Fuel pressure issue

[Ranger305]

Maybe I have finally found the source of my '92 3.0 struggling under load.

Truck starts and idles fine as well as rev with no load. On the road, it feels sluggish on takeoff or climbing hills and often feels like there is a miss/extra cylinder kicking in. It does feel better sometimes, but has gotten much worse lately. It has been a work in progress since I swapped the long block 3 years ago.

I put a fuel pressure gauge on it and got the following results. I recently replaced the fuel injectors(originals had 260k, so it was time) and the below readings mirror what I found before, so I don't suspect a leaking injector.

It has new o2 sensors as I was getting codes and they were due for replacement anyway. The TPS is also new. Distributor was replaced a year or so ago with new aftermarket because the bearings went out in the original. Plugs and new motorcraft wires have 4k miles. Just swapped the cap since the old one had pitting on the internal contacts (was a cheap one with aluminum posts). Coil is new within the last couple years.

Fuel pressure readings:

Key off/engine off - 0
Key on / engine off - blips to 10psi or so then back to 0. If I trip the key/pump a few times, it will climb past 40, but after a couple seconds of Key off, back to 0
Engine running- 30psi stable
Engine running, vacuum line to FPR removed and plugged- 40psi
Engine running, blip the throttle- quickly goes to 35psi, then drops to 28-29psi
Engine running at 2000-2500rpm- 28-29psi.

Pump is an airtex aftermarket that is maybe 10 years old. FPR was replaced around the same time.

I suspect the check valve in the pump assembly is likely bad. Would that cause running pressure to be a bit low and cause issues under load?

 

[RonD]

Fuel pressure would have to drop below 20psi, close to 10psi, to cause a running issue

Fuel Pressure Regulator(FPR) can leak pressure with key off
The Return fuel line is on the FPR
Disconnect this fuel line and put a towel or container under FPR port
Cycle key on and off two times, that gives it about 20psi pressure
NO FUEL should come out of FPR port, until fuel pressure is above 40psi
So you can check if that's where the pressure loss is

You could also Squeeze the Return line, carefully, to see if pressure holds, without removing it


Clogged intake air way, or clogged exhaust, either will show up as lack of power especially as RPMs get higher

You can remove the 2 O2 sensors and run the engine without them to see if it has more power, if so exhaust is clogged
Or a vacuum gauge can tell you if exhaust is clogged

 

[Ranger305]

I'll check the return line and double check the vacuum as well as the intake pathway. How does the MAF come into the equation? It is original 1992 issue.

 

[RonD]

MAF sensor Weighs the incoming air so computer can estimate how much fuel to add to it
Gasoline uses 14.7 to 1 air to fuel ratio
14.7 POUNDS of air to 1 POUND of gasoline
or 14.7grams of air to 1 gram of gasoline

Computer already knows its running a 3 Liter engine, so it knows exactly how much air(volume) is coming in at any RPM, its just Math, i.e. at Wide Open Throttle 3 liters of air comes in every 2 rpms, 1.5l per RPM(4 stroke engine)

What it doesn't know is how much that air weighs
"hot air rises", why? because its lighter than the colder air around it, think Hot Air Balloons
Air at sea level weights more than air at 5,000ft elevation, think Denver

People add Cold Air Intakes(CAIs), because the colder the air the heavier it will be so more fuel can be added and better power
All fuel injected Rangers came from factory with CAI, just FYI

MAF sensor can be cleaned, easy enough to do, but would set a code if it was "off weight" enough to cause poor running

And just an FYI, the 3.0l Vulcan engine is a high RPM engine, makes best power above 3,500rpm, other engines do that at 2,500rpms
So you NEED to keep RPMs above 3,000 at least to get any power out of these engine, closer to 3,500

Specs are here: https://www.therangerstation.com/tech_library/3_0performance.shtml
Torque is best power, for 1992 3.0l its 3,600rpm for best/highest torque

If you look at the 2.9l: https://www.therangerstation.com/tech_library/2_9_Page.shtml
Best torque is 2,600rpm, so low RPM engine

It just the design of the engine, short stroke

So "drive it like a rented mule", lol, it likes it, well NEEDS it

 

[Ranger305]

Thanks Ron. Here's my recheck with a fuel pressure gauge and a vacuum gauge both blipping the throttle and holding at a higher RPM for a bit.

With the vacuum gauge, I get a pretty steady 20.5 and the needle seems to return quickly.

FPR/Vacuum Test

youtube.com

I pinched off the nylon return hose from the fpr and it made no change in the running pressure and the fuel pressure still bleed off quickly when the jet was turned off.

I notice the problem most when accelerating from a slow speed up a hill. Power is uneven and noticeably sluggish. Sometimes it seems better. At 70 on the interstate, it often feels like it is struggling to maintain, then at other times it feels like it has power to spare.

 

[RonD]

If you pinch off the return line, running pressure should go higher than 70psi, so you may have a leak inside on the gas tanks plumbing, or pump is very weak
Any 1989 and up pump is capable of 80+ PSI

 

[Ranger305]

This is the line I pinched off and it seemed to make no difference.

If I pull the vacuum line, I get 40psi and no fuel in the vacuum line.

At 30psi though, it sounds like it should still run ok, so maybe I'm overlooking something else.

 

[RonD]

I don't have a 1992 3.0l to look at so can't say, but both fuel lines, IN and OUT, are over on drivers side of engine bay
One goes directly to Fuel Rail, usual hook up on drivers side of V engine in middle, or towards the rear, that's the IN from pump line
Other runs to FPR on passenger side of V engine, towards the front, that's the OUT, return line

The fact it only goes to 40psi without vacuum line connected could mean weak pump or pressure leak inside gas tank
Expected is 43psi, but could just be gauge is off a bit

But if pump is weak or there is a leak then pressure at engine could be dropping to under 15psi with higher demand, i.e. higher speeds and going uphill

Squeezing off the Return line, running engine, should see pressure climb quite high, 60-70psi +
Engine should stumble from the Rich mix as computer tries to compensate

But on that note I would also have expected "Lean" code from computer if fuel flow/pressure to injectors was dropping that low when engine is under a load

 

[Ranger305]

On this one, the fuel lines come in on the driver side. One runs to the rail in front of the fpr.the other runs to a connector under the intake. The vinyl line from the fpr test into that connector. My assumption is the one in front of the fpr is the inlet.

While something is wonky about the fuel pressure and it draining, I'm wondering if it is just onother issue.

I was thinking about the situation and I decided to double check the basics, timing, firing order, etc. Timing is 10deg TDC with the spout removed and I cab see it advance when the spout is in. Wires are in the correct firing order.

Remembered a problem where the truck would randomly start missing a few years ago. Back then I tracked the issue to some wires in one of the large harness connectors had fallen against the heat shield, melted through, and were grounding out occasionally. One was bad enough I had to splice it. I went searching and found a damaged wite in the harness that I believe is part of the tps signal. I put tape around it for insulation. It didn't help the issue, but I need to go back in better daylight and check my previous repair as well.

While checking the timing, I decided to clamp on near each plug as a quick check to make sure all were getting fire. I noticed what seems like a possible fire issue. At times, the timing light flashes are irregular. I put the clamp on the coil wire and I can see the same from it, albeit at the expected higher rate. Not sure what that means if anything yet. I'll test the light on my son's 3.0 to see if it may be the light itself, but I doubt it.

I may also try to find the old coil and swap it in to check and see if there may be some issue with it. I could also try swapping in the pickup coil from the old distributor as well to eliminate that from the equation.

 

[Ranger305]

While I do still need to swap the fuel pump for what I think is a bad check valve, I believe I found the misfire problem. After checking the individual plug wires and the coil to distributor wire and seeing erratic fire in all, I used a 12v test light and probed the signal wire from the distributor using the info at the link below. The dizzy signal did no show erratic flashes that I could tell. I put the 29 year old original coil back on and it now pulls more like it should.

How To Test The Ignition Coil (Ford 2.9L, 3.0L, 3.8L)

How To test the Ford Ignition Coil (2.9L, 3.0L, 3.8L). No Scan Tool required. Testing a No Spark No Start Condition. Ignition Coil Test. No Start No Spark Test. Testing The Switching Signal from the ICM w/ a Test Light.

troubleshootmyvehicle.com

The coil only had 3000 miles or less on it. I put a new one in a couple years ago as I figured the original was likely getting weak with 28 years and 260k miles on it.....

 

[RonD]

I guess I have to say it................"they just don't make them like they used too"

 

[Ranger305]

You can say that again. I just found the receipt. I bought the new one July 2020.

 

[O No 3.0!]

I guess I have to say it................"they just don't make them like they used too"

Yep. Ignition components are crap anymore. You should try having a throttle body injected gm truck or van. We have 3 where i work & have to replace the distributor caps yearly on them. Those & the modules go bad constantly. We are going to try the Napa gold stuff next time, ive heard they have better quality.