HELP! 1999 Ford Ranger 3.0L U vin, Flex fuel, misfire no codes

1999 Ford Ranger 3.0L U vin, Flex Fuel, automatic

I agreed to help out the son of a friend with a Ford ranger with multiple symptoms, but mainly claiming lack of power and misfire at idle.

Codes for Cam synchronizer and slow o2 sensor. Per the owner, previous mechanic checked cylinder pressure and was in spec. Owner had changed the IAC valve, and plugs and wires. There was a lot that went into the diagnosis, but the coolant temp sensor was reading out of spec, and was replaced along with the cam position sensor that was tested and non-functional which took care of the cam synchronizer code. MAF tests in spec. TPS Tested verified good. Throttle body cleaned. o2 sensor on bank1s1 was verified as slow and the b2s2 appeared to be reading low. Spoke to the owner who asked that I replace all 3 sensors, which I did. I noticed the engine load was slightly out of spec at 38% at idle, while motologic states 17-21 as normal. Also, LTFT1 and 2 both are in and out of the +12 to +15% range. Which I have been suspect of all along. But just saw the spec in motologic as +-20 as normal for this vehicle.
I checked fuel pressure which was 50psi KOEO and at idle, that bleeds down after shutdown to 40ish after about 4-5 minutes. Flood test, no ignition. Owner had a new filter so that was replaced. HOWEVER, motologic has a spec of 56-72. After scouring the web for more details, there was some disagreement from site to site. So I went with the motologic info and decided to very hesitantly change the fuel pump and pressure regulator that is in the tank. The new pump has the exact same psi and bleed down. (as I feared)
I ran an injector cleaner through 2 cycles via the fuel rail, then looked again at the ignition waveforms.

Exhaust back pressure test passed.

I have looked at the ignition waveforms throughout this process. And found cylinders 5 and 6 had what appear to me to be some form of intermittent misfire. But that's why I'm here..... I really don't know enough to track this down. I'm certain this will be very clear to someone who knows what they are doing. The waveform to me appears to be a slow burn, potentially from sticking injectors?????? I'm just throwing out a guess. I probably should have done my own cylinder compression tests in hindsight. I normally would.

I have attached a good waveform, which is what I see on all cylinders. And the two intermittent waveforms that I get on snap throttle. They happen fairly consistently. But not every snap.

I'm sure I've left out steps that I have gone through, and even gotten some things out of order as I'm pretty tired. Both physically, and of this vehicle. I would really really appreciate any help from this community. I have been following along on youtube and silently in the forum for a long long time and never wanted to bother you kind folks. But I'm at the end of my rope. I hope you all will be patient and kind for anything I've omitted and or any forum decorum I might have flubbed up. Also, please forgive the poor photo quality, and ignore the old bald guy you might see in the reflection.

Update: test drove the truck this evening and it runs much better after the injector cleaning. However the intermittent misfire/hiccups at idle remain.

Thank you all!

Motologic spec is out to lunch, +20 is definitely NOT normal. This thing is adding 20 percent more fuel for a lean condition.

Is this trim only at idle? Does it improve with RPM? or is it positive the whole rpm range of the engine?

Do a WOT run in 1st or 2nd gear, do the O2's peg Rich or Lean? and what is the MAF and Load pids at the highest rpm acheived under load?

What is the Alcohol Pid reading?

Thanks for the response!!!!

Fuel trims while driving or under heavy throttle go straight to zero or around it. So this is strictly at idle. I really didn't trust the motologic data. Sio glad to hear I'm not totally carzy. Load percentage under heavy throttle hits 70-100%. MAF is .1 to .5 or so I believe depending on the throttle.

I will try and do another run in the truck soon. Right now I'm trying to buckle down for this storm, since I'm in Tampa.

Smoke the intake, sounds like your have a vacuum leak. High trims at idle that improve with increasing rpm, telltale vacuum leak.

oh, yeah, STAY SAFE!!!!

If necessary, isolate the PCV system and plug the Purge Port, any trim improvement?

Thanks! I checked for vacuum, however, I didn't smoke it yet. Vacuum seemed to respond as I would want, although it was pretty shaky. Which made me question the condition of the valve guides? I did thoroughly go over the vacuum lines with starter fluid to see if I could find anything. But that was so long ago (diagnostic time wise), and several issues have been fixed since then. I definitely will smoke it and see, when this storm clears. I sincerely appreciate your help!

Finally got around to smoke testing today. And unfortunately, no leaks.

Here are the current readings at idle and at 2k rpm(images attached).

With the pcv system isolated and pretty much identical with the purge valve in place.

Snap throttle momentarily brings the LTFT down to the 6-7% range FWIW.

Took the truck for a drive just now. Fuel trims go to zero at full throttle. Back up at let off. ????? Truck actually is pretty smooth running now. But those fuel trims! Once parked and idling after a long drive the LTFT got up to 20, hovering around 18…… ugh. Am I missing a vacuum leak? Or could it be something else?

Update:

Decided to look closer at vacuum.

Disconnected the brake booster and hooked up a gauge there. At idle LTFT1 fluctuates between 9.4-10.2 and LTFT2 between 10.9-11.7. STFT1-2 both between -1.6 and 1.6. Vacuum at 18psi. Load 34.5.

At 2k rpm LTFT1 is 15.6. LTFT2 is 17.2. STFT1-2 between -3.1 and 3.1. Load 32.2

At 3k rpm the load decreases do 27.5.

Updated update!!!!!

Rapidly pumping brakes gets STFTs to rise the longer you pump, after about 12 seconds both STFTs reach 30. They return to “normal” once you stop. I’m assuming this may be normal?

Now I’ve disconnected the brake booster again and capped that vacuum line off at the intake. And now the load is 36.5 STFTs hovering around 0 LTFT1 at 12.5 and LTFT2 at 17.2. This is up from this same scenario earlier. And snap throttle no longer brings LTFT to zero momentarily. Except when in gear, or driving. I have no clue!!!! Ahhhhh!!!!!

Does all of this confirm a vacuum leak? Or am I just becoming more delirious? I’m really grasping here. Hoping you all can help.

Relative compression test image with ch1 being cylinder 1 ch2 being amp meter.
Cylinder 5 maybe ?????
Input pretty please?

One last addition. I’m hoping more info is more helpful.


When I turn the AC on, the LTFT immediately drops and stays below 10. It will fluctuate between 7-10.

Hope this helps find the issue. I’m totally at a loss here.

Smell the oil on the dipstick?

When you smoked the intake, was the PCV isolated from the crankcase? Did any smoke come out of the oil filler?

Put your vacuum gauge on the dipstick tube when running, there should be no vacuum (PCV must be isolated)

I don't really like the wavy RC test though, being a flex fuel maybe that sensor is askew.

I actually didn’t have the pcv isolated during the smoke test and no smoke from the tube. I will isolate the pcv now and check for vacuum.

The dipstick however, may have an exhaust smell to it.

Let me verify I’m doing this right. Vacuum gauge on the oil dipstick with pcv port capped? I don’t have pressure or vacuum at the oil dipstick with it capped, or with the pcv in place for that matter.

I’m actually starting to think this may be somewhat intermittent. Sitting here with it idling with the ac on and the LTFT’s were hovering around 10. Then started to increase up around 14-16. I shut off the ac and they go to 16ish. AC back on, hovering around 12. Totally unrelated???? Am I now hallucinating? Lol

I really do appreciate the help.

When you turn the AC on the RPM's are increasing.

Get a shop vac and blow it up the tailpipe and spray soapy water all around the exhaust manifolds, especially IN FRONT of the Pre-catalytic O2's, just curious if you a false lean condition, are the front O2's switching rich/lean, and what are the rear ones doing?

What happens to the trims when you introduce Propane into the intake airstream? do they start dropping? Does the Pre-cat O2 go full rich?

Try unplugging and capping off the Purge Valve too.

Rpm’s are actually remaining dead steady when the ac compressor kicks on.

As for O2 sensors. The two upstream are cycling between rich and lean consistently and the downstream one (once warmed up) appeared to be pretty constant between .070 and .1. However I’m looking at it now and it will intermittently (seemingly randomly) peak to .6-.7. From the low range.

Put a vacuum gauge on the dipstick tube, capped the pcv and the purge valve. No vacuum or pressure from the dipstick tube. Fuel trims still where they have been.

Will three consecutive throttle snaps make that rear O2 go rich?

Yes. Goes rich then oscillates back down to the low range.

Unplug both rear 02's what do your trims do?

Only one downstream O2 on this one. I’ll check in a few!