Absolute manifold pressure sensor bad?

redjeeper

redjeeper

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I have a 2004 4.0 automatic jeep and for whatever reason the jeep won't idle and it seemed to be running rich or lean.

So far I have replaced all four o2 sensors, crankshaft positioning sensor, and the air intake temperature sensor. None of them seemed to have solved the issue.

However, my friend remembered when we dropped the skid plate to put the front driveshaft in that there were wires just sitting there and found that there was a short in one of them. So he put it back together and the jeep was running better but still kind of rough and the rpms weren't going up.

We then disconnected the manifold absolute pressure sensor and although the car died more quickly the rpms were able to go up when putting the foot on the accelerator.

Could it really be the absolute manifold pressure sensor? I know the part isn't that expensive but I've been buying so many parts already.
 
Unplugging a sensor like that is rarely a proper diagnostic of the sensor.

Any codes?
 
idle issue. have you looked into the idle air control valve? might just need a pull and clean with some throttle body cleaner, a few cuties and a paper towel.

seem to remember my IAC issue caused a refusal to idle after a cold start but was fine once warm but also random stalls at lights. swapped the idle air control valve out with new oem part and problems cleared up.
 
03coloradowrangler said:
idle issue. have you looked into the idle air control valve? might just need a pull and clean with some throttle body cleaner, a few cuties and a paper towel.

seem to remember my IAC issue caused a refusal to idle after a cold start but was fine once warm but also random stalls at lights. swapped the idle air control valve out with new oem part and problems cleared up.
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Yes I took the IAC out and cleaned it and it looked fine.

Still idled rough and died
 
In my experience MAP and TPS sensors are replaced frequently even though there is nothing wrong with them. They are really quite tough and don't fail very often. More than likely you have something else going on that a good driveability technician could diagnose quickly. Do you have a scan tool with live data?
 
Jezza said:
In my experience MAP and TPS sensors are replaced frequently even though there is nothing wrong with them. They are really quite tough and don't fail very often. More than likely you have something else going on that a good driveability technician could diagnose quickly. Do you have a scan tool with live data?
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No I don't have a fancy scan tool. I just have a regular code reader and then turning on and off the car to get codes which I get zero.

I'm honestly at the point where I am about to bring it to a shop because it seems like whatever I replace doesn't do the trick.

I will say that I have not checked the fuel pressure or fuel injectors which may be the cause... I have also checked the coil pack and it was fine and my spark plugs didn't look that bad either but who knows, I know some people have replaced them and it was the last thing they did and it ended up being the culprit.

But do you think I shouldn't bother buying a new manifold pressure sensor? I've been throwing money towards this issue and I don't want to spend money on another part that probably isn't the issue.
 
without good live data there is no indication you need a MAP sensor.
 
You're not getting results because you are shotgunning parts at it. For what it is worth you can pick up a scan tool that will do live data for under $50.
 
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Jezza said:
You're not getting results because you are shotgunning parts at it. For what it is worth you can pick up a scan tool that will do live data for under $50.
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Hell, buy a random $10 Bluetooth obd2 tool on Amazon and the Torque Pro app.

And I'll recommend, again, checking the fuel pressure.
 
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freedom_in_4low said:
Hell, buy a random $10 Bluetooth obd2 tool on Amazon and the Torque Pro app.

And I'll recommend, again, checking the fuel pressure.
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Update: I checked the fuel injectors with a multimeter except one since the connector is hard to get off without taking other parts off.

Anyway, all of them read between 11.2-11.7 ohms and one read 8.6ohms while cold.

What’s the typical reading on this and could the one that read 8.6 be the issue?
 
Jezza said:
Mine were all 12.0 or more. 8.6 is quite low.
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I'm assuming the one that I didn't check is probably going to be around the same specs as the others. So should I just replace the 8.6 injector? But would only one bad injector cause my car to not idle?
 
redjeeper said:
Update: I checked the fuel injectors with a multimeter except one since the connector is hard to get off without taking other parts off.

Anyway, all of them read between 11.2-11.7 ohms and one read 8.6ohms while cold.

What’s the typical reading on this and could the one that read 8.6 be the issue?
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I can't find the spec in the fsm but an engine running on 5 cylinders still runs better than what you're describing.
 
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redjeeper said:
So should I assume that probably the other injector that was not tested is potentially bad as well and replace the two?
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You'll have to disconnect it to replace it anyway, might as well just pull whatever is in the way and check it first.
 
No. Even 2 poor injectors most likely would result in a misfire type condition but engine would still respond to throttle, most likely anyway.
Have you removed or tested your map sensor? Have you checked spark?

It still sounds similar to my sons ranger when the cat failed and was blocked. Easy way to check that is, since you just replaced O2 sensors, remove one and start it. If it an excessive back pressure issue it will run better with the new vent. But it could be anything causing it.
 
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Plugged cats are not usually an intermittent problem, and usually won't affect idle as much as higher rpm and load.
 
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