Running Slightly Rich While Cruising

Hey guys. Got a problem with my car running rich while cruising (and getting pretty bad gas mileage).

Car: 1998 Mustang GT (4R70W) with 2001 PI motor, SVO blower at around 8 psi of boost, MHS Stage II NPI blower cams, offroad Prochamber, Flowmaster catback, 75 mm TB, and custom CAI.

Problem: it's running somewhat rich while cruising. On average, the AFR is around 14.1-14.2, with a range from 13.7-14.6. (I've got an Innovate wideband). I've noticed the fuel mileage decrease by a solid 3 mpg over the last 4ish months. It used to easily get 24 mpg in mixed driving. Now, it struggles to break 21 under near-ideal conditions.

I logged the LTFTs, and both banks were smack on 1.00. The STFTs were jumping around somewhat, between .93 and 1.07, give or take. Also, at WOT, the AFR is exactly what it should be as per my most recent tune (11.9-12.0 across the RPM band), which suggests that the wideband is accurate.

Anyone have any ideas or other tests I should run?

Thanks!

A 14.1 average AFR while cruising is "good"?

trinity_gt said:

Stoich is 14.7 and your trims don't look bad. Are you also using the WB sensor as a feedback sensor for the PCM (via the simulator output) or are you still using the OE sensors in their OE positions?

When was the last time the WB sensor was calibrated?

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Yes, and the stock computer is calibrated to correct to 14.65, right? Assuming my wideband is accurate, why is it not doing that?

I'm still using the 125k mile OE sensors for the narrowband output. The wideband is located less than 2 inches from the driver's side factory O2 sensor.

I calibrated the wideband about 3 months ago.

N8Dogg98 said:

How are your injectors? I'm just spinning my wheels here, but if they have cracked/fragmented pintel caps and are gummed up from being old couldn't it be contributing to this issue?

Again, I'm just speculating but if you have a set of old injectors that are all flowing at different rates, you have some cylinders running lean and others running rich, no? Is it possible that this has an impact on your mileage and/or potential power output?

Otherwise, as trinity said a dragging parking brake, or a seized caliper pin on the rear brakes (very common problem on the SN95s) could be causing the decrease in gas mileage.

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AFAIK, the injectors are fine. They are 39 pounders from an 03 Cobra that had 5k miles on them, and I've put right at 30k on them. I get what you're saying about the mileage decrease from that, but why the running rich part? (as a whole system, and not having any LTFTs?)

I'm working on getting ahold of a set of Cobra brakes in the next couple of weeks, so I'll check the calipers then. But I don't have any good reason to suspect that they are dragging that significantly. There isn't any excess dust or anything on any of them, or are any of them exceptionally hot after a drive.

N8Dogg98 said:

I was asking not only for you, but for myself as well. When I installed my PI intake I didn't replace the pintle caps and I learned the hard way that you must replace teh o-rings (spilled fuel EVERYWHERE due to torn o-rings). Anyways, my car doesn't seem to run right and I was curious if broken/missing pintle caps + 14 year old injectors with 140k miles on them would cause uneven fuel distribution among cylinders, and thus impact my gas mileage and overall performance. Anyone who has input or knows from experience, I'd love to hear it!



I assume you're getting a set of front cobra brakes, correct? AFAK, the front GT brakes are not prone to any problems. OTOH, the rears frequently end up seized. What happens is the caliper pins stick into the caliper bracket. This will happen very fast especially if the rubber boot is compromised in any way.

If it were me I'd lift the rear, pull the rear wheels and remove both calipers to physically inspect the caliper pins. I've had seized caliper pins on my rear calipers (same model year as yours) 3 times since taking ownership 8 years ago. This has happened on each side, too. I also have friends with 94-04 cars that have had the same recurrent issue. Definitely worth checking out IMO.

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Yea, I've seen the 'gas everywhere under the hood' as well, back when I first did my supercharger. Best I can tell, everything is fine there.

Next time I'm home, I'll pull the rears off and check them out. I know exactly what you are talking about, and I know that I greased those pins pretty heavily on all four corners. Still definitely worth checking out.

wmburns said:

Question: Have you recently changed tire size or rear end ratio?

Consider the role that the narrow band O2 sensors "could" play in this case. I agree that LTFT values of 1.0 are perfect. But what if the O2 sensors themselves are wrong? The PCM makes adjustments based on the values returned by the front O2 sensors. If those values are wrong, then the PCM will adjust to an invalid AFR.

Garbage in, garbage out....(old timers computer saying).

However, 21 MPG doesn't seem bad to me. In fact, it seems outstanding for city driving.

But. Here's something else to consider. As O2 sensors age, their response time slows. This increases the time it takes the PCM to adjust LTFT. New O2 sensors repond faster thus limiting the time fuel trim is away from Stoich.

I have seen it make a 10% difference in gas mileage on my own cars by using new O2 sensors.

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Rear end ratio is still stock (3.27s), and tires were last changed over a year ago (from a stock size to a stock size).

I was also curious about the O2 sensors being wrong, but I wasn't sure if that was possible. That certainly makes sense.

Well....it doesn't get 21 in city driving, nowhere near that. City driving with a light foot will get me 16ish, really heavy foot will get me 12ish. 50/50 mix with reasonable driving is now getting me 18ish. And that's all fine and dandy. But the fact that it's only getting 21 on the highway is what has gotten my attention. My car has ALWAYS gotten 24+ on the highway. Right after I got it tuned last December after my new motor, cams, etc., I knocked down 24-25 mpg on EVERY trip to and from school (120 miles one-way, 77 mph, cruise-control, very few hills, no traffic, A/C off). I made that same trip yesterday, and it got 20 mpg, on the dot. That is a significant decrease.

Hmm, I might need to recheck the LTFTs. I reset the tune only about 60 miles before I took that datalog yesterday. I've since driven another 150 miles. Should that be sufficient for them to have learned the LTFTs?

2002BLGT said:

The tuner can change the stoich point in the tune , alot of tuners are now doing this because of the ethanol in the fuel , changing the stoich point to 14.10 or so instead of 14.64 , you should check with your tuner to find out if they did , but with O2 sensors that old would not hurt to replace them

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Hmm, interesting. I did not know that a tuner could change parameters like that, but I guess that makes since in case you have a car running on E-85, or something like that.

I will be seeing my tuner at the track on Saturday, and was planning to ask him about it then. I just wanted to get a few opinions before-hand so that I didn't sound like an idiot asking him about it.

Also, is there any brand of O2 sensor to get or to stay away from?