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Double Prime required..crappy idle afterwards

K

Kinslayer

NAXJA Forum User
Location
Mission, BC
Both me and my buddy have this issue - we have to crank the truck over, key off and then crank again for it to start..

Both 98s, both AW4s, both 2door. Mines black, his is green ;)

Double priming doesn't seem to help (key on, key off, key start) but key start, off, key start works.

Sometimes it's not required at all, sometimes it fires right up with no problems. The problem doesn't follow the temp of the vehicle, sometimes it fires right up cold in the morning, sometimes not, sometimes double key after a quick pitstop, sometimes not. USUALLY it's followed after a long period of being off (IE cold).

Most times, after starting up it will idle up, then down and sound like it's bogging out too.. and it will run like crap for a bit, usually stomping on it to 'clear it's throat' or so it seems... after that, runs fine.

I was thinking possibly a leaky injector..?
 
Maybe. The 97+ with the plastic tank and the relocated fuel pressure regulator/check valve on top of the tank frequently have problems with the fuel pressure/fuel draining off.

Put a pressure gauge on it, take a reading with the engine running, then shut it off and observe how long it takes for the pressure in the line to start dropping.

Post up what you find.
 
hmmmm kinda have same problem in my 90 aw4 was thinking fuel pump not as good any more or the one way valve letting the fuel back feed back into the tank
 
CromeY90 said:
hmmmm kinda have same problem in my 90 aw4 was thinking fuel pump not as good any more or the one way valve letting the fuel back feed back into the tank
Click to expand...

87-90 and 91-95/6 are a little different.

87-90 are Renix and the engine must "spin up" to 300+ rpms before a strong enough signal from the crank sensor will be sent to the ECU to fire the spark. For us, putting in an external check valve can fix the fuel drain off problem, but it doesn't add much in the way of a "speedy" start. To get a better start improve the battery main cables, check 5-90's website he makes some of the best I have ever seen. Have a fresh strong battery. A fresh starter helps too.

If you want to throw a fuel pressure gauge on and see how quick the pressure drops off that is always a good idea. Seller jbass195 on eBay sells a nice check valve for $14 + $2 s/h. Also, the FPR--fuel pressure regulator on the fuel rail can leak too. You can check it by pinching the return hose after shutting off the engine to see if the pressure drop off is reduced.

Good luck.
 
I have the same problem with my 97. If i double prime it, its fine. I asked around and I got the same response, fuel pressure check valve in the pump. I just cycle the key twice and usually im good to go. If I dont do that, I get extended crank and the thing sounds like it misses for a couple seconds until it smooths itself out. Apparently its a chrysler thing
 
The check valve is located on top of the gas tank and is part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When the check valve starts to fail, it reduces fuel pressure to a point where an extended crank is needed to start the engine. It is a very common failure. I experienced it on my 99 at around 75K.

A good (and simple) way to troubleshoot this problem is called "the poor mans prime"

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above a two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have the check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the bleed down and the stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

Also be aware that a bad battery can give you this symptom. Sounds strange but it definitely does happen. Have battery load tested if you have any questions; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times:
Hook up fuel pressure gauge on the schrader valve on the fuel rail
Start engine and bring to normal operating temperature.
Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)
Shut engine off.
Pressure should not fall below 30 psi for five minutes.
If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the
fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:

*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.

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

You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely.
 
birchlakeXJ said:
The check valve is located on top of the gas tank and is part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When the check valve starts to fail, it reduces fuel pressure to a point where an extended crank is needed to start the engine. It is a very common failure. I experienced it on my 99 at around 75K.

A good (and simple) way to troubleshoot this problem is called "the poor mans prime"

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above a two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have the check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the bleed down and the stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

Also be aware that a bad battery can give you this symptom. Sounds strange but it definitely does happen. Have battery load tested if you have any questions; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times:
Hook up fuel pressure gauge on the schrader valve on the fuel rail
Start engine and bring to normal operating temperature.
Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)
Shut engine off.
Pressure should not fall below 30 psi for five minutes.
If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the
fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:

*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.

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

You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely.
Click to expand...
not sure on the 95.5, all 95s and 96s were metal oldstyle tanks AFAIK, all 96s were the returnless fuel delivery system though. Pretty sure you mean either the check valve is part of the fuel pump assembly on 95.5 up (but may be on top or on the side of the tank depending on year) OR that the check valve is part of the fuel pump assembly on 96 up.

EDIT: yeah, 94-95 pump assemblies are listed as all being the same, while 96 is the new one and lists "comes with fuel pressure regulator". Fuel tank for all 96s is the same, gasket is the same, locking rings are all different but the 94 and 95 ones were superseded by the 95.5-96 one.
 
Last edited:
Thanks Birchlake..great tips.. I haven't gotten a pressure gauge yet though hehe.

I did find quite a decent exhaust leak yesterday though, cracked manifold where it goes from 2->1.. now I know the O2 sensors don't do anything until they are hot, which might explain why it runs shitty while hot at idle sometimes..
 
That could indeed explain it!

Try the "blowtorch trick"... grab a plumber's propane torch, turn it on but don't ignite it, and point the nozzle at the crack in the manifold. If the idle changes, you found your problem.

You can also use carb cleaner for this but I wouldn't want to spray it on a hot manifold, seems like a recipe for :explosion:flame:
 
depends on the brand/type. The stuff I buy says it's non chlorinated and the label on the back verifies this, it's just heptane and acetone.
 
Ok so an update on this.. i've been too busy getting married and going on vacation and doing wheeling trips to work on my heep heh..

I recently was working on a charging issue and noticed that the crappy idle strangely coincided with.... the #1 plug wire arcing out on the head. If replacing the wires resolved the idle issue (which it did) it MIGHT have resolved the weird start issue..we shall see.
 
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