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Unread 06-04-2010, 11:35 AM   #1
toyotapilot
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1997 ZJ 
 
Join Date: Oct 2009
Location: houston
Posts: 12
What should MAP sensor read with scanner?

I have been attempting to diagnose my '97 Jeep Grand Cherokee and find out why I am getting the random misfires and sometimes cyl 1, 4 or 6 misfire. The Jeep doesn't seem to be running bad, just the codes. And according to my scanner they always occur at 1,856 RPM, usually at 63 mph.

What should the range on the MAP sensor be with the scanner? For the last four codes the reading has been 44, 35, 46 and 47 kph. With the ignition on but the engine not running with the scanner connected the reading is 99 kph. With the engine running in park the number sits in the high 30's and drops when I increase the RPM's. Elsewhere on the web I have seen that the number should increase as the RPM increases.

What should the numbers be for the MAP with the scanner? Just trying to do some diagnostics before I take it to a shop. It just seems a little strange that the code always appears at the same RPM (1,856 rpm).

So far I have replaced plugs, wires, cap, rotor, then an OEM distributor assembly (distributor, cap, rotor, cam sensor) due to a worn bushing. The injectors are new, and the coil has been checked. The engine was a new engine installed about 10,000 miles ago.

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Unread 06-04-2010, 01:49 PM   #2
rm2001wj
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Sea level air pressure is close to 100 kPa and that's what your 99 (kph?) with engine off is indicating. That is also equivalent to 29.9 in Hg and 14.7 psi. Anything less than that is usually referred to as a vacuum. To calculate manifold vacuum, subtract MAP sensor reading (in kPa) from 100. With a MAP of high 30s, the vacuum is in low 60s at idle. That vacuum would be about 18.5 inches which is about what it should be at idle.

When driving, the MAP reading will correspond fairly closely to the load being placed on the engine. When accelerating hard, accelerator pedal on the floor, the MAP sensor reading will go up close to 100, corresponding to a manifold vacuum close to zero. If you are moving at a moderate speed and get off the accelerator pedal (load on the engine gets smaller than when idling), and the MAP reading goes down to the low 30s and even the 20s). As the vehicle speed increases, there will generally be a greater engine load, and that will produce higher MAP readings.

But, you can't really predict exactly what the MAP readings should be in various phases of driving of a particular vehicle; you have to measure it. However, using your readings, I would say that the MAP sensor is not the cause of your misfires.
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Unread 06-04-2010, 01:52 PM   #3
toyotapilot
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Thanks for the information! I understand a little bit better now....
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