I know there are some older threads on here about rough running on low rpm and idle. I wanted to start a new discussion about my particular truck, but if not appropriate, feel free to delete and I’ll resurrect and old thread.

Symptoms: lack of power, stuttering at low RPM especially under load. 800-1900 RPMs are the worst. Holding the brakes and gradually increasing throttle when in drive is one of the best ways to replicate. It actually sits at idle smooth and revs in park pretty smooth. Overdrive gear under throttle before it downshifts is rough as well. Once downshifting to over 2000 RPMs she’s real smooth.

I started with full spark plug replacement. Had a few coils with broken bodies that were loose and replaced. Tested all remaining coils primary and secondaries with an ohm meter. Tested at the harness and switching shows good 12v pulse at all coils. Boots look good and reinstalled with fresh dialectic grease.

Replaced air filter and fuel filter, cleaned MAF sensor, cleaned TB, cleaned PCV, and replaced IAC valve. Replaced a few suspect vacuum lines. Thoroughly tested for other vacuum leaks with carb cleaner method checking for revving at idle. I have also run premium gas (91 octane) for the last 5-6 tanks and ran at 3-4 bottles of Lucas and SeaFoam fuel injector cleaner. I have reflashed the ECU twice, trying stock tunes and the 91 Tow tunes from 5Star tuning. I have run hundreds of miles on each tune (currently road tripping).

I’m not sure what else to check here. Guessing still a vacuum leak, fuel injector issue, fuel pump issue, or a microscopic hole in a coil boot. Is it possible something like an AC clutch bearing or something could drag it down at low rpm?

Any other thoughts on what to diagnose next? Fuel pressure at the rail?

 

I've seen low rpm misses be cam/crank sensors. Just throwing an idea out.

 

Also what plugs did you use. When I had a gas it didn't like anything except Motorcraft. All others either missed from the start or after very few miles.

 

Interesting. Know along any good way to diagnose? I’ll starting googling about this.

 

Just did Autolite Platinum. While I don’t think this is the root cause, it’s probably worth trying. Truck ran great and had auto lite plugs before I replaced. It randomly started running poorly, and that’s when I replaced and started the other diagnoses.

 

Fuel pressure issues usually show up during periods of high power demand. We can probably rule that out.

Vacuum leak is a possibility. Bad IAC is another possibility. Yes, I know you put a new IAC on it, but don’t assume new means good. New just means it is clean.

MAF may need replacement, but that is an easy to check. If you unplug the MAF the computer is forced to use a default value. Yes, you’ll get a check engine light, but if the truck runs better with the MAF disconnected then your MAF needs more cleaning or replacement.

Otherwise, can you check and post your short and long term fuel trim values? That may be helpful.

 

Just ordered a Forscan from Amazon, so should be able to check fuel trim this week.

Assuming the sensor underneath the air intake plenum immediately after the air filter is the MAF, unplugging caused very erratic idle. There’s also another sensor just upstream. Idle was jumping up and down 300-400 rpms. I plugged it back in after 30 seconds or so. I can try driving around a bit to see if it adjusts.

 

I had similar issues, but it was only at idle. Once I hooked up Forscan, the MAF air volume reading was slightly off from spec. Once I got in there, it was a Duralast replacement. I bought another one from NAPA and threw it in to see if it helped. Unfortunately I got a defective one, the readings were worse and my stumble was way worse. I ended up getting a used Motorcraft (they are discontinued now) from a junkyard and its been gone ever since.

 

Got my Forscan today but having a hard time finding normal specs for MAF in g/s and MAF voltage. Also wondering what long term fuel trim specs should be?

It's a little iffy to reproduce. Sometimes it feels better than others, but I think it seems to be running better cold. After warming up and driving around a bit, then at idle here's what I'm seeing. It did seem to be running much smoother when I pulled these numbers however. I'll try again when I can reproduce.

RPM ~700
LFT1 = 5.86%
LFT2 = 3.52%
MAF = 7.25 g/s
MAF_V = 1.02 V
IAC = 31.08%

I don't think it's IAC valve issue at this point as I have tried thoroughly cleaning my old IAC and replacing with a new IAC valve.

 

Sounds like a tough one. What conditions are the boots in?

 

Boots seem good. I cleaned and inspected them. Bent and stretched them a bit to check for holes and they flexed well and didn’t seem to have any holes or white or brown spots.

Not reading any misfires on the forscan. I suppose I can try looking for escaping spark under load in the dark, that or just replace all boots and springs.

Also, thinking about attempting a smoke test for vacuum leaks.

Maybe if I can reproduce and record data in the forscan I’ll see something different on fuel trim or misfires.

Definitely discouraging to hear about things like particular brand plugs, MAF sensors, etc. Sucks to throw money at things to attempt to diagnose.

I still need to do an intake manifold / plenum vacuum leak test as I didn’t want to spray that much carb cleaner in that area since the engine was hot during my last test.