4.0 stumble issue and what fixed it for me

[obrut]

I bought a 1994 4.0 5 speed 2wd ranger a few weeks ago. I've been working on it and fixed the brakes and rear leaf spring shackles and mounts. Anyway it didn't have much power and really stumbled above 2500 rpms, no check engine light and no codes. I searched and read a bunch of post, went through and check all of my sensors, cleaned everything, made sure MAF, TPS, IAC etc were working and measured correct on a meter, ohm plugged wires, checked plugs. I checked everything twice but could not find any issues and I was not going to start throwing parts at it. So I started searching and reading more.

Long story short, it was the MAF sensor. I sprayed it with MAF cleaner and verifed the MAF was working with a meter and moved on to other parts of the engine. Then after not finding anything wrong with anything else I tried unplugging the MAF sensor and driving it and it was instanly 100% better. I started searching a little more on the MAF and more ways to test it and found a post from a guy that said something along the lines of excessive buildup causing a hard coating over the sensor. I pulled my MAF sensor back out and under a bright light sure enough it had something on it. I sprayed it with the MAF cleaner and it would not come off. I ended up spraying a q-tip and using that to clean it carefully. What ever it was covered the entire side of the sensor facing towards the air box but it was a very thin coating, hard to see and took some careful effort to get it off. I put it back in and it runs/drives great, revs all the way to redline.

I've cleaned several MAF sensors over the years and I have never seen something like this. I read so many posts about people having the same issue, replacing every part and not having any success fixing the issue. I hope maybe this helps someone when they search.

 

[ekrampitzjr]

Obrut, have you ever run an aftermarket air filter? Some of those with oiled filter elements are notorious for coating the MAF–sensor wire with the oil that leaks through the fabric.

 

[obrut]

I have but not in this truck, I use to run those filters and knew to clean the MAF regularly.

 

[RonD]

Thanks for the FIX

MAF sensor reports the WEIGHT of the incoming air, that's why its called a MASS air flow sensor
Engine computer already knows the amount of incoming air because it is programmed, in this case, for a 4 LITER engine, its just Math to calculate how much air is coming in at any RPM(crank sensor) and throttle position(TPS) at anytime

Gasoline air fuel ratio is 14.7:1 and that's a WEIGHT Ratio
14.7 pound of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline
Its why fuel injectors are rated in pounds/hour(lb/hr) of gasoline flow

But the MAF sensor can only sample 10-15% of the air passing by it
So any error is compounded up to 10 times in computer's calculation of air weight