2008 5.3L Fuel Pressure test procedure

[Mossyoakglock]

What is the correct procedure for testing the fuel pump/fuel pressure?

 

I suspect I have a fuel pump going out and I want to test the system. I have a Haynes at home but haven't looked at it yet to see if it has the procedure or if the shows the correct PSI numbers.

 

On a cold start the engine will stumble and feel like a miss. It will idle somewhat rough for about 2-3 seconds and then smooth out and idle normal. It never does it throughout the day no matter how many times I restart the truck. It doesn't do this every morning (or after a long sit for 7-8 hrs or more) so it is hard to tell when it might do it.

 

I'm not sure if there is a different procedure to test certain things. Since mine only does it on the first start of the day do I need to cycle the key to on with truck off or start truck right away? If I just turn the truck on (with engine off) wouldn't it prime the pump so when I do start it, it gives the reading like the pump is ok? Shouldn't I start it right away and see what the pressure does?

 

I get no codes either. The dealer wants $90 to test it so I want to see if I can test the pressure myself first before taking it to them.

Edited August 11, 2014 by Mossyoakglock

[Doug_Scott]

Have you tried just turning the key on while you put seat belt on, then continue turning the key to the start position? That will give the fuel pump time to prime the system.

When were the spark plugs last looked at? You may want to take a look at the ambient air temp sensor to make sure it is reporting the correct temp to the PCM. If it thinks the air temp is 80 degrees, and it is actually only 50 degrees, it will be lean on initial start up. May not set a code because it is still in the allowable range.

[txab]

Procedure is key on, engine off. Take reading

[Mossyoakglock]

Have you tried just turning the key on while you put seat belt on, then continue turning the key to the start position? That will give the fuel pump time to prime the system.

When were the spark plugs last looked at? You may want to take a look at the ambient air temp sensor to make sure it is reporting the correct temp to the PCM. If it thinks the air temp is 80 degrees, and it is actually only 50 degrees, it will be lean on initial start up. May not set a code because it is still in the allowable range.

 

Plugs were changed 3,000 miles ago and ambient air temp sensor is new too (went out at the beginning of the year) so that is new too.

[Mossyoakglock]

Procedure is key on, engine off. Take reading

 

So you don't start the truck and take the reading at first start up? If I did the key on, engine off it will prime the pump. If I go to start the truck after that it would start just fine and give a false reading, correct? My issue is when I first start the truck and get the stumble/miss (which it did again this morning). I don't prime the fuel pump. I would think I would need to start it first right away to get it to reproduce the same issue and take the reading.

Edited August 12, 2014 by Mossyoakglock

[00Silverado4x4]

To go along with what txab is saying, key on, engine off, take reading. Then turn key off, take reading and continue to monitor to see if pressure stays up, drops some and stops or continues dropping. What you will want to see is the pressure staying put and in the correct range. With the pump running I believe the pressure should be around 55-62 psi. When the key is turned off it will drop a bit but if it drops to less than about 50 psi you may be having a fuel pump problem. I apologize as I do not have the exact psi rating, just what I remembered from somewhere.

[Mossyoakglock]

I looked last night in my Haynes and I believe key on, engine off is around 55 psi and idle is around 45 psi, or somewhere around there.

 

I'm going to test it this weekend and if everything looks good I'll take it to the dealer and see what they say. It's weird because it will do it the following morning after sitting overnight (12 hrs or so) but it won't do it when I leave for work after it's been sitting (10 hrs) and it doesn't do it every morning. Although it did do it two days in a row and seems worse when it gets below a 1/4 of a tank.

Edited August 12, 2014 by Mossyoakglock

[Jsdirt]

My truck did this for a while at one point, and then cleared up. Was right around the time I had a gallon or so of bad gas I needed to get rid of out of a few mowers & weedwackers .. so I threw it in the tank.

 

Probably had some ethanol-separation going on, & sucked that crap up. After driving it a few times, the problem went away on it's own.

[Mossyoakglock]

I wish that's what was going on with mine but this has done it over a few tanks. It's probably my fault for letting the tank get too low a few times which has probably slowly killed the pump.

 

Could it have something to do with it that when the tank is full and the pump is fully submerged it works fine but when it gets down to a 1/4 of a tank and some of the pump is exposed it causes the problems?

[Doug_Scott]

I wish that's what was going on with mine but this has done it over a few tanks. It's probably my fault for letting the tank get too low a few times which has probably slowly killed the pump.

 

Could it have something to do with it that when the tank is full and the pump is fully submerged it works fine but when it gets down to a 1/4 of a tank and some of the pump is exposed it causes the problems?

 

If that causes an issue with fuel pump life, then there would be a lot more failures than there are. Majority of owners/drivers will put fuel in when it is needed, not when they are under half tank. There is a good flow of gas through the pump at any level, and that will cool the pump as well.

 

If the truck runs fine after the first start, I seriously doubt it will be the fuel pump. When you say it only happens on first start of the day, does it matter what time of day that is? For example, if you park the truck Friday night, then don't start it until 2 o'clock Saturday afternoon, does it still run rough? What is the air temp when you get this rough starting running?

 

It may be something like an injector sticking at certain temps. On the first start of the day try connecting the fuel pressure gauge to the fuel rail, then start the truck, and soon as it starts, shut it off and check fuel pressure. What you are looking for is the pressure to drop to near zero. If an injector is not closing at certain temps, it will give rough running, until the injector hits the temp that it starts working correctly at. I have no idea what the pressure should be, but I am confident it should not be zero. There are two ends in the fuel system, and both can cause a low fuel pressure.

[Mossyoakglock]

 

If that causes an issue with fuel pump life, then there would be a lot more failures than there are. Majority of owners/drivers will put fuel in when it is needed, not when they are under half tank. There is a good flow of gas through the pump at any level, and that will cool the pump as well.

 

If the truck runs fine after the first start, I seriously doubt it will be the fuel pump. When you say it only happens on first start of the day, does it matter what time of day that is? For example, if you park the truck Friday night, then don't start it until 2 o'clock Saturday afternoon, does it still run rough? What is the air temp when you get this rough starting running?

 

It may be something like an injector sticking at certain temps. On the first start of the day try connecting the fuel pressure gauge to the fuel rail, then start the truck, and soon as it starts, shut it off and check fuel pressure. What you are looking for is the pressure to drop to near zero. If an injector is not closing at certain temps, it will give rough running, until the injector hits the temp that it starts working correctly at. I have no idea what the pressure should be, but I am confident it should not be zero. There are two ends in the fuel system, and both can cause a low fuel pressure.

 

If it was the injector sticking wouldn't it run rough at every startup?

 

Air temp is roughly the same every morning (or at least it has been). It's been in the low to mid 70's the past few weeks in the morning with today being about 62*. It did it again this morning but not as bad. i didn't get home until about 8 pm when I usually get home around 6 and don't start the truck until 6 the next morning so it didn't sit as long as usual.

 

I'm thinking fuel pump (and this is based on what all I've read about fuel pump failures) because on the first start of the day (after sitting 10+ hours) it will run rough for about 2-4 seconds and them smooth out. If I let the truck idle for about 30 seconds, turn the truck off and wait about 10 more seconds and restart, it doesn't hesitate and fires up strong and normal. If it was an injector shouldn't it still run rough since that would not be enough time for it to heat up to the proper temp? With the truck running for 30 seconds and then shutting it off the fuel pump would have primed and brought the system up to full pressure and any restarts after that would not be affected (unless I was losing pressure rapidly). This goes the same throughout the day running around town. Start truck in morning and it hesitates (again, it doesn't do it every morning but it has been getting more consistent this week), run to store, etc and come out and restart the rest of the day with no problems. Even if I'm at dinner or something for an hour or more it will still start up normal when we leave. It only seems to happen when the truck sits for 10+ hours.

 

To answer your question about letting the truck sit it doesn't matter. I can park it Friday after work and start it 6 am Saturday morning or 4 in the afternoon Saturday and it still does it. There would be a large temp difference between 6 am and 4 pm. Probably about 70 in the morning and it's been in the mid to upper 90's in the afternoons here.

 

It might not be the actual pump but could it be the pressure regulator slowly losing pressure? Since that is part of the pump assembly I'd have to change the whole thing anyways.

 

I thought it could maybe have been the Range V8 causing the issue so I unplugged it this morning and it did the same thing. I wanted to eliminate all variables. I am going to replace the throttle body gasket this weekend since I've taken it off a few times in the last few weeks so maybe I knicked it accidentally or something.

 

Edit - Let me clarify that the truck is not hard to start or take multiple tries. It fires up the first time but just has a hesitate or miss for a few seconds.

Edited August 13, 2014 by Mossyoakglock

[Mossyoakglock]

I tested my fuel pressure and here are my numbers. How do these look?

Friday Morning:
1. Started truck without priming the fuel pump - ran at 60 psi for about 45-60 seconds and then dropped to 45 psi and held steady. Shut truck off and pressure did not drop.
2. Let truck sit for about 5 minutes and turned key on, engine off 3 times to get an average reading.
First time - 58 psi
Second & third time - 60 psi, max drop of 2 psi in 1 minute each time

Saturday Morning: I tested the pressure with doing the key on, engine off method first. I did it 3 times to get an average.
1. 56 psi, with a 6 psi drop in 1 minute
2. 56 psi, with 4 psi drop in 1 minute
3. 57 psi, with 3 psi drop in 1 minute

Then I started the truck and checked. At first start psi was 60 psi for about a minute and then dropped to 46 psi and held solid. There was no psi drop after 1 minute.

[Addison]

On 8/13/2014 at 10:06 AM, Mossyoakglock said:

 

If it was the injector sticking wouldn't it run rough at every startup?

 

Air temp is roughly the same every morning (or at least it has been). It's been in the low to mid 70's the past few weeks in the morning with today being about 62*. It did it again this morning but not as bad. i didn't get home until about 8 pm when I usually get home around 6 and don't start the truck until 6 the next morning so it didn't sit as long as usual.

 

I'm thinking fuel pump (and this is based on what all I've read about fuel pump failures) because on the first start of the day (after sitting 10+ hours) it will run rough for about 2-4 seconds and them smooth out. If I let the truck idle for about 30 seconds, turn the truck off and wait about 10 more seconds and restart, it doesn't hesitate and fires up strong and normal. If it was an injector shouldn't it still run rough since that would not be enough time for it to heat up to the proper temp? With the truck running for 30 seconds and then shutting it off the fuel pump would have primed and brought the system up to full pressure and any restarts after that would not be affected (unless I was losing pressure rapidly). This goes the same throughout the day running around town. Start truck in morning and it hesitates (again, it doesn't do it every morning but it has been getting more consistent this week), run to store, etc and come out and restart the rest of the day with no problems. Even if I'm at dinner or something for an hour or more it will still start up normal when we leave. It only seems to happen when the truck sits for 10+ hours.

 

To answer your question about letting the truck sit it doesn't matter. I can park it Friday after work and start it 6 am Saturday morning or 4 in the afternoon Saturday and it still does it. There would be a large temp difference between 6 am and 4 pm. Probably about 70 in the morning and it's been in the mid to upper 90's in the afternoons here.

 

It might not be the actual pump but could it be the pressure regulator slowly losing pressure? Since that is part of the pump assembly I'd have to change the whole thing anyways.

 

I thought it could maybe have been the Range V8 causing the issue so I unplugged it this morning and it did the same thing. I wanted to eliminate all variables. I am going to replace the throttle body gasket this weekend since I've taken it off a few times in the last few weeks so maybe I knicked it accidentally or something.

 

Edit - Let me clarify that the truck is not hard to start or take multiple tries. It fires up the first time but just has a hesitate or miss for a few seconds.

Did you ever figure out the issue I’m having the same problem with my truck

[silveradosid]

have you cleaned the throttle body

[Addison]

Yes and I was having problems with the evap system but I’ve changed every part on the evap system and it hasn’t threw another code since