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  #1  
Old 09-29-2009, 02:11 PM
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Default Fuel Pressure drops off, check valve or injectors

OK. For my 95 Jimmy 4x4 with the 4.3. I finally made a fuel pump gauge. (pieces from a canibalized a/c pressure guage, works great) well I tested my fuel pressure. when turning the engine on without starting the pressure shoots up to 60psi. But as soon as the pump turns off after 2 seconds, it drops off to under 10psi. When it IS running it remains at 54psi.

I've done reading. I haven't checked the pressure at the fuel filter yet. It'll be a bit of a bugger if I have to. But background. I've already replaced the fuel pressure regulator and the fuel pump. And there are NO leaks. I've checked the lines all the way up.

OK I see that this is a symptom of a bad pump check valve or bad injectors. Now the pump is new. And the old one did not seem to be bad before the change. But the symptoms are the same from before the pump replacement. I will probably end up doing the pressure check at the fuel filter. If the pressure doesn't drop off is it definitely the spider unit going bad? But just in case, where is the fuel pump check valve? Is it IN the pump? If it is that is NOT the problem.

I read it could also be fuel pressure regulator valve. As the FPR is new, where is this valve located? And how can I check it?

Now with my 98 I had to replace two of the injectors in the spider unit. Am I correct in understanding this unit in the 95 CAN NOT be serviced and it has to be replaced as one unit? I have a feeling I'm going to have to remove my plenum and do some checks there to be sure. And actually, removing the plenum might be an easier check than the fuel filter, and more info.

Is a way to check this to take off the plenum, pop out the injector tubes from the intake and see if anything shoots out when you turn it to ON?

I can handle taking the plenum off easier than dropping that damn tank again. I've done searches but just need these specific answers.
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Old 09-29-2009, 05:08 PM
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There is no other fuel pressure regulator valve than what is located inside the FPR itself.

Connect the fuel pressure tester to the pump output line at the fuel filter. If the pressure still drops off, the check valve in the tank is bad.
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Old 09-29-2009, 05:40 PM
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But where IS that check valve? And I'm not looking forward to undoing that fuel filter again. It's new but checking the pressure there is an issue. But again. Where is the valve?
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Old 09-29-2009, 05:46 PM
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From my previous reply:
Quote:
Originally Posted by swartlkk View Post
*snip* the check valve in the tank is bad.
More specifically, it is part of the fuel pump assembly. It is not independently replaceable so a whole new fuel pump would be required.

Are you sure you installed the FPR correctly? I'd atleast pop off the upper plenum again and have a look at the FPR and/or nut kit to make sure that it is not leaking. If you removed the CPI spider to replace the FPR and did not replace the internal fuel lines (nut kit), then that is a very likely leak point.

As far as replacing the spider as a whole, that would be the proper thing to do. It is unlikely to just have one or two poppet nozzles freeze up without the others having other wear/tear on them that would cause them to flow inaccurately. Replacing only a few injectors/poppet nozzles on the SCFI (96 thru mid-2002) engines is iffy at best. Most of the time, replacing more than one injector/poppet nozzle assembly is more costly than replacing the whole spider with the updated MFI spider (again only applies to the 96+ SCFI engines).

Why is checking the fuel pressure at the fuel filter a problem?
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Old 09-29-2009, 08:34 PM
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Eh, replacing those injectors on the 98 was a mistake. The shop I took it to made a mistake and said the 3 injectors on the driver side were bad. Turns out it was a bad o2 sensor. needless to say I didn't go to them again.

When I replaced the FPR I did it on-engine. I did not take the spider unit out. Yeah, I'm sure I installed it correctly. But I'm concerned there is another leak in there. I'm going to pop the flow valve off again to get a look at it at least, tomorrow. That'll be an easy check. And the fuel pump is new and it is working good so I doubt it's in that. And if the check valve is IN the pump, then that won't be it. So I'm back to the spider unit. Looks like I know what I'm doing this weekend. (AGAIN)

Disconnecting the fuel pump it tenuous. The lines were hard to get undone when I replaced the fuel pump. I had to use rubber fuel lines at the sending unit as is, but it'll require replacing the line from the fuel filter to the sending unit all out this time. Making THAT job bigger than it needs to be. And since I am now thinking the spider unit, it's a job that won't solve the problem, that can be done at a later time. I'm more concerned with this issue than replacing that line if I don't have to. Not only that I have to rerig my fuel pressure guage to get it to hook onto the fuel filter line. Presently I have it staying in the engine compartment since it's such a bugger to get a hand in back there. Yeah, after replacing those damn injectors on the 98 for no reason I have issues with doing jobs that don't solve the problem. If it won't matter I'd just assumed fixing that fuel filter line at a later time as it's not leaking or causing a problem now. (I inspected it thoroughly). Though I'm dreading the cost, it will be a relief to verify it is the spider. aside from the cost it really is an easy job to replace.
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Old 09-29-2009, 08:43 PM
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Quote:
Originally Posted by cage47 View Post
OK. For my 95 Jimmy 4x4 with the 4.3. I finally made a fuel pump gauge. (pieces from a canibalized a/c pressure guage, works great) well I tested my fuel pressure. when turning the engine on without starting the pressure shoots up to 60psi. But as soon as the pump turns off after 2 seconds, it drops off to under 10psi. When it IS running it remains at 54psi.

I've done reading. I haven't checked the pressure at the fuel filter yet. It'll be a bit of a bugger if I have to. But background. I've already replaced the fuel pressure regulator and the fuel pump. And there are NO leaks. I've checked the lines all the way up.

OK I see that this is a symptom of a bad pump check valve or bad injectors. Now the pump is new. And the old one did not seem to be bad before the change. But the symptoms are the same from before the pump replacement. I will probably end up doing the pressure check at the fuel filter. If the pressure doesn't drop off is it definitely the spider unit going bad? But just in case, where is the fuel pump check valve? Is it IN the pump? If it is that is NOT the problem.

I read it could also be fuel pressure regulator valve. As the FPR is new, where is this valve located? And how can I check it?

Now with my 98 I had to replace two of the injectors in the spider unit. Am I correct in understanding this unit in the 95 CAN NOT be serviced and it has to be replaced as one unit? I have a feeling I'm going to have to remove my plenum and do some checks there to be sure. And actually, removing the plenum might be an easier check than the fuel filter, and more info.

Is a way to check this to take off the plenum, pop out the injector tubes from the intake and see if anything shoots out when you turn it to ON?

I can handle taking the plenum off easier than dropping that damn tank again. I've done searches but just need these specific answers.

Came across this in another forum just before I saw your post. Interesting fix for what seems to be a similar problem:

http://www.automotiveforums.com/vbul...d.php?t=966478

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Old 09-29-2009, 08:43 PM
 
 
 
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2001, 2002, 96, bad, blazer, changing, check, chevy, filter, fuel, inline, jimmy, pressure, regulator, symptom, valve


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