That is an excellent question. Too bad my answer is not quite as good. I ran it on both pumps at different times, without noting much difference. My initial focus was to run on the front tank hoping to catch it filling the back one. As the problems accumulated, I switched to being most concerned with safety, avoiding damage to the truck, and trying to drive "just a bit more" hoping it would clear up while not getting stuck too far from home. The short answer is that I never killed it on the rear pump, but I never really tried to kill it, and ran mostly on the front pump. It was not really happy with either pump.

The few times it stalled were at low RPM during a turn or when crossing a speed bump. In favor of a friend mine (see below), the low RPMs could suggest a bad regulator. The bump/turn could point to junk in the fuel - but there really should not be anything that hasn't been there for a while, except maybe some condensation??

Re cross-filling, so far, I'm not convinced one way or the other. Of course, the valve might work some times and not others. The rear tank did overfill, arguably starting all of this, so that probably does point at the rear pump.

While I do not wish any ills on you or your truck, it is interesting that you report similar roughness after draining. Is the idea that you've made a big hole and the cross-filling adds a leak that drops the pressure until enough gas moves into it? How long does your front (playing the role of my rear) tank take to fill? If it is a fair question, how long to move an 1/8 to a 1/4 tank or so?

May I ask why you have not replaced your front pump? Feel free to not answer if it is none of my business. I would be mostly interested if it has something to do with difficulty of the repair. I gather the rear tank is easier to drop than the front, and that would be my guess looking at it. Any other insights you can add would be appreciated.

A friend has been encouraging me to replace the pressure regulator. For my part, I will read over the tests again, though you are adding to my resolve to not pull it "just in case." One question though: has anyone replaced a regulator on a 1996 4.9L, and did you find it necessary to remove the upper intake? I'm getting mixed signals. If it can be changed with the manifold in place, that would make it a lot easier (natch). It _looks_ like it should change with the intake in place, but it could be tight to impossible. It would be nice to know before starting the job.

Bill

 

The cross-filling takes time (several miles to accumulate much of anything). It's not something you could really notice over a 15 or 20 minute session in your driveway.

I think that as you said, there is a pressure differential between the two tanks that has some effect on what the engine gets from the active tank. I don't really understand it, but apparently no one else does either. It's all theory in my head at this point.

I haven't changed my front pump due to the cost. If you'd read my EGR thread, you know that I've spent a small fortune and every waking minute chasing a ridiculous mixture of issues that were related enough to keep me chasing my tail, but non-related enough to make me look back and wonder why I couldn't figure it out. That (I think) is behind me now, and the front tank pump will be my next victim. I'll do it like I did the rear, though, and replace everything while it's down.

Have you pulled any codes at all? The stalling could be something aside from a fuel issue. I hate to send anyone down the road I've been on, but the fuel pressure regulator, a stuck EGR valve, IAC valve, missing cylinder, etc. could be causing some of these symptoms too.

 

When you pulled the regulator vacuum, did the idle pick up? If the idle increased then the regulator should be good. My theory on the rough running after draining the tank is that the overfull tank will prevent any more fuel from being pumped in there, blocking off the cross filling. When it is cross filling there is a pressure drop, but with the tank overfilled and preventing this then the fuel pressure is stabilized.

 

One other thought... In another lengthy conversation with sean, we thought about the vapor lines running from the tanks to the charcoal canister being plugged or kinked.

You'd think, that if both tanks vent to the canister, that they couldn't build up enough pressure to overfill past the cap. I still haven't had a chance to go through mine to see if anything is obvious, but it might be worth a look on your truck too.

 

Codes are an interesting one. Based on Alldata's recommendation, I rented a scanner and found the truck giving itself quite happy with its operation. Consistent with what everyone except alldata told me, there was NOTHING in the way of a diagnostic mode.

That aside, I took the scanner back, got my refund, and THEN the truck stalled The check engine light came on, but it cleared itself on restarting. Might there still be something new stored? Either way, I plan to buy a cable and get some software to put on an old laptop vs. shelling out for a reader.

I never pulled the regulator hose with the engine running - with rail pressured, but not with the engine running. Now that leak is on standby, I am game to try engine running tests. Your test sounds simple enough. Do I need to block off the air "now" flowing into the manifold at the same time?

There is a line going (I think) from the canister to EGR or related equipment that certainly needs to be replaced, but the consensus seems to be that liquid in the canister is baaaaaad and to look elsewhere first.

Thanks!

Bill

 

When the truck stalls the check engine light will come on, and then restarting will cause it to shut back off. That is normal and doesn't necessarily mean there will be a code. You can get the codes read at a parts store, most do it for free. There will be no code for the cross filling but there might be for low fuel pressure. They don't always store codes for all faults. When you pull the regulator vacuum cap it off with you finger, the hose not the regulator. You may get gasoline in the canister with the fuel tank problems you are having.

 

Like sean said, fire it up and pull the vac line off the fuel pressure regulator. You'll either get gas running out or you won't - no big deal. If you put your finger on the vac line, the truck's idle will stay steady - if you leave it open, it'll idle fast. Gas coming out means bad FPR, no gas means it's good.

The line that you mentioned from the canister probably goes to your purge solenoid right in front of the EGR valve. Definitely check all of your vacuum lines and make sure they're solid. If not, replace 'em - sometimes it's those simple things that make your truck sound like it's sick...

Have you checked the IAC valve for smooth operation? That could be part of the stalling problem, and easy enough to clean out with some carb cleaner.

 

It's the regulator vacuum line that you want open to air? Blocking the hose keeps air from rushing into the vacuum system??

Would there be any harm in taking the vacuum hose off of the manifold and blocking the entry to the manifold there? That would keep air from rushing into the vacuum system, would leave the regulator open to air, and seems a LOT safer to me than reaching down between the parts of the intake to remove the hose with very little clearance, etc.

Otherwise, I could remove the hose with the engine stopped, plug the hose, and then crank it. But I think it would help to hear it running and then change when I remove the hose.

I'm a little aware/paranoid from my machining background. Part of learning to run a mill is being taught by a guy who _personally_ knows a guy named Lefty - really Ok, "Lefty" is often a mythical figure, but I've heard some pretty ugly stories. One had a happy ending: a dude was grabbed by his jacket and thrown over top of a lathe, with no serious injuries; if he had been running it forward, he'd have been killed. I have a lot of respect for running engines too.

Back to the stalling, do you think it is at all likely to get stuck someplace, or is the stall/crank cycle likely to repeat? I will test the regulator and drive a little more close to the house. AutoZone is about 4 miles from my house, and they will of course scan it.

Thanks,

Bill

 

It's the regulator that you want open to air, the vacuum leak will cause the truck to idle high anyways. So you want to pull from the regulator and immediately cap the vacuum source so the higher idle won't be confused with the vacuum leak high idle. It doesn't matter where you unhook the vacuum source, either directly from the regulator or further away, as long as you cap the source immediately. I'll have to go back and re-read about the stalling, because I can't remember the symptoms you listed. I think you said a possible fuel issue(?) probably the underpressure situation(?) if so then it may continue to restart unless a fuel pump goes out, but you do have a spare. If the stalling is an electrical issue then the truck will usually restart when it has cooled down.

 

Thanks for the clarification. I will check it at my next opportunity - at the rate this week is flying by, that will probably be Saturday.


I am the last one to ask about why it was stalling Your thoughts on why would be greatly appreciated. I was somewhat pleased with myself for quickly falling into fuel/air/spark thinking, and then for starting with the things I had changed - maybe there's hope for me yet. Anyway, everything that I had changed was where and how it should have been, and the selected pump was at least running. So, I did not find any obvious cause, tried the starter, and although it took a few seconds to crank, it started. On the other stalls, it cranked quite quickly.

A low-pressure condition is certainly possible. I did not have the pressure gage, and would not want to connect it with the engine hot anyway. I think it was Aliens who said that taking the excess fuel out of the back tank might have triggered pressure woes. Rhetorically, the part I don't get is how such an apparently small "leak" into the rear tank would so severely affect the front pump's performance. Confusion aside, that could easily be what is happening. Aside from a possibly twitchy regulator, my next best thought is gremlins.

My main concerns with the stalling are for safety, avoiding damage to the truck, and finally to avoid missing the obvious and getting stuck half way to AutoZone.

Thanks!

Bill