Well, my van is having some issues, and the shop figured a plugged exhaust could explain the symptoms, so they hooked up a backpressure gauge to sort it out. Guess what, the backpressure is so low they could not really get a reading. My backpressure is under 1 psi on the driver side. So it seems that even though the stock y-pipe looks kinda restrictive the way it is made, it is plenty free flowing for the engine. I do have the Bear River performance catback, and the stock muffler was a choke point.

They have not yet determined what is causing my problems. The basic symptoms are loss of power under heavy throttle. No backpressure, fuel pressure is normal, MAF is new OEM and is confirmed to be working fine, no vacuum leaks, no coolant leaks, all ignition components are tested to be working fine. I will let you know what they find out.

 

burned or leaking valves

 

I had a similar symptom years ago. During the initial tests the pressure was within specs and when I clamped off the return the pressure climbed up, pump and regulator good. Pressure held while sustaining a free rev of 2K proving the flow rate and filter good. Eventually I hooked up a fuel pressure gauge with a long hose to poke out the hood where I could read it while driving. Replicated the symptom and the gauge pressure was not keeping up. That proved the pressure regulator good but the flow bad. Replaced the filter no improvement on next test. Replaced the pump and the next road test passed. Just food for thought.

 

That would be food for thought, except the fuel pump is a brand new dealer pump. They tested the fuel pressure while driving the vehicle and replicating the problem. While the van is acting up, the fuel pressure is just fine. As I stated, this is being difficult to diagnose.

Burned or leaking valves doesn't explain why it produces tons of power under light throttle. It has lots of torque, it only acts up when you give it more than about 50% throttle

 

FWIW...I was told by my mechanic that you can have too free flowing an exhaust when it came to my 4.0l. Not enough backpressure at WOT (Wide Open) is as bad as too much with a restrictive exhaust system.

He gave me a cat back system that was under 2" because he said he's seen performance numbers decrease with a 2" exhaust. My exhaust - just a little bit bigger than stock, but with Mandrel bent tubes is much better than stock. Silent until WOT.

This year, going up the mountain passes with trailer in tow, I was still able to maintain a good speed and pass other trailers when needed. Couldn't say that the previous year, when I had the stock exhaust.

See the following if you don't know what Mandrel bent tubes/exhaust pipes are: http://www.muffler-tech.com/mandrel_bending.htm

 

I have had my exhaust for two years. Nothing has changed, so why would a free flowing exhaust cause a sudden loss of power especially at WOT. Some oversized exhaust systems cause a loss of low end torque, but that becomes irrelevant at WOT and high RPM.

He gave you a catback under 2". Thats what you get stock. Mine is 2 1/4" catback and I outrun everything in my class. I use my van for hauling up to a ton of scrap, and do so up the rocky mountain grades. I can hold OD going up a 5% grade at 6,000 ft. ( for the record that means I have to have less than 50% throttle to stay in overdrive and keep the torque converter clutch in lockup. If I encounter steeper grades than that, I switch out of OD.) The 4.0L is best with a 2 1/4" catback. Otherwise it just chokes. I know the difference, I actually raced my brothers stock '97 Aerostar, while his was stock, he couldn't outrun me, and I was hauling more weight, and he had the benefit of more gears. Ford does not choke it down to 2" because it produces the most power, it because it lets them use the same muffler they use on the 3.0L, which is the same muffler used on the Ranger. They opted for the one size fits all muffler, which by the was is designed for the 2.3L. It is only 1 3/4" inside.

Until recently, my van has had no real issues other than a rough idle. I had an injector replaced, it had cracked and was leaking externally. It still ran rough, so a bad MAF was found to be the culprit. It was an aftermarket MAF, so I replaced with a dealer one.

Ever since then it has run better at idle, has lots of torque, until you give it throttle, then it bogs down and actually produces less power at WOT than it does at 1/4 throttle. It did have an issue before where it would bog down if you tried to rev it when it was hot outside, but it was intermittent, and would only do it briefly. Now it does it all the time.

By process of elimination, it is looking like the computer itself might be the culprit. So far it has been determined the new MAF is fine, the TPS is fine, the ignition system is fine, the fuel delivery is fine, the engine does not have any coolant leaks, the compression is good.

Some things the shop has brought to my attention seem to support the faulty computer theory. One, the computer will not store codes. Not that it can't generate them, they have induced codes by unhooking sensors. It clears the codes from memory when you turn the ignition off. Two, it won't perform so of the self diagnosis tests, it just resets partway through. They tried to do a cylinder balance test, and it tests 3 or 4 cylinders before it resets and checks for codes.

The problem is worse when the engine is fully warmed up.

 

Khan I forget what year, motor you have!
Power...HHMMM thinking out loud...
You already know...Air in air out..Dog food in air cleaner...Make sure TB opening all the way. No rags stuck any where..Fuel gonna be controlled by maf only if it see's flow. I have had similar and always has been fuel or exhaust. Poping during WOT would be lean condition. Fuel or exhaust usually will exhibit an increase in power when you back off the throttle from WOT. any popping will be lack of fuel. Clogged exhaust won't do that. Also clogged intake valves will do that. Run seafoam through the engine vac line..

Thats all I can think of for now..I have a puter for a 94 4.0l

Dick

 

A little more food for thought.....Recently do any compression blead down tests?

 

Ok, just got a call from the shop last night. The did more drive tests with the fuel pressure gauge connected. Under heavy throttle, while it is acting up, the fuel pressure is registering at 35 psi, which is the extreme low end fuel pressure that is deemed acceptable. The shop feels that the pump may be culprit, even though it is a brand newer Motorcraft pump. They seem to shy away from the regulator being the culprit. They also finally captured a lean code. Even so, they only caught the lean code because the scantool was connected at the time the code was set. It did not store the code to memory.

 

Their conclussions from this most recent testing would be correct. This result points directly at a flow issue. Either the pump is not keeping up or there is a restriction in the flow such as a dirty filter or kinked supply line. The regulator restricts the flow on the return line so to maintain a constant pressure at the fuel injectors.
It would look like you are suffering the same issue I had several years ago. What a nightmare to diagnose, but now that we know how.

 

Don't want to be captain obvious here, but how about a dirty fuel filter? I read everything you said, and you did not mention that you replaced the filter. It could cause marginal pressure at high power settings. Another thing is how do you know conclusively that you do not have a vacuum leak anywhere under any and all circumstances? Just a thought, but I've seen mysterious vacuum problems occur from the vacuum line leading to the power brake unit more than once.

 

Ok, the shop deadheaded the pump to measure its pressure, and they checked its flow. The pressure is fine, but the flow is very low. Looks like the pump is in fact bad. At least it is a dealer part and has a 12 month warranty. What a run around. Hopefully this fixes the problem. The filter was replaced.

 

Is it possible that the sock filter (I assume Aeros have them) is just bad?

BTW, the new catalytic converter I got from you really made a difference on the loud Flowmaster the PO put on. The reverberation is down and the rumble has shifted more to the rear of my Suburban. Now I just have to adjust a hanger that is letting the pipe bang on the fuel tank cover. It's too late to ask who installed this system.

 

From reading the end of the thread, it appears your fuel pump is the culprit. Glad you got it diagnosed. I have a similar acting 3.0 L Aero, '90. It started a few weeks ago when pulling my boat. (related thread https://www.ford-trucks.com/forums/9...d-oil-pan.html) Having no sign of a problem for a few miles of driving, all of a sudden I lost power when applying the accelerator. THe van just bogged down and I had to pull over. After running some Seafoam through it, along w/ carb cleaner in the intake, the van responded normally until today. I'm on the bypass, trying to gain speed to enter the flow of traffic and the thing bogs down again. Fortunately there wasn't much traffic and I got down the road where I could exit safely.

What I am curious about is this. I have never had a shop diagnose how my van is working. I had them replace my heater core once but I've changed brakes, brake valves, alternator, thermostat, plugs, wires, etc. myself.
What will a shop do to troubleshoot my problem? Is is diagnosed by computer or brain power? What is the normal cost of a diagnosis?
I just put my van back on the road after a year sitting, and am perplexed by my erratically running van. I don't want to spend an arm and a leg to keep it on the road, if I can help it. My fuel filter is fairly new, so having a shop diagnose my problem is a consideration to speed up the solution. I just wanted to know what to ask for and what to expect.
Thanks
Tim

 

The shop can plug their code reader into you van and pull error codes from stored memory or from live test. Some diagnostic manuals will hint at potential problems based on the codes. But they (or you) will have to do detective work to really interpret what the error codes mean. You can start with what the diagnostic manuals suggest. This is where more experience helps.

I once fixed someone's car after they took it to a couple of mechanics who had no luck figuring out the problem. The primary clue to me was that the engine starts running badly after the it warmed up. I found the connector to his MAF sensor had not been fully plugged in. When the engine is cold, it runs in open-loop with some preset programming. When it warms up, it tries to run in closed loop, using more sensors to determine surrounding conditions. So if the problem starts up only after warm up, look for sensors that get used after warm up. Again, experience, or a good manual, will tell you which those are.