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Unread 12-08-2011, 07:13 AM   #1
95cherokee33s
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How to test fuel pressure?

1998 Jeep Cherokee with the 4.0. It started the other day when going up long hills, the jeep would cut out. I tested the fuel pressure and at idle it is 44-45 psi which is a little low but within the 5 psi of the 49 standard. Is there anything else I can do to check the fuel pump pressure to see if the fuel pump is the cause? Thanks

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Unread 12-08-2011, 07:32 AM   #2
maimedsk8er
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Does this happen on a full tank as well?
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Unread 12-08-2011, 09:23 AM   #3
the_weirdo
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To test fuel pressure you just connect a mechanical gage to the fuel rail.

What happens after the Jeep cuts out? It could be the CPS is failing. Going up hills creates lots of heat and can fry the CPS. Then once it cools down for a few minutes the Jeep will fire back up.
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Unread 12-08-2011, 09:32 AM   #4
maimedsk8er
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Quote:
Originally Posted by the_weirdo View Post
To test fuel pressure you just connect a mechanical gage to the fuel rail.

What happens after the Jeep cuts out? It could be the CPS is failing. Going up hills creates lots of heat and can fry the CPS. Then once it cools down for a few minutes the Jeep will fire back up.
If you read his post he has tested the fuel pressure and it's fine. I've never heard once of Crank Position Sensors "Frying" because of heat. If that were the case Jeeps everywhere would completely die when they overheat or are used for towing/offroading. They will fail eventually or even act up after years of being heated/cooled, but so will any other sensor on the vehicle; even the hottest, the upstream O2.
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Unread 12-08-2011, 11:39 AM   #5
the_weirdo
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Quote:
Originally Posted by maimedsk8er View Post
If you read his post he has tested the fuel pressure and it's fine.
If you read his post, he asks if "there anything else I can do to check the fuel pump pressure to see if the fuel pump is the cause?" The short answer is no - the only way to check the pressure is at the rail. That is exactly what I stated.

Quote:
Originally Posted by maimedsk8er View Post
I've never heard once of Crank Position Sensors "Frying" because of heat. If that were the case Jeeps everywhere would completely die when they overheat or are used for towing/offroading. They will fail eventually or even act up after years of being heated/cooled, but so will any other sensor on the vehicle; even the hottest, the upstream O2.
Sounds like you need to do more research on the CPS. Just because you haven't ever heard of it, doesn't mean it is not true. I've got tons of experience with the CPS (I battled this issue for 6 months and am very aware of how it happens) - they will absolutely get too hot and shut down (especially if they are old). Wait 15 mins for them to cool and they function again. The issue with the CPS is that it is directly behind the downpipe - when the exhaust heats up, it heats up the CPS and causes it to fail. A major contributor to heat is also cracked exhaust manifolds which outlet hot exhaust right on the sensors.
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Unread 12-08-2011, 10:32 PM   #6
motoxxxman
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actually yes there is other ways to test the fuel pump, going deeper into pressure testing at the rail:
-test fuel pressure at idle with the fpr vacuum hose connected, then disconnect the vacuum hose and hold your finger over it to stop the vacuum leak (still idling) and look at the fuel pressure now. remember both numbers. the higher number (vacuum hose removed) is the one you use to compare to stock spec.
-now connect the vacuum hose again and find a way to mount the pressure guage somewhere it can be seen while driving. like remove the hood and stick it in the wiper if it's long enough, or something similar. the more throttle you give it, the higher the pressure should get, and vice versa. basically, you're looking for a drop in pressure as rpms climb. ie: cruise at like 15-20mpg, lock the tranny to 2nd gear, give it heavy throttle and watch the guage; fuel pressure should be near that high number from idle with vacuum removed, and should stay at that pressure all the way to redline. full throttle should be exactly the same pressure as that high pressure at idle, again, all the way to redline. if the pressure drops off at any point as rpms climb, especially if it's a considerable amount, fuel pump is shot.

and fyi, a fuel pump can be darn near 90% shot and still provide perfect test results doing the idling test, because idle uses near zero fuel volume, like 99.9% of the fuel the pump is pushing will be just going straight back to the tank.
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Unread 12-08-2011, 10:35 PM   #7
motoxxxman
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oh, and to add another thought, if it's only cutting out on heavy loads, ie climbing a hill, it could simply be something like spark blowout, due to the spark plug gap being too big. re-gap the plugs to stock spec and see what happens. (or replace the plugs and make sure the gaps are correct. champion or ngk plugs only, anything else is gimmick or junk)
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Unread 12-08-2011, 10:43 PM   #8
the_weirdo
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Quote:
Originally Posted by motoxxxman View Post
actually yes there is other ways to test the fuel pump, going deeper into pressure testing at the rail:
-test fuel pressure at idle with the fpr vacuum hose connected, then disconnect the vacuum hose and hold your finger over it to stop the vacuum leak (still idling) and look at the fuel pressure now. remember both numbers. the higher number (vacuum hose removed) is the one you use to compare to stock spec.
-now connect the vacuum hose again and find a way to mount the pressure guage somewhere it can be seen while driving. like remove the hood and stick it in the wiper if it's long enough, or something similar. the more throttle you give it, the higher the pressure should get, and vice versa. basically, you're looking for a drop in pressure as rpms climb. ie: cruise at like 15-20mpg, lock the tranny to 2nd gear, give it heavy throttle and watch the guage; fuel pressure should be near that high number from idle with vacuum removed, and should stay at that pressure all the way to redline. full throttle should be exactly the same pressure as that high pressure at idle, again, all the way to redline. if the pressure drops off at any point as rpms climb, especially if it's a considerable amount, fuel pump is shot.

and fyi, a fuel pump can be darn near 90% shot and still provide perfect test results doing the idling test, because idle uses near zero fuel volume, like 99.9% of the fuel the pump is pushing will be just going straight back to the tank.
Absolutely agreed, but he must measure it at the rail - there is no other place to measure.
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Unread 12-08-2011, 10:51 PM   #9
motoxxxman
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Quote:
Originally Posted by the_weirdo View Post
Absolutely agreed, but he must measure it at the rail - there is no other place to measure.
correct
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Unread 12-09-2011, 05:47 AM   #10
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note to weirdo, notice you're the ONLY one who mentioned the Crank sensor??
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Unread 12-09-2011, 08:18 AM   #11
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Just a thought, but there's a semi-permeable bag in the tank that holds gas in it when in conditions where the incline or decline you are on will cause your gas to be on one side of the tank, no longer keeping your fuel pump pickup line submerged. If this bag falls off, or has a hole in it, it will cause a dry spot in your fuel line when it contacts the air in the tank, thus causing a stall.
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Unread 12-09-2011, 12:33 PM   #12
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Quote:
Originally Posted by maimedsk8er View Post
note to weirdo, notice you're the ONLY one who mentioned the Crank sensor??
Notice that I offered possible solutions, for a similar issue that I had (stalling when going up hills).
Notice that you have offered no solutions.
Notice that everything on this board thinks you are a jackhole.
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Unread 12-09-2011, 02:05 PM   #13
Azzy
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A failing Crank sensor WILL do exactly what problems he is having, it looks like a fuel problem but is not. Just went through the exact crap last week. They will warm up and then stop working. in fact, after getting a tow home after the last time we tried to get it running again, my XJ fired right up after we pushed it into the driveway.
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Unread 12-09-2011, 10:42 PM   #14
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Quote:
Originally Posted by Azzy View Post
A failing Crank sensor WILL do exactly what problems he is having, it looks like a fuel problem but is not. Just went through the exact crap last week. They will warm up and then stop working. in fact, after getting a tow home after the last time we tried to get it running again, my XJ fired right up after we pushed it into the driveway.
I had the exact same scenario - got a tow home on a flatbed after it stalled. Once I got dropped-off at home (30 minutes) I started it back up and drove it into the garage. Swapping the CPS solved the issue. It would be nice to hear back from the OP and run a 2 minute test when the vehicle stalls or at least get some more info to help diagnose.

The OP can test for a failed CPS with an ohm-meter and about 2 minutes of time. If it is bad it may test good at cold and bad at hot. Here is the procedure in post 3: http://www.jeepforum.com/forum/f11/c...-maybe-889010/. Try it next time the vehicle stalls.

It may or may not be the problem, but it is easy to test and include/exclude.
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Unread 12-13-2011, 09:18 PM   #15
motoxxxman
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Quote:
Originally Posted by maimedsk8er View Post
Just a thought, but there's a semi-permeable bag in the tank that holds gas in it when in conditions where the incline or decline you are on will cause your gas to be on one side of the tank, no longer keeping your fuel pump pickup line submerged. If this bag falls off, or has a hole in it, it will cause a dry spot in your fuel line when it contacts the air in the tank, thus causing a stall.
um, NO
you're talking about the sock. that thing is nothing more than a filter to keep larger debris out of the fuel pump. it has absolutely zero effect on fuel supply when on inclines. not to mention it's volume is less than 0.25 fluid ounces
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