Long crank when engine warm

[aquaman788]

Have a 97 XJ 4.0. When I crank my car over in the morning, its starts up almost instantly, like it barely even cranks before it fires up. But almost every time I crank it after that during the day, I takes a much longer crank, like 3 to 5 seconds, and the engine runs rough for about 2 or 3 seconds before it starts to idle smooth.

When I start up in the morning, the RPMs will usually rise to about 1200 on initial turnover, quickly drop to 1000, and then after a minute will stabilize at 700 to 750. When I start it after that, the engine will usually try and go straight to around 700 RPM, run rough almost like a misfire for a few seconds, and then stabilize.

Once she's up and running, she runs great; plenty of power, no stalling, smooth idle and acceleration. Just trying to figure out why it will start up fast in the morning with a higher idle, and then struggle to turn over when warm and have a lower initial idle.

Work done so far. replaced fuel pump and filter assembly, upgraded 4 hole fuel injectors, have run a pressure test on the fuel rail and it is holding well, so I don't think its a fuel pressure issue. Also checked the injectors for signs of leaking and found no evidence of such. New spark plugs and wires, inspected and cleaned distributor cap and rotor contact points. Battery is about a year and a half old. New starter. Removed throttle body and IAC valve and thoroughly cleaned both.

I know people often associate long cranks with a faulty crankshaft or cam shaft position sensor, but if that were the case I would think I would have a no start situation or issues when the car is running. Don't think its a heat or vapor lock issue either, I live in Alaska so my engine cools off fairly quick, and even after sitting for 6 hours while I'm at work it will still struggle to turn over.

I have found recently that if I tap the accelerator when it's long cranking that it will fire up and idle smooth right away.

Any ideas you may have are much appreciated. I know there are alot of forums on this issue but none seem to address my particular circumstances.

 

[Tim_MN]

When I start up in the morning, the RPMs will usually rise to about 1200 on initial turnover, quickly drop to 1000, and then after a minute will stabilize at 700 to 750. When I start it after that, the engine will usually try and go straight to around 700 RPM, run rough almost like a misfire for a few seconds, and then stabilize.

I have found recently that if I tap the accelerator when it's long cranking that it will fire up and idle smooth right away.

Are all the engine sensor genuine Jeep parts and not cheap Chinese clone parts ? O2 sensors are NTK, not Bosch or Chinese generics ? Inspection of the spark plugs or Live Data on your OBD-II code reader shows good clean combustion and no signs of excessive fuel ?


There should be no reason at all to fiddle with the gas pedal to start a fuel injected Cherokee engine, unless the PCM is re-booting and deleting the idle settings due to low voltage. Causes of low voltage could be corroded wire connections and ground connections, damaged wires, a faulty battery, or a faulty alternator.

Inspect the start and charge system, Load Test the battery and alternator. Test the Coolant Temperature Sensor and inspect its wires and wire plug.




Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Battery terminals and battery wire connectors should bright silver, not dull gray/black and corroded. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, wire brush, and clean until shiny the cable/wire ends and whatever they bolt onto.

Jeeps do not tolerate low voltage, bad wire connections, or poor grounds.


Place your DVOM (Digital Volt Ohm Multi-Meter) on the 20 volt scale. First check battery voltage by placing your multi-meter's positive lead on the battery's positive post ( the actual post, not the clamp ) and the negative lead on the negative post. You need a minimum of 12 volts to continue testing. Next, leave your meter connected and take a reading while the engine is cranking. Record this voltage reading. Now connect your positive lead to the battery terminal stud on the starter and the negative lead to the starter housing. Again, crank the engine and record the voltage reading. If the voltage reading at the starter is not within 1 volt of battery voltage then you have excessive voltage drop in the starter circuit.

Typical voltage drop maximums:
• starter circuit (including starter solenoid) = 0.60 volt
• battery post to battery terminal end = zero volts
• battery main cable (measured end to end) 0.20 volt
• starter solenoid = 0.20 volt
• battery negative post to alternator metal frame = 0.20 volt
• negative main cable to engine block = 0.20 volt
• negative battery post to starter metal frame = 0.30
• battery positive post to alternator b+stud = 0.5 volt with maximum charging load applied (all accessories turned on)


Test the output at the alternator with your volts/ohms multi-meter. You should be measuring 13.8-14.4 volts. Have the battery, starter, and the alternator Load Tested for proper function in a test machine that applies a simulated work load. Handheld testers are inaccurate and will often pass faulty parts.

 

[aquaman788]

Appreciate the input, but I'm doubting this is an electrical issue . If that were the case, why would it start great in the morning. If I had a faulty charging system, that wouldn't explain why it turns right over in the morning. I can literally let the car sit for a week and it will turn over instantly, but if I drive it 5 miles and then let it sit anywhere from 5 minutes to 8 hours it struggles on the start. I am an electrical technician for the military so I have done my due diligence when it came to inspecting the electrical system. The battery, starter and alternator have all been replaced within the last year. Battery voltage always checks out good, roughly 14 while running and well over 12 volts when off. The cables are the originals and show their signs of aging, but when I perform voltage and resistance checks across them, they check out to par. I do plan on upgrading them to a heaver gauge in the near future though.

Also, if it were an electrical issue, then I don't think tapping the gas would have an effect on the crank. If the PCM were losing power and resetting, again this would cause problems on the morning cranks as well I believe.

As for the sensors, many are the originals. I had considered the O2 sensor as I know the previous owner said he had changed it shortly before selling it to me, and based upon the state the Jeep was in when I bought it, he likely bought the cheapest one NAPA had available.

 

[Tim_MN]

Newish starting/charging parts doesn't automatically mean good parts. Warm start issues suggest a sensor that is heat sensitive.

 

[Unclewolverine]

Mine does the same thing, even across three different engines, rebuilt alternator, new distributor, high energy coil, 8.8 wires, every sensor replaced except the map sensor. I'm pretty sure the only things I haven't changed over the ownership of mine is the the fuel pump and injectors. I suspect my issue are 300k injectors, but that's where I'm at.

 

[RCP Phx]

I think I would take a closer look at the fuel injectors and fuel pressure. I would also consider testing the oil to see if there are traces of fuel?

 

[RCP Phx]

What's the injector part #? What's the flow rate (GPH@PSI) ? Where did you get them?

 

[old_man]

This is a common problem related to heat soak and your fuel rail/injectors.

 

[aquaman788]

Newish starting/charging parts doesn't automatically mean good parts. Warm start issues suggest a sensor that is heat sensitive.

Like I said, I went thru all of the starting/charging system components and everything checks out. Forgot to mention that the alternator has been recently replaced as well. The engine has plenty power when it turns over, even when its warm. It does struggle at all to crank, it simply just doesn't turn over, almost like its running to lean.

I think I would take a closer look at the fuel injectors and fuel pressure. I would also consider testing the oil to see if there are traces of fuel?

I have hooked it up to a pressure gauge and it has plenty of pressure after the initial fuel pump prime, and bleed off is minimal when the engine shuts down. I have pulled the injectors while they were warm to look for any signs of leaking fuel and none could be found. Oil has looked fine on the last couple changes, no signs of oil fuel that I can tell.

What's the injector part #? What's the flow rate (GPH@PSI) ? Where did you get them?
I purchased a set of rebuilt Bosch 4 hole injectors from K Suspension that they sell specifically for the XJ 4 hole upgrade.

This is a common problem related to heat soak and your fuel rail/injectors.

I am aware of this issue and I dont think this is the case. I live in Alaska and drive less than a mile to work in sub 40 degree temps, so the engine barely reaches operating temp. It then sits in that cold weather for 6+ hours while I'm at work and will still have the rough start. If it were a vapor lock issue, I would think that that length of time in those temps would allow any vaporized fuel to recondense.

 

[RCP Phx]

K is known for shoddy injectors!

 

[aquaman788]

K is known for shoddy injectors!

I've seen you make this claim before in your previous posts, but I have never seen many negative reviews of them while searching across the internet. Do you have any experience with them?

 

[RCP Phx]

I've seen you make this claim before in your previous posts, but I have never seen many negative reviews of them while searching across the internet. Do you have any experience with them?

Just research them here, you'll find plenty of people that replaced them with good ones.

 

[aquaman788]

Just research them here, you'll find plenty of people that replaced them with good ones.

Well this issue did come about not too long after I had swapped out the original injectors for the 4 hole. Luckily I still have the originals, which I only swapped out to try and improve fuel economy. Might be worth cleaning the originals up, throwing them back in there and driving around for awhile and see if there is any change.

 

[maxbraketorque]

Have the originals professionally cleaned and tested. If there is no one nearby who can do it competently, I recommend Fuel Injector Clinic. https://fuelinjectorclinic.com/ I've worked with them on my other project car, and they do great work.

 

[aquaman788]

Have the originals professionally cleaned and tested. If there is no one nearby who can do it competently, I recommend Fuel Injector Clinic. https://fuelinjectorclinic.com/ I've worked with them on my other project car, and they do great work.

The originals worked fine, I only swapped them out to try and gain the supposed benefits of the 4 hole ones. I shot some carb cleaner thru them and scrubbed them clean before throwing them back in, but I will look at those guys if I do stick with the originals and start to have issues.

 

[aquaman788]

So I went ahead and installed the old injectors, started the car 5 times yesterday throughout the day and didn't have a single hard start. So it looks like it was something with the 4 hole injectors. I contacted K Suspension and they are processing a replacement set for me. We will see if the new ones work any better.