Oil cap causing lean conditions?

Hello. My 99 Mountaineer fuel trims was B1 26% and B2 30% and no CEL I did a smoke test and found the bottom of the EGR valve was letting out smoke so I replced it. Started the truck had it idle for awhile but no change on the fuel trim. I reset the KAM and at startup it immediately climbed to 26 and 30. Smoke test again and this time I see smoke coming out from the engine oil cap. Turn the cap over and there is a gasket. Can a worn gasket cause lean conditions? Thanks.

I would suspect a bad intake manifold gasket.

It's definitely possible, BUT I think it'd take more than just a leaking oil cap gasket to cause the kind of trims you're seeing. Really, I don't think most cars of that generation would have 30% trims with the cap completely off.

Oil caps are cheap, but you could prove/disprove this by covering over the valve cover while watching your trims. If you don't see a big improvement, then continue your search.

Do your trims remain high at idle and off idle? It sounds like you've covered vacuum leaks pretty well, wondering about a possible fuel issue.

I know you said you have no codes, but this video shows how to use fuel trim to ID the type of lean condition you've got going on.
I'd be interested to see what the trim numbers and O2's look like at different loads, and maybe even get a fuel pressure reading.

I guess if the PCV system is pulling vacuum on the valve cover and the oil cap isn't sealing, it's conceivable that it could potentially cause a lean condition.

5.0l Mountaineer?

It's high during idle and trims improve on a test drive on higher rpms indicating a vacuum leak. SFT are hovering around 0 and O2 are switching as expdcted. During idle when I turn on the heat and the clutch engages the fuel trims improve halfway. Clutch disengage trims spike. I also hear the IAC valve click as well. Perform a bidirectional on the IAC valve and trims improve. But I guess that is to be expected with more air coming in. Bought a new oil cap. It sealed better with no play but no improvement. PCV located back of the lower intake manifold but I don't see smoke coming from there. For a while now my trims has been in the 15 to 20 range. I've found a broken plastic vacuum tubing between the intake and EGR regulator solenoid. A torn hose between purge valve and intake and when I think these two findings solve my problems there appears to be more lol This leak has got me stumped. It's 5.0 engine

Old vacuum lines can be a pain like that.
Maybe disconnecting the battery and shorting the terminals together for few seconds would help make the computer forget about that learned fuel trim more effectively. Fords will keep the learned fuel cells for a while even after the problem is fixed. Maybe for some reason the scan tool KAM reset wasn't enough.
Did you happen to check under the dash for a tricky leak, or around the brake booster area?
Does your smoke machine have a flow gauge that would indicate if the system being tested is sealed?

Sure sounds like a vacuum leak. What was engine temp when you did your smoke test? Do it with it cold.

My smoke tester doesn't have that feature. Its one of those smoke mods I got from ebay consisting of a paint can, 1 psi regulator hot wire and adding mineral oil. It helped me find those two leaks I mentioned earlier. The brake booster seemed fine. Depress the brake pedal and pinched the hose no change at all. I couldn't tell you what the engine temp was. The car was off for several hours and I was changing the EGR valve during the time. Turn on the car for five minutes. When the trims dicnt improve I shut it off then smoke test it for about ten minutes. I'll keep digging and see what I can find

Good move with the brake booster hose! That's one more thing off the list :lol:

Any lack of power noted on the test drive? I'm still wondering about a fuel problem, despite the improvement in trims that you noted. Not trying to poke holes in your analysis, just thinking about other common lean issues these trucks had.

How much of an improvement between idle and 2500 RPM do you see? Maybe post the numbers?

I didn't record the results on the test drive. Under load there was drastic improvement s on LTFT. It aS under seven on both banks.

tribal599 wrote: I didn't record the results on the test drive. Under load there was drastic improvement s on LTFT. It aS under seven on both banks.

Yeah, that sure sounds like a significant improvement! Unless the short terms were super high at the same time, and I figure you'd have noticed that.

A quick and dirty test for fuel pressure would be the WOT throttle run, watching for the upstream O2 sensors to go rich during wide open throttle in first and second gears. If you see that, then I say we're really not worried about fuel pressure.

An outside chance would be a stuck open PCV valve, sucking hard on crankcase vapors, and pulling in unmetered air through seals and gaskets. You could test for this by pinching off the hose from the valve to the intake.

Sorry for late reply. Been busy. Here is the snapshot of test drive concerning WOT and O2. i took voltage and current injector waveforms done at the ECM since they were below the intake. All eight injectors averaged between 8 - 8.9 amps and the waveforms look exactly like this snapshot posted.
There was a small vacuum leak that I found when spraying water. I can hear it get sucked in. It was located at the hose at the back of the intake along bank 2 side. Put a clamp on it but LTFT B1 26 and B2 30 only went down 19 & 25 and on some days 15 & 19.
Did a cold intake smoke test in the morning and no smoke was showing. Next is to check the PCV system. Even though I don't have a misfire I'm going to check for internal leaks. I'll update as soon as I have the results.

I blocked the breather hose, plugged the PCV grommet and the PCV hose. I feared crimping the hard hose will damage it I I blocked it off with my finger. Then put in a vacuum gauge in the dipstick tube and I was slowly getting pressure and no vacuum.

No intake leak, then And your WOT run confirms that the fuel system can provide enough at high load to keep the engine rich.

The trims never improved by having all the PCV stuff blocked off?

It never improved.