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1999 Suburban 5.7 P0300 trouble!


camaro1970
12-07-2009, 10:27 PM
Alright, so my suburban has 249,000 miles on it, but it has a new long block with maybe 20,000 miles on it. a couple months ago i realized it had a slight miss, so i did a compression test and sure enough, cylinder #3 had only 50 lbs of compression. I located a new head because after i took the thing off i found that it had a crack in it. after putting the new head on now all 8 cylinders were at about 150 lbs, and the P0303 code went away. the day i had finished putting it back together i tried to drive it and it gave a new code P0300, this code was accompanied with a rough idle, no power on any kind of incline and the smell of fuel out the exhaust. The random misfire code has me stumped. I put new plugs, wires, cap and rotor, both upstream 02 sensors. I checked the Egr valve and it seems to be fine. The roughness of the idle is strange, it will idle fine for a few seconds then you can feel a definate miss, then fine for another period of time. the interval between the miss is never the same either. At first i thought it might be that the distributer is off, but it is not throwing the code that tells that it is not within +/- 2 degrees (p1345 i believe). is there anything more i can do or does it need to go see a mechanic before this problem can be solved? i know people have mentioned the distributer ring gear and the fuel injectors, and fuel pressure regulators. is there anyway to determine what its problem is without replacing everything listed?

P.S. I miss carbeurators...

j cAT
12-08-2009, 09:03 AM
fuel pressures need checking. the 300 dtc is common with low pressures.

camaro1970
12-09-2009, 01:21 AM
fuel pressures need checking. the 300 dtc is common with low pressures.


do i need to have a mechanic do this? or can i do it? if so how?

j cAT
12-09-2009, 09:20 AM
do i need to have a mechanic do this? or can i do it? if so how?


the fuel rail has a test port fitting the fuel gauge can be had at like auto zone etc... or buy for 35.00 connect with engine off and the start and see what pressures are there.

you need around 60psi of pressure..if the pressure is ok then I would look for intake leaks vacuum leaks.


the random missfire code as mentioned can be many things.with this mileage many more things.

does this vehicle have good power at full throttle at hywy speeds ?
if not the cat converter would need checking for restrictions...

camaro1970
12-12-2009, 05:05 PM
the fuel rail has a test port fitting the fuel gauge can be had at like auto zone etc... or buy for 35.00 connect with engine off and the start and see what pressures are there.

you need around 60psi of pressure..if the pressure is ok then I would look for intake leaks vacuum leaks.


the random missfire code as mentioned can be many things.with this mileage many more things.

does this vehicle have good power at full throttle at hywy speeds ?
if not the cat converter would need checking for restrictions...


If you put the gas to the floor it has power, it just doesn't have any low end torque, and it has a rough idle. i will check fuel pressure tomorrow, and report back, i also may check the distributer gear for wear since i have heard that can be a problem. if it is the fuel pressure, how do i tell if its the regulator or the pump?

j cAT
12-12-2009, 07:33 PM
If you put the gas to the floor it has power, it just doesn't have any low end torque, and it has a rough idle. i will check fuel pressure tomorrow, and report back, i also may check the distributer gear for wear since i have heard that can be a problem. if it is the fuel pressure, how do i tell if its the regulator or the pump?

if the fuel pressure is low it is most always the fuel pump...when engine is running...it could also be however poor connections to the fuel pump both electrical and fuel ....


with the key on engine running if you have good fuel pressure then key off ,,,engine off ,, fuel pressure drops rapidly. with the poor idle this can be fuel pump, fuel regulator diaphram leak , and dirty injectors leaking down...this can be your problem as well...
then the intake/other vacuum leaks are possible..

if you have good power at the high speeds passing power, probably the cat converters are good..

camaro1970
12-13-2009, 08:38 PM
if the fuel pressure is low it is most always the fuel pump...when engine is running...it could also be however poor connections to the fuel pump both electrical and fuel ....


with the key on engine running if you have good fuel pressure then key off ,,,engine off ,, fuel pressure drops rapidly. with the poor idle this can be fuel pump, fuel regulator diaphram leak , and dirty injectors leaking down...this can be your problem as well...
then the intake/other vacuum leaks are possible..

if you have good power at the high speeds passing power, probably the cat converters are good..

so there was a new development with the truck today. i went out to start the car this morning and it cranked over fine, but would not start. my dad who gave me the truck last year said that is what it was doing right before the fuel pump went out and now he is almost positive it is the fuel pump. so here is my new question, is there some way of testing it without spending the 500 bucks on a new one? and secondly if i don't want to spend 500 bucks do i have any other viable options besides the autozone crap? ac delco is the only way to go am i right? thanks in advance

777stickman
12-13-2009, 09:37 PM
Fuel pressure testers are available for rent at A/Z. Pressure needs to be 55-60 psi while cranking.

Dist test: Pull cap. Gently push down on rotor and gently rotate. Any rotation of rotor is not good. Any side to side play is not good. Up and down play of .005-.020 is ok.

Fuel pump: If it comes to that get a Delphi/AC Delco pump that is matched to your VIN

camaro1970
12-15-2009, 09:10 PM
so i just checked the fuel pressure on the truck, and it was 55 lbs at idle, is that too low, in the manual it says 60-66 is the right amount? could this be causing my problems?

j cAT
12-15-2009, 09:31 PM
so i just checked the fuel pressure on the truck, and it was 55 lbs at idle, is that too low, in the manual it says 60-66 is the right amount? could this be causing my problems?


55psi is ok with the 5.3L engine ..the 5.7L requires higher pressure...
if the pump installed last year was an aftermartket pump [not delphi/oem] this is common for this to occur after a few month to a year..

when the key is off how fast does the pressure drop ?

when replacing again make sure you have good pump pick hose secuity..tight not loose..

keep fuel level above 1/4 tank at all times...replace filter and relay..

camaro1970
12-15-2009, 09:39 PM
55psi is ok with the 5.3L engine ..the 5.7L requires higher pressure...
if the pump installed last year was an aftermartket pump [not delphi/oem] this is common for this to occur after a few month to a year..

when the key is off how fast does the pressure drop ?

when replacing again make sure you have good pump pick hose secuity..tight not loose..

keep fuel level above 1/4 tank at all times...replace filter and relay..

it is the 5.7 liter.
when i turned the car off, i watched the guage for about 20 seconds and it appeared to be holding pressure, it never got above 60, the only time it came close was when i reved the motor up a little bit, and it spiked to nearly 60.

yea i am pretty sure the pump was replaced about a year ago with something from autozone. i am going to have to order one online somewhere (acdelco).

could this explain how my left bank catalytic converter is running about 100 degrees cooler than my right, and specifically at idle cylinder number 1 is about 100 degrees cooler than the rest of them? i used a laser thermometer to measure them.

so i need to always keep the fuel level above 1/4? i am going to check the filter tomorrow and probably just replace regardless because its pretty cheap. where is the relay?

j cAT
12-15-2009, 09:49 PM
it is the 5.7 liter.
when i turned the car off, i watched the guage for about 20 seconds and it appeared to be holding pressure, it never got above 60, the only time it came close was when i reved the motor up a little bit, and it spiked to nearly 60.

yea i am pretty sure the pump was replaced about a year ago with something from autozone. i am going to have to order one online somewhere (acdelco).

could this explain how my left bank catalytic converter is running about 100 degrees cooler than my right, and specifically at idle cylinder number 1 is about 100 degrees cooler than the rest of them? i used a laser thermometer to measure them.

so i need to always keep the fuel level above 1/4? i am going to check the filter tomorrow and probably just replace regardless because its pretty cheap. where is the relay?


first the cat converters should be the same temp...the cooler one may be bad...this could also be because of the lower than required fuel pressures so that fuel pressure needs attention..the fuel pump relay should be in the fuse block...I don't have the exact name or location ..should be in the owners manual or the fuse box cover.

the fuel must be kept above 1/4 tank...if this has been running often with low fuel your problem is quite the norm..as the pump runs hot and then the motor winding short out causing a slower rotation..with your reving and getting a higher pressure this all fits...

when cranking the voltage will drop below 12volts with a weak pump this is how most find out that the pump is bad...

when you order on line be careful ! there are scam oem/dephi pumps listed out there...chinese crap clones !

camaro1970
12-16-2009, 12:47 AM
first the cat converters should be the same temp...the cooler one may be bad...this could also be because of the lower than required fuel pressures so that fuel pressure needs attention..the fuel pump relay should be in the fuse block...I don't have the exact name or location ..should be in the owners manual or the fuse box cover.

the fuel must be kept above 1/4 tank...if this has been running often with low fuel your problem is quite the norm..as the pump runs hot and then the motor winding short out causing a slower rotation..with your reving and getting a higher pressure this all fits...

when cranking the voltage will drop below 12volts with a weak pump this is how most find out that the pump is bad...

when you order on line be careful ! there are scam oem/dephi pumps listed out there...chinese crap clones !

yea i would be ordering it from the acdelco website, its only 260 from them where chevrolet would charge 465 for the same thing, so you feel that it is probably the pump if its not the filter or the relay then?

j cAT
12-16-2009, 08:59 AM
yea i would be ordering it from the acdelco website, its only 260 from them where chevrolet would charge 465 for the same thing, so you feel that it is probably the pump if its not the filter or the relay then?

sometimes the connections both power/ground are not secure this causes a voltage drop..pump does not get the full 12volts...

this bad connection maybe in the fuel tank...then in the fuel tank loose hose connections..

MT-2500
12-16-2009, 09:45 AM
Proper testing will give you the answer to that.
And as Jcat said check power and ground also.



The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

mishalah
01-14-2010, 08:31 PM
Any further results on this? Did the regulator get tested...or replaced?

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