I have a 2000 E250 4.2L that's been throwing lean bank codes (P0171 and P0174) which if I'm correct means a PCV vacuum leak. It was loping at idle, almost stalling sometimes, and after looking for about an hour for a problem it decided to run fine. The only time I was able to replicate the issue was when I got in to drive it and put my foot on the brake pedal. Can the power brake booster leak sometimes and work correctly the rest? It holds vacuum, I get a couple of assisted pumps of the pedal with the engine off, and when I pull the vac line off the booster with it running I get the exact same loping idle.

 

yes it can

 

I had a 2003 e series 4.2V6 with the same two codes.
That engine is known for "isolator bolts" going bad and causing vacuum leaks and those codes (and corresponding symptoms).
I recommend you remove the doghouse in your van and try to flex the upper intake forward/back and up/down. I bet you'll hear vacuum leak. Very very faint.
I have a long thread on here fixing mine. A little hard to find but you can find it if you try.
Check it out, you'll see.
i replaced them and fixed an egr port issue and then it ran great forever.

 

Thanks, I'll look into that. I still suspect it's an issue with the power brake booster and I already ordered one from Rockauto, but I can always return it.

 

no offense intended guy, but after rereading your original post; I feel more certain that your booster is not the problem. My humblest apologies in advance if I'm wrong though.

For your own sake, please check into my thread about the isolator bolts. When I discovered it, the vacuum leak was unbelievably faint. But by slightly flexing the plastic upper intake, the faint hissing would change.

"Slightly flexing" = applying a good amount of pressure with my hands, NOT feeling it move more than a few thousandths of an inch <can the human body detect that small ? >

please keep us posted.

 

No offense taken, I intend to check your suggestion before I try to replace the booster (Which I'm sure will be a huge PITA). I only think it might be the booster because it ran fine while looking for a leak until I had my foot on the brake for a couple seconds.

I'll pick up some starting fluid and spray it around the intake.

 

Typically in my experience the diaphragm goes bad
YMMV

 

If it is intermittent enough, can you reproduce it in the driveway without removing the brake booster vacuum line? Was thinking if so, then take the vacuum line off, plug it, and see if you can reproduce the the loping. OR if intermittent enough, perhaps there is a safe place like a large empty parking lot or safe road with no traffic where you could test drive it. That's assuming one can safely stop a E250 without power brakes..

 

The only way I've been able to reproduce it is by stepping on the brake pedal.

I also tried spraying starting fluid around the intake manifold. There was no change in the idle at all. I suppose if I could get some help and make it idle bad by stepping on the brakes I could spray ether around the booster. I got the new one already, just haven't had a chance to work on it.

 

it's the diaphragm
99.9% sure

 

I'm still voting for isolator bolts.
And I believe the tie-in with the booster is:
when you apply the brake pedal, that's a large volume of vacuum to "fill" and it exacerbates the isolator bolt issue.
("Exacerbates" - did I use that word correctly?)

 

P0171 and P0174
those mean Lean Bank 1 and Lean Bank 2
(I'm pretty sure)
which usually amounts to a large vacuum leak (after the MAF) making both engine banks run lean.

 

Yes, you used it correctly.

And you could be right, it makes some sense too because if the bolts are bad and the top cover on the intake is being held closed by vacuum, a loss of vacuum pressure from the brake booster working would let it open and leak.

 

same thing happens with the diaphragm
either way if you have a leak, you need a to replace it

 

Update on this issue:

It wasn't the power brake booster.

I'm sure it's not the intake or the isolator bolts, although I still plan to pull it apart, clean the EGR tubes and replace the bolts.

It's gotten more consistent, pulling up to a stop it may idle fine for 5 seconds or 15 seconds or a minute, then it either idles really rough like it's only firing on half the cylinders or it just dies. Same codes, P0171 and P0174. I need to look around the EGR valve, maybe the tube is cracked and when it opens it's creating a vacuum leak.