Need Help With Silverado Lack of Power, Hard Starts, etc.

[mriggs90]

Hi guys,
ok, so I've been reading in your forums about problems similar to mine, but it seems like no one has had the same experience as me. I have a

2000 Chevrolet Silverado 4.8L V8 with about 150,000 miles on it.

As of late, it feels like it is lacking power on acceleration. It also has a problem wanting to start at times. The engine cranks fine, but it doesn't seem to want to fire consistently. When it doesn't start I just let off, then try cranking again and it will fire up. In the summer, i thought I noticed a flooded smell when this happened, but I'm not 100% sure. As for the driving, as I said it just seems like it lacks power and hesistates when I give it gas. Also, and I know this is very subjective/inaccurate due to many factors, but it seems like my fuel mileage has gone down about 3 miles/gal. for a few fillups. So here's some more info:

Spark Plugs should be good, I replaced them at around 117,000 miles.
Air filter is fairly new (K&N factory style replacement), about 120,000 miles.
New fuel filter less than 200 miles ago.
New fuel pressure regulator today.

At first I thought it was a fuel delivery problem, so I bought a tester. Here's the results before and after the fuel pressure regulator was installed.
Before After
Key on, engine off: 56 psi 60 psi
Engine Idle Pressure: 48 psi 51psi
Engine Idle w/ vaccum
hose disconnected
from pressure reg. 58 psi 60psi

I read in another thread that the pressure should stay above 50 psi in the first test for 3-5 minutes. Mine drops below in 6 seconds.

Right now I'm trying to clean my MAF sensor, I heard that could make things sluggish. I'll try and post if that helps, but right now I feel like I'm shooting blind, and I just can't afford to start replacing everything. Any and all help would be appreciated.
Thanks,
-Matt

[old_master]

What does fuel pressure drop to after the pump shuts off and how long does it take to stabilize?

[ColoradoSilverado]

Fuel Pump! Don't buy the $300 Delphi piece of crap! I got a pump off Ebay for $45, works just fine!

Bert

[mriggs90]

Fuel pressure (before new regulator was installed, but is about the same now) dropped below 50 psi in 6 seconds, below 30 psi in 44 seconds, below 20 psi in 1:30, and below 10 psi in 4:00. I don't know if it ever stabilized after that, I just assumed it would continue to drop.

I didn't think it could be the fuel pump, because it will supply the spec pressure initially, and I would think if it couldn't supply enough pressure it wouldn't run well at higher rpm, but it seems to, it's just the lower rpm, and the pressure just leaks back down. Unless maybe it's a check valve somewhere in the system. I'm trying to rule out the simple stuff before I drop $300+ into a fuel pump which may or may not solve the problem. It seems like this is more like practicing medicine....1000 things can cause certain symptoms, and I can't afford all the remedies to figure out which one it is.

P.S. I cleaned the MAF sensor, it seemed like it may have helped a little with the sluggishness, but I'll have to drive it a while before I'm sure. I don't want to think it works just because I want it to. But I have a question...if the MAF was running in closed loop mode, would it try to make the same air/fuel ratio when starting as it does with engine running, and make the long starts and sluggish driving? Or am I totally off track?

[old_master]

There's a check valve right at the fuel pump outlet, and there's also a flex line that goes from the pump outlet to the module housing. If either one leaks, pressure at he injectors will leak down. There are four common problems with this style pump: noisy, check valve leaks, flex line leaks, and insufficient maximum output pressure. Many aftermarket pumps fail one or more of those areas right out of the box. Lifetime warranty is nice but, having a pump that meets GM factory specs, and lasting 100,000 miles is worth something too.

There two ways to determine if the leak is in the tank:

1) Install a shut off valve, either permanent or tempoaray, in the fuel pressure line anywhere between the fuel pump and the service port on the engine. With the valve open, allow the pump to pressurize the system, then immediately shut the valve off. If fuel pressure does not drop, the leak is in the tank.

2) Connect the fuel pressure gauge directly to the fuel filter, (fuel pressure can not reach the engine). Allow the pump to pressurize the line and gauge, (pressure should be 78psi to 108psi) then shut the pump off. Pressure should hold, (usually around 70psi), if not, replace the pump.

[j cAT]

the fuel pressure spec is 53-62 psi. the pressure should only drop 3-5 psi over 15 min...as mentioned check at the fuel filter for pressure drop...if it drops the pump is in need of replacement or as some have had success install a check valve at the fuel filter. this is much cheaper..since you have good pressures when running..

[rjeffery]

Definitly the fuel pump check valve. I have replace 2 OEM pumps for this same problem.
BlackBerry8310/4.5.0.110 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102

[mriggs90]

Thanks for the input,
I will try to perform that check as soon as possible, but right now it's pretty cold and I have tons of work to do. I'm sure I can figure this out when I do have the time to do it, but does my fuel pressure gauge hook directly to the fuel filter without an adapter (i.e. does it have the same hookup as the fuel rail port had?). If not, then what kind of adapter do I need to get?

Also, will this check valve help with the sluggishness of the truck? I don't really understand why this problem causes these symptoms. It seems like if the truck gets the initial pressure then it should start right away and drive fine unless I left the key in the run position for a little while and then tried to start it. btw I have very little experience with in-depth maintenance on vehicles...not trying to question anyone's judgement, just trying to get a better understanding of how it works.

Anyway, thanks again for the info.

[old_master]

To check fuel pump maximum output and the check valve, you need to disconnect the fuel filter outlet line and modify the tester to connect directly to the fuel filter. A good hardware store should have the stuff you need, along with a fitting from an auto parts store. Here's a pic of a shut off valve that I made, it can be modified very easily to measure fuel pump maximun output.

[mriggs90]

I apologize for being so lax about testing my fuel pressure. I ended up taking my truck to a local shop we trust....I think.... Since all my problems were intermittent, I didn't know if they could find anything or not. They ended up telling me that I had a slight leak in my intake gasket, they cleaned out my throttle body and cleaned my MAF again, and told me my spark plugs were shot. I didn't really read reviews on the Bosch Platinum Plus 2 before I bought them, I just assumed that it was good technology....definitely wrong. 33,000 miles and they looked like crap, so I put a new set of AC Delco's in with the Iridium tip....truck runs a little better now.
My problem is the mechanic told me they couldn't find a problem with the fuel pressure, and they didn't know what was causing the hard starts (it didn't happen apparently when they had the truck).
Now the mechanic told me that all the silverado's drop fuel pressure with the ignition in the run position test, which I know is wrong because I tested my dad's '01 Silverado and it held. My question is now I'm out about a hundred bucks and I still have a truck that I half-expect not to start now and it's getting worse... Before I spend any more money, would me putting in this check valve have any chance of fixing my starting problem, or is it most likely not the cause?
Sorry for the long post....

[old_master]

Your leakdown is excessive... you need to isolate the fuel pump and watch leakdown to determine if the leak is under the hood or in the tank. This is what you need to test it:

[j cAT]

replace the fuel pump ! how many times do you have to test the pressure and with the low readings still have doubts...

the fuel pressure spec is 53-62psi ...if when running at idle it can not do this its N/G ...

if you did have intake leaks you will get check engine light very fast...

with your comments about these so called mechanics , they apparently don't know much about this engine and its controls...

this forum has so many owner posting exactly like yours ...

95% fuel pump !

[old_master]

Originally Posted by mriggs90

Hi guys,
At first I thought it was a fuel delivery problem, so I bought a tester. Here's the results before and after the fuel pressure regulator was installed.
Before After
Key on, engine off: 56 psi 60 psi
Engine Idle Pressure: 48 psi 51psi
Engine Idle w/ vaccum
hose disconnected
from pressure reg. 58 psi 60psi

Fuel pressure (before new regulator was installed, but is about the same now) dropped below 50 psi in 6 seconds, below 30 psi in 44 seconds, below 20 psi in 1:30, and below 10 psi in 4:00. I don't know if it ever stabilized after that, I just assumed it would continue to drop.
Thanks,
-Matt

Fuel pressure is fine, leakdown is where the problem is. Could be the pump or it could be an injector leaking or the regulator bypassing internally. If there are no external leaks, that's all it can be.

[j cAT]

Originally Posted by old_master

Fuel pressure is fine, leakdown is where the problem is. Could be the pump or it could be an injector leaking or the regulator bypassing internally. If there are no external leaks, that's all it can be.

I have to disagree with you on this fuel pressure ...his idle pressure is below 53psi..The OEM shop HELMS manual states that the pressure must be above 53 psi...

my pressures at idle are above 53psi ...other find this true..

I know these books do have errors ..

[old_master]

Originally Posted by j cAT

I have to disagree with you on this fuel pressure ...his idle pressure is below 53psi..The OEM shop HELMS manual states that the pressure must be above 53 psi...

my pressures at idle are above 53psi ...other find this true..

I know these books do have errors ..

I agree, 53psi to 62 psi is the spec range however, according to the GM dealer issue shop manual, GM does not publish any specific engine running fuel pressures. There are far too many variables involved that can affect engine running pressures. The 53psi to 62psi range is with the key on, and the engine off, fuel pump running. The OP found the key on engine off pressure to be ~57psi with his old regulator, and 60psi with the new one, both are within spec. With the engine at idle, he unplugged the vacuum line at the fuel pressure regulator. With both regulators, the pressure increased to the key on engine off readings, just as it should provided the pump max output pressure is within spec, (78psi to 108psi). The problem could very well be the pump, but not due to a lack of pressure, it would be due to the inability of the check valve to hold the pressure after the pump shuts off.