Runs worse after cleaning MAFS.

You gotta Love these old VG30E/i powered beasts, raw truck DNA to the core.

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Runs worse after cleaning MAFS.

Postby PackPath » Sat Oct 19, 2019 9:57 pm

Warning: This post is kind of long.
My question is at the bottom.

Hi All,

I just came across this group via internet search because I need a bit of help.

My 95 VG30E was running great, but got the worst gas mileage I've ever seen at <9 mpg and seemed to have a lot of exhaust smoke. I thought it was running rich. Not wanting to fail emissions testing, I decided to start with basic maintenance to see if the mpg improved any before moving on to more complicated measures.

I started by replacing the air filter. Simple enough.
At the time, I didn't know that I had to disconnect the hose post MAFS in order to change the air filter, and so I think I knocked some dust loose because the engine intermittently started to act like it was dying after the air filter change.

After doing a lot of research, I learned about MAF sensors and how they could affect gas mileage. Okay, I'll test and clean the MAFS.

On my initial look, I noticed the MAFS connector has already had a ground wire attached to the engine block.

I tested it by starting the truck up. Started and ran great. No signs of dying.
Leaving it running, I disconnected the MAFS connector and the truck almost immediately stopped running. So I learned that this means the MAFS is good. Please correct me if I'm wrong.

Then I loosened the post MAFS hose clamp, lifted the air box lid off the base, removed the air filter, wiggled the whole thing off the air hose, and carefully took it apart.

To my surprise, the MAFS was fairly clean, as was the screen body and the supporting hoses and mounting surfaces. I carefully cleaned it with electrical contact cleaner anyway and let it dry for about 15 minutes before putting it all back together.

* I tried to start it.
* It hard started like it has never done before and then went into a slow, loping idle. It felt like I was on a horse.
* A very strong smell of gas was apparent about 10 seconds after starting, so I thought it was getting flooded.
* I let it idle for a bit and gave it a small blip. It did not like that.
* When I press the accelerator slowly, it falters and lopes so hard, the whole truck shakes.
* Tons of white smoke coming out of the tailpipe, enough to fog my entire front yard.
* The smell of gas in the air was like I was swimming in it.
* I shut it down.

Finally, here is my question:

If the MAFS has to learn the engine again, how do I help it to do so?
Do I let it idle, or should I take it out around the neighborhood (it won't accelerate)?

I have another possibly related question regarding a cracked section of small line going into the throttle, but since the truck ran fine with that cracked line, I'll keep this post to the MAFS issue.

If you've stuck with me this far, I thank you.
Any tips are appreciated.

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Postby smj999smj » Sun Oct 20, 2019 1:48 am

There were issues with a bad ground circuit to the MAS and in those cases, a supplemental ground was installed going to the ground circuit pin in the MAS harness connector, so that makes sense. The WD21's were pretty basic compared to later models and the ECM has a base setting to default to if the self-learning memory is erased. That means there is no re-learn procedures like those needed for later models, such as idle air volume re-learns or closed throttle position switch re-learns, etc.
You might start looking at things other than the MAS. Check the hose to the fuel pressure regulator to make sure it's not disconnected or split and make sure there is not gas inside of it, which would indicate a ruptured regulator diaphragm. Check the fuel pressure to make sure it's within specs, which is typically 34-36 PSI regulated and around 45 PSI static (engine not running). You'll also want to do a fuel pressure leak down test to make sure one or more fuel injectors aren't leaking. A bad oxygen sensor can also cause the engine to run rich, as well. VG30E engines usually have two EGI ground points near the center of the engine on the intake plenum; there will be several black wires going to two, 10MM head bolts. Remove the bolts and clean-up the eyelets and the contact point on the intake manifold to make sure they are getting a good ground. Also, make sure there are no splits in the rubber intake duct. Try those things and let us know how it goes.

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Postby PackPath » Sun Oct 20, 2019 4:22 am

Thank you, smj999smj.

I read about the bad ground circuits, so I was glad it had already been taken care of on mine. Apparently, Nissan used to sell an harness adapter that was plug-n-play, but it is no longer available.

Is there a reason why cleaning the MAFS would cause such a big reaction? Did it trigger something else to go off/bad? This makes me leery of touching anything else and rendering the truck unmovable, but I'll try. I have to get this running before the snow season.

I will try all of the things you suggested. I have to research where the components are and how to perform the tests. (Can you tell I haven't done any work on my engines besides changing oil and topping off the fluids? :oops:) I purchased new shocks for it because the truck sags and still has the original OEM shocks. :shock: I need to get it running right first though.

I'll have to take it to a shop to have the fuel pressure tested. Looks like I can't do it myself as it's not in any of my manuals. I guess it might not be necessary if it's running though, right?

Can you tell me where the rubber intake duct is please? I cannot find anything on that in my manuals.

I guess I should also check the ECC canister filter, since I know it has not been checked in the few years I've owned it. I also got a new PCV valve that I need to install as soon as I can figure out where it is.

Thanks again. I'll update when I make progress.

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Postby smj999smj » Sun Oct 20, 2019 2:04 pm

The factory service manual is the best manual to use. While I prefer the paper manual, which you can sometimes find used copies on Ebay, you can access a free, online manual at Nico Club's site. Engine Control will be in the "EC" section: ... athfinder/

If you need a fuel pressure gauge, some parts stores, like Autozone, have one in their tool loaner program.

The intake duct is the rubber duct between the air cleaner box and the intake plenum. The cleaning of the MAS hotwire usually doesn't create a big "reaction" unless it's sprayed too hard and the hotwire breaks. The hotwire is a very fragile filament that can be easily damaged if not careful.

Bad shocks can cause a bouncy or harsh ride, but they will not cause the vehicle to sag; weak or broken springs will cause the vehicle to sag.

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Postby PackPath » Sun Oct 20, 2019 5:08 pm

Thank you very much for the FSM section guidance! That was one of the first things I downloaded, and I use it along with the Chilton and Haynes. Since I think I'm below the assumed mechanic skill level these manuals are written for, the procedures are difficult to follow when I don't know what or where the parts are.

Anyway, I'm fairly certain I found the problem, and makes sense to me as to why I did not have this issue before I cleaned the MAF sensor.
The middle wire is broken. I think it's the ground because it was connected to the workaround spliced ground that was run to the engine ground.

The problem I have now is that it broke only about 1/4 inch from the black MAF connector (or right at the top of the red cap).
Not enough room to strip the sheathing and re-solder.
I wish I could post photos...

Do you know offhand if a new connector can be purchased?

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Postby PackPath » Sun Oct 20, 2019 7:59 pm

I found one at RockAuto for $4.00.
Hope it fits.
In the meantime, I'll try some micro-surgery. Can't hurt to try!

I'll update once I get it installed.

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