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Driveway Diagnostics - MAP sensor

Posted in How To: Engine on December 18, 2013 Comment (0)
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Got a brain and a decent set of hand tools? If the answer is yes, then consider yourself capable of fixing most of the common maladies your Jeep is sure to experience over its lifecycle. By design, Jeeps are relatively simple critters. Sure, you can't fix the more modern variants with a butter knife and a hammer fashioned from a rock, but chances are you won't need a $100,000 Scantronicbot machine either.

Any fool can chuck new parts at a mechanical problem until it goes away. But why throw good money out the window when you don't have to? That's why over the next year (or maybe longer, who knows) we'll highlight some problems common to Jeep models, walk you through the diagnostic tests to pinpoint the issue, and show you the solution.

MAP Malfunction
All fuel-injected Jeeps use a MAP (manifold absolute pressure) sensor to determine how much fuel the engine requires. The MAP sensor monitors engine manifold vacuum and translates that reading into an electric signal sent to the ECU. Depending on the voltage from the MAP sensor, the ECU will fire the injector for a longer or shorter burst to deliver more or less fuel, as the case may be. It's a fairly foolproof and simple design, but they can have issues. If after the engine has come up to operating temperature your Jeep suffers poor drivability, surging, rough idle, excessively rich exhaust, or pinging even with mid-grade fuel, chances are the MAP sensor has begun to go out of range or has failed altogether. The thing is, a bad MAP sensor won't always trigger a check engine light or cause the computer to register a DTC (diagnostic trouble code).

Diagnostics
Newer Jeeps, especially '96-newer OBDII models, have the MAP sensor mounted either on the manifold or right next to the throttle body. Older Jeeps, especially '95-older OBDI models, will have the MAP sensor mounted remotely, usually on a firewall-mounted bracket.

First, you can check for codes, but like we just said, many times a failing MAP sensor won't throw a code until it has died altogether. It doesn't work with all models, but if you don't have a code scanner you can sometimes turn the key from Off to Run three times in succession and then leave the key forward in the Run position after the final time. Watch the digital odometer display or look for flashing indicator lights. Either note the code that is displayed or count the flashes. The OBDI code for a faulty MAP will be either DTC 13 or 14, while the OBDII code is P0108. But chances are you won't have a scan tool or be lucky enough to have your Jeep flash codes.

Test for voltage at the plug connector as indicated in the photos and captions, and make sure the vacuum line to the MAP sensor isn't kinked, crushed, or leaking. The MAP sensor should also hold vacuum without bleeding down. Follow the photos and caption for the full rundown.

The Fix
With a MAP sensor the only fix is to replace the unit if found faulty. Thankfully, in addition to offering tons of really cool aftermarket gear to personalize your Jeep, Quadratec offers a whole array of high-quality OE-spec replacement parts. After determining ours was faulty, we ordered a new MAP sensor and had it installed in minutes. The Jeep immediately regained its drivability and no longer surged, bucked, or idled harshly. We haven't calculated a full tank yet for actual numbers, but the mileage seems improved as well.

Parts and Special Tools
Quadratec offers replacement parts for all Jeep models from current JK all the way back to the '41 Willys, as well as certain specialty hand and diagnostic tools. Our tools were part of our current inventory, but if you don't want to buy special diagnostic tools, your local auto parts store will sometimes rent or loan what you need.

• Multi-meter with DC scale
• Hand-operated vacuum pump
• Quadratec PN 55122.2001 MAP sensor for '87-‘95 Jeep Wrangler and Cherokee with 4.0L or 2.5L engine

Using our ’89 Wrangler’s OBD1 MAP sensor as a guide, there will be three wires in the harness plug: a power in, a negative ground, and a signal out. The wiring colors and locations may be different, so with the key in the Run position and the engine off, unplug the harness and test for voltage with a multi-meter. There should be 5 volts at the power wire. If not, the problem is upstream of the MAP sensor in the vehicle’s wiring harness. Be careful with older plugs—the plastic gets as brittle as ice. Using our ’89 Wrangler’s OBD1 MAP sensor as a guide, there will be three wires in the harness plug: a power in, a negative ground, and a signal out. The wiring colors and locations may be different, so with the key in the Run position and the engine off, unplug the harness and test for voltage with a multi-meter. There should be 5 volts at the power wire. If not, the problem is upstream of the MAP sensor in the vehicle’s wiring harness. Be careful with older plugs—the plastic gets as brittle as ice.
After verifying there’s 5 volts to the power wire, probe the signal wire (usually the middle one) and check the voltage on your multi-meter. At 0 in.Hg vacuum the output should be 4.7 volts. Hook a vacuum pump up and check voltage while applying vacuum. At 5 in.Hg the signal should be 3.9 volts, at 10 in.Hg it should be 3.0 volts, and at 20 in.Hg it should be 1.1 volts. More voltage equals a richer condition, less is leaner. Keep in mind even a small variance (0.2 volts) can make a big difference in air/fuel ratio and drivability. After verifying there’s 5 volts to the power wire, probe the signal wire (usually the middle one) and check the voltage on your multi-meter. At 0 in.Hg vacuum the output should be 4.7 volts. Hook a vacuum pump up and check voltage while applying vacuum. At 5 in.Hg the signal should be 3.9 volts, at 10 in.Hg it should be 3.0 volts, and at 20 in.Hg it should be 1.1 volts. More voltage equals a richer condition, less is leaner. Keep in mind even a small variance (0.2 volts) can make a big difference in air/fuel ratio and drivability.
Our Jeep was incredibly rich at lower RPMs, wouldn’t rev past 3,000 RPM, and idled very poorly. Our voltage readings were all over the place at every measured value. So at higher engine speeds when manifold vacuum was low and more fuel was required, our MAP was still reading like the engine was at or just off-idle, starving the engine of fuel. A new MAP sensor from Quadratec (PN 55122.2001, $52.99) cured our problems. Our Jeep was incredibly rich at lower RPMs, wouldn’t rev past 3,000 RPM, and idled very poorly. Our voltage readings were all over the place at every measured value. So at higher engine speeds when manifold vacuum was low and more fuel was required, our MAP was still reading like the engine was at or just off-idle, starving the engine of fuel. A new MAP sensor from Quadratec (PN 55122.2001, $52.99) cured our problems.
Multi-meter with DC scale,
Hand-operated vacuum pump, and Quadratec PN 55122.2001 MAP sensor for '87-‘95 Jeep Wrangler and Cherokee with 4.0L or 2.5L engine Multi-meter with DC scale, Hand-operated vacuum pump, and Quadratec PN 55122.2001 MAP sensor for '87-‘95 Jeep Wrangler and Cherokee with 4.0L or 2.5L engine

Sources

Quadratec
West Chester, PA 19380
800-745-2348
www.quadratec.com

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