I had this happen once before but I had a major hole in the resonator and eventually had RP (P0171, P0173) until I was instructed to look at the resonator and found it in the same spot that others have had. That solved the problem.
However before I got to the RP I was noticing that my Instant Fuel reading on the trip computer was running amok and bouncing around from a normal reading all the way to 56 mpg at 75 miles an hour or up to the 85 or all the way to 99mpg range if I took my foot off the pedal and coasted. Once I replaced the resonator- that went away.

Now today out of the blue the same thing is happening. So after about a 30 mile trip where my fuel average showed to be 43 mpg I took out the Jag Icarsoft. It showed everything to be normal, have no error codes (P1111). LT Fuel trims at idle are +3.7 and +4.1 which seems fairly normal (ST were running at -2 to -3). At 2500 rpm LT goes to +2.5. So nothing there indicates a leak.
I did find the bolt holding the air-box in had come out, but got another bolt and re-tightened that. The airbox was still closed tight around the filter.

Sometimes the reading is normal or where I would expect to be then I hit a bump or go up a small hill or down a small or around a curve and get crazy high readings. I can't see anything that is loose, after I put another bolt on the air filter housing. The speed and mileage part of the trip computer are in sync with the car. At lower gears the reading seems very normal but once in 4th or above it can show normal to crazy high figures.

So what gives. Has anyone else had the problem?

My BIG question is whether or not the misread of the instant and average fuel is indicative of some other more serious problem in the car or is something become loose that is shifting around and causing the misread (I though perhaps the airbox being loose-but no such luck). I can tolerate the higher readings, but worry it is a symptom of something else. Would running the ETM accomplish anything with this problem?

Perhaps someone can tell me how this instant and average fuel work in the first place.


Thanks- as always

Tom in Dallas/Plano
2005 S-Type 3.0 128k

 

How consistently can you duplicate the fault? If easily repeatable, try changing the electrical load and see if the values change significantly. For example, turn on the headlights if off, or visa versa. The rear window defroster and HVAC fan are other big electrical loads.

Mostly just thinking out loud, but perhaps you're seeing the side effects of a power supply problem, either high or low. Changing other loads might be just enough to change the equation, giving you some direction to pursue.

You also mentioned the same symptoms were present when the resonator had previously failed. Have you checked the replacement? If still good, inspect the rest of that duct for leaks, especially within the bellow folds.

 

After I bolted back down the air filter box I checked all over the resonator. Tight as it was new . The bellows and the attachment to the throttle body are new as that was the way the resonator came. I have driven the car three times today and saw it all times. It could have happened yesterday, but I was not paying attention to it due to heavy traffic. I am sure it will do the same in the morning. I will try and play with the loads. I did turn the AC on while driving and that did nothing. The lights were on for 2 of the trips today. I will try the rear windows defrost and see what happens. I had it on some of last week when very cold, but not the spike in average or instant fuel.

I will report back and appreciate your thinking.

TBB

 

I have played around with every item I could turn on and off and in different combos . No change in the amok mileage. I wish I were getting what it says I am.

One thing I did notice, at the suggest of NBCAT, was my battery voltage at the battery after a 15-20 mile highway run was 12.4 at the battery. When I ran the ETM it showed at 11.9-12 volts. Normally when I have a failing battery and this has been consistent, I would get the ABS and DTC to flash on briefly and a beep to the effect to let me know. I do not have that, but could a battery that is stating to go south cause this type anomaly?

Your opinion on AGM batteries- which is what I use, vs standard and vs the EFB batteries. I have a tendency to go to the EFB.

Thanks

Tom

 

Hey tom

i know that injector pulse width is part of the equation along with load and multiple other readings and then the instant math is done. If one of the ingredients is a little wonky at times but not enough to set codes then you will have funny mileage at times. Yes i did have the same issue once a couple years back. Once in a while cruising at about 75 mph on freeway i would check the instant and it would be like 60 plus then go to normal about 20 and back and forth. After i pulled the charger for the valley hoses i looked and replaced a lot of gaskets and cleaned hidden sensors. Either i had a minute vac leak or a dirty sensor because i have never seen it do that again

 

Thanks: Do you know what sensors you cleaned? Is there plaything on the scanner I should look for? Will the injector pulse width show out of range?

Tom

 

as far as pulse width goes do a movie with sdd and compare them one to another. You might even have a clogged or dirty injector or two. I know i cleaned the map sensor and the entire egr valve. The entire throttle body and a couple others that relate to the supercharger. I think i had a slight vac leak somewhere that came and went with engine torque or movement. It probably was either the brake boost line or a gasket that smoke test could not find

 

Thanks.My LT fuel trims don't really indicate a leak. When warm and running at 40-70 I stay consistent at +2.5 on both sides. The map sensor is a good idea. My problem is I do a lot of short trips. The fuel injector not working properly is a real possibility, but if it got out of range- do you think it should throw a code or perhaps it is just on the edge? Maybe one of the better fuel injector cleans and a nice trip of some miles will help.

Thanks

T

 

if you have a schrader valve on the fuel rail you can do a nice cleaning yourself. I think a pulse way out of range will code. I think a dirty injector is where to start. There are kits available to pressure clean and have a tank additive. I got one at autozone for my 2000 chevy and it made a difference. My 06 str does not have an access valve so i put in a big can of berrymans at low fuel sometimes to clean stuff

 

I may try the BG44 that Jag recommends or the Berryman or even I have heard good things about the redline.

It is an easy starting point. I will check on the fuel rail schrader valve or perhaps someone will chime in about 3.0 engine.

Thanks

T