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Disconnecting MAP sensor (wiring) makes engine run great


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jscharpf
New User

Oct 27, 2012, 12:25 PM

Post #1 of 11 (29829 views)
  post locked   Disconnecting MAP sensor (wiring) makes engine run great  

1989 Ford F250 460 V8. 89000 miles, truck runs great, always starts, etc. Today it started up but would not idle properly. It acted like a vacuum leak, (you can keep it running by feathering the throttle). Runs great off idle, no black smoke, feels like it's lean at idle as if a hose were disconnected. No Engine light warnings.

After poking around, I noticed that if I disconnect the MAP sensor (just the electrical connector), the truck idles great (better than I remember) and runs fine on the highway, no loss of power, no black smoke, really seems to run fine.



My first thought was that I should just replace the MAP sensor but I'm wondering could this be covering some other issue. With the sensor disconnected, of course the check engine light is on. Do I worry? I don't drive it much, not really worried about poor mileage as long as I don't damage the engine.

Any ideas, thoughts? I don't have a diagnostic meter but the check engine light never came on and goes off if I re-connect the MAP sensor wiring (of course it won't idle properly then).

Jeff


(This post was edited by jscharpf on Oct 27, 2012, 12:50 PM)


Tom Greenleaf
Ultimate Carjunky / Moderator
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Oct 27, 2012, 12:51 PM

Post #2 of 11 (29801 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

Do check for vacuum leaks and vacuum to MAP sensor is in state of vacuum at idle - should be. Codes may help but you have one now just from unplugging MAP sensor but I don't think it will recall that once plugged in again - not certain.

VERY few of these known to me with the 460 monster. I do recall an older one the right rear spark plug was impossible to get at nicely and may be a problem if so close to heater box you can't touch the plug wire boot by hand?

T



Hammer Time
Ultimate Carjunky / Moderator
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Oct 27, 2012, 12:53 PM

Post #3 of 11 (29798 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

That could go either way. If the sensor is bad it will default to a predetermined signal if the computer loses communication to it. It could also be responding to other sensor readings that are corrupted also. That vacuum leak you hear could be robbing the map of vacuum also. You really need a high grade scan tool to analyze all the sensor data to look for an issue.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



jscharpf
New User

Oct 27, 2012, 2:07 PM

Post #4 of 11 (29777 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

Thanks for the responses! I want to clarify, I did not hear any vaccum leak, but my thought was that the engine ran as if there was a vaccum leak (won't stay idling but you can feather the throttle to keep it running).

I pulled the vaccum hose off of the sensor and it was a very strong vaccuum at the hose. I don't remember how the truck responded with the hose disconnected and plugged.

I DO know that leaving the hose connected, but disconnecting the electrical plug made the engine run fine (in all modes, idle, off throttle, full throttle, cruise).

I guess I'm willing to spend the $80 or so for a new sensor. I was more or less wondering what would happen if I just left it disconnected. Nothing seems to be going wrong.. yet lol..


Hammer Time
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Oct 27, 2012, 2:23 PM

Post #5 of 11 (29771 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

Leaving it disconnected just leaves the computer in open loop or a type of limp mode and your fuel mixture will be way too rich. That's not the way to fix it.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
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Oct 27, 2012, 8:41 PM

Post #6 of 11 (29749 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

Vacuum was present and took off the hose and plugged it. Great but then said you didn't NOTICE any change with just that? Why did you bother if you weren't looking for a change from vacuum to no vacuum?

I'd have to search the world to find out what values to expect at what vacuum it sees and what signal it sends out but apparently sends out a wrong signal no matter what and default it better. That's not a fix and will not just waste fuel but cause more troubles later.

460s are such a block of heavy iron you don't notice much with small issues but you do so it needs attention. If springing for a new MAP on a guess is acceptable to lose that $$ without more testing than go for it.

The idea is it is monitoring intake manifold actual pressure (a vacuum reading) which is telling of the load on the engine and adjusts fuel delivery and probably timing for the load at the moment.

Found a new one for $38 bucks. Cheap enough to lose but I'd at least apply different vacuum to it via a hand vacuum pump w gauge now and see if it will react or dig for the values it sends on and test it out.

IMO you are risking if not too late wrecking the converter(s?) with improper fuel delivery or anything making it pass thru unburned fuel,

T



jscharpf
New User

Oct 28, 2012, 6:41 AM

Post #7 of 11 (29734 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

Thanks again for the replies. I will absolutely buy another sensor and install it. I'm not worried about the cost and if the new sensor does not fix the problem I will take it in to a professional. The truck was my father's vehicle which he purchased as a retirement gift for himself in 1989. It never saw a day of winter until I bought it from him so that's why it has low mileage and runs like new. He's probably rolling over in his grave the way I take care of this thing.

Anyway, I don't want to damage the engine so I'll get it fixed. I appreciate the feedback!
I was more or less just wondering why there was such a big difference in the way the engine ran with or without the sensor connected. I used to work as a mechanic in the early 1980s and I remember the first GM computer controlled cars. In those days, the "limp mode" would make the car barely run.. it was obvious to the driver. Here, it seems like it runs fine, but I get your point about the engine being so big that it might be hiding this.


So today I will order a MAP sensor, get it installed, and go from there.

Thanks again for the help, this is a great forum, and I like the up front answers, there's no beating around the bush with you people! :)

Jeff


Hammer Time
Ultimate Carjunky / Moderator
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Oct 28, 2012, 7:03 AM

Post #8 of 11 (29729 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  


Quote
I used to work as a mechanic in the early 1980s and I remember the first GM computer controlled cars. In those days, the "limp mode" would make the car barely run.


Don't even try to compare 80s technology with current systems. You didn't even have fuel injection in the early 80s. You had an 0/2 feedback system with a carb and an M/C solenoid.

I agree with your plan on how to proceed from here though.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
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Oct 28, 2012, 7:13 AM

Post #9 of 11 (29726 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

I hate to suggest parts tossing vs testing out to be certain. In this case it's age and some very low cost items probably are not a waste if you are keeping this truck.

Just do one highly suspect item at a time. That's because if you do multiples you don't know which fixed it and totally complicates if it doesn't fix it if one new thing is defective.

89 and earlier used the term "computer controlled" but that was stretching the use of the word. Start replacing vacuum hoses around the thing if not done already. Watch out for breaking off plastic spouts on items if you do just cut hose with razor and peel them off of those. No glue I've found will fix the broken vacuum trees here and there on this and other vehicles.

460s were tough engines just impossible to make very fuel efficient,

T


loweboy
New User

Feb 10, 2016, 5:05 PM

Post #10 of 11 (18473 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

hello, I have a 460 efi 1990. If I disconnect the new map sensor it idles great. when I plug it up it runs like crap.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Feb 10, 2016, 5:08 PM

Post #11 of 11 (18470 views)
  post locked   Re: Disconnecting MAP sensor (wiring) makes engine run great  

This thread is nearly 4 years old. If you need help with something, please create a new question of your own.

This one now locked



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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