My girlfriend has a 2008 Jeep Grand Cherokee (5.7l Hemi) that won't start. It rolls over but doesn't fire (or even sound like it's trying to start). However, a little starting fluid sprayed in the air intake and she fires right up. After getting it started, it will continue to start (if warm). Cold starting is the issue. So I'm going to break it down the way I thought it out to eliminate possible culprits. Please let me know if I'm missing something or incorrect about something.

1. Fuel Supply Issue
I. Fuel Pump - The main reason I don't think this is it is because it starts fine warm. If it was a bad fuel pump, it wouldn't just not work when cold starting and work when warm starting.

II. Fuel Filter - Again, it would have to be totally plugged to not push the needed fuel to the engine but then again it starts fine warm and runs fine once running (no hiccups or loss of fuel).

III. Fuel Pressure Regulator - since this regulates fuel to the engine, having it start warm and run fine after starting doesn't really make me believe this is the issue. BUT, I am not to sure on symptoms of a bad regulator other than what I've googled (which don't match the issue at hand).

IV. Pinched/clogged fuel lines or vapor lines - again, it starts fine warm and after I spray either.

Notes: The one thing I am unsure about is the fuel vacuuming idea. Fuel Pressure needed to start and idle is different than it is driving. After it's started the lines vacuum from the pump (in theory). Could this explain anything as to the difference in cold/warm starting?

2. Spark & Compression
I. Spark Plugs - spark is fine. Plus, again, it starts fine warm.

II. Compression - tested good but again, starts fine warm.

Other than a small issue that I haven't thought of or some electrical issue. I am at a loss here.

Anyone have any ideas?

. Check the Engine Coolant Temperature (ECT) Sensor--its thermistor may be bad. It's supposed to provide a rich mixture when cold and a lean mixture when hot, by controlling fuel injector pulse width. Resistance should likely be around 40,000 ohms at 68 degrees F and 1,500 ohms at normal operating temperature--get the exact specs. This is a prime source for problems.

. Replace the IACV. Otherwise, you can try to remove and carefully clean the IACV, if the car has one, with non-chlorinated brake cleaner or throttle body cleaner. This is the next most likely cause of your problem. It regulates the amount of air the engine receives at start-up and idle, through use of a step motor--this is air that by-passes the throttle valve. The air valve and mechanism become cruded-up, which interferes with the motor precisely controlling the air flow, by impulses from the PCM. If it sticks closed, the engine will idle low, rough, or stall. Also, ensure that the throttle body's upper by-pass port is cleaned. This is a good time to clean the complete throttle body and replace the air filter. If you don't have an IACV, you may need to thoroughly clean the throttle body--the concept, here, is the same; i.e., an air control issue.

Therefore, the IACV controls the amount of air and the ECT Sensor controls the amount of fuel, based on engine coolant temperature, at start-up.

. Check the battery voltage. What is it? Best to remove and fully charge the battery.

. Check the fuel pump relay. Battery voltage should be received by the fuel pump.

. Ensure the MAP Sensor is connected and functioning. It also helps control air/fuel mixture.

. Check pressure going to the fuel rail--compare to specs.

. Check for air leaks.

. Replace spark plugs (Champion only), if they have over 40,000 miles on them.

A comp will be hooked up to it later today. Hopefully that gives us something. From what I've read, the ECT controls fuel mixture. Assuming it's bad and can't recognize a cold start from a warm start would explain why the mixture won't fire when cold but will when warm (as a richer mixture is needed for a old start).

I will look into this more and update accordingly.

Any solution?