Hi

I’ve been surfing YouTube all day and feel dumber than I did in the morning.

im trying to diagnose low power on a 99 ranger XLT 4.0 4x4 with 170k. She idles beautifully, has a new trans and brakes snd fuel filter.

I checked the TPS, cleaned the MAF with MAF cleaner and can’t find any vacuum leaks.

uphill acceleration is dismal. Maintaining, let alone achieving 70 is a workout but the engine isn’t over revving. So it seems I’m on to exhaust obstruction. Here’s the MAF. inside the tube it’s bright clean. No CEL.



here is a video of my vacuum readings.

instructions say if vac drops SLOWLY on acceleration, it could be plugged exhaust. But it’s quite responsive. Any help is welcome and appreciated.

 

At idle the vacuum should be 18"-21", in the picture it looks to be 21", which is good

Run engine up to 2,500rpm approx. just high RPMs and hold it steady
Vacuum gauge would drop at first then come back up to say 18" and hold steady, if its slowly dropping or under 14" then exhaust is most likely partially plugged up

Blip test is when you open throttle WIDE OPEN, then let it snap closed
Vacuum should drop to 0"-2" then pop back up INSTANTLY above 25" then settle back at 21"
If it slowly comes back up to 21" then clogged exhaust

You compression should be good if vacuum is over 18"

1999 Ranger needs 55-65psi fuel pressure, but you should get Lean Codes if pressure gets too low
Or Rich codes if its too high

Spark plug gaps should be 0.054"
You can pull a spark plug or three and have a look at the tips, light brown is expected, darker means Rich or oil leaking in

Low compression or clogged exhaust are about the only things that can cause low power without setting codes
I assume CEL(check engine light) comes on with key on?
So you would know if any codes were set

 

Ok. THANKS!!! I’ll try the blip test next and then remove the plugs and examine.

Exhaust smells great, no sulphur or oil smell, just truck. oil is leaky but I don’t see any evidence of burn. No smoke at all. Keeping fingers crossed

id try to relieve o2 pressure but they’re really amazing terrible spots. We’ll have to check the forums on that too