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MAP sensor voltage at idle?

G

grahamtwatson

NAXJA Forum User
Location
Woodinville
1990 Cherokee Ltd, 4.0L, Auto, 200k miles...

I'm trying to track down why I'm only getting 11mpg, and just looked at the output voltages from the MAP sensor. At hot idle I'm getting 1.9~2.0 Volts. It will go down to about 1.0V on overrun. The Jeep Renix MPI manual says this should be 0.5V~1.5V, but I've seen elsewhere on NAXJA that 2.0V is good. Input voltage and output voltage with engine off are in spec.

I can see that if the voltage is high it would cause a rich idle, and possibly make the engine run rich at part open throttle as well, which in turn would cause high fuel consumption.

So, is my MAP in spec or not? If it's out of spec, would the high voltage at idle be a valid reason for bad mileage?
 
My factory manual says at hot idle, the output from the map should be .6 to 1.5v. Make sure you are measuring to the ground at the sensor, not the chassis ground. A bad map sensor will definately cause poor performance and crappy mileage. Most Autozone's can test the map for you.
 
Goddangit! I replaced the MAP sensor with a new one and it reads the same (actually a fraction higher at 2.1v). I know I have a little blow by - the engine has done 200,000 miles after all. I'm wondering now if it might help to put a 1k variable resistor in line with the +5v so that I can adjust the idle voltage to within range, although this would mean that I'd have a low WOT voltage.

Oh, and I'm measuring the voltage across pins 1 and 2 on the sensor itself.

Of course, the ideal thing would probably be to deal with the blow by, but a head job and re-ring is a lot more than I want to have to deal with...

Any thoughts or alternative suggestions as to what to do here would be very much appreciated.....

Cheers,
Graham
 
Not sure what your connection is with blow by it should have nothing to do with your MAP sensor.
Do you have a vacuum leak? That would effect the MAP.
O2 sensor, CAT plugged, no overdrive, Trans in Power mode, temp sensor are the things to look at also.
Oh BTW MAP sensors very rarely go bad.
 
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I was thinking that blow by could reduce the manifold pressure a bit, but perhaps not, now I think about it.

I've got a new O2 sensor, the cat is clean - I got to look through it when I did some exhaust work recently, I can't find any vacuum leaks (tested with carb cleaner sprayed on all the joints), trans in power mode doesn't seem to make any difference to mpg and the temp sensors test in range. The vehicle hardly ever goes fast enough to use overdrive - it's all side roads work.

Graham
 
grahamtwatson said:
I was thinking that blow by could reduce the manifold pressure a bit, but perhaps not, now I think about it.

I've got a new O2 sensor, the cat is clean - I got to look through it when I did some exhaust work recently, I can't find any vacuum leaks (tested with carb cleaner sprayed on all the joints), trans in power mode doesn't seem to make any difference to mpg and the temp sensors test in range. The vehicle hardly ever goes fast enough to use overdrive - it's all side roads work.

Graham
Click to expand...
What size tires?
Did you check the +5 volts.
 
langer1 said:
What size tires?
Did you check the +5 volts.
Click to expand...
Standard tires - 235 75R/15s, and the supply voltage is 4.9v.

I'm *fairly* sure the problem is the 2.0v at idle - unless someone can convice me this is OK. I'd love to hear suggestions as to why it's that high...
 
grahamtwatson said:
Standard tires - 235 75R/15s, and the supply voltage is 4.9v.

I'm *fairly* sure the problem is the 2.0v at idle - unless someone can convice me this is OK. I'd love to hear suggestions as to why it's that high...
Click to expand...
Whats your vacuum at idle?
With your engine off and the ignition on what reading do you get?
Low vacuum will give a high MAP signal reading.
Also the 0.5 to 1.9 is at sea level
 
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langer1 said:
Whats your vacuum at idle?
With your engine off and the ignition on what reading do you get?
Low vacuum will give a high MAP signal reading.
Also the 0.5 to 1.9 is at sea level
Click to expand...
Don't have a vacuum gauge, so can't tell you the idle vacuum. Suspect this may be the problem though..
Engine off, ignition on is 4.9v
I'm about 300ft above sea level.
I thought it was 0.5 to 1.5v - if it's really 1.9 then I'm (just) in spec and need to move on to something else :-(

Thanks for all the suggestions - much appreciated!
 
grahamtwatson said:
Don't have a vacuum gauge, so can't tell you the idle vacuum. Suspect this may be the problem though..
Engine off, ignition on is 4.9v
I'm about 300ft above sea level.
I thought it was 0.5 to 1.5v - if it's really 1.9 then I'm (just) in spec and need to move on to something else :-(

Thanks for all the suggestions - much appreciated!
Click to expand...
Good the 4.9 is correct so it's working correctly.
Which temp sensors did you check there are three?
You have one for the gauge and one for the coolent and one for air temp.
212=185ohms 160=450ohms 100-1600ohms 70=3400ohms.
 
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grahamtwatson said:
1990 Cherokee Ltd, 4.0L, Auto, 200k miles...

I'm trying to track down why I'm only getting 11mpg, and just looked at the output voltages from the MAP sensor. At hot idle I'm getting 1.9~2.0 Volts. It will go down to about 1.0V on overrun.

So, is my MAP in spec or not? If it's out of spec, would the high voltage at idle be a valid reason for bad mileage?
Click to expand...

Those numbers are within spec so your problem's elsewhere. Since the O2 sensor is new and the cat isn't plugged, you could have a problem with one of the other sensors (coolant temp. or CTS, manifold air temp. or MAT, throttle position or TPS).
If your engine starts easily, idles smoothly, and generally runs well, that usually rules out the load sensors (MAP & TPS).
 
Unfortunately the temp sensors (all three of them) are in spec, as is the TPS (and it's a nice smooth change in voltage as you move the throttle).

As I said, according to the Jeep Manual I've got, the voltage should be 0.5v to 1.5v across pins 1 and 2 on the MAP sensor at hot idle. I've seen several people say it's OK up to 2.0v though, so I'm just confused.

Would anyone be willing to let me know what voltage they are getting on their Jeep? (One that's doing more than 11mpg, of course!)

Thanks Guys!
 
grahamtwatson said:
Unfortunately the temp sensors (all three of them) are in spec, as is the TPS (and it's a nice smooth change in voltage as you move the throttle).

As I said, according to the Jeep Manual I've got, the voltage should be 0.5v to 1.5v across pins 1 and 2 on the MAP sensor at hot idle. I've seen several people say it's OK up to 2.0v though, so I'm just confused.

Would anyone be willing to let me know what voltage they are getting on their Jeep? (One that's doing more than 11mpg, of course!)

Thanks Guys!
Click to expand...
Is working correctly, you need to check what the map sensor is reading and thats your vacumm.
Do you thing that if your oil pressure was low that jumping the sender would increase your oil pressure?
You may have burnt valves and low compression, it's time to move on to mechanical problems in my opinion.
A very small leak in the line or a plugged line going to the MAP could be a cause, but the MAP is only a vacuum sensor and yours is ok.
 
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Well, there is still some confusion over whether or not my MAP sensor IS in spec or not - according to my Jeep manual it's not, but opinions are definitely varying....

And no, I realise that dinking with the sensor isn't fixing a low manifold pressure - I'm going to try and get a guage to test this asap - but unlike the oil pressure the low manifold pressure could possibly also make the engine run rich. Hence the short term "fix" of adjusting the voltages to get the engine to not run rich.

I have a nasty feeling I'm going to have to do some head work at some point soon, but in the meantime...

This leads me to three questions: -
1) What voltage are you actually getting out of your MAP at hot idle, so I can deal with the ambiguous spec.
2) What is the correct manifold pressure anyway - once I get a gauge I'm still going to need to know what it should read!
3) Recommendations on things I could to do to improve the idle manifold pressure short of taking off the head..

Thanks all.
 
grahamtwatson said:
Well, there is still some confusion over whether or not my MAP sensor IS in spec or not - according to my Jeep manual it's not, but opinions are definitely varying....

And no, I realise that dinking with the sensor isn't fixing a low manifold pressure - I'm going to try and get a guage to test this asap - but unlike the oil pressure the low manifold pressure could possibly also make the engine run rich. Hence the short term "fix" of adjusting the voltages to get the engine to not run rich.

I have a nasty feeling I'm going to have to do some head work at some point soon, but in the meantime...

This leads me to three questions: -
1) What voltage are you actually getting out of your MAP at hot idle, so I can deal with the ambiguous spec.
2) What is the correct manifold pressure anyway - once I get a gauge I'm still going to need to know what it should read!
3) Recommendations on things I could to do to improve the idle manifold pressure short of taking off the head..

Thanks all.
Click to expand...
Darn your not getting it, your MAP reading is high, but why?
When you tested it with the engine off that was correct, which means your MAP is working correct.
Now you need to check your vacumm which is what the MAP is reading.
Here are my readings
1987 Xj 4.0L
21 mpg
MAP 1.7v
Vacumm 20 in hg
 
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I was getting it, but didn't have a vacuum gauge, so was trying to eliminate the things I COULD test... I've got one now though, and I've only got 16 inch at idle, and 20 at ~2500rpm.

So it's back to question 3 from above - what can I do to improve the manifold vacuum, short of a head job? As I said, I don't *think* I've got a manifold leak - I've tested by spraying carb cleaner over every joint I could see - no effect. I've also just seafoamed the head through the manifold to see if I could clean things up a bit - plenty of smoke as advertised, but no other effect as yet ...
 
grahamtwatson said:
I was getting it, but didn't have a vacuum gauge, so was trying to eliminate the things I COULD test... I've got one now though, and I've only got 16 inch at idle, and 20 at ~2500rpm.

So it's back to question 3 from above - what can I do to improve the manifold vacuum, short of a head job? As I said, I don't *think* I've got a manifold leak - I've tested by spraying carb cleaner over every joint I could see - no effect. I've also just seafoamed the head through the manifold to see if I could clean things up a bit - plenty of smoke as advertised, but no other effect as yet ...
Click to expand...
Ok now you know why the MAP was off. You now need to cap all your vaccum lines. And check your timing.
Still thinking, you could still be in Open loop mode. make sure it warmed up good and your sure the coolent temp sensor is good, not the one for the gauge.
Also check the CKP.
 
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