How much does a high MAP effect mileage w/ a MAF?

[Haulin' 8.1]

I'm trying to track down a slight rich condition and fix a poor fuel mileage condition on my truck. Engine is in excellent tune and no mechanical defects present. I'm getting about 9.5-10 mpg and things like a tune up or replacing a weak thermostat helped tremdously with power and driveability but never help mileage. Long term fuel trim was at -12 with the good tune and went to about -8 with the thermostat (went from ~187* up to ~198*).

 

It's a 2001 2500HD with the 8.1 / Allison. I've been checking everything with books, scanner, and tools and can't find any problems. The only thing that raises an eyebrow is the barometric pressure reading, 15 PSI is a bit high even for 200' like my altitude. I'm not used to a fuel injection that uses a MAP and a MAF. I would think the MAF would override the MAP but I'm not sure so I asking.

 

Thanks for any advice,

Vernon

[vhato]

Hmmm,

I agree. I always thought the MAF reading was used for 0-98% throttle and MAP was ignored. I do know at 99-100% throttle the MAF is ignored and only MAP is used.

 

A descreened MAF can cause a rich condition.

 

Incorrect O2 Readings can cause a rich condition to if the PCM thinks the Catalytic Converters are getting too hot.

 

When engine temperature hits a threshold temp (I dunno what it is) the PCM will cause a rich condition in an attempt to prevent Catalytic Converter damage also.

 

The Engine not switching Loop modes due to a stuck open thermostat will cause a rich condition because the PCM thinks the engine is always warming up.

 

A 180/160 degree thermostat upgrade will cause a rich condition also if the PCM has not been upgraded to know the engine is warm at 160 degrees because it is looking to be around 195 degrees.

[Haulin' 8.1]

Nevermind, it's right. I found a section on checking the MAP readings based on local barometric pressure. So I called the airport about 8 miles away. You have to multiply the actual baro by 3.4 to convert to Kilopascals and compair it to the scanners BARO readings. I had to switch the scanner read-outs to metric to see it in kilopascals. Long story shorter there's a lot more graduations of Kilopascal than PSI in this system. Airport kp was 102.9 and my MAP records a baro of 101+ which is within specs of +/- 3kp. EDIT: Since this information may prove helpful to someone in the future doing a search or whatever I wanted to point out that the actual barometric pressure you multiply by 3.4 to get KP is the barometric pressure in inches of Mercury (In.Hg.) like the weather man uses. I'm sure you could stick to In.Hg. / PSI if you know the formula but with more graduations KP is the most accurate.

 

Back to the drawing board. Thanks Vhato, my MAF is stock and cleaning it hasn't reduced the rich condition. Maybe it's the Champion plugs with the slightly reduced gap, 0.55" vs. 0.60"? The manual calls for Denso's at 0.60". I'm not finding a significant injector leakdown and fuel pressure is within specs. No misfiring that the scanner nor I have noted.

 

Vernon

[vhato]

Not a total loss....I learned how to correctly use MAP readings in the real world. Thank You!!!!

[Butch 02 Denali]

Didn't GM change their mind and now recommend .040 for all plugs? I thought I read that somewhere!

 

Have you tried it without the Hypertech?

[Haulin' 8.1]

Didn't GM change their mind and now recommend .040 for all plugs? I thought I read that somewhere!

 

Have you tried it without the Hypertech?

 

<{POST_SNAPBACK}>

 

 

 

 

I'll look into the plug recomendation, 0.060" is very optimistic anyways for consistancy under heavy loads. It's specifically for lowest emmissions.

 

I haven't tried it without the Hypertech but I did make some headway by trying a different MAF. It has reduced the long term fuel trim by about 1.5-2 points. It seems my throttle responce is stronger and slightly more HP across the board. I know one side is averaged at -5.5 now and my fuel mixture is more stable. Before Short term was constantly readjusting for current conditions but now the Short term fuel trim average is at 0.0 with current Short term trim never going more than three points from 0.

 

I had a '98 Suburban come into the shop that had just started dieing, twice at idle and once on the road. Both times at idle the engine started right back up and on the road it restarted from rolling in gear before she reached the shoulder. I was checking all the usual suspects and not finding anything wrong (fuel pump and wiring, ignition, etc.) The 'burb idled slightly rough and was a little low on power. Then it died on me just idleing in the shop and restarted then ran about an hour. I checked the alternator for leaking AC voltage and found 4.8 Volts AC. This is way above the 0.300 Volts A/C limit guideline. The alternator had a diode(s) failure causing electronics to go berserk and occasionally just loose it. This got me to go and check my truck just in case, it's usually overlooked. My truck's alternator produced 0.649 Volts A/C so I thought it could be a possible cause of my problems. I then checked the freshly installed reman alterator on the burb and got 0.645 Volts A/C. So what the heck. I tried to locate some new diodes to put into my alternator but found they aren't serviced separatly. I don't feel like spending $140 on another alternator just to find it at the same level as the one I just took out so I gave up on that trail.

 

I'm tracking down a posible gear indication problem with the PCM right now and I'll post what comes out of it. If there's nothing to it I probably will remove the power tuning and see what happens.

Vernon