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Ivan, ASE Certified Master Technician
Category: GM
Satisfied Customers: 4871
Experience:  ASE Master Tech.20+ years exp. 9 years self employed. Participant on AutoLab Radio broadcast
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1998 GMC: 4.3L V6 Vortec, with CSEFI (SPFI..fuel filter..started doing

Customer Question

I have a 1998 GMC Jimmy, 4.3L V6 Vortec, with CSEFI (SPFI some call it).    It was acting like it had a dirty fuel filter, about a year ago.   So i replaced the filter, and everything was fine, for a year.   It started doing it again about 3 weeks ago.   (if you're stopped, and you apply accelerator to go, it will hesitate, bog-down, then instantly rev-up and go)    To me this is exactly the experience of having a dirty fuel filter.    But 3 weeks ago, i replaced the filter again, but now i'm having the same problem again.   Also, it is not consistent.   The problem seems to come and go, whenever it wants to.    What's really going on?
Submitted: 8 years ago.
Category: GM
Expert:  Ivan replied 8 years ago.

Hello Wyatt, these truck are extremely sensitive to fuel pressure. Check fuel pressure with a guage. You have a test port on the fuel rail on the intake. Fuel pressure needs to be 55-61psi with the key on. It runs around 55 psi running and pressure increases under load. Make sure fuel pressure does not go down from 55 ever. If you have less then 55 with the key on, or fuel pressure does go down, the fuel pump is your issue and is very common with this complaint.


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Customer: replied 8 years ago.
Yesterday, i slapped-on the fuel pressure gauge.   With engine at normal temp.... With key on and pump running, engine off, pressure was 62.   When pump turned-off it dropped to 54 but increased back-up to 62 within about 5-10 seconds.   I then cranked the engine, and with engine running it stayed at 54 psi.   If i stepped on the gas, it increased, as it should.
Customer: replied 8 years ago.
p.s.   I meant to add.......      According to what i see on the pressure gauge.   I don't think it's the fuel pump, even though the pressures are slightly out of spec, my gauge could be slightly off.   If you think it is the pump, please explain so i can understand why.

I meant to add this, but i got stuck doing some acceptance thing, and questionaire, and totally lost the q&a page.
Expert:  Ivan replied 8 years ago.

The way these systems are, they require and absolute minimum of 55 psi to open the fuel injector tappets. The tappets work like a mechanical valve and release at a certain pressure. If the pressure is even 4 psi lower, the system will start having issues. The fuel pressure regulator's job is to increase fuel pressure when load is applied raising fuel pressure towards that 66 psi mark. If fuel pressure decreases, the fuel pump is the reason why. Regulators can fail, but it's usually a case where the fuel pressure will not hold, it sinks with the key on

Expert:  Ivan replied 8 years ago.
I'd like to add, you need to tape that guage to the winshield and see what pressure is doing when it starts acting up
Customer: replied 8 years ago.
Sorry, little confused by your answer.   Are you saying you think it's still gonna be my fuel pump?
Expert:  Ivan replied 8 years ago.
Could be, see what the pressure reads when the truck starts to bog and hesitate. It's possible the pressure that you are reading at idle is going to be very different under load when fuel consumption and demand rises.
Customer: replied 8 years ago.
I did have the gauge on, in the driveway, trans in drive, holding the brake, and tapping the gas.   Pressures did as i expected, and increased up to about 64, or 65 under demand.   I will Tape the gauge to the windshield, and try it for a while.   But when i disconnected the gauge, and drove around, i had no problems, and it seems that the problem could dis-appear for another six months.

I've been reading alot online about a lot of problems with the CSEFI systems, and that the fuel meter body should get replaced the new "MFI" (Multi Flexible InJection.... ).    What do you think about that?   
Expert:  Ivan replied 8 years ago.

These intervals can drive you crazy, the key is to have the guage on during the bog. You won't see anything when the truck is OK. If fuel pressure is truly ok during the bog, then we can focus elsewhere. This post will remain open for a long time, so you can jump right back with me as you need to. That maxi-injector set up, is a common failure, but I would want more evidence before condemning it.