Sorry if this has been discussed but, recently my 86 5.0 really ran great for a few days and idled perfectly. Something it rarely does in spite of feeding it numerous parts. I had to fix an alternator issue and while there noticed the Map sensor was loose and laying on the wheel well. Screwed it back where it belongs but the idle started to be problematic once more. Actually worse than usual. Pulled all codes nothing unusual. Ok... had an idea. Pulled off the Map from its mounting position on the right fender and stuffed it into a hole near the mount. The idle has been perfect since. Opinions? This is at least the 3rd sensor I have had on the truck but 2 are Autozone products. Just wonder if engine compartment heat is causing problems? Always runs better in Winter also. Thinking seriously of moving it inside the cab even though I know Fords have had these mounted in the engine bay all along.

Thx

 

I would check the connector. They like to corroded in the fenderwell area. Sometimes if you turn the wheels just right they can throw water up in that area.

 

I would also suspect the electrical connection at the device or the wiring leading up to it.

 

Yes I did also and re cleaned and tested continuity. Also tested the sensor with a HZ meter and vac pump, stupid thing is in spec. No codes other than the usual 44 because there is no air pump. Just funny when its exposed to the heat of the engine compartment the idle degrades the longer the truck is driven. Left this out that when I start driving all is well and idles beautifully but after time starts to degrade and when it does, its running rich. I actually turned it off in a Dairy Queen drive through the other day so I would not gas the poor workers inside. Just another mystery from the "truck from hell" My theory is that the electronics may be cheap and not able to take the heat. Just trying to avoid buying yet another Map sensor. So thought Id ask here to see if anyone thought this could be an issue. Also the hose to it is new along with all the under hood vac hoses.

 

There is nothing wrong with letting it sit there and idle, and then while it's idling grab the sensor and start playing with the wiring to see if you can make it mess up.

 

ttech, where are you located? Thanks to a friend [Ford F834], I have a few used Motorcraft map sensors, taken from various trucks and cars. Look on the case for numbers and I'll see if I have a match. If I do, it is yours... free!

 

Forgive me if I'm in left field here, as I'm not well versed in the Ford version of the MAP sensor. The troubleshooter in me noticed the behavior changed when you relocated a sensor. In the stock location, the sensor reads absolute air pressure inside the intake manifold. You then relocated the sensor so it's reading ambient air pressure, right?

If my hunch is correct, the sensor itself is fine, and the root problem lies elsewhere. The computer is expecting an input of actual pressure inside the intake manifold. Instead, it's now getting a different value so outputs (fuel metering, ign timing, etc.) are changed to match. That's probably why the idle is good.

In a stretched analogy, think of a carbureted vehicle with a too-lean idle, caused by who knows what, such as a vacuum leak or improperly set idle mixture. Meanwhile, you monkey around with the choke instead, causing just enough richness to compensate for the lean condition. If one error was just enough to compensate for the other, the engine might idle fine. You'd swear the choke adjustment fixed the original issue, when all it did was induce a second error that coincidentally compensated for the original error.

Now don't read too much into my carb analogy, other than you may have inadvertently caused the computer to react in such a way that the idle smoothed out. Meanwhile, you now have a new issue, such as the very rich idle you mentioned.

You may not be getting any fault codes because under some conditions, manifold pressure can be the same as ambient pressure. The input value isn't far enough out of range to trigger a fault code. These first gen EFI systems aren't super sophisticated, and may not have a very tight range of acceptable input values.

Now if I had only been more helpful and suggested what the fix could be, instead of suggesting what it probably is not. A leak in the induction system is a likely culprit, anything downstream of the MAP sensor. Unmetered air being drawn into the intake can really wreak havoc with the computer's calculations.

If I had to throw a part at it, I'd try the O2 sensor first. It is a very critical input to the computer. I wouldn't feel bad if that didn't fix it, as it's a normal wear and tear item.

After that, the idle air valve could be at fault.

Humbly submitted, knowing the sum of my first gen EFI knowledge could fit in a thimble. A very small thimble...

 

He can move it around, it has a vacuum hose going to it.

 

In the immortal words of Emily Litilla:




But you've got to admit I talked a really good game there, even if it was totally wrong.

 

Oh that's ok. Yup it's still connected electronically and to its vacuum hose off the intake. If you look at my earliest posts you can probably get an idea where I've been in trying to make this engine run correctly. I'm not a professional but have an extensive knowledge of automobiles up into the 80s. Learned a lot of things the hard way lol. That's why this truck has been kicking my butt. And add to the fact that I hate giving up. I will get it running perfectly for awhile then back to some of the same issues I already covered. The map thing was an idea but this has proven not to be the case I think. Now on a thought it may be the IAT. Always seems to be running a little rich. Also am wondering about the fact the intake hose to the air box from the radiator support may be contributing to my trouble

 

Have you run across the capacitor thing inside the ECU? Apparently when they get old they leak all over the circuit board and cause problems.

 

Yes. Know what you are talking about. I now have 3 ECUs and have visually inspected them for leaking caps and corrosion. All work the same. But thanks for the thought

 

Mechanics hate working on Fords. There is no way to get real time streaming data from the computer. If you could do that, you could probably find what's causing it.

 

Very true and this is early production EFI. I hear in 1986 there were a bunch of issues that had to be sorted out. Think it was a little rushed. Don't understand why Ford didn't wait until 1987 to introduce EFI when there was a major facelift. But I always can go with a carb. Right now it's more of an annoying problem. The last issue was intermittent engine shut down. Traced to the " new TPS" I had installed a couple of months earlier.

 

You have to admit the EFI is a little bit better than all those vacuum lines with a carbed engine. I like the way they run without messing around with a choke also. But when they run good they run good, when something goes wrong it's hard to figure out sometimes.

I have a 86 ranger with efi and it starts and runs excellent, except it has always had a rough idle to it. I have been running it for over 6 years and after doing lots of work to it, still haven't got it figured out yet. It's just a nuisance though, it never stalls. Maybe I need to look at the capacitor thing, something I have never done yet.