Are you an OBD I convert? Do you have the option of setting a switch to change into SD (speed density) mode? That might get you through for awhile while you sort out the MAF issue.

 

Yes I am an 'OBD I convert' hehe. Just checking the 'Speed Density' box will do it eh? That'll just take a PCM reflash.

 

Okay found out a couple new facts:

New MAF makes little difference - with stock tune and MAF ends w/ screen in place BLMs fall to about 110 instead of 108 (w/ stock tune and descreened MAF).

Car now spits raw fuel at deep throttle in closed loop.

Everything seems normal in open loop, good idle, good O2 readings, no foul smells, no raw fuel. Rules out injectors, fuel pump, pressure reg, coil, opti, and other ignition items...

Fuel pressure holds at about 43psi with the engine off, about 40psi at idle.

O2s are only a couple months old, genuine AC/Delco pieces.

Plugs are 2 months old, AutoLite 104s.

Wires are 2 months old, Taylor SpiroPro OVC customs.

Again everything is fine in open loop, but as soon as it reaches closed loop things go crazy. I've seen clear evidence of this in cells 16 and 18. I almost want to blame the PCM itself, but wouldn't *any* malfunction be a complete disaster, not just a fuel delivery issue? I'm confused and don't know what else to check.

 

You realize how much better 110 is than 108 right? 108 is the bottom of the barrel, that means the PCM is taking out as much fuel as it can, and it's still running rich. 110 means yes, it's taking out fuel, but at least it's got you to a decent A/F ratio.

You got different readings between the two MAF's... clean yours, see if you can get yours to read 110. My MAF was dirty and I was running 110 BLM's, cleaned it and (without any PCM changes at all) I'm running 124-128's.

 

the car cant run rich in closed loop if your BLMs are 110. now if you BLMs were 108 then you could say it is running rich b/c there is a possibility the computer would correct down into the 90s if allowed. if your BLMs are 110 in closed loop then in open loop you are running pretty rich so you should smell fuel when you start up... it will also run rich at WOT.

im not sure how accurate this is but what you could test is how the computer corrects in SD mode(when MAF unplugged) this way the computer calculates how much air is entering the motor from feed back of several sensors. if you BLMs jump up into the 120s or 130s then that is telling me the MAF is miscalculated and you can either buy a new one or adjust the MAF tables to compensate... what i would do since its quite a bit cheaper

 

Yes you're right but its still not anywhere near the 120s where the car was until just recently. Even at 110 in cell 16 if I rev the heck out of it, it spits raw fuel. If I then immediately unplug the MAF and let it stabilize, then rev it again, everything is fine. I swapped MAFs with a good working one right out of a friend's 97 Z28. I pulled my MAF a couple nights ago and cleaned it (though it looked clean) and I scanned it immediately before and after the cleaning with duplicate results.

The BLMs do indeed stay in the 127-130 range at idle with the MAF unplugged (been running this way since I found this problem). I cannot see how the MAF is miscalculated - it's running the factory '95 tune. And when I swapped with a friend's working unit, the idle BLMs changed very little. I assume the difference between the figures is that he has an absolute stock MAF whereas mine is descreened.

If there were an exhaust leak before the O2s, would this cause them to read much more/less oxygen and cause my BLMs to plummit? I see no signs of obvious leakage but it's a thought.

 

It sure seems like it is has something to do with the MAFS setup.

Does the car run ok in closed roop in speed density mode (MAFS unplugged or PCM programmed for it)?

If so, and you've tried a known-OK MAFS, then perhaps the wiring to the MAFS is faulty. You might consider a new connector for it.

 

an exhaust leak before the O2 sensor causes what is called a "false lean" condition. this is where the computer thinks you are running lean from the extra oxygen leaking in. in this case the BLMs go UP because the computer adds fuel to compensate, actually causing you to run rich. your BLMs are off in the wrong direction.

another thing... had the result of an exhaust leak actually lower BLMs(of course we know it doesnt). you would be assuming an equal volume of air, from equal sized cracks, seeping in on both banks which is highly unlikely! this leads me to believe the solution is probably gonna be found before combustion... ie. intake or fuel system related.

i am quite confident you can fix your problem by simply decreasing all the values in your 4 MAF tables by the same increment until you get the desired BLMs.

 

Yes the car runs fine in SD mode. I even get really good gas mileage! The power seems a bit off on the top end and it cranks up sorta weak when its cold.

 

Dave,

I switched to Speed Density mode on my '95 because of the same symptoms.

With the MAFS with stock tables, I am running around 100-108 BLMs (lowered my max to 80 to see how bad it was).

With speed density, I am actually 135-140 with stock VE tables.

After tuning with VEmaster and by hand, I've got them near dead-on 128, and the car runs a LOT better. Seems to be better gas mileage and a lot more power throughout the RPM band, even at part-throttle.

Seeing how my car has 110K mi, I am wondering if the MAF sensors ever just "wear out." How many miles were on your friend's that you tried?

 

JSK,

Thanks for the reply. Thats a lot of helpful info. My car is about to reach 100k, and my friends is at about 74k and running strong. So it's perfectly safe to run my car in SD mode? Does that mean I can remove the MAF completely and make a nice bypass tube to put in its place? Not having to worry about the MAF (for N2O purposes) and the cooresponding tuning tables would make things a lot easier. The weather down here doesnt change that much anyway - pretty much always hot and humid. If I can tune it in just right once, it should work well 11 months out of the year.

With the MAF unplugged (PCM not set to 'SD mode') are the INTs and BLMs still active? I just find it odd that the BLMs can go from 108 with it plugged in, immediately to 128 when I unplug it. Almost as if it's defaulting to 128 like during WOT...

Let me know if you have any other tips regarding SD mode and tuning. Thanks!

*** Also I'd like to change the 'topic' of this thread to "BLMs steady 108 in closed loop" since it's happening on the stock tune also. ***

 

Yes, speed density mode doesn't affect closed loop operation, so your BLMs/INTs will be actice once the engine reaches operating temp (just like with the MAFS). It's like your car would be a '93. They have closed loop, etc.

You don't need to remove your MAFS, no. I have mine still in for now until I figure out a good way to connect my custom 4" CAI w/o it there. But there will be less of an airway restriction if you remove it!

I think it's going to 128 because your tune is just good for your current combo. When I first switched to speed density mode, my car was running 128 in some cells before I started tuning it.

My tips for tuning SD are to take a long log of all types of driving, part-throttle of various levels (1/4, 1/2, 3/4, etc.) at as many different RPM ranges as possible.

Then run this log with your current PCM image through VEmaster. If you can't find that program on the web by searching I can email it to you.

Then continue to log and run VEmaster until you're near 128 in all cells. You can fine-tune by hand as well. The higher the VE table value the leaner it'll run, the lower, the richer.

Ait temperature shouldn't affect the tune because speed density inherently takes it into account with the IAT module.

HTH!

 

I am now officially running my PCMforless tune in SD mode. Everything seems good except the right BLM going way lean (high 130s, low 140s) sometimes when I lift the throttle. At idle BLMs are 127/129 give or take. I can hear an exhaust leak that's become louder since a friend and I yanked on the exhaust system to make the 3.5" alum. DS squeeze between the torque arm and the 4" Loudmouth resonator...so I think that could be where the lean condition is coming from.

When the PCM refers to 'right' and 'left', is right=odd or =even cylinders?

JSK333 - I drove for about 15-20 min at different TPS while logging like you said which resulted in a 5MB CSV file. I'm going to fix this exhaust leak or at least try to find the cause of the lean right side and then run it through VEmaster. Thanks for the tips.

 

Cool, I'm sure fixing the leak will help!

I also wanted to correct my erroneous statement--increasing VE values fattens, and decreasing leans, not the other way around!

Glad I could help some. VEmaster should help things out!

Solomon