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CSmith1128 05-15-2010 08:48 PM

Fuel Pressure testing...
Hey.. I just checked my fuel pressure and it is rapidly fluctuating between 42 psi and 50 psi while it is idling. My repair manual says the pressure should be between 44 and 54 psi. Is the pressure supposed to be fluctuating, or is it supposed to be steady? Also, if it is supposed to be steady, what could cause it to fluctuate?


MR.CLIFFORD 05-15-2010 08:49 PM

I'm gonna go with your key word of... fuel pump. How many miles on the pump you currently have?

CSmith1128 05-15-2010 09:54 PM

I don't think it has been changed so I'm going to say it has about 94,000. I was thinking it was the fuel pump, but I want to get some ideas on what else I should check before I go replacing the fuel pump.

jpdocdave 05-15-2010 10:07 PM

it should not be fluctuating, only thing i'd check is look at the fuel line under the hood for a kink or bend. its a long shot, but i've seen a couple before where someone damaged the fuel line, causing similar problems, but i'd bet on a pump.

doclouie 05-15-2010 11:54 PM

I am pretty sure there is a fuel pressure regulator and I would check that before I tear into a fuel pump.

rrich 05-16-2010 02:40 AM

Check the intake manifold vacuum - I'd bet it's fluctuating too. The pressure regulator tries to keep a constant differential between manifold vacuum and fuel pressure. If vacuum varies, so does FP.
Your gauge compares FP to atmospheric pressure.

Are you having a fuel starvation problem or just curious what the pressure is?

CSmith1128 05-16-2010 01:53 PM

It keeps stalling out on me while I'm driving. I checked the ignition and I have a good spark. Changed the distributor cap and rotor, the ignition coil and the spark plugs. Now I'm moving onto fuel and seeing if I have a problem there.

rrich 05-16-2010 03:56 PM

You might try revving it up to about 3000 and hold it - if it's still holds the same pressure it's not likely a fuel problem.

Need more information to be of much help.
There's jillions of reasons it could stall.

Engine hot, cold?
Hot outside or cold?
Going uphill, downhill, flat?
Low speeds, high speeds?
How many miles between stalls.
Burps and bucks before it stalls or just cleanly shuts off.
Between stalls -- runs nice as always or intermittently burps?
Predictable, not predictable?

How do you get it restarted - wait awhile, instantly starts again like it never stalled, threaten it with a towtruck? Kill 2 sheep at midnight? (I know a guy that when his stalls he props a For Sale sign in the window - it instantly starts! Go figure.)

Check Engine Light On, Off, Flashes, sometimes on and off.
Did you check it yet for codes?

I assume that you replaced those items AFTER you had the problem, not before.

Please answer all the questions as best you can. Otherwise you'll get lots of wild guesses on here that could be expensive.

And - BTW - what is it? Year, 4, 6 and any engine mods?

CSmith1128 05-16-2010 05:43 PM

I revved it up and it stopped fluctuating.. It held steady around 50 psi.

-It stalls when it is hot and when its cold.
-Its also been both hot and cold outside.
-Flat land.
-Both high and low speeds. Sometimes right after I start it, it stalls. Sometimes when I'm going 65. It usually stalls when I'm going 65/highway speeds, then after it stalls its hard to start unless I wait a couple minutes. After I wait I can sometimes drive it without it stalling. Then, for the next couple days after I stall, it stalls about 10 seconds after it starts.
-When it stalls, it just cleanly shuts off.
-Not predictable and it runs a little rough between stalls. Not noticeable to other people probably, but I can tell.
-I wait about a minute or two before I start it up again and sometimes it runs fine, sometimes it wont start at all.
-No check engine light, no codes.
-And yes, I changed everything after I stalled.

-It stalled on me a few months ago and my mechanic put in a new distributor pickup and it ran fine for about 2 months before it started acting up again.
- Before that it would do the same as it does now.

Its a 1997 6 cyl 4.0L no engine mods.


rrich 05-16-2010 10:38 PM

Good answers!
Since FP is holding steady at higher speeds it's very doubtful its fuel
Just for grins - check the coil wire - probably not it, but it can't hurt to look. Make sure it's fully seated both ends.

No codes, clean shut-off, restarts sometimes quickly, sometimes not.
It sure sounds to me like the Crankshaft Position Sensor - those are the symptoms.

Before replacing it, try something.
Find the connector to it - it's on the back of the engine behind the end of the valve cover. The sensor itself is on the bellhousing - where it's hard to see, even harder to reach.

Next time it stalls -- Find the connector - black plastic - find the red tab on it - pull the tab out about 3/8" - that's a lock. Open the connector by pulling it apart. Wipe the dirt off the pins and put a dab of Silicone Dielectric grease on the pins and the plastic. Reconnect. The grease is available in tubes and small little envelopes at parts houses.

The signal in that wire is very small and any resistance in the connector can be enough to give trouble. For the techies - it's a very high impedance signal.

It will probably start fine. But just sitting a moment often lets it start too - so you won't be sure you fixed it. Time will tell.

If it continues the stalling, replace the CPS.
It looks terrible to replace, but it's not. About a 10 minute job with a helper.

Let us know and we can guide you through the replacement - hopefully you won't need to do it.

Doing the connector trick works about 1/2 the time.

The CPS runs from $30 all the way to over $100. I think yours is the cheap one.

DO NOT GET ONE FROM A DISCOUNT STORE - the best place is from the dealer. You'll get a quality part that will last, and the latest version. AutoChina and the like only sells knock-offs - IF they work at all, they will fail soon.

Buy quality, expect quality results.

Let us know.

CSmith1128 05-17-2010 01:45 AM

Hey... thank you for the help.

Here's the crazy thing...

- About 6 months ago I replaced the crankshaft position sensor.
- I replaced it because it was idling rough and would sometimes stall out when I stopped and I received an error code.
- Also, about 3-6 months before I replaced the CPS, the previous owner replaced it.

When I replaced the sensor, I didn't apply any grease. I will try to disconnect it and clean it and replace it with grease.

However, since I have already replaced it, and since my mechanic replaced my distributor pickup about 3 months ago when it was stalling the same way (and that fixed it for 3 months), is there anything else I should check while I'm checking the sensor? I like to do everything at once, if possible.

Thank you for the help. I appreciate it!

rrich 05-17-2010 03:00 AM

Where did you get the sensors?

Could it be they all came from China or Mexico?
The only sensors I'll install are ones I know are from the dealer. Even dealers may cheat - make sure it's in the original Chrysler box.

I've seen too many knock-offs give trouble to use anything else.

A high impedance circuit like that the signal is easily shorted by dirt, water etc. The Dielectric grease keeps that stuff out - forms a barrier.

Something else to look at too - that sensor signal goes into the harness, then to the plugs on the computer. Engine off, battery disconnected - remove and clean those big connectors on the computer. Then use a light coating of that Dielectric grease on them and reconnect.

Only use Dielectric grease - other greases like Vasoline, wheel bearing grease etc. will not work properly.

CSmith1128 05-17-2010 03:16 AM

I bought the sensor from AutoZone, which is the closest parts store to my house.

Ok, so I will clean a grease the connectors tomorrow.

Is it possible for the sensor to go bad without throwing any codes?

Other than the connectors, is there anything else that can cause these problems?

If not, I'll check back after tomorrow and let you know what happened.

rrich 05-17-2010 08:23 AM

The CPS can set a code, but rarely does. A condition to set a code is it has to be there awhile - when a CPS fails, it usually causes the engine to shut down, faster than it can set a code.

The signal from the CPS is super important to the computer - it tells when to shoot fuel through the injectors and when to make the spark. A momentary loss of signal has a profound effect.

The Autozone part - close doesn't make the part any better. When it comes to important things like sensors, NONE of the aftermarket suppliers have decent sensors. NAPA might, they are usually better, but I won't chance it.

I've probably been involved with at least 40 CPS ailments - I've learned not to bother with the junk. The dealers part is very close to the gyp-joints price - maybe even a little lower sometimes.

If you take a CPS apart you'll find a very fine coil of wire wrapped around a magnet. The wire must be wrapped too tight or the insulation rubs thin. In any case, non dealer CPS's have proven over and over to be unreliable.

No, I don't, and never did work for or with a dealer - I just have to answer to my customers and friends when I fix something.

It's well worth the effort to drive a few hundred miles to get one that works - all the time. Most dealers will send you one by Fed Ex.

Chrisbob 05-19-2010 03:47 PM

I've been having the same EXACT problem. I think I have my answer and I really appreciate your responses. I read them all and now I will lube the CPS. I also will avoid buying important stuff from Kragen. It's all from China or Mexico. I don't shop at WALCRAP either...never have.

rrich 05-19-2010 06:04 PM

I hope you mean lube the connector, not the CPS itself! I'd bet that would cause problems.

It really isn't lube - Silicone Dielectric Grease, it's a silicone barrier that keeps moisture and dirt out. It just feels like grease. As a lubricant it would be terrible.

It's that white stuff you find on lots of electrical connectors. A tube will last you a lifetime - you'll lose it or step on it before you use it up.
Also good to put on plug wire ends.

It doesn't always work, but often enough to try it. Else the CPS needs replacing - with a real part, not a knock-off.

Chrisbob 05-20-2010 04:31 PM

I lubed the connector not the CPS. It turns out it was the wrong connector. I did use dielectric grease. I'll lube the right one now and see what happens. The jeep did start this morning. When I held the accelerator pedal at 2100 rpm the speedo fluctuated about 500 rpm up and down but could not hold a steady rpm. Engine was cold and A/C was on. When I turned off the A/C the rpm's stabilized but still jumped around slightly. Any thing to be concerned about there? Connections to the coil are good.

CSmith1128 05-26-2010 08:43 PM

New symptoms...
Hmm.. I now have another symptom.

-I just noticed that when I stop at a stop light, my oil pressure drops. And the longer I drive, the more quickly it drops when stopping, and the lower it drops. Its around 40 when I'm driving, then it drops to 20 when I stop, and eventually it drops to around 5.

-It also has a little shake to the car when I stop. Its not a constant shake, just a little harder wobble every few seconds.

- Also, I've been driving it normally all week and it hasn't stalled out on me yet. So, thats a plus.

Is this all a problem, or is this normal?


Indy 05-27-2010 11:58 AM

Oil pressure drop is normal .. nothing to worry about.

Harder wobble every few seconds may be because of dirty throttle body and IAC. Try cleaning them and see if it helps.

rrich 05-27-2010 01:03 PM

The oil pressure - maybe you are using multi vis oil that's too thin? What are you using, and where do you live?

Remember, the additives that are added to make it multi vis are NOT LUBRICANTS. Best to use a straight viscosity oil, 20, 30, 40, 50 etc.

The unstable idle - the computer has been getting unreliable signals from the CPS for a long time. It will probably take 100 miles or more before it regains stability.
Or - reset the computer back to default. Leave the battery disconnected overnight. That's the only way to get it back to factory default. Other methods only clear codes but do not deep clean the memory.
-- For those that want to argue - try it their way - drive it. Then do it my way - notice the big difference.
Unless of course you "like it to run bad."

I'm still suspicious of that Chinese CPS - it may work for awhile, but it will eventually fail!

When you use the right quality parts, you get quality performance.
Cheap junk is just cheap junk.

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