5.7 vortec troubleshooting

C.Berry

C.Berry

Bad News
Joined
Aug 10, 2008
Location
Blacksburg, Va
I'm hunting down a sluggish first gear in my new to me 99 suburban, seems it's lacking the initial acceleration most 5.7 engines of my past vehicles have had "put you back in seat"!! Well I checked fuel pressure today and with key on engine offi was around 55 psi and it held I started the truck and at idle pressure dropped down to 49 and held if I opened throttle to 2k it stayed at 49psi then jumped to 55ish and back to 49! Is this a bad pump or do I have a bad signal from one of the many sensors on these engines?
I thought it was trans at first but it's smooth no hard shifts or late shifts I did notice on hills I have to run 2800-3100 rpm to stay at 70 but that's not bad in my opinion cruising 70 on flats I'm around 2200rpm operating perfectly! Any ideas orielly code reader showed a soft code for o2 sensor but they say that crap for every vehicle on the planet! Thanks fellas I'll be trying to trouble shoot this over the weekend! I made a 3 hour round trip in it today with no issues really besides sluggish takeoff from stop!
 
Fuel pressure is about right. Changes in pressure is the action of regulator which is vacuum sensitive. The best pressure check is first block the return line with engine off , turn on key and see what the pump alone will do. Then with gauge connected drive vehicle around changing conditions. My gauge has a long hose that I zip tie gauge to driver side wiper. Acceleration may be more of a factor of rear end ratio Do you have a 3:90 or a 4:10
 
I see I'll have to get a longer hose for pressure gauge to drive and be able to monitor it! It's got a choppy idle almost it seems doesn't drop low doesn't go high just seems off gonna do some sensor cleanin and try a few things to try and bring some life back to this thing thanks
 
I'll check axle ratio and post back!
 
So this thing has gt5 code for 4.10 and I found a g80 code!!! Are le lockers in these semi float 14 bolts good or similar to timex lockers in z71 10 bolt?
 
Also found this burb has a floor shifted 241 I could have sworn it was push button on 99s?
 
My 98 Hoe was push button.
 
I'm pretty sure it just depends on the trim level


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G80 would be a gov loc or gov bomb depending on your perspective. It has a governor preventing locking at highway speeds . It senses one wheel spinning faster than the other and engaged clutches to lock up. Normal sensible driving allows it to work as intended. If wheels are spinning really fast and are allowed to slow down and are spinning at different speeds when it locks up it can and will break stuff. Check for vacuum leaks regarding rough idle
 
Yeah I was hoping they had done a better locker in the 14 bolts oh well it'll hopefully just be used in snow and muddy back roads so not concerned with the g80 till it blows up on me! Yeah this one is a LT trim I figured the higher end would be push button to keep soccer moms from breaking nails!? I'm good with it tho! I've read that fuel pressure should be at 60-65 psi while idling is that for the 5.3 and someone just put wrong engine down? Didn't get to Che k pressure while driving or with return pinched hopefully Tom and I'll give that reading!
Is there a dummy how to for checking crankshaft sensor with volt meter so see if it's causing bad timing?
 
SFI: 56-62
CSI: 60-66

Lower numbers under vacuum. Check pressure under load & leak down after 10-15min & if you have an inspection camera, look behind the injector module and see if the regulator is dumping fuel back there. If you slam the throttle open, pressure should immediately jump to the higher end. if not its likely either a faulty pressure regulator (which is serviceable on these motors) or the pump is worn out and isnt delivering enough fuel. could be restricted fuel filter, but I havent personally seen too very many fuel filters cause enough restriction unless the pump was failed or failing. Return line will be difficult to pinch off since IIRC its all hard line (plastic or metal)

CKP - I don't know of any tests for the CKP but if it was the issue it would act as though it were misfiring and likely not run at all. Since it is an ON/OFF sensor, its not prone to wearing like the TPS is. It would also set a code if it were failed.

You can test the TPS with a scan tool or voltmeter, and I would recommend checking it as well for smooth, linear response without dead spots. My money is on the fuel system, especially since you mentioned pressure being low and not jumping when you snap the throttle. it ought to respond nearly instantaneously. You should never see 49 psi on these motors. 56 is the lower limit. (with the poppet valve injectors, any less than what you have and the poppets will NOT open)
 
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Further inspection shows I have a loud whining noise coming from the rear end of the truck at idle! I'm sure it's the fuel pump making the noise should I drop tank or cut a inspection plate out to do this fuel pump I'll be doing it by myself last tank I dropped wasn't much fun...
 
I drop my tanks to change fuel pumps. I'm not worried about a whiny pump if its working. I did learn to pressure check a new pump before installing. I've accumulated extra GM fuel lines that I soldered 1/4 flare fittings to use to block off lines or connect a gauge to check pressure of components.
 
Mines working just not sure how good lol it has spider injection from what I understand and if that's the case then I'm def under on the pressure side gonna double check to make sure tho
 
Airtex/carquest pumps are notorious for being noisy. If it is a airtex or carquest pump and your local advance store is good to you, you can probably get it exchanged. You can also check dead head pressure on the pump, should be at least 90 psi.
 
I'm running another pressure test right now what I'm seeing is this

Truck off engine on gauge jumps to 59 then drops to 52 and and slowly crawls up to 55 psi and holds strong! Then when I start truck it drops to 49psi and holds lil rough idling and seems to clear up it will stay 49 until I turn off engine then climbs back up to 55 and holds I'm waiting to see if pressure drops now after off but so far it's holding At 55 tho! Honestly it's probably the original pump in this truck a lot of engine sensors accessories are originals I'm going to dealership Tom to get maintenance records that they show to tell me what's been logged as replaced so I can know in advance!
 
Anyone else cut a inspection hole for fuel pump work on a burb? I've done it on a Zj with great success did whole pump replacement in about 2 hours including cutting and riveting new metal back in! Rear carpet is ruined in the truck already so won't hurt my feeling to cut it and eventually replace it and I'll probably not sell this till it's not worth anything to anyone else!! Maybe I'm just being lazy haha
 
15 mins and has dropped to 52psi from 55!? And fluctuated back up to 55-57 lol good ole GM
 
Check valve in the pump and pressure regulator is not leaking, you can also do a volume test on the pump. I'll look up the specs on it, I suspect it may fail that. Sounds like it isn't supplying enough fuel when you start it. What does the pressure do when you get on it from a stop?
 
From what I've read it only takes 50psi to start one so it's getting that it'll shoot to 59-60 then drop to 55 when idling! Pressure just sits at 49 when I get on it it'll bounce to 55 then to 49 but 90% of the time it's on 49 running
 
C.Berry said:
Anyone else cut a inspection hole for fuel pump work on a burb?
Click to expand...

I did on my old 2500 Burb because I knew when the pump failed, the tank would be full (44 gallons)... and wasn't going to haul an empty 55gal drum & pump for that crap!
When it did finally die, @obxfisherman (got it from me) was VERY happy to turn a multi-hour repair into 20 minutes!
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How did you figure out where to cut hole on it? The one one my jeep didn't look quite that good not gonna lie lol
 
I'm down to it has the be pump one reason besides low psi is at 65 when it goes to passing gear up hill you can feel a jerk as if running out of fuel!
 
And that's when you need to see what the fuel pressure is. It's worth checking the cap and rotor too. They pretty often go out, usually result in a misfire code though.
 
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