No accelerator Pedal Response at Operating Temp

[BobFunGhoul]

2nd owner 1997 Suburban K1500 / Auto / currently 170,000 mi., well maintained by previous owner.
New injector pump @ 100,000 mi.
Remote heat sink and "tested" PMD installed in bumper @163,000 mi.
Fuel tank imploded and replaced with new @ 165,000 mi.
New lift pump @ 170,000 mi.

After I installed the new lift pump, she was smoother and had more power since the tank implosion. 100 miles after the pump install, I was driving on the highway for about 30 min and she started to stammer but not stall; sporadic response from the throttle pedal. I exited the highway, stopped at a light and she held a steady idle with intermittent response from the pedal. About 1 mile later I lost all pedal response and I idled her home into my driveway. Shut down and immediate restart with same results. Let her cool for a very short time, restarted and had intermittent pedal response. The next morning, cold start, she ran fine. Before I get her up to operating temp again and get stuck, I'm just looking for some suggestions as to where to look first. No idiot lights illuminated nor did any trouble codes set. I'm leaning toward replacing the PMD with new, but before I drop the cash.........
Any ideas would be greatly appreciated and much thanks in advance.

[greatwhite]

Tank implosion?

As in collapsed?

Did you find the cause of this and was it the fuel cap?

Diesel fuel caps and gas fuel caps are different on these trucks.

A gas cap on a diesel tank can cause "implosion" and poor running because it doesn't vent the tank properly.

In other words: if it doesn't let enough air in, you can't get enough diesel out which can cause lack of power, stalling, bucking, etc. Go long enoguh and it actully "sucks" the tank in on itself.

Starting and running fine after sitting also matches the fuel cap issue. As it sits, the vacuum bleeds off and then it will run fine until the vacuum builds up to a point and starts your issues all over again.

Have a look if you haven't already. It's a quick easy check and a cheap fix if it is the problem....

[BobFunGhoul]

The tank implosion was due to a clogged fuel cap. That issue has been corrected with a new tank, fuel cap, float and strainer, and filter. I should have done the lift pump at the time, but it seemed to be working fine. Nevertheless, she gave out awhile later and I replaced it.

When I idled home the other day, I opened the air bleeder and the water drain while running and I had plenty of fuel flow without the engine skipping a beat. I don't believe this is a fuel delivery issue.

Thank you for your interest.

[Warren96]

Could be air in the fuel.

[Warren96]

Have you opened the air bleeder, not the water drain, on the fuel manager to see if there is just fuel coming out? Save your PMD money for another day.

[JohnC]

If the problem is electronic, there will be codes set. Did you scan for codes or are you going off the fact that the check engine light did not come on?

Idle only is generally an electronic problem - the PCM put the engine in limp mode.

If it's not electronic, it almost has to be fuel related.

[BobFunGhoul]

I have opened the air bleed valve and I get a steady flow of fuel with no air. I also checked for codes with a scanner and there are no codes set. I believe if the accelerator pedal sensors were at fault, I would get codes. I was told that if the PMD is bad it will not set any codes, but I don't know if a dead accelerator pedal is a symptom of a faulty PMD. I've experienced a faulty PMD previously and the symptoms included: surging, erratic idle, and stalling, but once I shut down and restarted, the vehicle ran fine until the PMD heated up again. With my recent experience, when I shut down and restarted I had the same issue...no accelerator response.

Thank you for your assistance.

[racer55]

I would start by cleaning/repairing all grounds.
Battery to intake,frame to block,body to block and the cluster of grounds on the rear passenger side stud just behind the intake.

Also verify that the PMD harness ground is secure and attached to the IP(located near where your extension plugs in at the IP).

[BobFunGhoul]

I took her out again last night. She drove beautifully for about 20 minutes, then started to act up. Slightly different symptoms occurred.....erratic idle, sporadic pedal response. I got her home and shut her down. She restarted without any issues and ran smoothly, but I didn't want to chance going out again. I will check all of the grounds previously stated and see if that helps. Thank you.

[racer55]

Time to try a new PMD and resistor,the used tested PMD's are poor substitutes for new(mostly junk),especially since you can often find new dorman(flight systems)PMD's for about $160 on ebay.
Even some parts places apparently have lifetime warranty on the replacement PMD's so shop around-lifetime warranty is worth more up front to me.

[Hooptybass]

I know Oriely's offers a lifetime PMD . not sure what brand but i was going to go with one before i switched back to a 4911 pump

[racer55]

Most chain stores carry dorman-rebadged flight systems PMD's.
If it doesn't say stanadyne then it is a flight systems no matter who's name is stuck on.

There was another maker many years ago but I forget what they were called?

[DmaxMaverick]

Most chain stores carry dorman-rebadged flight systems PMD's.
If it doesn't say stanadyne then it is a flight systems no matter who's name is stuck on.

There was another maker many years ago but I forget what they were called?

Sol-D.

[racer55]

Thankyou.

[BobFunGhoul]

Grounds have all been cleaned up and still the same issue. I ordered a new pmd, and I should have it by the weekend. When I install it I will let you know what happens. Thanks again.

[BobFunGhoul]

Okay, so I installed a brand new gray PMD, harness, and resistor. Took her out for a ride and she ran like a champ. After about 20 min of driving, she starts to act up again. I get her home, pull in the driveway and she stalls. This time I can't start her back up. I open the air bleed and water drain and there is no fuel. Just a puff of air that came out of the air bleed. I loosen the fuel cap and I hear a rush of air (fuel cap is less than a year old). I don't know if it is vacuum or pressure. I tried starting it with the fuel cap loosened, but no luck. I let her sit for a couple of hours, attempt to start and she fires right up. The lift pump is operating both before start up and while running. I thought maybe one of my fuel lines is internally collapsed, but all of the flexible lines are firm when pressure is exerted on them. I also believe if I had a collapsed line it would act up all of the time....not just at operating temp. I read in another forum where someone had an identical issue, changed his water in fuel sensor and that corrected the problem. I can't imagine why that would cause stalling. I checked her again this morning. Opened the air bleed and water drain, energized the lift pump and got tremendous fuel flow. She started right up. Before I take her out again and not make it home, any suggestions would be greatly appreciated. Thank you.

[racer55]

Well with any temp related problem a temp sensor is something to think about.

A scan tool that will display sensor values is a big help with problems like this.

[JohnC]

I also believe if I had a collapsed line it would act up all of the time....not just at operating temp.

Unless it collapses due to vacuum in the tank... Try running with the cap loose to see if it still dies.

[BobFunGhoul]

Well with any temp related problem a temp sensor is something to think about.

A scan tool that will display sensor values is a big help with problems like this.

Unfortunately, I don't have a scan tool that reads sensor values. Is there a temp sensor on the vehicle that controls the lift pump? Is it possible the oil pressure switch is opening at operating temp.? Thank you.

[BobFunGhoul]

Unless it collapses due to vacuum in the tank... Try running with the cap loose to see if it still dies.

I'll give that a shot in the morning. I'll just keep her close to home. Thank you.

[BobFunGhoul]

Alright......so I ran her around a little bit today with the fuel cap loosened. Picked the kids up from school. Stammered once on the way back. Got into the driveway, brought the kids inside and settled them in. I went back out to the Suburban and started her up. I was going to take her for a drive until she stalled. Instead, I opened the hood and opened the air bleed and the water drain.....no fuel coming out but engine still running. I could feel the lift pump running. I checked voltage at the output of the ops.....14.4v. Shortly after, she stalled and wouldn't start again. I cycled the plugs and the lift pump numerous times with the air bleed opened, no fuel. I'm stumped. It seems the trips I am able to take before she stalls are getting shorter. One thing I noticed is that the lift pump used to run a little bit after the engine was shut down. That doesn't happen any more. I don't know if that has anything to do with my problem. Any ideas would be greatly appreciated. Thank you.

By the way....there are approximately 34 gals. of fuel in the tank.

[BobFunGhoul]

A few minutes after my last post, I went back out, cycled the lift pump with the key, opened the air bleed and fuel came out like a geyser. I closed the air bleed and started the truck......no fuel at the air bleed, but the truck was still running. I went under and felt the lift pump running. I disconnected the lift pump connecter, and the truck continued to run. I re-connected it and gave the lift pump a few raps, thinking something maybe stuck in there....no change; still no fuel at the air bleed.

[racer55]

Plugged tank strainer or a dead LP-the LP's can fail and still act like they are pumping.

I would lean towards the strainer being plugged or defective,it can pump fuel for a short while until it becomes clogged with debris again.

[racer55]

You mentioned in the first post that a new sending unit was installed,a great many aftermarket sending units have been turning up causing problems because the strainer they are sent with is for gasoline-not diesel.

The diesel strainer is much coarser and the gas strainer is limiting fuel flow- abig debate has happened over this.

My advice is to drop the tank and remove the strainer,then install an inline fuel filter pre LP-this is much easier to change and monitor.

The debate was about that very advice so decide for yourself.

[BobFunGhoul]

You mentioned in the first post that a new sending unit was installed,a great many aftermarket sending units have been turning up causing problems because the strainer they are sent with is for gasoline-not diesel.

The diesel strainer is much coarser and the gas strainer is limiting fuel flow- abig debate has happened over this.

My advice is to drop the tank and remove the strainer,then install an inline fuel filter pre LP-this is much easier to change and monitor.

The debate was about that very advice so decide for yourself.

I think you're right. But with 30 some odd gallons of fuel in the tank, I wonder if it would be more feasible to cut an access panel in the floor of the storage area and work like a gentleman. I'll have to take a look today. I don't know if there are any supporting cross members running over the sending unit assembly. Thank you

[DmaxMaverick]

Replace and inspect your old fuel filter. If the filter and fuel is clean, the problem isn't the strainer, or the fuel. The fuel manager will show your fuel condition long before the strainer/sock plugs, regardless of which you have. This is an old argument, the "sock" thing. It seems some of the aftermarket sender units have a finer screen, but it won't matter, if the fuel isn't contaminated. If the fuel is contaminated, removing it doesn't fix the problem. Even the "fine" strainer is coarse, compared to the fuel system and filter. Removing it only moves the problem from one place to another, and will likely, over time, be worse than leaving it in place. The screen increases the surface area of the fuel uptake. Removing it reduces that area to the cross-section of the pick-up pipe, and then it takes only a pea size contaminant to stop the flow completely, while it would go essentially unnoticed with any screen in place. Adding an inline filter isn't a bad idea, but it's NOT a replacement for the tank sock, regardless of which you have.

It sounds like your fuel lift pump is failed or is not getting consistent power, and/or you have an air intrusion leak. If it's both, the leak can be on either side of the lift pump. If you have no air while the LP is running normally, but do when it isn't, I suspect you have a suction leak between the pump and fuel manager. A suction leak doesn't immediately mean it will leak fuel under pressure, or enough you may notice. The fuel pressure is (or should be) relatively low, typically 4-8 PSI at idle. If you have installed a pump that isn't OEM/Delco, then all bets are off, in any case. Many of the aftermarkets are DOA in the box.

[racer55]

Replace and inspect your old fuel filter. If the filter and fuel is clean, the problem isn't the strainer, or the fuel. The fuel manager will show your fuel condition long before the strainer/sock plugs, regardless of which you have. This is an old argument, the "sock" thing. It seems some of the aftermarket sender units have a finer screen, but it won't matter, if the fuel isn't contaminated. If the fuel is contaminated, removing it doesn't fix the problem. Even the "fine" strainer is coarse, compared to the fuel system and filter. Removing it only moves the problem from one place to another, and will likely, over time, be worse than leaving it in place. The screen increases the surface area of the fuel uptake. Removing it reduces that area to the cross-section of the pick-up pipe, and then it takes only a pea size contaminant to stop the flow completely, while it would go essentially unnoticed with any screen in place. Adding an inline filter isn't a bad idea, but it's NOT a replacement for the tank sock, regardless of which you have.

It sounds like your fuel lift pump is failed or is not getting consistent power, and/or you have an air intrusion leak. If it's both, the leak can be on either side of the lift pump. If you have no air while the LP is running normally, but do when it isn't, I suspect you have a suction leak between the pump and fuel manager. A suction leak doesn't immediately mean it will leak fuel under pressure, or enough you may notice. The fuel pressure is (or should be) relatively low, typically 4-8 PSI at idle. If you have installed a pump that isn't OEM/Delco, then all bets are off, in any case. Many of the aftermarkets are DOA in the box.

The only advantage of having the sock is that it takes more algae or debris to completely block off the fuel intake,it also helps prevent a vortex from being created when the tank is nearly empty-you are out of fuel anyway the sock will allow air to be sucked in just the same since it is easier than drawing fuel-path of least resistance.

The inline filter pre LP will still trap any debris before it reaches the LP and I wonder where a pea sized piece of trash comes from that you are so fond of referring to?

If someone uses dirty containers to fuel their truck,they are bringing a problem onto themselves,service station fuel is normally only contaminated with water if at all.

There are a great many posts of people retro fitting the GM strainer to a new sending unit to remedy a fuel starvation problem,or going the inline filter route with no strainer and having positive results-the new strainers are a problem.

[DmaxMaverick]

The only advantage of having the sock is that it takes more algae or debris to completely block off the fuel intake,it also helps prevent a vortex from being created when the tank is nearly empty-you are out of fuel anyway the sock will allow air to be sucked in just the same since it is easier than drawing fuel-path of least resistance.

The inline filter pre LP will still trap any debris before it reaches the LP and I wonder where a pea sized piece of trash comes from that you are so fond of referring to?

If someone uses dirty containers to fuel their truck,they are bringing a problem onto themselves,service station fuel is normally only contaminated with water if at all.

There are a great many posts of people retro fitting the GM strainer to a new sending unit to remedy a fuel starvation problem,or going the inline filter route with no strainer and having positive results-the new strainers are a problem.

Been there. Done that. The sock is necessary to maintain reliability. As I said, removing it reduces the uptake cross-section, creating a greater possibility of blockage, full, or enough to cause starvation. The pea-size contaminant I'm so fond of can be any number of things. Congealed algae, ice, wax, flake of paint/sealant, marble (toy), or anything a child or disgruntled ex may decide to drop in your tank. Still, removing the sock isn't addressing the root problem, if there is a problem with restriction, due to the sock, any sock, of any porosity. If you "fix" a problem by removing the sock, you didn't "fix" anything.

This is not to say that some aftermarket senders may or may not have issues. That's a question of quality, not design. That's the gamble of saving money on non-OEM, or inferior quality parts.

[racer55]

Agreed on all points but 2,the cross sectional area of the intake pipe is the same with or without the sock.

The advantage of the sock is that it gives a larger surface area to become plugged before complete inability of flow.

Very similar to the pick up tubes in a dry sump oil pan,their length allows them to pull oil from a much larger area,but if a tube gets plugged after the scavenge area-it is still plugged.

Also removing an improper sock to repair a restriction caused by the sock is a repair.Idealy the correct OEM sock should be used.

[BobFunGhoul]

I pulled the sending unit today. The tank is less than 6 months old, so there is minimal debris. The fuel appeared quite clean. The sock actually held fuel in it when it was removed from the tank. It took a few minutes for the fuel to drain from it. Although the sock increases the surface area from where the pump draws from, the problem with this sock design is flexibility. It's too soft! Therefore, when it gets a bit contaminated, the pump draws it flat and now the area of suction is reduced to the circumference of the pick up tube. I replaced it with an AC/Delco sock.....a much different design with 3 different levels of filtration built in. The Delco sock is dual layers of rigid mesh in a cylindrical shape. The rigid material reduces it's chances of being drawn flat. The 2 layers of mesh provide the finest filtration. On the side of the sock, one layer of mesh is cut out into the shape and size of a dime. Then the open seam is sealed shut, providing a small area of coarser filtration. Finally, on the end is a spring loaded gate, also about the size of a dime. If the rest of the sock gets clogged, the gate will then open up and let in unfiltered fuel.

I got her buttoned up this morning and have been driving around all day....no problems at all.

[racer55]

Thanx for the great post.

[BobFunGhoul]

I Just want to thank everyone for their concern and excellent advice. It was much appreciated.