Hello all,

I've got a 97 f150 Scab 4x4 4.6l, 5-spd that is the perfect truck except for the poor gas mileage (11 city 13 hwy). I've followed all the suggestions to improve MPG and some seem to have worked temporarily, but it seems that the PCM module factors these gains away and brings me back to the 11/13 mpg.

Also, every couple of fill-ups I clean the MAF so the truck doesn't stall starting off.

Last week, I took the truck to the dealer and had them run diagnostics and they said evosition: form-data; name="message"

Hello all,

I've got a 97 f150 Scab 4x4 4.6l, 5-spd that is the perfect truck except for the poor gas mileage (11 city 13 hwy). I've followed all the suggestions to improve MPG and some seem to have worked temporarily, but it seems that the PCM module factors these gains away and brings me back to the 11/13 mpg.

Also, every couple of fill-ups I clean the MAF so the truck doesn't stall starting off.

Last week, I took the truck to the dealer and had them run diagnostics and they said everything was "in-spec".

Here's the question(s)

1. How does a person know it's time to replace the MAF rather than just keep cleaning it?

2. Is there a way to test if the PCM needs to be replaced? One online source has these for $238 which doesn't seem like much if it worked.

Thanks for any assistance or advice. My ultimate goal in life has been to get the MPG up to 16 highway.

thanks

ihilani

 

One more question, is there an easy test to see if the CATs are clogged or need to be replaced?

Thanks again for all your past advice.

ihilani

 

There is something wrong with the website. Big parts of this post are duplicated and I can't fix it. Please bear with the replicates. I'll try three posts with part of the answers on each.

Last question first, three problems with cats.

(1) they plug up. Hook a vacuum gauge to manifold vacuum (For instance the EVR line). Run at idle. Then rev to 2500 RPM in neutral. If the vacuum decreases at 2500 or decreases over time the cat is plugged. If the vacuum at 2500 isn't greater than at idle, the cat or exhaust may be starting to plug. A vacuum gauge is probably <$20.

(2) They wear out and don't reduce emissions as well as they should. If drivability is your concern, don't worry about it. There is not performance or drivability issue with a worn out cat (unless plugged). If you care about mother earth, there is a post cat O2 sensor for each cat to monitor its efficiency. The PCM runs a monitor (program to test) the cat every drive cycle (well almost but close enough). If it exceeds 150% FTP (EPA standards), it will light the MIL and set a code. You can use a scan tool, oscilloscope or DMM to monitor the post-cats for slow switching to verify cat performance.

(3) They rattle (or start grass fires). Wait until you can't stand it anymore and drive off a bridge. Jump out just like James Dean in Rebel Without a Cause.

 

Second question. You don't replace a computer. It is almost certainly not your problem.

First question. The MAF is used for fuel calculation only in open loop. This is only before the O2 sensors warm up (say first 2 minutes after a cold start) and when at wide open throttle. The rest of the time the PCM is running in closed loop, calculating fuel based on feedback from the pre-cat O2 sensors. That is to say the MAF only affects drivability right after a cold start and at wide open throttle. However, the PCM computes fuel trims to determine if the MAF fuel calculations should be adjusted based on O2 feedback when in closed loop. These adjustments are called long term fuel trims. Again, when the O2 sensors are used to calculate the fuel requirements, the computer does the MAF fuel calculations based on MAF also but only uses the MAF calculations to store adjustment factors, the fuel trims. These fuel trims are still not used in closed loop, only when you run again in open loop. This adjusts for many parameters that change over time, even correcting for MAF calibration.

So there are three cheap ways to check for MAF functions (these are things your Ford techs might do since they get paid by the job, not the clock hour)

 

So there are three cheap ways to check for MAF functions (these are things your Ford techs might do since they get paid by the job, not the clock hour)

1) Use a scan tool to check the fuel trims. If they are within a few percent, say + or - 5%, the MAF is OK. There are fuel trims in a matrix of RPM vs. throttle position so check them at several operating conditions. If they stay within +/- 5% you know the MAF is good. If they are outside +/-5%, something is wrong but it may not be the MAF. This only clears the MAF, it doesn't condemn it.

2) Does the engine run smoothly right after a cold start, before the O2 sensors warm up? Again, if it does, the MAF is OK. If not something is wrong but it may not be the MAF. You can unplug the electrical connector to either pre-cat (front) O2 sensors and the truck will always run off the MAF (the MIL will light). If the truck runs smoothly off the MAF it is OK. Don't use wide open throttle to verify the MAF. Too many other things go wrong at wide open throttle.

3) Unplug the MAF and see if it runs significantly better without it. If it does, the MAF is probably bad. You can also unplug one of the pre-cat O2 sensors so it stays in open loop even when warm and see if it is better with the MAF unplugged. Again, if it does, the MAF is probably bad.

Finally, there are far too many potential causes of poor mileage to guess what it is. The MAF is almost never a cause of poor mileage. The reason is it is NOT used when in closed loop mode which is 99.9% of most driving. Only if you drive very short distances between cold starts or are frequently running at wide open throttle could the MAF affect you mileage. Or if some fault kept you out of closed loop, like a bad O2 sensor.

 

well considering that its a 4x4 x-cab, its not going to get too good of gas milage anyway, but you should be able to get 16-17 on the highway. My truck is identical to yours only its a 2wd, and i get 15 town, 17-18 highway. How many miles do you have? That could play a factor in your poor milage. if its got alot of miles, clogged cats could possibly be a problem. First, Id check for carbon build-up in your intake--i thought my truck ran great until i clean all that out--WOW felt like a brand new truck. Search the Engines forum for "Rough Idle Problem", and youll find 3 pages of good info and lots of pics to help you do this, its not too bad.
Could also be a PCV or MAF problem, but youd probley get a check engine light if these were bad.

Ill post back tomorrow, im off to bed...long day of class tomorrow

 

hmmm...double posted on me too and i only hit submit once...