98redjeep

So, I have a new problem.

The jeep has started to become hard to start. It will either crank and crank and not start, then start instantly the second time you try. Or, it will slowly crank and then start.

I have recently installed a red top optima battery, which I thought might be defective or something, but I tested it with my volt meter and measure 12.5v when vehicle is off and 13.89 when running. I do not have a load tester, however all the lights in the cab are very bright and do not seem to dim when cranking.

I thought maybe my wife got a tank of bad gas (water in it?) so when the tank was at 3/4 I added premium to the tank. This may or may not have helped anything.

I do not currently have a way to measure the fuel pressure.

SO, I am thinking the starter is going bad; or the gas is bad; or the fuel filter is clogged, or the fuel pump is going. Any other ideas or tests I could do to try out mine?


thanks everyone, you are always so helpful on this forum.

RTorrez1

Next time you start it try the poor mans prime. Turn the key so you hear the fuel pump prime do not try to start it do this 2 or 3 time then try to start it. If it starts then more then likely you anti-flow back valve is going bad. Give that a try and let us know if it starts any better.

tjwalker

To elaborate a bit on what RTorrez1 is getting at with your XJ....
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The check valves (there are two of them; the primary one is on the fuel pressure regulator and a secondary one can be found on the fuel pump itself) are part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When a check valve starts to fail, it can reduce fuel pressure to a point where an extended cranking time 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) first step in helping you 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 about 2 seconds. Be sure to wait until it stops running.
3. Turn key to OFF position
4. Repeat above steps 1-3 two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have a check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the fuel pressure leaking down and the subsequent 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 starting symptoms as well. Have your battery load tested (any parts store will do this for free) if you have any doubt; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times. You will need a fuel pressure gauge. Many parts stores will rent you this tool inexpensively.

*Hook up fuel pressure gauge on the schrader valve on the fuel rail under the hood

*Start engine and bring to normal operating temperature.

*Observe fuel pressure gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)

*Shut engine off.

*Observe pressure on gauge. Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, a check valve within the
fuel pump assembly, or a fuel tube/line is leaking. An adaptor tool/hose normally included with the fuel pressure gauge can help you with this. Consult the manual that comes with the fuel pressure gauge and the Factory Service Manual for your year XJ for more information on this exact procedure, 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 – most likely a check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

**Note. Where check valve is suspect, a quick loss of fuel pressure is often the fuel pressure regulator check valve and a slow loss of pressure is often the fuel pump check valve. But keep in mind that either or both check valves can be to blame.

You can limp a check valve problem along indefinitely if you like, as it doesn't mean the fuel pump itself is going to fail. Perform the “poor mans prime” before cranking to speed up your starts. However, if you have a check valve issue and want to resolve it, it is recommended that you replace the entire fuel pump assembly for two reasons. First, you have to drop the gas tank to access the assembly. Secondly, with two check valves, replacing just the fuel pressure regulator where one of the check valve resides may or may not resolve your problem; remember there are two check valves; the other check valve is part of the fuel pump.

The fuel pump assembly consists of fuel pump, fuel pressure regulator, fuel gauge sending unit, fuel gauge float, pickup filter and pigtail wiring harness. If you do replace the fuel pump assembly, purchase a high quality assembly. Know this….you do get what you pay for here. Some cheap aftermarket assemblies have been known to fail prematurely. I personally prefer Bosch fuel pumps as they manufacture the best fuel pump assemblies for the XJ. Carter is my second choice. Airtex is my least favorite choice and I will not recommend or install them. A search will show you why.

98redjeep

Okay so the "poor man's prime" seems to have worked! I will say that the engine was not cold though; not sure if that would have made a difference.

I did turn the engine back off and then started it up again without doing the poor man's prime and it started instantly.

So what is this anti-flow back valve and where is it located? I hope it is not inside the gas tank.

tjwalker

The check valves are part of the fuel pump assembly, located in the fuel tank.

You can either replace the assembly or live with it and perform the "prime".

Re-read my post; it's all in there.

98redjeep

thank you for your informative post! I wrote my reply to the previous poster; Your post did not show up yet when I wrote my reply.

Looks like I really need to get my hands on a fuel pressure gauge!

98redjeep

Is this the entire fuel pump assembly? http://www.rockauto.com/catalog/more...456&cc=1315943

It looks small. The regulator and check valves are all inside of this? It is also a lot cheaper than I expected.


How hard is it to drop the fuel tank?

sumax

no the whole assembly looks something like this.

98redjeep

My pressure has dropped to 12 within 3 minutes. Probably looking at a bad fuel injector I guess? Is it best to change them all? I had thought about upgrading them anyway. It would be nice to know for sure it is an injector.

98redjeep

I should have mentioned I started out at 48 psi

tjwalker

Don't guess and I would definitely not assume that your problem is fuel injectors. Your problem could certainly be a check valve problem within your fuel pump assembly as this is a much more common problem. If you have an injector leaking, usually you will have a "stumble" when the engine starts, where the engine has to burn off the extra raw fuel. Test your way to a solution here. Don't throw parts at this problem; that is inefficient and becomes expensive very quickly!
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If pressure falls below 30 psi, it must be determined if a fuel injector, a check valve within the fuel pump assembly, or a fuel tube/line is leaking. An adaptor tool/hose normally included with the fuel pressure gauge can help you with this. Consult the manual that comes with the fuel pressure gauge and the Factory Service Manual for your year XJ for more information on this exact procedure, 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 – most likely a check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

**Note. Where check valve is suspect, a quick loss of fuel pressure is often the fuel pressure regulator check valve and a slow loss of pressure is often the fuel pump check valve. But keep in mind that either or both check valves can be to blame.