Have just recently picked up this 1979 F250 Ranger XLT with 400CID V8 and C6 Auto Trans, all original California truck built in San Jose CA, all components match VIN tag on drivers door. It runs great around town, and as part of the CA emissions thing I know it recently passed smog with flying colors. I also know a smog tech recently touched my engine

All's good except under load above 2000-2500 rpm... engine chugs and sounds like a miss and loses power, at such time I back off the throttle and get back under 2000 rpm and all's well. In park, rev the engine up over 2000rpm no problems. Hrm.

Okay so I have some work to do including checking my plugs and tracing vacuum lines (and ?) ... my question right now is I've looked over the vacuum lines and all looks okay as far as I can tell... except the following two items (pics included);
1. The front vacuum branch/manifold/? with four ports has third and fourth ports equalized or short circuited. This looks like a modification by someone. Thoughts?
2. There is a pressure regulator or some sort of pressure device (big flat pancake) with a tee-in line that looks like this is some sort of "switch" or something... with a big open-to-atmosphere hole. Is this a break off from a mounting stud or another vacuum tee or just design to be open to air?

Thanks for help, here's two pictures of items above, along with truck vacuum diagram.

Appreciate any tips on the vacuum deal OR the dying under load deal. I have a 73-79 Ford F-Series/Bronco Haynes manual on order. Hoping that will have some good troubleshooting procedures related to my primary issue of dying under load.

Thanks!

Short-Circuited vacuum:


Open-to-Atm Vacuum Device:


Vacuum Diagram:

 

Come on, no one?

Update; After poking around at some things including firing order, I noticed the vacuum motor and damper (for the positive crankcase venti system) is stuck wide open on the carb housing inlet. Stays open and doesn't move while pulling throttle cable and letting off. What gives?

Again, Thanks for your input!

 

Gotta give people more than 4 hours on a Sunday to respond.

The four-port item in the thermostat housing is a ported vacuum switch. This controls the vacuum signal to the distributor vacuum advance; it's purpose is to route either ported or manifold (or in some cases, a delayed or reduced version of either) based on engine temperature.

You can perform a sanity check to see if the vacuum routing is related to your bogging issue by connecting the distributor vacuum advance directly to a ported vacuum port on the carburetor; depending on your base timing you may need to increase the initial timing. This will help point you to a yes/no answer. Since it's only dying under load but NOT out of gear, it's reasonable to hypothesize that the issue is timing advance related and NOT fuel related.

I don't understand your question. Are you asking why something is broken? Or are you stating your understanding of how something should work, and asking why the truck's behavior is not inline with your understanding? Can you show a picture of what you're working with? The PCV system consists of only a valve and a breather; there is no vacuum motor. There's a vacuum motor at the end of the air cleaner snorkel that serves a different purpose - is this what you're asking about?

 

My bad

That's what I'm thinking. Was just happy to find out these motors don't have points to gap/maintain. But it does have original electronic ignition which I've read on this forum can be trouble...

This. Why would this be stuck open or is that question the answer, it shouldn't be open always? Is this related item or unrelated. Priority is fixing the 'bogging' under load.

Thanks!

 

The door and vacuum motor at the end of the air cleaner are unrelated to what you're dealing with. There is supposed to be a hot-air shield on the passenger exhaust manifold with a dryer hose running up to the mounting flange on the bottom of the end of the snorkel. There's also supposed to be a dryer hose running from the end of the snorkel out to the radiator support, mounted to a fresh air intake. Both are long gone on most people's trucks.

The door (and vacuum motor) are controlled by a manifold-vacuum-sourced temperature switch inside the air cleaner. Below a certain temperature, the switch is CLOSED which makes the vacuum motor pull the door shut. This directs warm air wafted off the exhaust manifold into the combustion stream to keep the carburetor from icing over. After the switch reaches a certain temperature, it OPENS, the vacuum motor lets go, the warm-air path is shut off, and the door goes sideways so that outside air from the fresh-air intake can move past the door and be drawn into the engine. It has nothing to do with the position of the throttle.

Almost any truck on the road will have had this stuff removed at some point; if it remains, it most likely no longer works.

 

AH, got it... the carb de-ice. Forgive a Californian for not recognizing that

For the record it's still hooked up to the dryer hose off the exhaust manifold. I guess that's why I thought it was part of the re-circ system but yes in retrospect the PCV vents off the crankcase not the exhaust.

Thanks!


I'll get around to testing the vacuum advance soon and let you know what happens.

 

Just follow-up post in case anyone wanted to know... it turned out the starter relay had a loose ground connection... previous owner thought a spade connection was good idea to save a couple bucks instead of using ring terminal. Welded the split ring washer and nut in place, spade just slid out. New relay $18, had the ring terminal on hand. Problem solved, runs like a champ. Seems like all my problems are always electrical....


I would like to get smarter about vacuum systems one day, my previous tinker toy was an aircooled 1965 beetle... no vacuum anything on that beast!

Cheers!


Mike

 

Glad to hear you got it worked out...but what does a starter relay ground connection have to do with missing at high RPM under load? The starter relay is completely out of the picture once the engine is running. Was this the right thread?

 

No, you're right, it didn't. I don't know why it seemed to run better, but re-drove this afternoon and still doing the backfire pop thing and losing power. On to the next thing, glad I caught the loose starter wire now vs. cold morning but didn't solve the main issue.