How to fix the lean condition CEL codes on a supercharged VX?

[VehiGAZ]

I need to tap into the collective wisdom a little more directly than searching the forums. I found several references to these problems over the years, then posts claiming they fixed the problem by doing this or that, then more posts 2 weeks later saying the problem is back, so my forum research has been inconclusive. Plus, it isn't too clear which posters had a supercharger and which did not.

So, here are the relevant facts of the case...

1) I installed a supercharger last summer, along with a free-flowing exhaust. Whoo-hooo!! What fun! The bigger exhaust (2.5" from the cats back) helped as much as the S/C, by the way... it definitely improved torque off the line, whereas the S/C only helps the high end of the rev range.

2) Starting this winter, I have been getting check-engine lights fairly often. They are codes P0101 (MAF System Performance) and P0171/P0174 (Fuel Trim System Lean Bank 1/2). They keep coming back over and over, sometimes at idle when I start the engine, sometimes after being on the road for a while - there doesn't seem to be any pattern to it.

3) Sometimes the CELs are accompanied by an intermittent "stutter" of the throttle, and sometimes not. What I mean by that is that you hit the gas pedal partially, and instead of getting power, the engine kind of bogs down for a second or two, then the power kicks in. It is similar to the way the engine behaves when you are running out of gas.

4) The MAF code may be triggered by an out-of-spec reading on the MAF or the manifold absolute pressure (MAP) sensor, or from air/vacuum leaks in the intake. I replaced the MAF with an Autozone rebuild, but that didn't help at all. My mechanic tested the MAF and found that it was reading out of spec, but that doesn't mean much since the MAF that cured my 928 tested out of spec, too. Nevertheless, I will bring it back to Autozone and replace it. He checked for air/vacuum leaks but didn't find one. I have not checked the MAP sensor in any way.

5) My mechanic found that the throttle position sensor wire harness was loose, so he tightened/reattached it about 3 weeks ago. Since then, I keep getting the lean codes (P0171 & P0174), but not the MAF code (so far).

My mechanic and I are not sure what to try next. He keeps thinking in terms of the S/C kit is maybe not engineered right and these problems are part of that. I disagree, because I ran it with the S/C on there for about 6 months before I had any problems at all. I am suspicious of the fuel delivery system, and want to replace the fuel filter and fuel pressure regulator next. But before we start chasing our tails in a circle, I want to know if anyone here can provide some more conclusive advice.

So with all of that in mind, have you had this problem with your VX? Is it supercharged? How did you finally fix it?

Thanks in advance!

[Ldub]

Had the exact same symptoms...thought it was due to cracked intake manifold gaskets, so I replaced them. Even though they were cracked & needed replacing, on the first test drive the cel came on & the same codes appeared.:sighwgray

In desperation, I emailed Jerry Lemond, who was gracious enough to reply quickly. He indicated that there was un-metered air entering the system somewhere (vacuum leak) OR the little sensor wires in the MAF might be crudded up with K&N filter oil residue.

After replacing all my cool lookin', but poor sealing, stainless braid vac & brake booster hoses with rubber, I bought a can of MAF cleaner & de-crudded the sensor wires.

Result...cel free for over a year.:dance:

[VehiGAZ]

Thanks for the tips Ldub!

I just went down to Autozone to trade in the MAF and, while they had one waiting for me, their comuter system was out and they couldn't process the return, so I have to go back tomorrow (to pick up the second front brake rotor they didn't have on the shelf).

If that doesn't do the trick I'll have my mechanic check all of the hoses again.

Has anyone ever found an adjustable fuel pressure regulator for the VX?

[Ldub]

Thanks for the tips Ldub!

I just went down to Autozone to trade in the MAF and, while they had one waiting for me, their comuter system was out and they couldn't process the return, so I have to go back tomorrow (to pick up the second front brake rotor they didn't have on the shelf).

If that doesn't do the trick I'll have my mechanic check all of the hoses again.

Has anyone ever found an adjustable fuel pressure regulator for the VX?

IIRC, CYRK is the only one that I can think of who was experimenting.

[WormGod]

I know Cyrk ended up having to go with a custom fuel management. I would fire him a PM and see what he did. Sounds similar.

Though it sounds peculiar that a wiggle of the wiring brought about a CEL. That at least sounds easy enough to remedy with some splicing. God knows, the re-wiring during S/C install was a tedious POS.

I get engine studder occasionally in my VX, but it's only when I have it running, turn it off long enough to run into a store or something, and then start it back up. Only happens on restart. I rev it up to about 2k rpms for about 20 secs and it purrs on its own again. I am sure that is fuel related. I still have yet to replace my fuel filter so it may have somehting to do with it (I am hoping).

Good luck.

[VehiGAZ]

Gary, that's not quite what happened - he didn't get a CEL from touching the loose TPS wiring - it stalled when he did. Prior to that, I was getting CELs almost daily, but since he tightened up thew wiring, I am only getting them once a week or so, and it has (so far) only been the lean condition codes.

The replacement MAF goes in next. Then comes the new fuel filter.

[WormGod]

Ah, well, still sounds like some loose wiring in there. It's all color coded so should be easy enough to resolve. :bwgy:

BTW, what octane are you using?

STarting with the MAF is a solid measure. Baby steps towards a solution. God knows, I am in need of a new EGR. Even after taking it out twice and cleaning it up real well, it still ends up looking like it came out of a 50 year old car.

[VehiGAZ]

I generally run 93 octane - only occasionally do I get stuck with 91 - but it's always "high test" since I supercharged...

The new MAF went in last night and it sort of seems to run a little smoother and stronger. I'll see if I pop any CELs in the next couple of weeks before monkeying with the fuel system (other than changing the filter).

[Hotsauce]

Our stock fuel pressure reg is adjustable.

Its just not a factory approved adjustment.

In the top of the FPR is a tamper resistant torx screw that adjusts fuel pressure. Its factory set, and secured with loctite.

Clockwise increases pressure.

John C.

[VehiGAZ]

Thanks hotsauce! That's a pretty valuable piece of info!!!

No CELs since the new MAF went in on Wednesday, and I think my mileage has improved, too. We'll see what tale time tells...

[AlaskaVX]

I'm having the same problems I think... also some smell of fuel on startup. Let me know if that fixed it!

[VehiGAZ]

Alaska,

Yes, I was also getting a noticeable gas smell in the exhaust on cold startups. It lasted for a minute or so until the ECU switched over to "warm" operation mode. I thought it particularly odd that I would be getting lean codes when the smell of gas suggested it was running rich! Anyway, it is not gone since I replaced the MAF.Anyway, I'm not saying I'm cured just yet, but I haven't gotten any more CELs since replacing the MAF last Wednesday, and I also think the engine is running a little smoother and stronger since. And although I haven't finished a full tank yet, I think my mileage has improved as well.

So here are my lessons learned...

1) K&N filter oil can gunk up your MAF elements and throw off its operation.

2) Be careful if you buy a rebuilt MAF and make sure you can return or exchange it if it doesn't work well.

As my mechanic explained to me, they will rebuild the MAF, test it, and if it is anywhere within spec range, they will sell it. That may be fine for stock engines, but my supercharged engine may not be happy with a MAF that operates at one end of the spec range or the other.

Btw, if anyone is reading this and wondering what the heck an MAF is and what it does, here's a quick explanation... the Mass Air Flow (MAF) sensor is located on the air intake piping and tells the engine control unit (ECU, the engine computer) how much air is being sucked into the engine. The ECU uses this measurement to decide how much fuel to spray into the intake in order to have the right fuel-to-air ratio for the given operation of the vehicle (e.g., idling vs. wide-open throttle acceleration).

If the MAF is under-estimating the amount of air going into the engine, then it will mix in too little gas, resulting in lean codes P0171 and P0174 (as has been my case). Conversely, if the MAF is over-estimating the amount of air, then it will mix in too much gas, resulting in rich operation. Neither one of these is good for your engine and may pop check-engine lights.

Good luck Alaska!

[VehiGAZ]

Two weeks and no CELs!!! Whoo-hooo!!!! No more stuttering throttle, the smell of gas on startup is gone from my exhaust, and my mileage has improved about 1-2 mpg (although that last one might be because of the warmer weather). I'm considering myself in remission for the time being - if this keeps up for another 2-3 months, I'll call my CEL problem fully cured.

In the meantime, I ordered a Scan Gage II from Amazon which just came in. I'm going to use it to display instantaneous mileage, which will hopefully cure the S/C-induced spasms in my right foot. ;Dr;

Thanks for the winning tip, Ldub!

[Jolly Roger VX'er]

Our stock fuel pressure reg is adjustable.

Its just not a factory approved adjustment.

In the top of the FPR is a tamper resistant torx screw that adjusts fuel pressure. Its factory set, and secured with loctite.

Clockwise increases pressure.

John C.

I did this too when I replaced my FPR....the tamper-proof torx bits were available at Advance Auto when I bought them...I bumped mine up a little (rich=clockwise). Been running great!

Just bear in mind that cat's don't like overly rich conditions!

[USMCvxOFFROAD]

Any one have a problem with their vx running way to rich with out miss fires? because i do and have no idea whats going on, and it was fine till i had her smoged here in cali and a new exhaust put on. idles at about 1000 rpm too. oh and buy the way anyone have an extra electric radiator fan laying around for a 99

[USMCvxOFFROAD]

fixed my own drunken late night workin on the VX problem missed the groves on the mass air sensor and intake hose both sides

[FelixFisher]

I did this too when I replaced my FPR....the tamper-proof torx bits were available at Advance Auto when I bought them...I bumped mine up a little (rich=clockwise). Been running great!

Just bear in mind that cat's don't like overly rich lingeries (http://www.robustbuy.com/womens-clothes-lingerie-sets-c-1083_1085_1088.html) conditions!

Pretty nice thought yes cat's dont link overly rich conditions