Hi all, I have a 2002 Mazda B3000 3.0 V6 auto. I’ve had it for about 6 months and have done a ton of work to it. It had the classic exhaust valve recession into the heads where it lost compression on 2 cylinders and a bunch of other issues. I’ve replaced the heads, fuel pump, fuel injectors, fuel filter, mass air flow sensor, upstream and downstream O2 sensors, evap canister purge valve, spark plugs, PCV valve, various vacuum lines, all intake manifold and sensor gaskets, spark plug wires, and coil pack.

After all of that work, I’m still getting very high short term and long term fuel trim values (~30% or more) at idle. At 2500 RPM, STFT and LTFT values fall off to around 6-10%, still not great. I’m also getting engine pinging or detonation during acceleration and cruising on the freeway. Fuel pressure was measured at ~50 psig and compression readings were excellent after heads were replaced. Pretty much every vacuum connection has been checked or had a hose clamp put on it and everything is tight. I have sprayed starting fluid and carb cleaner all around the engine and have not found a vacuum leak.

Any ideas on what could be the culprit here? I’m pretty much out of ideas. See screenshots of my scan tool at idle and 2500 RPM.

 

Welcome to the forum

First Check the VIN numbers 8th digit, V is Flex Fuel, U is gasoline only

1999-2003 B3000's could be either

If computer is Flex Fuel and fuel injectors are gas only then you could get what you have.
Engine itself didn't matter, computer and fuel injectors need to match


Pinging AFTER engine is warmed up is from pre-ignition, 87 octane gasoline is self igniting just before spark plug fires, the "noise" are those two explosions colliding inside the cylinder, very bad for the engine, and once it starts it heats up that cylinder which causes even more pinging/knocking

Pre-ignition can be caused by a Lean air/fuel mix
Your high fuel trims means the O2 sensors are showing Lean Voltage(too much oxygen in the exhaust) so computer is opening the fuel injectors longer to add more fuel

0 fuel trim(STFT) is the injectors open time that the computer has calculated based on RPM, MAF sensor, Air Temp, Coolant temp and throttle position

If O2 shows too much oxygen in the exhaust then computer adds more fuel, +1 fuel trim
Too little oxygen then -1 fuel trim, less fuel added

So yes a large air leak would cause computer to miscalculate 0 STFT and it would have to ramp up open time for injectors, the larger the air leak the higher the STFT.
Computer does have a limit, and if it reaches that limit and engine is pinging then there is a BIG AIR LEAK, or another problem.

Simple test for vacuum leak is to unplug IAC Valve after engine is warmed up
So after engine is warmed up let it idle
Unplug the 2 wire connector on the IAC Valve
IAC valve will close and idle should drop to 500RPMs or engine may even stall, either is GOOD, it means no vacuum leaks
If idle stays high then there is a leak

Fuel Pressure on a 2002 is speced at 65 PSI but you should be fine at 50psi as long as it stays at 50psi when engine is under load, higher fuel use
Computer bases fuel flow out of a fuel injector on pressure and injector size, both programmed into its software, there is no Fuel Pressure sensor, nor does computer have "eyes" to see what injector size is being used

So low fuel pressure will cause less fuel to flow out an injector in 100millisecond open time
If computer is Flex Fuel it "knows" there are larger injectors installed, so it "knows" it will open injectors for LESS time using gasoline because more fuel will flow in for larger injectors

 

Great response, I appreciate all of the info. To answer your question, it's a "V" - flex fuel. The new injectors were a spot on match to the old injectors as far as P/N and appearance go. Also, the truck was doing exactly the same thing before the fuel injector swap as it is now.

I did the IAC connector unplug test at idle after it was warmed up, no effect whatsoever. I traced the engine RPM when I did it and it did not budge or run rough at all.

I put the fuel pressure gauge on it again and got 59 psig, so I would definitely say there is no fuel issue (which I pretty much knew anyway as pretty much the entire fuel system has been replaced).

I also decided to try tightening the torx screws on the lower intake while it was running to see if that made any difference. I was watching STFT's and they dropped noticeably when I started tightening the screws. With that clue, it became pretty obvious to me that I had a vacuum leak between the lower intake and the heads. I made an effort to spray starting fluid along that joint and saw that STFT values shot into the negative immediately, although I didn't notice any difference in how it ran or engine RPM.

Armed with that knowledge, I disconnected from the upper intake (again) and yanked it off to get to the drivers side lower intake bolts. I torqued them all up as much as I felt comfortable, reassembled everything, and fired it up. STFT values were hovering around 0, going back and form from -0.8 to +0.8. I thought I had finally nailed it. I took it on a little test drive and about 10 minutes into the test drive, I pulled over and hooked up my scan tool. STFT was ~10 and LTFT was ~20. Yikes. My only thought now is that when I put the new heads on, maybe they are just slightly off from the intake, so I'm not getting a good seal there. I'd have to rip everything off, including the heads and redo everything to fix it if that is the issue.

As far as the pinging noise, it sounds a bit more like a metallic rattle from the engine internals. It happens really quick and goes away just as fast if you stab at the throttle quickly and then let off. It almost sounds like a loose rocker arm stud or a pushrod rattling. Not sure what it is. What I can say is that when I stab the throttle and let off quickly, I hear the metallic rattle and there is no impact on STFT at all (when I'm charting STFT). Maybe I should pull everything off.

 

One other little tidbit, I put a vacuum gauge on the engine before torquing the lower intake screws and had 17 in Hg, after torquing the lower intake screws I measured 17.5 in Hg. An improvement, but still a little low, consistent with a vacuum leak.

 

Good work

Engine warm with IAC Valve unplugged
Pull off 1 intake vacuum hose at a time and plug that port with your finger
Start with Power Brake hose and PCV hose first, then go to the smaller hoses

Now the PCV valve is a controlled/planned vacuum leak, but with engine at idle, high vacuum, the PCV Valve should be pulled closed so very little vacuum loss.
A PCV Valve has that rattly weighted valve inside, part you hear when you shake one.
That weighted valve is made in such a way that high vacuum will pull it up to close the valve, as vacuum drops(throttle opens) the weight will drop to let more vacuum suck "Blow-by" from the engine.

The power brake booster only uses vacuum when you press brake pedal, so should be air tight when you are not pressing brake pedal

When you plug a port with your finger and RPMs drop, you found a leak.
Could be cracked vacuum reservoir tank or failing EVAP or EGR solenoid or ???


You could pull lower intake and use some sealer(RTV) on the gasket then bolt it back down again.
No reason to get a new gasket, the bolt torque is not that high, 21ft/lb

 

Hey Ron, random question for you. Another vehicle of mine is giving me a headache also...2004 Lincoln Navigator with the 5.4L DOHC 32V Intec engine (no cam phasers like the F150). It has recently developed a rough idle and what feels like a misfire. I plugged the 'ole scan tool in and noticed erratic ignition timing - see attachment. What do you make of that?

 

15.5deg BTDC at idle doesn't seem to far off

I would warm up engine and unplug IAC Valve to see if you have a vacuum leak

Also check connection on throttle position sensor if you have one, and watch it position on the scanner with engine idling to see if it is changing
Around the mid-2000's Ford was changing over to "drive-by-wire"