98 4.0 hard to start when cold

Background:
98 4.0, AW4.
4 month old battery. Clean terminals, decent cables.
Brand new OEM plugs, wires, cap & rotor, air filter... full 30K and then some within last month
Symptoms:
When cold, the engine has to crank for a little before starting. I have let the pump prime a few seconds, no difference. Crank sounds strong. Once it runs, the idle can be slightly rough but is generally strong at ~1K till warm, ~750 once warmed up. These RPMs are taken at the factory in-dash tach.
When warm, the engine starts up like it was never shut down. Every time - reliable as the tides.
I'd started thinking "fuel pressure regulator" right off the bat.
My old man said "Clean the throttle body & TPS / idle air control / whatever sensors the Jeep has"
My boss, a mechanic, says "Check your fuel control system - whatever crap a Jeep has in it, anyway..." (We are Volvo / Toyota mechanics... my jeep is known as "The #$%Box" around the shop)
From searching, I read that the CTS being bad can lead the jeep to a lean condition making it hard to start when cold. What are the values I should be reading on the DVM when it is cold?
Other common failures?
Thanks!

Perform your preventative maintenance. Clean the the IAC and Throttle Body. Use some injector cleaner. Perform a complete Tune-up.

Perform your Diagnostic Tests: Fuel Pressure, Fuel injectors, Throttle Position Sensor, MAP sensor.

The CTS doesn't come online until you are warmed up and in closed loop.

A failed check valve allows the fuel to drain back to the tank. This results in the following symptoms:

SYMPTOMS

· Longer than normal cranking times
· Never starts on first try, almost always starts on the second try
· Rough idle for a few moments of idling
· Rough running for first ½ block of driving
· Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45 PSI.


POSSIBLE CAUSES

· Failed check valve on the fuel pump in the gas tank.
· Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky injector.

There are a few solutions to the problem –

1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.

2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.

3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.


If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies, like Airtex, don’t hold up and fail completely within 12-18 months. .

a vacuum leak could be the cause also. might check the torque on your intake manifold bolts. if those are good then the gasket maybe bad...or the intake could have a crack.

Will smoke the intake to check for leaks.
Full tune up performed.
Keying the ignition 2 or even 3 times produces no change in hard start when cold. Starting, shutting down & trying to start again immediately also produces no change. Does not seem to be a check valve issue.
I'm burning a can of Sea Foam through this last 3/8 of a tank right now. No change yet.
Throttle body is mostly shiny metal, though I'll clean up what carbon there is.
I have spotted what looks like dried fuel residue on the intake manifold. That's another thing I will be checking out when I have spare time in the shop.
Meanwhile, what values should the TPS / MAP / IAC sensors be reading?
Hard to believe any of them are too far buggered, since it runs & drives fine without throwing any codes.

It is time to do some fuel pressure testing with a fuel pressure gauge. That is one of the first things I would do if I were working on your XJ. It is simple and needs to be ruled in or out before moving onto other possibilities.

As you are having trouble COLD, fuel pressure "leakdown" is certainly high on the suspect list. There are specific numbers in the factory service manual that outline what is acceptable as far as a drop in pressure over a certain amount of minutes. It also outlines further fuel testing. But if you refer to Tim's post under possible causes, perform 1-2 and see what you get; that will certainly get you started and may yield a clue to what is going on.

It is possible that you have a leaky fuel injector. A leaky injector will introduce raw fuel into that cylinder, and will drop your fuel pressure. That results in an extended cranking time and running a bit rough for a short while, (you mention that your XJ does this) as the engine burns off the extra fuel.

Gotta test your way to a solution and fuel testing comes first. After that, I would test the coolant sensor (located on the thermostat housing) as a bad sensor will send an input to the computer that affects cold testing. There is a chart in most manuals which outlines the acceptable resistance of that sensor at different temperatures. The best way is to remove the sensor, place it in a jar of water, use a calibrated thermometer and heat the water and test resistances. Too much work for this kid. What I do is to test the sensor while it is on the vehicle. I get a reading with the engine stone cold, then a reading after the engine is warm. Compare your results to the resistance chart. If you are close, you are okay. If the resistance is out of spec, it usually is way out of spec...time to replace that sensor.

Good luck and be sure to update us to what you find!

According to the 2000 FSM, the Intake Air Temp sensor is the one that the PCM checks to richen the mixture for starting. The manual doesn't give resistance values but does say that it reacts inversely to the temperature. As temperature goes up, resistance goes down. I suspect that if it is busted, it won't react to temp changes at all.

Not sure on our Heeps, but some older systems that I worked on had the identical sensors for coolant and intake air. On the old Cadillac, I had to switch the electric connector over to the functioning CTS so that it would tell the computer that it was cold out, to get it to start.

More news:
Performed some tests after work last night.
Once I got the engine started (typical long crank) and moved it into the shop, pressure was at 49 PSI with the engine off. Started it up, still 49 psi.
I shut down the engine and left the gauge connected while I removed the throttle body and shot it with lots of carb cleaner. It is now carbon free. I also shot cleaner at the sensor openings though I did not have a tool handy to clean the ports. Removing the sensors for cleaning was deemed too risky when I tested the tightness on those screws.
The rail kept over 30 PSI during the throttle body cleaning process, longer than the FSM's 5 minutes for leakdown testing.
So, I'm getting 49 +- 5 when running, within 1-2 seconds of key on, holding acceptable pressure for 5 minutes and the thing still cranks when cold.
FSM says 0 PSI on a cold vehicle is normal, though I may still get some information by testing pressure key ON but before I start it. The pump is still suspect, and I have yet to test the volume it puts out.
Any other ideas?
Will keep at it and report.

Replace the coolant temp sensor. I know this sounds odd, but a bad sensor can make the ECU think engine think it is already warm when it isn't. Thus resulting in hard starting.
I had the exact symptoms with our '99 and that fixed the issue.

The CTS doesn't come online until you are warmed up and in closed loop.
FSM says that the PCM received input from the CTS in open loop startup and warmup modes.
Don't have the tools at the house but will test sensors early next week. I would love for this to be a cheap 10 minute fix...

AIT, Air Intake Temperature sensor is online while starting. They are NOT the same on our Jeeps as the CTS. $27 at rock auto. Check the resistance with a meter first. Heat it up and see if the resistance decreases.

winterbeater said:

AIT, Air Intake Temperature sensor is online while starting. They are NOT the same on our Jeeps as the CTS. $27 at rock auto. Check the resistance with a meter first. Heat it up and see if the resistance decreases.

Click to expand...

The CTS is online as well during startup mode. Will check both.

I've held off on updating until I was fairly sure the problem was solved. Here is what I did:
Verified that the fuel system was operating properly. No change.
Cleaned TB & shot carb cleaner in the TB sensors. No change.
Shot contact cleaner in the coolant sensor & applied a wire brush. Voila - starts with minimal cranking.
Never even got around to resistance testing the sensors in question!