MAP Sensor bench test - JeepForum.com

 
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post #1 of 6 Old 01-09-2016, 06:22 PM Thread Starter
InterpreDemon
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1998 ZJ 
 
Join Date: Jan 2013
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MAP Sensor bench test

Thought I would pass this one along for those of us (probably all) who enjoy chasing ZJ gremlins all over the place.

My Niner had the random high-idle, immediate reset upon restart a few times last year, cleaning and resetting the MAP sensor connector took care of it. Couple days ago I ran down to the bank and on the way noticed high idle on coasting, fluctuating RPM, etc., and within a couple more minutes started bucking and intermittently losing all power, basically 80% coasting into the bank. Revving with no load still intermittent cut-out like a bad coil, etc. Popped the hood and fiddled around with connections and after a couple minutes the idle settled in and return home was unremarkable. The next day everything seemed fine all day, but being an old hand at this I had the "Holy Water" Innova scanner ready in the glove box, which doubtless is why the demons were hiding.

On a bank run the following day, and almost at the exact time and place as the first event, the problem came back... but I was prepared. Got a system lean code, which suggested MAP or fuel pressure, but I also saw some bizarre and erratic MAP readings, like as low as 2" (27" vac, which is impossible) and dicking with connections didn't seem to immediately point to anything but it settled down to the point I thought I could limp home bucking and stumbling with my foot on both the gas and the brakes. Just in case it was something thermal, I decided to leave the hood unlatched so as to ram some cool air into the engine compartment... and sure enough within a couple miles things settled back to normal.

Because of the bizarre MAP readings (impossibly high vacuum) and a fuel pressure check was nominal, I determined that it either had to be a bad MAP sensor or bad wiring thereto, or PCM, but since the wiring is all in perfect shape and there seemed to be a reliable indication of a thermal issue, I decided to shot-gun the MAP sensor with a MOPAR replacement. Unlike many, I have no problem replacing a $90 item that has served me for over 100k miles even if the old one is not a confirmed kill... if the problem is not resolved I know I have a good spare. Running for an hour or so in the driveway and a few turns around the block after the swap, all systems seemed normal again, but for greater confidence and a wider radius of operations (beyond running down to the bank or supermarket) I needed to be sure the old MAP sensor was bad.

SO... I set up a test on the bench. Using a 1/2 x 12" rod-less pneumatic cylinder, vacuum gauge and tubing I was able to deliver precise, variable and steady vacuum across the normal operating range to the sensor. Next I re-tasked a 5 vdc wall-wart for an old router, alligator clipped it and a meter to the sensor and set about checking and plotting its response curve, which was disgustingly normal and linear, just the way it is supposed to be.

But now it was time for the thermal testing, which was accomplished with the aid of a 40 watt goose neck lamp systematically positioned at various distances over time with the vacuum held steady at 20" (idle). Sure enough, I found a very narrow "Goldilocks" range where the output, instead of being a steady 1.50V, started erratically roller-coastering further and further down to the point where it was bouncing off zero, then back up again. The thermal time constant was very long and it took multiple cycles and tests until I got the lamp positioned for maximum continuous crap-out, but it was definitely a confirmed kill. I don't have an IR thermometer, but I'm going to guess the temp to be around 110F. I have not tested to see if that varies with different vacuum, but I suspect not since when the demon surfaces and it starts twisting its head around and spitting pea soup, I can vary the vacuum all over the place while chanting "The Power Of Christ Compels You" and it appears to have no effect.

So that's it... a MAP sensor CAN have an intermittent thermal failure mode, not just dead or stuck lean or rich, and in this case the failure was to the lean mode (high vacuum, low voltage).

UPDATE: After two days and a couple hundred miles I can report a few other very minor driveability issues that have gone away that I did not really notice until they were gone. First is that the low throttle response is noticeably smoother and precise and the dashpot action much better. I always thought the difference between my '96 5.2, which is extremely smooth, and the Niner was due to the greater torque of the latter, but now the light throttle response of the two is identical and the shift points under light load as well. In particular the TCC, which would sometimes chug in and out at 40mph under light and variable load is now a solid lock with greater hysteresis.

I also had previously noticed a slight variability in the exhaust tone under light load sometimes, which I merely attributed to the freer flow (louder) combined with road or wind noise, but now that has disappeared completely as well, so it is clear to me the MAP was doing some minor jitters for some time before the disease became severely debilitating. Normally when one experiences no-code light load issues it points to the TPS, plugs/gaps, plenum plate, O2, etc., and I have dealt with all of those over the years, but if you are experiencing the occasional high idle or slight "hunting" under your foot under light steady loads, or the engine does not go to idle or the rpm fluctuates around significantly if you put it in neutral while coasting at 20-30 mph, you may be dealing with an ailing, but not yet mostly dead, or all dead (as Miracle Max would say) MAP sensor.


UPDATED Update: Still going strong, running great. Going to swap out the MAP sensor on my daughter's 4.0 this weekend just for the heck of it since I'll be passing through her town. She had the fast idle quirk two or three times over the past couple months, and the 4.0 always seems a tad rough anyway, so why not?


Last edited by InterpreDemon; 01-11-2016 at 12:02 PM. Reason: Update
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post #2 of 6 Old 01-09-2016, 07:53 PM
dharwood
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Finally, someone with as much OCD as myself (-:

Nicely done using common sense troubleshooting with on hand materials.

Kudos,

Dale
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post #3 of 6 Old 01-09-2016, 11:01 PM
MrRoundel
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Quote:
Originally Posted by InterpreDemon View Post
I have not tested to see if that varies with different vacuum, but I suspect not since when the demon surfaces and it starts twisting its head around and spitting pea soup, I can vary the vacuum all over the place while chanting "The Power Of Christ Compels You" and it appears to have no effect.
Odd, it always works for me.

Kidding aside, nice work on setting up your test apparatus and procedure. Can't argue with the empirical. At least I can't.

Next step, if a new MAP sensor doesn't fix it: "Quick, the Holy hand-grenade." Oh, wrong movie.
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post #4 of 6 Old 01-10-2016, 09:17 AM Thread Starter
InterpreDemon
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Quote:
Originally Posted by MrRoundel View Post
Next step, if a new MAP sensor doesn't fix it: "Quick, the Holy hand-grenade." Oh, wrong movie.
Oddly enough, when I was plotting the MAP readings I did try to convert them to the air speed velocity of an unladen swallow, but as the Niner was not made in either Africa or Europe the numbers were as irrelevant as the pet shop owner's complaints about being a qualified brain surgeon but preferring to be his own boss.
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post #5 of 6 Old 01-10-2016, 09:23 AM
MrRoundel
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Quote:
Originally Posted by InterpreDemon View Post
Oddly enough, when I was plotting the MAP readings I did try to convert them to the air speed velocity of an unladen swallow, but as the Niner was not made in either Africa or Europe the numbers were as irrelevant as the pet shop owner's complaints about being a qualified brain surgeon but preferring to be his own boss.
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post #6 of 6 Old 02-08-2016, 05:38 AM
jswerve
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Anyone have the oem map sensor part number?

Sent from my SM-G900R4 using Tapatalk

1998 5.9 |3.5" IRO lift w/adj CA and track bars | Bilstein 5100 | TJ Rubi wheels 245/75/16
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