Did you know our trucks have an AFPR?

Haulin' 8.1 · March 25, 2005

AFPR = adjustable fuel pressure regulator, long read but good for techies.

I got a '04 2500HD, 6.0, auto in the shop yesterday. It needed a tune-up and to get an inspection sticker. With the scanner I noted radom misfiring and severe lean out during WOT. So I went to town with the usual plugs and fuel filter, injector flush etc. But the engine was still leaning out to the point it would detonate the fuel after shifting into 3rd during WOT accelleration. I checked out the fuel pressure and sure enough it was about 2 lbs under specs. I have a trick I came up with for bumping fuel pressure by tapping on the regulator using a socket and hammer to slightly collapse the housing thereby increasing pressure on the spring inside which raises the fuel pressure. Works great on Fords. I was going to try it on this HD. Well I was looking at the regulator to determine what tools to use when I noticed a very small set screw right in the middle of it. Having dealt with AFPR off and on for years with my race cars and the occasional factory type I recognized it's purpose right away. I then thought it was too good to be true. So after grabbing the correct bit (T10H, tamper proof torx 10, H=hollow) and giving it a gentle twist I was able to raise the fuel pressure. It took a few turns to bring it into specs. I did several adjust and drives while watching the Short Term Fuel corrections on the scan tool to get it just right. Once I had it just tipped into rich I stopped. The good news was that power picked up under WOT due to much less timing retard from knock. There was still a bad WOT lean down after going into 3rd which is either due to poor calibration programming or the owner installed Airaid filter but it was now less severe and had less knock retard.

I was so really pleased about this tid-bit of knowledge because I immediatly went over to my truck, an '01 2500HD, 8.1, auto and looked at it to find that mine has an identical regulator. This is great because I've been working on my poor fuel mileage condition for months. One thing that has plauged me is that my MAF and PCM don't agree, that is the reading the PCM gets from the maf ends up as too much fuel at the injectors. In the case of the customers 6.0 his PCM had a positive fuel trim because it was having to increase injector firing time from the MAF calibration to get enough fuel. But mine was having to decrease injector time to reduce excessive fuel, having a negative number fuel trim. This doesn't mean much for part throttle cruise because the PCM is constantly correcting based on the O2 sensors, this is fuel trim, and once it's learned it remembers it. So unless a major change is made it's usually very close anyways. But at WOT things are different, the PCM overrides the O2 sensors becuase they are going to be reading rich anyways (at least they should) and goes mostly by MAF reading compared to calibration. So if fuel delivery is off by a large enough margin you can run horribly lean, like the 6.0, or way too fat, like my 8.1. One kills the engine and power, the other wastes fuel and gives away power. Adbrupt throttle changes cause the PCM to deliver fuel based on MAF reading also because the O2's aren't fast enough to correct in transient conditions.

So you could safely bet that I had all our equipment hooked up to my truck on the way home today. By relieving a few PSI I have most of my fuel trims hovering a 0.0%. This is good, more power and less fuel consumption. Now my PCM and MAF match each other almost perfectly, what the MAF says the PCM matches with the correct amount of fuel, not too much not too little.

EDIT update. I've since noted that maximum HP is acheived with fuel trim slightly rich (-2 or 3%). Fuel economy is actually better due to less knock retard and the adaptive learning can handle it fine.

I don't know why I haven't noted this before. I've seen no mention of the adjustment mechanisim online or in my diagnostic manuals, and I've looked. It made my day.

Vernon

WARNING: Adjustments should only be made by those who know what they are doing, you could ruin your engine. Just adjusting fuel pressure as a diagnostic guess will make things worse in most every instance. Make absolutely sure you have everything else in order, and check your pressure as decribed in factory literature before deciding to adjust pressure. Then only do it if you can verify the outcome immediately with the proper equipment.

MrQuadrasteer · March 25, 2005

AFPR = adjustable fuel pressure regulator, long read but good for techies.

I got a '04 2500HD, 6.0, auto in the shop yesterday. It needed a tune-up and to get an inspection sticker. With the scanner I noted radom misfiring and severe lean out during WOT. So I went to town with the usual plugs and fuel filter, injector flush etc. But the dang thing was still leaning out to the point it would detonate the fuel after shifting into 3rd during WOT accelleration. I checked out the fuel pressure and sure enough it was about 2 lbs under specs. I have a trick I came up with for bumping fuel pressure by tapping on the regulator using a socket and hammer to slightly collapse the housing thereby increasing pressure on the spring inside which raises the fuel pressure. Works great on Fords. I was going to try it on this HD. Well I was looking at the regulator to determine what tools to use when I noticed a very small set screw right in the middle of it. Having dealt with AFPR off and on for years with my race cars and the occasional factory type I recognized it's purpose right away. I then thought it was too good to be true. So after grabbing the correct bit (T10H, tamper proof torx 10, H=hollow) and giving it a gentle twist I was able to raise the fuel pressure. It took a few turns to bring it into specs. I did several adjust and drives while watching the Short Term Fuel corrections on the scan tool to get it just right. Once I had it just tipped into rich I stopped. The good news was that power picked up under WOT due to much less timing retard from knock. There was still a bad WOT lean down after going into 3rd which is either due to poor calibration programming or the owner installed Airaid filter but it was now less severe and had less knock retard.

I was so dang happy about this tid-bit of knowledge because I immediatly went over to my truck, an '01 2500HD, 8.1, auto and looked at it to find that mine has an identical regulator. This is great because I've been working on my poor fuel mileage condition for months. One thing that has plauged me is that my MAF and PCM don't agree, that is the reading the PCM gets from the maf ends up as too much fuel at the injectors. In the case of the customers 6.0 his PCM had a positive fuel trim because it was having to increase injector firing time from the MAF calibration to get enough fuel. But mine was having to decrease injector time to reduce excessive fuel. This doesn't mean much for part throttle cruise because the PCM is constantly correcting based on the O2 sensors, this is fuel trim, and once it's learned it remembers it. So unless a major change is made it's usually very close anyways. But at WOT things are different, the PCM overrides the O2 sensors becuase they are going to be reading rich anyways (at least they should) and goes mostly by MAF reading compared to calibration. So if fuel delivery is off by a large enough margin you can run horribly lean, like the 6.0, or way too fat, like my 8.1. One kills the engine and power, the other wastes fuel and gives away power. Adbrupt throttle changes cause the PCM to deliver fuel based on MAF reading also because the O2's aren't fast enough to correct in transient conditions.

So you could safely bet that I had all our equipment hooked up to my truck on the way home today. By relieving a few PSI I have most of my fuel trims hovering a 0.0%. This is good, more power and less fuel consumption. Now my PCM and MAF match each other almost perfectly, what the MAF says the PCM matches with the correct amount of fuel, not too much not too little.

I don't know why I haven't noted this before. I've seen no mention of the adjustment mechanisim online or in my diagnostic manuals, and I've looked. It made my day.

Vernon

WARNING: Adjustments should only be made by those who know what they are doing, you could ruin your engine. Just adjusting fuel pressure as a diagnostic guess will make things worse in most every instance. Make absolutely sure you have everything else in order, and check your pressure as decribed in factory literature before deciding to adjust pressure. Then only do it if you can verify the outcome immediately with the proper equipment.

<{POST_SNAPBACK}>

Nice read, Haulin'. Quite educational for this dummy .

I laughed at your disclaimer. Not because it's there or what it says, but becuase you gotta cover your butt so bad these days, that even sharing a find on the internet can haunt you !!

Haulin' 8.1 · March 25, 2005

Thanks, I was beginning to think nobody would comment because it was over the top, lol. But the other trucks on this site don't have this feature so I put it here. You never know when your mechanic might tell you that you need a new regulator and you can ask him if he tried adjusting it first. Plus spark knock is a infamous problem in the 6.0's, this information could help.

I like disclaimers, too many people think you're responsible for their own actions nowadays. In other words the adjustment is there but use it properly. These trucks cost a ton of money.

willieboy · March 25, 2005

So, do the 5.3l guys have this same adjustment? If so, maybe I will have to swing by your neck of the woods and have you check my Burb out.

JRKRACE · March 25, 2005

Reminds me of all the adjusting I used to do on my Grand National. I had an adjustable regulator and a gauge right next to it...

Haulin' 8.1 · March 25, 2005

"So, do the 5.3l guys have this same adjustment? If so, maybe I will have to swing by your neck of the woods and have you check my Burb out." Willeboy

I'm not sure, I looked at an '05 5.3 truck today and it didn't. It has a returnless fuel system though which I think is new for '05. Returnless fuel injection systems regulate the electric pump to controll pressure. If you pop your hood and look at the drivers side of the plastic engine cover, just under it, below the edge, about half way, you should see a small round puck about the size of a silver dollar with a small vacuum hose attached to it. This will be mounted to the fuel rail, which is the metal pipe attached to all the fuel injector tops. Another tip is that you will see two fuel lines attached to the fuel rail, supply and return. These are the silver braided lines with quick connects coming out of the trans tunnel just next to the brake booster. They will be connected to the fuel rail also. If it's returnless there will be only one.

By the way I checked my girls '99 LS1 Trans Am and it has a very similar looking regulator but without the adjuster screw. Hmmm, wonder if the truck unit would swap on.

"Reminds me of all the adjusting I used to do on my Grand National. I had an adjustable regulator and a gauge right next to it... " JRKRACE

Hehe, yeah me too. Except mine are Thunderbird SC's. I have my fuel pressure guage on the dash. Lets me run on stock head gaskets forever and a day. LOL. I have a special allen wrench that is really long, I can adjust pressure without opening the hood through a hole I drilled in the cowl cover. I must have made adjustments a hundred times on the butt dyno LOL.

Neil Patterson · April 1, 2005

Hello! I just read you're find on the 6L fuel delivery issue. My 99 6L Silverado L.Box Ext Cab is doing the same thing. Changed plugs, dealer said change gas stations, nothing. It is noticeable during shifts up or loading up climbing a hill on the highway without putting you're foot into it. However I also have a hell of a whine from the fuel pump, it's loud! I think you may have found my problem. Could the dealer correct this or is it likley not ( hard to say) I would like to do it myself however I would need the correct tool. I will ask a GM dealer mechanic friend of mine. Thanks for the help. Neil P Thunder Bay

Greg · April 2, 2005

Very interesting. Everything was fine until the section on the 8.1. Sorry, my brain is not fully powered up on Saturdays.

Haulin' 8.1 · April 2, 2005

However I also have a hell of a whine from the fuel pump, it's loud! ---- Could the dealer correct this or is it likley not ( hard to say) I would like to do it myself however I would need the correct tool. I will ask a GM dealer mechanic friend of mine. Thanks for the help. Neil P Thunder Bay

<{POST_SNAPBACK}>

Niel all the electric pumps make some noise but if it's getting too loud that usually means it's going out. Make sure your pump can maintain full pressure, whatever it is currently set at, before trying to raise it in order to correct your issue. It could be a weak pump.

About the dealer service adjusting pressure, I can't say. I use the factory service manuals (Helms) to work on my truck and like I said the adjustability isn't ever mentioned. In fact anytime the steps get to a problem with the pressure it always says to change out the regulator. So I don't know if it is something a dealer tech can/will do to resolve a pressure related problem.

Vernon

Haulin' 8.1 · April 2, 2005

Very interesting. Everything was fine until the section on the 8.1. Sorry, my brain is not fully powered up on Saturdays.

<{POST_SNAPBACK}>

Still trying to recharge after Friday night I assume.

Cliff notes:

The 6.0 needed more pressure to match the MAF to injectors, raised it, much better.

My 8.1 needed less pressure to match the MAF to injectors, lowered it, much better.

Does that help?

Vernon

Haulin' 8.1 · April 2, 2005

Here's a update on my truck with more results:

On my 8.1 I did a lot of tuning over last week. When I had my pressure set low enough so that fuel trims were at or near zero (just right) I noted occasional leaning out at WOT and some knock triggered spark retard. I never actually heard any like on the 6.0s. So I continually raised pressure in an attempt to prevent this. I got to the point that the pressure was as high as possible, about 64 PSI, and it didn't help. So I put higher octane fuel in the tank, that didn't help. Evidently the lean out during WOT is programmed in as it was about the same no matter what. It never ran low enough to misfire but it did lean down a few times during WOT no matter how much fuel the PCM had at it's disposal. I've put the pressure back down to match the MAF and am driving it around enough so that if it can learn to adjust the WOT fuel trim it will in time. I will scan it again in a few weeks to see. I also seems like much of the witnessed spark retard was automatic, maybe torque management? It does seem to be using slightly less fuel but I haven't been able to do accurate testing as of yet.

More 6.0 tuning:

Another 6.0 HD came in Friday at 100K with the same symptoms as the other 6.0 above. Same things were done for the tune-up. This time I went straight for the jugular and raised the pressure to the maximum allowed specification (62 PSI, vacuum line off). On the subsequent test drive the power was as high as I've ever felt on a 6.0 HD and I neither heard or saw any evidence of spark knock. I decided to let the O2 sensors correct for mileage during part throttle operation and set the pressure to prevent an overly lean WOT mixture and left it there.

Afterwards I noted a rougher idle, on a hunch I ran a "Crankshaft Profile Learn" test. This recalibates the PCM's crankshaft sensor readings. I figured that at 100K wear on the sensor may have changed it's output enough to cause roughness at idle or that the complete tune up had changed crankshaft rotation characteristics enough to throw the PCM off. To my amazment it smoothed out within seconds. Pretty cool. Regrettably, I didn't get a chance to drive it again afterwards because the customer showed up right then and took delivery.

Vernon

keithstef · April 8, 2005

Here's a update on my truck with more results:
On my 8.1 I did a lot of tuning over last week. When I had my pressure set low enough so that fuel trims were at or near zero (just right) I noted occasional leaning out at WOT and some knock triggered spark retard. I never actually heard any like on the 6.0s. So I continually raised pressure in an attempt to prevent this. I got to the point that the pressure was as high as possible, about 64 PSI, and it didn't help. So I put higher octane fuel in the tank, that didn't help. Evidently the lean out during WOT is programmed in as it was about the same no matter what. It never ran low enough to misfire but it did lean down a few times during WOT no matter how much fuel the PCM had at it's disposal. I've put the pressure back down to match the MAF and am driving it around enough so that if it can learn to adjust the WOT fuel trim it will in time. I will scan it again in a few weeks to see. I also seems like much of the witnessed spark retard was automatic, maybe torque management? It does seem to be using slightly less fuel but I haven't been able to do accurate testing as of yet.

More 6.0 tuning:

Another 6.0 HD came in Friday at 100K with the same symptoms as the other 6.0 above. Same things were done for the tune-up. This time I went straight for the jugular and raised the pressure to the maximum allowed specification (62 PSI, vacuum line off). On the subsequent test drive the power was as high as I've ever felt on a 6.0 HD and I neither heard or saw any evidence of spark knock. I decided to let the O2 sensors correct for mileage during part throttle operation and set the pressure to prevent an overly lean WOT mixture and left it there.

Afterwards I noted a rougher idle, on a hunch I ran a "Crankshaft Profile Learn" test. This recalibates the PCM's crankshaft sensor readings. I figured that at 100K wear on the sensor may have changed it's output enough to cause roughness at idle or that the complete tune up had changed crankshaft rotation characteristics enough to throw the PCM off. To my amazment it smoothed out within seconds. Pretty cool. Regrettably, I didn't get a chance to drive it again afterwards because the customer showed up right then and took delivery.

Vernon

<{POST_SNAPBACK}>

I just looked for the pressure regulator. I have a 2004 2500hd with the 6.0l.

I couldn't find it.

The pressure line that comes from the tank, goes to the drivers side rail first then splits to the passenger side.

It doesn't seem to have a return line were I thought a pressure regulator should be.

Did they change something?

Actually, I thought when I looked into a Supercharger, that the tank needed to be dropped to modify the fuel pump/pressure regulator.

Pic's of this regulator would be great.

Thanks

Keith Stef

Haulin' 8.1 · April 8, 2005

Keith your truck has the new returnless style fuel system. It is electronically regulated at the pump. That's why you can't find a find a vacuum referenced spring pressure diaphram type on your fuel rail. The vacuum/spring type onlys holds enough pressure at the rail to maintain the setting and bleeds the extra off into a return line going back to the tank. Since your returnless pump only sends enough to maintain pressure it doesn't need a regulator and return line at the fuel rail.

The only way you will be able to accurately modify fuel delivery is with PCM programming.

Vernon

Haulin' 8.1 · April 8, 2005

The supercharger kit will have a pump with higher capacity but the pressures will stay the same. In effect it can maintain the needed pressure while delivering more fuel.

Vernon