97 E350 5.4 l

I'm having problems with this 97 Ford Econoline. It runs great in my opinion but the long-term fuel trims stay at 25% positive and it's hogging up a lot of fuel. I changed out the mass air flow sensor and that really helped Short-term fuel trims,they are alternating plus and minus below 10% as they should. No smoke coming out of the exhaust. When you put on the vacuum gauge it stays in the green area on the higher end closer to 23. The other thing to take note of is the O2 sensors operate perfectly until you floor it they run dead lean. Pretty sure I repaired all the vacuum leaks and did do a smoke test but only a small amount came out of the EGR valve. I sprayed some brake cleaner inside of the yard EGR valve and did see some change. There is a small leak coming out of the last bolt of the exhaust manifold but I couldn't imagine it would be that. I checked the fuel pressure regulator there was no fuel coming out of the vacuum line. I've tried disconnecting and plugged the vacuum line to the charcoal canister to see if there were any leaks coming from back there. No change long-term fuel trim stay at 25. I haven't done any fuel tests what the fuel filter fuel pump fuel injectors or anything fuel related yet. I'm just wondering if I'm wasting my time or not sure what direction to go in. Any insight would help greatly.

O2 sensors operate perfectly until you floor it they run dead lean

Sounds like you covered some bases pretty good, but this one test spells lack of fuel delivery.

If your trims were positive at idle then improved with higher rpm, that is indicative of a vacuum leak.

Leans O2's at Wide open throttle usually mean lack of fuel.

My favorite trick with this is to go for a run with the scan tool and a propane can with a nice long hose into the air intake, when cruising observe the trims while adding the propane fuel. If the trims start improving, you got a fuel issue.

I agree with you I would like to do a propane test, unfortunately these Econoline Vans are very hard to get near the intake in the front of the engine to test that area because the breather box is in the way. I'm going to try to test it again though with propane. I hooked up the fuel pressure tester this morning and it's reading at a steady 32 psi and climbs almost to 40 PSI at acceleration. Doesn't that eliminate the fuel delivery right, this thing's driving me crazy but like you said it would have to be a vacuum leak I would imagine somewhere I'm not finding.

You can have pressure, but no volume.

That pump should produce 0.5 - 0.7 Gallons per minute.

You stated at idle the front 02's were switching correct?

It might not hurt to shove a shop vac in the tailpipe to blow mode and spray soapy water around the front O2's.

And the rear O2's are lean as well when the front ones go lean?

So you suspect a leak may be coming from the 02 sensors? It only has upstream no downstream. How about the exhaust manifold leak would that do it? Another thing to take note of is the fuel trim are perfect at cold starts. It seems they start climbing instantly 2-3 mins after start up ltft goes straight to 25+

fuel trim are perfect at cold starts. It seems they start climbing instantly 2-3 mins after start up ltft goes straight to 25+

So after that 2-3 mins what do the trims do when you brake torque in reverse? Trims don't really count until youre in closed loop operation.

Do you have High trims at idle and through to WOT?

I don't suspect, I want to make sure your not getting a FALSE lean condition.

Here's is some food for thought.

Under load, what are the trims doing at low to high engine speed?

DO you have a CALC. LD pid or an ABS LD. pid on your scan tool? if so, how high do they go at Wide Open Throttle in first gear? and note the highest rpm achieved.

I'm going to try some of those things you mentioned. Yes the long-term fuel trims do improve the harder you accelerate, they don't improve much at regular to mediocre acceleration but when you really get on it they definitely go closer to zero. So I did the Shop-Vac test you recommended and the main areas it foamed up at was the backs of the manifolds on both sides. I soaked everything else down with soap and water and there was nothing else appearing to leak. But they were definitely getting pretty good just in the back that I can see of because you really can't see the front of these manifolds on the econolines you can't see crap in the front. I don't think I could get these manifolds off I and sure it would cost at least$1,000 to have it done if anyone will do it. hate to have to sell this vehicle over this I'm really shocked at they would make that much of an impact if this is the case. I'm going to have to look into some of the things you recommended and see what happens. If you think of anything else please let me know thanks.

I'm not super knowledgeable in this area but I have heard, not yet witnessed, that with exhaust manifold leak you can get a venturi effect drawing fresh air in at the manifold and causing a lean reading at the o2 sensor. Given he sees a difference after it warms up it seems plausible to me. What are your thoughts on this?

@fixn_junk

That fresh air will cause a lean condition, but usually at all rpm ranges. The OP stated his trims improve off of idle, so this is very indicative of a vacuum leak.

The fuel trims long term stay at 25 while it's at idle, once you accelerate hard they go close to zero. Yes they constantly change all the time while driving especially the harder you accelerate. The short-term fuel trims look fine all the time they rarely go above 10% positive or negative. It's just a long-term fuel trims while at idle stay at 25%.

I did the Shop-Vac test and the only place I could find bubbles from the soap was at the rear manifolds. However I did a couple vacuum tests today by pulling the brake booster line and capping it off of my thumb and that definitely made a difference with the long-term field trims they went down to about 10/11 which isn't that great but it's much better than 25. I also tried another test with the other port on the intake that goes to the fuel pressure regulator that little canister underneath on the passenger side and also the AC vacuum line. Once I plugged that line off the directly on the intake port LT fue trims also went all the way down to about 9 or 10 This is really confusing between this and the manifolds leak I'm not sure which is the main culprit I have to say I'm going to have to do more tomorrow but I would love to hear some input tonight of anyone sees this.