Does anyone know what the vacuum should be when cranking over a 1987 F-150 4.9 Liter straight 6 that won't start? What should the vacuum be if I got it going and it was at idle? Its electronic fuel injection.

 

I don't know about cranking. At idle it should be aprox 14 to 17, and fairly steady.
Good luck.

 

Thanks. I get a zero vacuum reading when I turn the key and crank it over trying to start it. I wondered if a zero reading automatically meant the vacuum is wrong.

 

If I were you, I would check it out on a friend's car that's running good, and see what readings you're getting off the guage. They can vary in quality, and usually have a **** to adjust the guage. You sould have a vacuum tree on the intake manifold. They usually have some lines hooked to it, and some with caps on that you can take off, and hook up the vacuum line on your gauge there. I would think you would get some reading while you're trying to start it. If you have your foot on the gas, alot of vacuum will be lost at the butterfly valve on the carb or throttle body. Hayne's or Chilton's have a page on using the vacuum guage to diagnose. If you can get your hands on one, I would read it over. If times are tight, they usually have some manuals at your library. Can't beat the cost there.
Good luck

 

Thanks for the help. I'll do it. I read something that said oxygen sensors should be replaced after 30,000 miles. I never replaced any sensors, and it has over 200,000 miles (I bought it new way back when I was in much better financial shape). I wonder if a bad oxygen sensor could cause it not to start, or could be connected to zero cranking vacuum. Don't have money to replace sensors that might have not been the problem.

 

The o2sensor won't stop it from starting. They send a signal to the computer after the truck warms up.

 

Hi Riverside,
The first things to check are compression, spark, and fuel pressure. these things will tell you alot. also how old the gas is that's in it. Have you been running it right along, or had you bought it because the old owner couldn't get it going. A little more history on it would be helpful. Also, there is a section on this site that tells you how to pull any codes by using a paper clip. Do a search for it.
Good luck..keep pluging at it.

 

O is what you should have if it is not running.
The question is What happened , Did you let it sit for awhile or did it just stop running , Thats what we need!

 

Thanks. I'll keep pursuing this zero cranking vacuum reading. I was just hoping there was some bad sensor somewhere that caused lost cranking vacuum that I could just replace. I guess life isn't that simple.

 

I posted that last before I saw the new ones. Thanks everyone. Some history. It has compression, spark, fuel pump o.k. Started running rough one day, and the next morning it would not start. Thought the injectors were plugged because the tank was low, put some cleaner in the tank, next morning got it apart and the injectors seemed squirting okay. Put it back together but still wouldn't start. Spent all my savings at the parts store new egr valve, new wires, plugs, TFI module, idle air control valve, not knowing what i was doing. A couple times I was able to get it started on a warm dry day if i had someone unplug the idle air control valve when I cranked, then plug it back in when it caught. It drove great, but idled terrible. I tried to that again, but it wouldn't work.

 

I also denied I had problems with my fuel injectors. I took a spray bottle with gas and sprayed gas into the throttle body. It cranked and ran very badly. I pulled the plug wires off to see which cylinders were firing. Mine ran on three cylinders. 4,5 & 6 ran the thing. I removed the injectors out of 5 & 6becouse I knew they were firing, and swapped them with the injectors from 7 & 8. Wow! The engine ran on 4, 7 & 8. I called Summit and got a whole set of ford Racing Parts #19 fuel injectors and about 1 hour later the engine was Grawling like a Lion !

 

That has got to be it. Can't describe how relieved I feel. Thanks.

 

Auto Zone has then for 40 bucks each. I called Summit and got mine for 209 for the set of 8.

 

Thanks. Summit is the way to go. I'm going to do what you did.

 

Hey when you going to get those , and let us know how it turns out!