fuel pressure gm 454 1998 motorhome

rognelson777 · 02-12-2013, 12:15 AM

Posted for transmission help on here for motorhome and need help with fuel pressure diagnostics 1998 gm 7.4l 454 8 injectors.

Checking fuel pressure today, turn on key, pressure goes to 54 then drops quickly to 49 and stays there, start cold engine 32 degrees outside and pressure goes to 50 / 51 and when you rev engine, moves 2/3 psi put under load in drive full throttle and stays same. shut off engine needle jumps for a second up then back to 49. will stay there with key of for 1/2 hour.

Do you think pressure is okay? Engine starts and runs great, just dealing with this transmission vibration towing, going to change wires, plugs and cap/rotor, but do not want to install if I have to take it all off to do other repairs

tj310 · 02-12-2013, 10:04 AM

I am working on a 1999 3500 GMC and those numbers look about the same. I would hook up a gauge and test under load. ---Trevor

rognelson777 · 02-12-2013, 01:26 PM

new data, I did replace the regulator in 2011 with BWD pn 24027, Noticed change next spring going to charlotte from phila. just did the following diagnosis

I can pinch vacuum line going to regulator. The following has happened

1 Start and pinch line goes to 57 psi and stays there. Under load drops 1psi
(in drive with 1/2 throttle) Release vacuum line drops to 46psi
Squezze again and no change release
Shut off goes to 57 then drops to 46 and stays there.

2 Start and pinch line goes to 57 psi and stays there. Under load drops 1psi
(in drive with 1/2 throttle) Squezze and shut off and sounds like a solenoid kicks in and drops to 47 psi and stays there

3 .Start and pinch line goes to 57 psi and stays there. Under load drops 1psi (in drive with 1/2 throttle) Release vacuum line drops to 46psi
Squezze again and no change hold and shut off drops to 37psi sounds like a electric solenoid releases
Release pinch and goes up to 40psi.

Thoughts? other tests?

Rich Biebel · 02-12-2013, 08:16 PM

I had a '96 GMC with the same driveline as your MH...

Started to have engine issues when it was only a few years old.

Intermittent loss of power and stalling.....

I did the fuel pressure test and your numbers are fine...

I changed a bunch of parts including the fuel pressure regulator....and the problem would come and go....

Problem persisted....and got worse...

I attached a fuel pressure gauge and taped it to the windshiled and drove it around untill it started acting up.....Fuel pressure was dropping.... a lot...

It turned out to be the fuel pump pickup filter sock....It had dirt.... and red silt in it and was causing the problem....

I replaced the pump....filter sock and can filter......and stopped going to the gypsy gas stations I was going to......Problem never occured again....

I kept that truck for 14 years and bought it new and towed with it all those years......

02-12-2013, 12:15 AM	#1
rognelson777 Member Join Date: Dec 2010 Posts: 372 Likes: 20 Liked 132 Times in 72 Posts	fuel pressure gm 454 1998 motorhome Posted for transmission help on here for motorhome and need help with fuel pressure diagnostics 1998 gm 7.4l 454 8 injectors. Checking fuel pressure today, turn on key, pressure goes to 54 then drops quickly to 49 and stays there, start cold engine 32 degrees outside and pressure goes to 50 / 51 and when you rev engine, moves 2/3 psi put under load in drive full throttle and stays same. shut off engine needle jumps for a second up then back to 49. will stay there with key of for 1/2 hour. Do you think pressure is okay? Engine starts and runs great, just dealing with this transmission vibration towing, going to change wires, plugs and cap/rotor, but do not want to install if I have to take it all off to do other repairs

02-12-2013, 10:04 AM	#2
tj310 Member Join Date: Sep 2009 Location: Dresden On. Canada Posts: 460 Likes: 0 Liked 0 Times in 0 Posts	Re: fuel pressure gm 454 1998 motorhome I am working on a 1999 3500 GMC and those numbers look about the same. I would hook up a gauge and test under load. ---Trevor

02-12-2013, 01:26 PM	#3
rognelson777 Member Join Date: Dec 2010 Posts: 372 Likes: 20 Liked 132 Times in 72 Posts	Re: fuel pressure gm 454 1998 motorhome new data, I did replace the regulator in 2011 with BWD pn 24027, Noticed change next spring going to charlotte from phila. just did the following diagnosis I can pinch vacuum line going to regulator. The following has happened 1 Start and pinch line goes to 57 psi and stays there. Under load drops 1psi (in drive with 1/2 throttle) Release vacuum line drops to 46psi Squezze again and no change release Shut off goes to 57 then drops to 46 and stays there. 2 Start and pinch line goes to 57 psi and stays there. Under load drops 1psi (in drive with 1/2 throttle) Squezze and shut off and sounds like a solenoid kicks in and drops to 47 psi and stays there 3 .Start and pinch line goes to 57 psi and stays there. Under load drops 1psi (in drive with 1/2 throttle) Release vacuum line drops to 46psi Squezze again and no change hold and shut off drops to 37psi sounds like a electric solenoid releases Release pinch and goes up to 40psi. Thoughts? other tests?

02-12-2013, 08:16 PM	#4
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: fuel pressure gm 454 1998 motorhome I had a '96 GMC with the same driveline as your MH... Started to have engine issues when it was only a few years old. Intermittent loss of power and stalling..... I did the fuel pressure test and your numbers are fine... I changed a bunch of parts including the fuel pressure regulator....and the problem would come and go.... Problem persisted....and got worse... I attached a fuel pressure gauge and taped it to the windshiled and drove it around untill it started acting up.....Fuel pressure was dropping.... a lot... It turned out to be the fuel pump pickup filter sock....It had dirt.... and red silt in it and was causing the problem.... I replaced the pump....filter sock and can filter......and stopped going to the gypsy gas stations I was going to......Problem never occured again.... I kept that truck for 14 years and bought it new and towed with it all those years...... __________________ Rich Biebel S/C 1479 Last edited by Rich Biebel; 02-12-2013 at 08:19 PM.