Fuel pressure jumping under load... help please

[Scotts Elle]

My truck started hesitating and stumbling under partial throttle load. I initially thought it was the COPs but after installing some Weapon-X COPs and new TR-6's the issue is still there. The pumps are Walbro's with a steel Y fitting and are relatively new (just over a year old with less than 10K on them)

Under heavy engine load my fuel pressure gauge jumps between 40-50psi IIRC.

A/F gauge reads 14.8 at idle and around 11-12 under moderate to heavy load. Haven't tried WOT. I still make decent boost.. Checked for vac leaks and its all good.

Any ideas?

My truck is being a grumpy bi*ch since arriving from cold Calgary to Houston.

 

[Scuba-Matt]

Scott have you tried bypassing the fuel pump resister?

 

[Scotts Elle]

I've read about doing it but I don't know what this does. What does this do?

 

[Z7What]

It sends full voltage to the pumps all the times if bypassed
Wayne

 

[wydopnthrtl]

The resistor was my thought too.

Rich

 

[MMCustomworks]

Make sure your O2 sensors are not going out. I had the same issue and it was my pumps going out and 1 O2 sensor not working.

 

[Scotts Elle]

Didn't work. New resistor is in and it still runs like crap. Under load it sputters bad and the power just isn't there. Getting tons of pressure at higher RPM (50-60psi). The fuel pressure needle jumps up and down like a wiper blade. Between 30 and 40 psi.

Oh and Steve, the O2s looked fine. I pulled the front 2 this evening. PS, are you ready for my console now.

Any other ideas?

 

[Fade 2 Black]

Check and be sure your Maf is clean and dry. Did you wash underhood recently or anything? If you run a Mafia take the cover off of it and make sure it's dry also. I'd also make sure that the fuel pump relay next to the backup power box is good, if it's latching/unlatching when open loop is commanded then that could cause your issue. You can swap that relay with another right next to it, they're the same part.

 

[Scotts Elle]

Thanks Fade. I just checked the relays and they are working fine. The MAF is dry and clean... looks brand new in there.

 

[Fade 2 Black]

Retrace your steps, what have you recently done (if anything) that could cause this? Have you had the truck all apart or anything? Can you log/check the TPS throughout it's range? For some reason I'm thinking the PCM isn't staying in open loop mode while it should and it's commanded off of TPS and load (airflow). Could also be a bad ground somewhere. You could try jumpering the fuel pump relay inside the backup power box under the hood so the pumps run full speed full time and see how it does under WOT, although it will most likely be pig rich at idle so I wouldn't run it too long that way.

 

[MMCustomworks]

PS, are you ready for my console now.

Thought you had it done? PM me what you are thinking about, Ive been doing some stuff with the console lids that look oem.

 

['88347coupe]

My truck did the exact same thing. I ended up having to wire in a second dropping resistor in parallel. Took me a month to figure it out because, I to put a new dropping resistor in,and it didn't fix my problem. Cheak your voltage at your inertia switch relay (the one beside the hi/lo relay). You should have 8-9volts at idle and cruising. When the resistor is bypassed you should have battery voltage (13- 13.8v) at said relay. Your hi/lo relay just alternates between a direct chasis ground and an alternate ground going through the resistor. This is done by a ground wire coming from the computer that triggers that relay. Hope this helps:beer:

 

[Scotts Elle]

Thanks. Great info. I will give it a try.

*EDIT* Just wired them together. Still no change.

 

[Cichlids]

Your truck has turned into mine with all those issues oke: