Hey gang, I have a problem that has been stumping me awhile. 98 4.0 ohv ranger 5 speed with 145xxx miles. Ever since I got it it has had a bad ping between 2-3500 rpm under moderate throttle. Goes away toward WOT. Also has a shaky idle, but not real bad. Just shakes lightly. Never stalls. Finally it threw p0171 and p0174 codes the other day, so I hooked up a scangauge and found both my LTFT banks at +25% idle. Under moderate throttle, it drops to sub-10% numbers, and 0 at WOT. Both banks stay within 2% of each other. I replaced all intake manifolds this fall, so that shouldnt be the issue. I did the propane and carb cleaner tricks and didnt find anything. I plugged all vacuum outlets from the intake to isolate the vacuum systems. Still idled rough and +25%. Truck consistently pulls 16mpg winter, 19mpg summer, mostly city driving. I have cleaned MAF and IAT, reset computer, and replaced some vac lines. The IAT and ECT temps are both correct on my scangauge. IACV was new this fall. MAF reads .4 lbs at idle, seems right. Plugs and wires were new this fall. About the only thing I haven't checked is EGR, it's been too cold to pull it.

If I saran wrap the intake closed, and plug the little IAC breather, I can still suck air in through the brake booster hose. Where else could it be coming from? Like I said, I disconnected all 3 vac ports leaving the intake.

 

Just guessing here, but did you check the PCV valve and hose? That is another potential vacuum leak.

Can you pressurize the intake with the brake vacuum hose, and try to listen for a leak?

 

Hopefully it's not the intake manifold gasket.

If it's a partly clogged injector, you're ready for the "Italian tune-up"
Add a double shot of injector cleaner to the gas.
Find an open road and red-line it a few times.

 

I cant seem to figure it out. My fuel pressure is 62psi at idle and throttle blips. Tps numbers smooth from idle to wot. Maf shows 4.4 at idle. Iat and ect temps correct. I cannt find anymore vacuum leaks. Replaced intake gaskets last summer.

 

One clue about injectors is the fuel trim.
If it is unequal between the left & right banks, then either an injector is under-preforming or an O2 sensor is weak.
This ECU runs closed loop almost all the time from idle to acceleration. Only at WOT does it ignore the O2 sensors.

 

correct me if I'm wrong, but wouldn't underperforming injectors be worse at higher rpm? My ltft is maxed out at idle, and comes down with rpm. At an easy cruise it's around 9-11%, and at heavier throttle it drops to near zero. I may be wrong but I though fuel starvation would get worse under more throttle/load, that's why i assumed it was an air issue.

 

I just re-read your original symptoms. Idle fuel trim of +25% is high. EG: My cold start STFT and LTFT show -2% & -5% (at 50F IAT).
I still am betting it's a partly clogged injector.