Hey. So, my son's 98 xj has the classic symptoms (I think) of a bad fuel pump check valve. Slow to start, stumbles a bit till he starts to drive it, then it runs like a champ. Here's what I don't get, if the check valve's bad, and is sending inconsistent fuel pressure to the engine, why would the symptoms disappear over time?

 

It may not be the check valve, but the pressure regulator. A bad check valve will cause a crank-no start condition until fuel pressure finally reaches the rail. This can be verified by turning the key on and off a couple of time before trying to start and it should start perfectly. You need a pressure gauge to test pressure. When you turn on the key the first time on a cold engine the pressure should jump up to full rated pressure then the pump shuts off until you try starting it. IDK what your jeep should be, but my WJ is 50 psi. When the engine starts the pressure should remain at near full rated pressure. If the pressure drops low and remains low, the pressure regulator is at fault or the pump is not putting out enough volume, OR the filter is causing a restriction. If the pressure drops to zero quickly after shutdown, that's a bad check valve or leaking injector.

A check valve only stops flow in one direction and that's all it does. It's designed to maintain pressure in the fuel system after shutdown, but may leak down slowly after a few hours. It may be all the injectors are seeping a bit until they warm up and causing the idle stumble. Another common problem is what we call "heat soak." that's caused by fuel boiling in the fuel rail after shutdown when it's really hot.

Another thought. Try cleaning the idle air controller. It may be sticking and causing the stumbling idle until the cylinders heat up.

 

A check valve only stops flow in one direction and that's all it does. It's designed to maintain pressure in the fuel system after shutdown, but may leak down slowly after a few hours. It may be all the injectors are seeping a bit until they warm up and causing the idle stumble. Another common problem is what we call "heat soak." that's caused by fuel boiling in the fuel rail after shutdown when it's really hot.

Another thought. Try cleaning the idle air controller. It may be sticking and causing the stumbling idle until the cylinders heat up.[/QUOTE]

Gotcha. So once the engine is started, the check valve is out of the equation. Using the "key prime" method, it'll start after turning over 4-5 times. Then like I said it stumbles for a bit, then runs like buttah. Warm start up much better than cold. I've cleaned the IAC already, but maybe it's time to put a new one in. Also sounds like it could use a new fuel pump assembly... You agree? Thanks for your help!

 

Check the pressure before you go dropping the tank. I thought I had a problem with my 2000 WJ since the pump was 255K miles old, but it delivers 50 psi all the time. My problem was a TPS that was scratchy around the idle position. It had a lumpy idle when cold and a stumble just off idle.

 

Some simple diagnostic tools would take a lot of the guess work out of this. If you're going to do your own work on these Jeeps a few tools pay for themselves to have on the shelf and none of them are expensive.

Fuel Pressure Gauge
Vacuum Gauge
Compression Gauge
Multimeter

All of these items can be purchased on Amazon for not a lot of money and will help you get info to either help yourself or help the forums help you. These tools can also help cut way down on money wasted on un needed parts and wasted labor time.

Get a fuel pressure gauge and hook it to the fuel rail. Start the Jeep and get it warmed up. 49.2psi +/- 5 psi at idle. If it's too low you have If it's too low you might have a pump issue if it's too low you have a regulator issue.

If it passes that shut down the Jeep and watch your fuel pressure gauge. Pressure should not fall below 30 psi for 5 minutes.

This is straight from the FSM in regards to the leak down test.

There is also a volume test, hot wire the pump and you should see at least a 1/4 liter (250ml) in 7 seconds from the fuel line at the rail.

 

Problem with a 98 is the fuel pump is in the gas tank...have to drop the tank to get at it...before you jump the gun...buy a fuel pressure test kit and verify you have a fuel problem before you change pump for no reason

 

Honestly was gonna just replace the pump but you guys have convinced me to do the fuel pressure check first. Your insights are much appreciated!

 

DO NOT buy a Harbor Freight tester! I just made the mistake of buying one and I had to do some special plumbing work to not have the hose fitting that screws on the gauge leak! The Chinese weren't too careful about holding thread tolerances and made the female fitting too big. I had to buy a brass pipe nipple and trim a little off the end to make the threads short enough for it to tighten up before it bottomed out, then buy a connector to connect the pipe end onto the gauge. Also, you'll have to use pliers to tighten up the fitting on the schrader valve. Spend a little more than $25 for a good one. I had to gut the plastic case in order to get the assembled unit into it.

 

Dude so funny you said that. Was so close to picking one up but the reviews were horrifying... Some s*** about the gauge filling up with fuel, etc. Products are hit or miss there. Bought a chipper that's been a work horse. Other stuff has been 100% junk. Ended up with a highly rated guage from Amazon... Heck, we just put a new IAC in and I haven't seen him since...I might not even need it (or, he might need a tow).

 

Believe me, if you own a jeep, you will need it sooner or later! I may be quoting the wrong person, but I think it's Boxburn who says, "If it's not broke, it will be!"

 

Learning this fact quite quickly. Great rolling project for my son and I. I tell him, owning an XJ is a lot like life. Great father-son stuff. However, difficult to understand for others.. My wife the other day asked me, " did you know there were gonna be this many problems when you bought it?"

 

There are no more problems than any other 20 year old car ...but you can fix most of them yourself without a mechanic...her 2002 year Sata Fe was thankfully scrapped at 15 years old and it barely made it there My old 92 just bit the dust at 25+ years and if it was anything less than a busted cam I would still be driving it...Died at 335K...I don't think the Hundai made it past 150k

 

From the looks of Hyundai's in the junk yard, they fold up like beer cans in accidents. Kia's too. One thing I have to say about Jeeps is parts are a lot cheaper than most other cars. My friend with a Ford Exploder spent $250 for a front wheel bearing! Ouch!

 

Alright so I used the tester and here are the results:
When cold, just priming: pressure jumps to 47 then when the key is off it immediately drops to 20 or so.

​​​​​​When starting from cold, pressure jumps to 47, hold steady, then when we switch it off it immediately drops to like 6.

So based on what you guys have said, and what I've read, that means the fuel pump is good but it's probably the check valve on the regulator or leaking fuel injector (s). What do you guys think?

 

Im trying to think of an easy way to check for a leaking injector yourself without sending them to a shop...First...try the same test on a fully warmed engine...if the pressure holds it might be an injector issue...I don't see a check valve warming up and getting better. but an injector ..easily could. You should be able to pull the rail and injectors out of the head without disconnecting the fuel feed line...If you then PRIME only you could see if any of the injectors leak thru....DO THIS ON a COLD engine ONLY.AND MAKE SURE THE INJECTOR LOCK COLLARS ARE INTACT.. Have you pulled the plugs if one or more are leaking that/those plugs might seem wetter or sootier than the others....Do you have an air compressor...take each injector out completely and slide some rubber tubing over the inlet side hook the other end to your compressor and crank the pressure to 40-50 psi...see if any of the injectors pass air...The check valve is only there to trap pressure for the start before there is a constant run signal to the pump once the pump is running the check valve is wide open and serves no purpose so it would not cause a rough running engine when cold...depending on your mechanical abilities you could disconnect the fuel line from the rail and hook up a gauge directly to the feed line...if it holds pressure after prime then check valve ok...I think a whole new set of stock injectors is cheaper than dropping the tank to change out the check valve so you have a 50/50 shot even if you do nothing...but you should be able to isolate with just a minimal amount of work...AND its not winter