My 2007 grand cherokee misses when the A/C is on, what could cause this?

 

Have you checked for any codes with a scanner, or by the key on/off method?

A misfire can be caused by a lot of different things.. Anything that has an effect on the air/fuel ratio is going to be one of the more possible likely suspects. Since you say it is only happening with the air conditioning on and running.. This makes me think to first suspect, test, and clean up the IAC valve. The IAC can cause issues with rough idle and possible misfires. The IAC valve is the built in compensatory mechanism for when the air conditioning is on and running. With the a/c on and the compressor engaged at the pully.. this requires the engine to supply more horse power. The IAC valve is what supplements the air/fuel ratio to match the increased horse power demand from the engine, by the compressor, for when the air conditioning is in operation.

There is of course a lot of other possibilities.

 

If the engine is missing, report it to the authorities.

 

I replaced the IAC, ran okay for a couple days, but still missing. The engine light comes on and off and the engine seems to run fine once the engine light comes on. Still getting the same old '300' code as before. Replaced all the sensors, cleaned the fuel injectors, replaced coil overs & plugs. Stumped!! Help!!

 

Oh yeah, when it starts to miss I give it more gas and it runs fine.

 

You replaced all of the sensors on your Jeep?

Have you thoroughly cleaned all the electrical connections, and groundings.. over the whole entire Jeep?

Maybe when you are pushing on the accelerator it is allowing a better electrical current to flow?

 

I font know where to start that. Is there a wiring diagram I can download?

 

There might be a diagram for the grounds somewhere on this site? I am not really sure? Maybe someone else would know how to direct you for that? They are not too difficult to find. You just look to anywhere an electrical wire terminates at a metal grounding point. You want to undo the grounds and use a wire brush, or a wire wheel (on a drill) to grind off the tarnish.. Corrosion is not always ussually something you can see. It is most likely going to be there on vehicles of this vintage.

You can start at the battery. I would start there at the battery. Check it to see if it has 12Volts+ static while at rest. Check it with a load (headlights and high beams on with the a/c blower motor fan on high, while holding the engine RPMs at approximately 1850rpm). You can also check the battery to see if it drops voltage when turning the starter motor to crank the engine. This would indicate a worn out battery that needs replacing. The easiest way to do these tests would be to drive it up to Advance or AutoZone. They will run these exact tests on your battery for you if you ask them to. The battery tests done while under a load will give you a general idea of your charging system - the state of your alternator too, which is important.

The battery is the primary electrical concern. Without it, none of the other electrical really matters.

From here, you can go immediately to the side of the battery.. to the main chassis ground located right there at the inner fender wall. Disconnect it to clean it up with a wire wheel running on a drill (or, elbow grease and a wire brush), and then tighten this grounding point back down again tight.

The engine block has grounds on it at the side and back near the firewall. You could go there cleaning those next, etc... Until you find and locate all of the grounds in your Jeep, to clean them with a wire wheel, or a wire brush.

Having good clean grounds is one of the most basic important baseline things! If the grounds are not good, a ton of unusually difficult to diagnose problems arise!

While you are looking to locate all of the grounds, look closely at any and all of the other wiring... Anything that does not look right? Over time, a portion of the main wiring harness may rub through with how it sits right next to bolts, or other sharp metal pieces in the engine compartment (it especially happens where it rubs through a bolt located near the backside of the engine, on some Jeeps). Look over the insulation you see on all of the wiring everywhere, to see if it has become brittle, torn, frayed, or burnt?

There are a lot of grounds and other electrical connections to look over (and clean!). For example, the one where the cable runs from the battery to the alternator is one of them. Anywhere a wire terminates...

 

Check your fuel pressure.

 

Did a fuel pressure check, range is supposed to be 56-60. It is steady at 64, is it being too high an issue?

 

No that should be ok. When was the last time you did a tune up or checked the spark plugs? Does it burn oil?

 

Tuneup was done. Coilovers replaced, plugs replaced, injectors cleaned and every sensor replaced including IAC. Checking vacuum next.

 

Code P0300 is a code that is generated when your engine has a misfire that is not related to only one cylinder.

What causes many, or all cylinders to misfire?

 

Apparently you didn't see this?

 

Fuel pressure is at 64, rang is 56-60.