Strange AF reading...could my FMU be shot?

[newgsrdriver]

Ive noticed over the past couple days that sometimes during driving (it seems to get worse as I drive longer), my autometer guage goes from its normal stoich/rich spot to stoich, then leaner and leaner as driving continues.

If I turn the car off for a bit, then start it again, it seems to go right back to its stoich/rich position. The car still seems to drive the same...not getting any knocking or anything when this happens.

Could my 02 sensor be giving an incorrect reading, or do you think maybe my FMU (Jackson Racing POS) could be finally shot?

[Blown98GSR]

The JR FMU usually dies in two ways...the upper or lower diaphram's rip. In both cases the wheep hole usually leaks fuel and the fuel pressure will tend to stick at a certain level without adjustement.

How lean are you readings? Do these readings occur when you are in boost? Do they persist when you are driving under a regular load(not on boost)?

Do you have a fuel pressure gauge inside the car?

The regular 02 reliant A/F gauges are so inaccurate that I don't pay much attention to the one I have.

[waveoff]

Do you have the TPS trick installed or any other kind of wiring that required hacking the TPS wires? When my TPS wiring starts to short out my a/f gauge shows funny readings.

[Blown98GSR]

I was trying to figure out how lean he was going.....as with our setups, at cruising speeds on the highway after our 02's have warmed up and all, we usually get a reading very close to stoich on the Autometer gauges.

if you are going into the red, then you have a problem

[newgsrdriver]

Well, it does it during WOT and normal driving. It seems to like two positions...one at 1 o'clock on the guage, directly in the middle of stoich, and another just a couple bars stoich.

It normally bounces back and forth between the 1st lean bar and the 1st rich bar, and sticks on the 1st rich bar at WOT. No TPS hack, no wiring modifications. And it has gone into the red...very scary, but no change in driveability!

Thats the wierd thing, is then when I finally get to my destination to test it out, I swear it goes back to normal.

This just started happening the other day, with no new modifications and no changes. Its never been like this either, always consistent with the description above.

[JRCivic]

If what you are describing happens during normal "cruise" type driving, and not really "spirited" or WOT driving conditions... then the JR FMU would have absolutely nothing to do with the problem. Remember that it only affects the fuel delivery in boost... not in vaccuum.

[Marc]

Jim is right. In fact, there is nothing wrong with your car. Under light throttle and maintaining a consistent TPS the ECU will start to lean out the mixture in order to get better gas mileage. This is part of the reason why you get higher mileage on the highway than during city driving.

[newgsrdriver]

Today I started her up...same problem. Stoich to lean. The car drives totally normal.

Then, the lights go out. They stay out. No reading, just blank for a good 15 minutes. Get to a gas station, fill up, play with the power and ground, both wires look good, and the guage still isnt on.

Drive away, and after a couple minutes, it pops back on with its normal Stoich/Rich setting and stays that way for the rest of the day.

Could the guage itself be faulty or what???

[waveoff]

That's too weird... What do you have as the power source and ground for the gauge? My gauge used to give messed up readings when I had the wires tapped into the cigarette lighter wires. Are your connections soldered? You could possibly try other ground and power sources than you have now. I suppose it could be the gauge. Hope ya figure it out.

[newgsrdriver]

Thanks guys [img]smile.gif[/img]

FYI, it was my ground wire. I had it under one of the battery tie-down bolts, and it was a weak connection. Relocated it and the guage worked like a charm once again.

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