1995 351w Giant Vacuum Leak?

[Kiraisuki]

I've got myself a 1995 E350 Econoline with a stock EFI 351w that behaves as if it has a gigantic vacuum leak in it that we can't find for the life of us. We've traced and replaced any vacuum lines we found, smoke tested the intake and the vacuum tree, and vacuum tested the cooling system, and we cannot find any new leaks. Nonetheless, the engine idles real low and stumbles over itself a lot, twitching at idle. The engine idles up and seems to run better if almost the entire intake is blocked off, indicating it is dumping air in from somewhere we can't find. Does anyone have any idea where we're not looking so we can fix this massive leak?

The smog pump and EGR were completely siezed and we were unable to find good replacements for either of them, so they have been removed and the associated lines and holes plugged and capped. The engine runs better without them, but still seems to have this massive lean condition. The timing is set to the Ford factory recommended 10°btdc for this engine, verified by timing light.

[Steve83]

I've got myself a 1995 E350 Econoline with a stock EFI 351w...

You should put ALL the truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

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Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location in your profile & upload an avatar of the truck. The more pics you post of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you. Not all its details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now.

...behaves as if it has a gigantic vacuum leak...the engine idles real low and stumbles over itself a lot, twitching at idle.

That's NOT the behavior of a vacuum leak; large or small. A vacuum leak would cause a HIGH idle, and an obvious hiss from the leak. This is how to pinpoint a vacuum or exhaust leak:

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The engine idles up and seems to run better if almost the entire intake is blocked off...

Where exactly are you blocking it, and exactly how much is "almost the entire"?

Does anyone have any idea where we're not looking...?

No because you didn't post any pics of the van, the engine, the intake, or any of the places you ARE looking.

The smog pump and EGR were completely siezed and we were unable to find good replacements for either of them...

Where exactly did you look, because they're VERY common & affordable.

...they have been removed and the associated lines and holes plugged and capped.

It ran VERY well when it was new & had all that stuff working. So if you want to get it running closer to that way, you should try to get EVERYTHING closer to that condition, including the EGR & 2ndry air systems. They HELP the engine run better.

The timing is set to the Ford factory recommended 10°btdc for this engine, verified by timing light.

Did you follow the instructions on the VECI label? You can't simply connect a timing light - the engine must be prepared for the test to be accurate & useful.

[Kiraisuki]

You should put ALL the truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

I have updated my signature with details of the van. As for history, I bought it almost a year ago off a friend. He got it from some guy out in Tucson, AZ, who traded some goats for it. We weren't told anything about it, so all we know of its past life was constructed from documents found in the vehicle. It lived its life as a shuttle for a retirement community in California before being parked for an unknown period of time less than 8 years (how old the tires were when we got it) and was handed off in quick succession to three other people before getting to Goat Guy. Since then, me and another friend have been working on it to try and convert it into a camper. The engine has always run poorly with the twitching and stumbling at idle the entire time we've owned it. The exhaust has never smelled right, and in fact changed smell as we've worked on it, but it has no color to it. No sweet smell, no black soot, it just smells like an engine that isn't running well with no catalytic converter because someone stole it. (We have a replacement, we have just not installed it out of fear of it being stolen again. We plan on devising a method to protect it at a later date.) The PIP sensor on the distributor went out as well, and that was replaced with an MSD distributor because we could find no OEM replacement and the only other choice was a WAI GLOBAL brand one known for failing unexpectedly.

Later on we discovered the engine would get hot quick because the cooling system was chock full of mysterious brown sludge that we slowly flushed out until we discovered the radiator leaked like a sieve and the sludge was stopping it from leaking. The radiator, heater core, all coolant hoses, thermostat, and both temperature sensors have been replaced as a result. The oil intercooler has been removed as well because it leaked. We have not replaced it yet, but plan to at a later date. That caused the engine to not be able to come up to temperature at all, so we installed an aftermarket electric radiator fan to attempt to remedy it. Unfortunately the engine still doesn't get hot enough while stationary even when the electric fan isn't running, and severely overcools itself when driving at any speed (the fan never comes on when driving and only comes on for a second at idle.)

While replacing the two coolant sensors, we accidentally removed the air temperature sensor and found it to be coated in oil. That was replaced as well.

Where exactly are you blocking it, and exactly how much is "almost the entire"?

We block the entire open hole (highlighted in white) in the airbox here with everything hooked up, and it causes the engine to idle higher and smoother than without the blockage. The little plastic air scoop that connects to the airbox was missing when we got the van. We installed a tachometer and discovered under normal conditions, the engine idles around 500-600 and flutters slightly. With the airbox plugged it idles up to around 700 or so.

No because you didn't post any pics of the van, the engine, the intake, or any of the places you ARE looking.

(Throttle body with airbox and hoses removed)

We removed the airbox and associated hoses and shoved a smoke testing machine into the upper half of the throttle body with the throttle held open, and plugged the lower half. We then meticulously went around the engine bay and underside of the van looking for smoke. We found a few broken vacuum lines going to a reservoir that we traced back and replaced, verifying correctness with the vacuum diagram under the hood. We also found the charcoal canister hose had come undone, which we fixed. We spent another couple hours looking for any more smoke and found none. After replacing all the hoses and radiators in the cooling system, we used a pneumatic cooling system vacuum tester and bubble remover to test the cooling system and it held vacuum overnight, and has not leaked any fluid since.

Where exactly did you look, because they're VERY common & affordable.

We checked the big four auto parts stores (AutoZone, Oreilly Auto, Advance Auto, NAPA) as well as RockAuto, eBay, and CARiD. AutoZone had the EGR valve and sensor, but the EGR pipe snapped while taking out the old valve and none of the stores had a replacement at the time, so we deleted it. The smog pump started making horrible grinding noises and spitting clouds of rust later on. We tried to find a replacement, but all we could find were expensive remanufactured units and salvaged units with no guarantee of functionality.

It ran VERY well when it was new & had all that stuff working.

I am aware of this, however as stated above we didn't have much luck finding replacements. We soon plan on installing a custom computer into the vehicle so we can install more sensors, have better readout from the computer, better control over tuning, and access realtime datalogging, allowing us to not have the emissions components be a problem. We just want to make absolutely sure the engine isn't a dead man walking before we start doing that.

Did you follow the instructions on the VECI label?

Yes, we followed the instructions about warming the engine and removing the SPOUT jumper before hooking up a timing light and messing around.

Thank you for your help and please let me know if you need anything else.

[Steve83]

MSD is known for setting vehicles on fire, so you should keep an extinguisher &/or good insurance (which is also the best/only way to protect the cats). The coolant sludge is called "rust".

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Blocking the filter inlet doesn't block ALL airflow - the PCV & vapor systems still allow air in. So that suggests that the MAP may be going bad. It's the only part that I recommend swapping instead of diagnosing. But don't toss the current MAP until you're sure it was bad. IDK where it is on a van, but it looks & is connected like this:

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This is the EGR tube with LLT warranty: https://www.amazon.com/dp/B002YCE22Y

Used/reman air pumps should outlast the van, but I usually just unstick them & keep running the original:

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