I have a 1987 f-150 inline 6 that will not idle. Idle goes up and down then dies in about 10 seconds after starting. I replaced IAC and cleaned out the EGR valve. There was no change. I disconnected a red vacuum line from the small manifold on the top of the plenum, it goes to a part that I do not know the name of. When I disconnect that, it runs. I don't know what that part is, and can't figure out the abbreviations on the vacuum diagram under the hood. The Chilton manual doesn't help me. Please help!

 

It isn't FPR, is it? Fuel pressure regulator is what that is, and it could be not working correctly. Is there any fuel in the vacuum line when you take it off? It could be that the fuel pumps are not providing enough pressure to run the truck for more than just a few seconds. Does it stay running if you start it a second time, or does it stall out every time? I would definitely check fuel pressure.

 

I'm Matt's wife, checking his posts...It never stays running. Sometimes he's thought when it's warmed up that it ran a little better. But normally it never idles, you have to keep your foot on the gas. When this line is off, he said it runs, but rich. I've never seen fuel in that line, it seems to be empty. He's blown on it to see if it's clear of obstructions (I think? was the reason!) Does that make sense?

 

I would check the fuel pressures to get some actual numbers. With the vacuum line attached, it lowers the fuel pressure from where it is when it is disconnected. If the fuel pump is not able to get it up to the regulated pressure, it still may be able to get it up high enough with the vacuum disconnected (maybe even too high).

 

Does this vacuum line that is getting disconnected attach to a small black box mounted on top of the plenum for the heater core?

 

Yes, it does. Why?

 

That would be the MAP sensor (manifold absolute pressure sensor). The box on top of the heater/ac box. I am surprised it will run with it disconnected. It reads the manifold vacuum to determine the fuel mixture required. It could be bad or you may have a vacuum leak. A vacuum leak will confuse the MAP and computer. The Haynes manual tells you how to check the MAP with a tachometer (or multimeter that reads rpm). The FPR should be mounted on the back side (driver) of the fuel rail with a fuel line going in and out and a vacuum line on it. The FPR allows fuel to flow unrestricted with no vacuum and reduces the pressure from about 40psi to 30-35 when you have vacuum at idle.

 

Here is a picture of the FPR.

http://bellsouthpwp.net/b/r/broncoii/f150/FPR.jpg

Here is a picture of the MAP.

http://bellsouthpwp.net/b/r/broncoii/f150/MAP.jpg

You may also want to run the computer codes. Go to the following website for
instructions. Click on getting codes. http://fordfuelinjection.com/index.htm

The computer may tell you what is wrong, or at least what it thinks is wrong.

 

These pictures are on a 90 f150 w/ 302. But I imagine the 300 I6 will be similar.

 

The truck reads about 55 psi when running with the FPR unhooked. When you connect the FPR it drops to about 45psi. The truck is now starting and running, but the idle will come up and then drop down and almost die. then it will go back up and back down, etc. After about 2 minutes, the idle evens out and it seems to idle fine. when you hit the gas and back off quick, the idle dips a little below where it should (maybe 50 or 100 rpm below where it should), but then pops back up to what sounds like maybe 600-650 rpm (guessing by ear right now). When the truck has warmed up, if you shut it off and turn it right back on, it idles up and down, like it always does and then levels out after a few minutes again. Also, we have tried pulling the codes, but there is no check engine light. The wiring panel (flat metallic one) on the back side of the gauges goes to where the light should be and the other end goes to the terminal where the SES light wire should be, but there isn't even a SES wire coming into the gauge cluster! We tried tapping into the DLC connector under the hood (like all of the websites say to do to pull the codes) and we get nothing! Any ideas to pull the codes any other way? Please help!!!

 

I didn't even think about the MAP sensor. The fuel pressures you are getting are right where they should be for the straight six. My '87 didn't have a CEL hooked up in the dash either. Jump the SIGRTN(black wire/white stripe in big diagnostic plug) to the STI (single wire) and then use a volt meter to go between the positive battery terminal to the STO wire in the big diagnostic plug. You should have a signal of 12V with the key off. It should drop down to 0 soon as you turn the key on when you have the jumper in place for the test, then it should start sweeping between 12V and 0V blinking out the codes (analog volt meter). You can also use a small test light in place of the volt meter. Also, I think that you have to hold down the clutch pedal (if you have a standard transmission) for it to enter the test properly.

 

I have just replaced the MAP sensor for my 90 4.9L. I also replaced the ECM earlier (bad decision).

First try blocking off the vacuum line with your finger while the truck is running - mine stalled out.

Second remove the electrical connector at the MAP sensor and you can either
reattach the vacuum line or block it off - mine ran as if I disconnected the
vacuum line.

I replaced the MAP sensor and it runs fine, hope it takes care of your problem.

 

Hello all. I am having the same problem with my '89 F150 4.9L I6 man trans and was wondering if the MAP sensor, or maybe the FPR valve alleviated your problems. The fuel gages haven't worked since I bought the truck. All my vacuum lines look good, and I just did a full tune up. rotor button, cap, plugs, plug wires, air filter, EGR valve, and in-line fuel filter, starter, oil change. Now the truck has taken to running really rough, but only at low rpm, like it's not getting gas? 1st gear only? Doesn't make any sense. I was even starting to suspect the fuel dellivery system.

Thanks for your input!!

 

Check the egr system. If the valve is stuck open at idle, it will run terrible and have little power. Once the rpms get up, it is less noticeable.

 

Thanks to all who had advice. Here is what I ended up doing. I took the whole freakin' thing apart (dashboard, trying to find the check engine light that wasn't there, tested egr, map sensor etc.,) put it all back together, and it wouldn't fire up. Checked for fuel pressure and had none. Replaced the fuel pump relay, no luck. I could jump a line in the self test plug to make the pump operate, but it wouldn't come on with the key. God I love cars. Went to the junkyard to see if I could find a computer for the thing, thinkin maybe that might be the problem, to no avail. Put the old computer back in and..... wait for it, it started right up. So, to recap, I put no new parts in it, adjusted nothing, and what do you know, it now idles fine once it warms up. Maybe next time I have a free Saturday to completely waste, I will see if I can fix the coldbloodedness. Untill such time, I will rev it until the cows come home!

P.S. On the upside, my brother the die hard Chevy fan had his truck die from alternator failure on the way back from the junkyard, requiring a call to AAA, and firing up the old warranty. I said nothing, but smiled on the inside.