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Unread 06-02-2013, 06:51 PM   #1
wsigo
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1996 ZJ 
 
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Video of my fuel pressure while driving- 96 ZJ 5.2- Input welcome

Yes, I've searched and read and read and searched. My eyes hurt, and my searching ability tends to leave me with 30 unrelated tabs saved for later reading.

I've been having problems with my ZJ stumbling, not revving, falling on its face, etc. this has been intermittent but as of late has been happening more frequently especially after it has been driven/ warmed up.

I bought a fuel pressure gauge from Harbor Freight, connected it to the Schrader valve on the fuel rail and used a metal coat hanger to hold it up while I drove down the road and back.

Just after connecting the gauge I turned the key to run three or four times to prime the lines. I noticed immediately it seemed too slow to build pressure up to about 40 PSI.

Started the Jeep and let it idle for about five seconds in park- gauge read approximately 46 PSI. Revved it a couple times and noticed the gauge dip to around 30 and back up.

Turned the engine off and went inside for about 10-15 minutes. Came out and pressure had not dropped but a couple PSI.

Test drove and shot this video.

With 249K+ miles I think I should just go ahead and replace the fuel pump, regulator and filter since I have to pull the tank anyways it seems.

Thoughts?



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Unread 06-02-2013, 07:07 PM   #2
dellis
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The fuel pressure should not be dropping like that ... I've driven around with a gauge on my car just like that, and it will barely moves off its pressure at idle (except for that rapid fluctuating). The fact that it occurs during hard acceleration (as you need for more gas) and not intermittent (e.g. electrical problem) seems to clearly indicate a restricted fuel filter or weak pump.

Make sure it is at least a Bosch pump (the OEM supplier), although some people believe that it should be Mopar - I guess that they are assuming that Bosch used higher quality parts for the Mopar pumps. You want to avoid Airtex, because there's been a lot of reports of failures with those pumps.
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Unread 06-02-2013, 07:15 PM   #3
ZeeJay1997
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given the fact that the pressure doesn't bleed off quickly when shutdown, ruling out injector leakage, I would be looking for a pump module.
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Unread 06-02-2013, 08:15 PM   #4
Rich93ZJ
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I noticed in the video that you are in a mountainous region. (Just a thought) I wonder if altitude is playing a factor. Have you tried this test at low altitude?
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Unread 06-02-2013, 09:43 PM   #5
wsigo
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Quote:
Originally Posted by dellis View Post
The fuel pressure should not be dropping like that ... I've driven around with a gauge on my car just like that, and it will barely moves off its pressure at idle (except for that rapid fluctuating). The fact that it occurs during hard acceleration (as you need for more gas) and not intermittent (e.g. electrical problem) seems to clearly indicate a restricted fuel filter or weak pump.

Make sure it is at least a Bosch pump (the OEM supplier), although some people believe that it should be Mopar - I guess that they are assuming that Bosch used higher quality parts for the Mopar pumps. You want to avoid Airtex, because there's been a lot of reports of failures with those pumps.
Yep, I knew it should be keeping pretty close to about 49 PSI even under throttle. Just never had a gauge on it before today and I've read so much about different things I was thinking at first it was electrical. (PCM connections) It's pretty obvious now that it's in fact fuel related. SOMETIMES if I kill the engine and restart- the issue is cleared up. That makes me lean toward a pump on its way out versus a bad filter- but I'll throw a new one on since they are so cheap anyways. I did also see people talk about going with a Bosch and not the Airtex. Rockauto.com has a Bosch P/N 69544 for $62.79. It is listed as "Fuel Pump Only"- this does come with the little "sock", correct? I've changed out many a Chevy Vortec fuel pump to know I need that little guy.

Quote:
Originally Posted by ZeeJay1997 View Post
given the fact that the pressure doesn't bleed off quickly when shutdown, ruling out injector leakage, I would be looking for a pump module.
Do you mean a fuel pump? Or the relay? I swapped the horn and pump relays today- didn't change anything.

Quote:
Originally Posted by Rich93ZJ View Post
I noticed in the video that you are in a mountainous region. (Just a thought) I wonder if altitude is playing a factor. Have you tried this test at low altitude?
I'm in the Ogden, Utah area. I have wondered this as well. I just got back from a trip to Indiana and the whole time I was there (a week) it ran great and had a noticeable power increase from Utah altitude. As soon as I came back here it started running poorly more often. On the other hand, it wouldn't take long (20-30 miles or so) for the computer to relearn anything so I think I can rule out the altitude difference. Not to mention it shouldn't even run this bad just based on that anyways, in my opinion. Everyone would be calling for The Big 3 to recall millions of cars if they were that susceptible to variation.

I appreciate everyone's input!
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Unread 06-02-2013, 10:50 PM   #6
Afdude88
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I thought those mountains looked familar. I was stationed at hill for about 3 years. I'm not in the service anymore but the wife and I are moving back later this year.

Ot...my guess is weak fuel pump

Dana 35c behind a 242, behind an AW-4, behind a 4.0L Inline Six sitting over a Dana 30, all turning stock 215/75/15 tires and moving a 93 Grand Cherokee to places your truck only dreams of.
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Unread 06-02-2013, 10:59 PM   #7
Afdude88
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I second the don't get a airtex. I currently have a pump from Napa. Looks different than the original I took out but so far, about 1.5 years later, it hasn't failed me. Though it does get noisy after about an hr and a half straight of being on.

Dana 35c behind a 242, behind an AW-4, behind a 4.0L Inline Six sitting over a Dana 30, all turning stock 215/75/15 tires and moving a 93 Grand Cherokee to places your truck only dreams of.
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Unread 06-03-2013, 12:17 AM   #8
wsigo
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Quote:
Originally Posted by Afdude88 View Post
I thought those mountains looked familar. I was stationed at hill for about 3 years. I'm not in the service anymore but the wife and I are moving back later this year.

Ot...my guess is weak fuel pump

Dana 35c behind a 242, behind an AW-4, behind a 4.0L Inline Six sitting over a Dana 30, all turning stock 215/75/15 tires and moving a 93 Grand Cherokee to places your truck only dreams of.
Quote:
Originally Posted by Afdude88 View Post
I second the don't get a airtex. I currently have a pump from Napa. Looks different than the original I took out but so far, about 1.5 years later, it hasn't failed me. Though it does get noisy after about an hr and a half straight of being on.

Dana 35c behind a 242, behind an AW-4, behind a 4.0L Inline Six sitting over a Dana 30, all turning stock 215/75/15 tires and moving a 93 Grand Cherokee to places your truck only dreams of.
AFSC? I just did 7 years 2W0- AMMO. When are you coming back? We'll have to grab a "strong beer"

I really am thinking weak pump. She's got a whole lotta miles and I have no idea on the MX history other than a receipt I found for trans service about 25K ago at AAMCO.

From the prices I came up with, I think the Bosch from Rock Auto is going to be the hot item to grab. I can also get a filter for like $3.18 from them and they ship pretty quickly from Arizona on a lot of stuff.

I just need verification that the pump I mentioned in my OP is the one to get.
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Unread 06-03-2013, 06:44 AM   #9
Afdude88
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2A3x3. I was a 4th AMU crew chief

Dana 35c behind a 242, behind an AW-4, behind a 4.0L Inline Six sitting over a Dana 30, all turning stock 215/75/15 tires and moving a 93 Grand Cherokee to places your truck only dreams of.
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Unread 06-03-2013, 10:10 AM   #10
a70eliminator
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Not sure I would trust those pressures. If you tap off the schreader valve you know that is just a very small orifice and a the slightest drop in system pressure will indicate a major drop on your guage, unless you completely removed the schreader core, then I guess it would be accurate.
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Unread 06-03-2013, 03:54 PM   #11
wsigo
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Quote:
Originally Posted by a70eliminator View Post
Not sure I would trust those pressures. If you tap off the schreader valve you know that is just a very small orifice and a the slightest drop in system pressure will indicate a major drop on your guage, unless you completely removed the schreader core, then I guess it would be accurate.
I see no reason to doubt the gauge readings.
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Unread 06-04-2013, 08:09 AM   #12
a70eliminator
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Well I guess I didn't explain what I was thinking very well. Not doubting guage accuracy, only that pressure in the fuel rail has to get to the guage through that tiny schreader valve (unless you removed the needle valve) in the hydronics field we call it a snubber, the restriction will cause a lag in pressure readings at the guage.
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Unread 06-04-2013, 08:20 AM   #13
dellis
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Quote:
Originally Posted by a70eliminator View Post
in the hydronics field we call it a snubber, the restriction will cause a lag in pressure readings at the guage.
I understand the concern, but I don't think there's a snubber problem there. The pressure goes down during acceleration and stays down with the higher fuel demand, and the pressure then rises only when demand is released.

That looks like a fuel pressure problem rather than a snubber problem.
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Unread 06-04-2013, 09:36 AM   #14
a70eliminator
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Ok yea I see that now the pressure does stay down your right.
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Unread 06-04-2013, 04:51 PM   #15
wsigo
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Thanks guys.

So, Bosch fuel pump part number 69544- listed as "pump only"... does this or does this not contain everything I'll need? I don't believe I need the entire assembly with float etc. Do these Jeeps have the little "sock" pre-filter on the fuel pump? If so, does this Bosch pump come with it?

Edit- I just looked at the pump on Amazon and the photo doesn't show one, but I realize it could just be a representation. It's also almost one dollar cheaper than Rock Auto and has free shipping, in case anyone else needs one.

Double edit- "Frequently bought together" on Amazon shows the fuel strainer (Sock, as I call it) for $7.35, so I guess my question has been answered.
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