronze sending unit pickup "sock" for any type of debris large enough to clog fuel lines, second was on most Rochester equipped models was a small filter located in carbuerator body.For particulates,impurity's and the like. Based upon the mechanical fuel pumps design,which is more pumping than sucking, having an additional filter element installed anywhere on this system effects the delivery of fuel to the engine. Either before the pump,or after,it wasn't designed to have this extra element installed.my truck ran like crap both when I had the filter after and before the pump. My conclusion for my configuration was that, if the tank ain't dirty and I don't introduce crap to the system the original design is sufficient. I actually did need an inline filter when I originally purchased the vehicle because I had an auxiliary aftermarket tank,which had a lot of crap accumulated in it from the installation and maintenance of it. Once I eliminated all the outside factors,and got it back to the stock system.there really wasn't a need for the extra filter.#2 thing that caused similar behavior was wiring to the starter,which I note that you did mention as a possibility for the occurrence. Yes run unfused from ignition to starter.I recently rewired my truck with an American Autowire Classic Update kit. It provides this by means of a fuseable link in series with connection. How I found this to be an issues was I modified the recommended installation by making this connection with a easy connect/disconnect terminal connection rather than the spade metal to metal connection. (This decision left me on the side of the road at midnight for almost two hours before a good Samaritan with automotive knowledge happend upon me.) What I'm basically trying to say is try it without the inline filter as long as there's no visible crap in your clear filter,replace in-carb filter and fire It up. seems like you may have resolved the electrical side of it hopefully. Good luck.