Hey, I'm a first-timer here, though I've been reading this forum for a while. I've learned so much from all you guys. Many thanks.

Now I'm stumped, and don't know where else to turn. I have a 1979 F150 with a 302. It was running great till last week. All of a sudden, it would not run at idle, either in gear or in park. I could keep the engine going by keeping my foot on the accelerator, but as soon as I took my foot off, it died instantly. My thought was a fuel delivery problem, so I changed the fuel filter. No difference. Then I rebuilt the carburetor. Same problem. So then I suspected the gas pick up in the tank. So I bypassed the tank and ran the truck from a portable can... same problem. To keep the engine running in park I have the idle screw all the way down without the spring. The throttle position solenoid will hold it running too, if I set it right. But I don't think that's its job.

The fuel pump is pretty new... less than 1000 miles on it. Most of the ignition system is new also.

There must be something wrong with some carb setting for that fast idle screw to have to be run down so far just to keep it going in park. It is set lower than where it would be with the spring bottomed out. Even so, when I put it in gear, she dies.

Is it possible my new fuel pump is working, but not delivering enough pressure to keep it going at idle? I don't know what else to try at this point. Any help appreciated.

 

Do a thorough visual inspection. Look for a cracked or disconnected vacuum hose. Also, a clogged PCV will also lead to make up or compensate for the "missing" air by using the throttle.

 

Thanks for the quick reply. Yes, I have checked all lines. I had replaced all of the vacuum hoses earlier this year. Also, the PCV valve is new, and the PCV system is operating correctly. When I got the truck, the spacer under the carb was so clogged that the PCV system would not work. That was my first project. So I know it is working well now. I had checked vacuum a few months ago, and had plenty. I guess I should check again. Thanks for the reminder that this could be vacuum related. But that curb idle screw is really confounding me. I didn't change any settings on the throttle or choke when I soaked the carb and rebuilt it. So something has changed somewhere.

 

Are you running an EGR set up? Could be some carbon buildup in the passages or the EGR valve itself isn't moving when vacuum is applied.

 

Try bypassing the fuel pump and see what it does.
If the fuel pump is from the McParts store, then it might've failed. You can pull the line at the carb and crank the engine and see what kind of volume you're getting. Put a tall bottle around the line to capture the fuel, or connect some hose to the line and run it away from the truck.

 

Thanks for the tip! I thought about giving that a try, but wasn't really sure what to expect. I'm not sure how much volume is supposed to come out of the pump, and I don't have a fuel pressure gauge. Probably cheaper to replace the pump, which is about $15. The pump is providing plenty of few at higher RPMs. I suppose if it has an minor leak in the diaphragm, it might not be able to keep up sufficient volume at lower RPMs. Does that sound logical? I'll try your suggestion and see what I see.

 

Hey Filthy Beast... thanks for the reply. My EGR does move when vacuum applied, and it's not caked with carbon. But I really don't know if it's good or not. It's difficult to test with a vacuum gauge, because it's one of those positive pressure EGRs, which actually acts like it has a diaphragm leak when you put vacuum on it. I've taken it off and cleaned it, and had considered replacing it before this problem. I've been seeing posts about squirrelly idle and stalling being caused by the EGR. Mine is the original EGR, with 94K on it. Maybe it's time to replace it anyway. Thanks so much.

 

If it runs fairly well with your foot on the gas, meaning you could drive it on the road, I wouldn't think it's a fuel delivery problem.

I do think it's either a vacuum leak, plugged idle jets, or a malfunctioning idle solenoid. Or maybe the timing slipped somehow or the EGR is bad.

Do you have power to the idle solenoid? Does it extend when powered? It's job is to set the idle speed with the ignition on and to let the throttle plates close farther with power off to prevent run-on. The normal idle speed is set with the screw sticking out the back of the solenoid, the other screw is used to set the solenoid power off idle speed to 500 RPM. This is slow enough that the engine is unlikely to run-on or diesel.

 

peterfak: You're welcome.

If you can get your buggy to stay idling, try to manually depress the EGR valve with your hands....without killing yourself. If it (the engine) croaks, the EGR should be OK. If no change in engine idle speed when doing this, it gave up the ghost.

You could also try spraying some carb cleaner around the EGR valve and base while observing any change in engine RPM.

You probably have resolved the cause of the illness by now....if not, lasermike has some good ideas.

 

A while back, my yellow truck backfired and blew out the check valve in the power steering booster. It wouldn't idle, either in park or drive, but ran fine-as long as I kept the RPMs up. That was a big vacuum leak

Try spraying carb cleaner on the hoses while you wiggle them. Also the base of the carb and base of the EGR plate. If the idle changes, you've found a leak

 

Wow, thanks for the great suggestions guys. Idle solenoid works correctly, jets should be good, as I just rebuilt the carb. Fuel delivery seem okay. So I am back to suspecting vacuum issues. I tried the propane trick around the EGR and found that the engine runs at higher RPM when propane is around the valve, so maybe I have a bad EGR. I'm going to change it, even if that tuns out not to be the problem. It's the original valve, and looks pretty grungy. And I never thought to check the check valve on power steering booster. Plenty to try here. Many thanks.

 

Vacuum operated power steering booster is new to me but a leaking diaphragm in a brake booster will create a good sized vacuum leak that's near impossible to find without disconnecting at the manifold and plugging the fitting.


Just be glad you don't have a '70s Mercedes. The W114 Coupes have vacuum operated seat back locks in addition to the fuel door and door locks.

 

Yeah, i don't know about the vacuum to the power steering either... haven't looked. But I did replace a bad brake booster a few months ago, so I am pretty sure that is not the source of the leak. I am still betting on a bad EGR. I ordered one today and will plunk it in tomorrow. I also tightened down all my intake manifold bolts in case there was a leak there. I've had a bit of oil seepage there, right near the carburetor. I thought maybe some air might be getting in there. A couple of the bolts were not the 25 lbs the book recommend. We shall see. Thanks for the tip. I'll report back after I put this valve in tomorrow.

 

Looking forward to hearing you tell us, "I'm good to go!"

 

oops - meant power brake booster. sorry