'98 XJ Hard Start after Heat Soak

[PaulJ]

My '98 occasionally doesn't like to start the morning after a good heat soak the previous day. When the weather is moderate or otherwise, it starts up immediately whether the engine is hot or cold. My guess is that I have an injector that leaks when the engine gets a very hot heat soak after shutdown. Any thoughts?

 

[PaulJ]

FYI, it always cranks fine.

 

[Tim_MN]

Probably a failed fuel pressure check valve, and maybe a leaky injector or two.



When a 1997 to 2001 Cherokee won’t start on the first try day after day, the first thing to suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located in the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of 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 45-50 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 pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, don’t hold up and fail completely within 12-18 months.

 

[ehall]

My '98 occasionally doesn't like to start the morning after a good heat soak the previous day. When the weather is moderate or otherwise, it starts up immediately whether the engine is hot or cold. My guess is that I have an injector that leaks when the engine gets a very hot heat soak after shutdown. Any thoughts?

Fuel from an injector leak should evaporate away pretty quick, so overnight would seem to cancel that out. A temperature-sensitive injector leak should also close up after the car cooled off for a couple of hours. You can diagnose this by pulling the plugs and checking for wetness. A better test is to pull the rail and lay the injectors over cardboard, and watch for leaking under pressure, but it would be hard to do that if it only shows up when engine is hot.

A defective check valve usually shows up as overnight problem (fuel drains out of the rail) but that should happen all the time not just after running hot. The pump could be on the way to failing and not quite there yet, maybe the pump gets hot and the check ball fails to seat. The usual test for check valve is put a pressure tester on the Schraeder valve and see if fuel drains out rapidly when you turn off the vehicle (fuel drains out of the rail back into the tank). Watch the pressure when you prime the pump, when it runs, and after you turn it off. If you only have problem when engine is warmed up, you could let it idle for a while before turning it off.

Option 3 is the IAC gets stuck or something. Try holding the pedal to the floor when you start it. That will bypass the IAC.

 

[Dragonslayer]

Vapor lock.

When you shut down the engine after a hot run, the fuel pressure drops, then engine
heat causes the fuel in the fuel rail, and injectors, to vaporize and form bubbles.

On the next startup, the engine will cough and sputter and misfire until the vapor
clears out and the injectors start spitting liquid fuel as they're supposed to do.

Next time after a hot run, soon after shutting off the engine, prop open the hood
and let the engine cool down more quickly with the hood open. If this is actually
your problem, the engine should fire up smartly on the next cold start.

Some folks install a fuel injector heat shield to alleviate this problem. Some heat
shields were installed by the factory on later model Cherokees.

 

[Tim_MN]

Vapor lock only happens when the engine is hot. Once it cools down, the fuel condenses back to liquid.

As already suggested, simple fuel system testing should pinpoint the actual cause of the symptoms.