Help interpreting map sensor readings - JeepForum.com
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post #1 of 12 Old 08-21-2019, 12:03 PM Thread Starter
KiloWatts
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Help interpreting map sensor readings

Hello all,

I've suspected some fuel/air issues for a while and I'm wondering if anyone can help with interpreting my map sensor readings.

It's showing 12.7 inhg at idle, and about 8.5 wot. For kph, it's 43 at idle and about 30 wot. From what I'm reading this is too high overall. My symptoms are long cranks in the AM, and infrequent stalls. Normal operation otherwise, so it's been hard to nail down. Do these numbers look off?

Thanks in advance.

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1996 4.0 L6 RWD
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post #2 of 12 Old 08-21-2019, 12:34 PM
zjosh93
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For stockish stuff idle vacuum should be around 18-20 inhg. At WOT you should be under 3 inhg down to nearly zero depending on how restrictive your air intake system is. So I'd say your idle is too low and the WOT too high. Probably easy enough to double check with a cheap mechanical gauge.
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post #3 of 12 Old 08-21-2019, 12:47 PM Thread Starter
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Quote:
Originally Posted by zjosh93 View Post
For stockish stuff idle vacuum should be around 18-20 inhg. At WOT you should be under 3 inhg down to nearly zero depending on how restrictive your air intake system is. So I'd say your idle is too low and the WOT too high. Probably easy enough to double check with a cheap mechanical gauge.
Hey zjosh93, thanks! I see. Everything is 100% stock, so I'm wondering if it's a problem with the map sensor, or what else could cause such readings.

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post #4 of 12 Old 08-21-2019, 01:00 PM
zjosh93
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Been a while since I had to remember vacuum gauge testing results. Low idle vacuum can be worn engine components, valves, rings, etc. Low WOT can be a dirty air filter or an exhaust restriction.

Could also be a bad sensor I suppose. I know the later 4.0 has the MAP sensor on the throttle body but I'm not sure where it's getting the vacuum from so I can't really say.

Most auto parts places have vacuum gauges for $10-$15. I'd think about buying one. Weird that the OBDII isn't picking anything up. I assume you don't have a check engine light and have pulled any codes?
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post #5 of 12 Old 08-22-2019, 05:39 AM Thread Starter
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Yep, no CEL, and no codes. It's such a "barely-there" problem. I guess I'll have to wait until I'm stranded. Kind of nerve wracking because I'm trying to get it in tip-top shape before a 10-day driving trip around New Hampshire. I'll pick up a vacuum gauge in the meantime.

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post #6 of 12 Old 08-22-2019, 07:47 AM
jtec
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I would put a vacuum gauge on - confirm the readings, then old school* vacuum leak check .

* a can of carb cleaner as it idles - intake manifolds get the dbl ck.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #7 of 12 Old 08-22-2019, 08:59 PM Thread Starter
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Quote:
Originally Posted by jtec View Post
dbl ck.
Sorry, what is this?

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post #8 of 12 Old 08-23-2019, 09:15 AM
jtec
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dbl ck = double check - sorry I get lazy...

Did you try the vacuum leak check?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 12 Old 08-23-2019, 10:29 AM Thread Starter
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Not yet. It's actually started normally the past 3 days and run with no issues. So uh... I guess it's fixed! 😂. I'll try that vacuum trick though.

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post #10 of 12 Old 08-23-2019, 04:52 PM
Benthal8
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You probably know this stuff but this guy is amazing at explaining the interrelationship between sensors and air/ fuel mixtures. Iím still checking his 2 videos about fuel trims that preceded this one.
... just in case something in here spurs some ideas.
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post #11 of 12 Old 08-24-2019, 03:08 PM Thread Starter
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This guy is amazing! I'm watching this one first, as suggested:

Benthal8 likes this.

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post #12 of 12 Old 08-25-2019, 11:06 AM
Benthal8
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Yes! Same info but he really knows how to lay it out.
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