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Bad fuel mileage/slightly high fuel trim

Juanhmi

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City, State
Louisville
Year, Model & Trim Level
'99 XLS V6 OHV
Been getting steadily worse fuel mileage on my '99 OHV, down to 10-11 in town. I've been all over looking for vacuum leaks and I don't think I have any, or any bad intake gaskets, although the carb cleaner trick might just not be hitting the right spot. New MAF sensor, new air filter, new PCV valve, I "think" I replaced the idle air control valve a year or two ago but can't remember. Long term fuel trim at idle on both banks is +7-10%, scan indicates a "pending" lean code on both banks but hasn't tripped a check engine light yet.

At 237,000 miles I'm wondering about O2 sensors. They seem easy to reach and aren't terribly expensive, but I could also be missing something real dumb. Aside from this being a really hard-to-find vacuum leak, am I missing anything?

I did just have valve cover gaskets done by a shop I trust and I've taken a look at any hoses they could have knocked loose. If I can't figure it out myself I'll take it into them, but I'm not blaming them yet as the fuel consumption issue has been going on since before that job.

Edit: fuel filter is also less than a year old. I have not read fuel pressure yet, need to buy a gauge (I have one but for low pressure carbureted vehicles only).
 


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J_C

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Fuel pressure is the next thing I'd check. You can get a gauge to do that as a free loaner tool from some of the auto parts stores but beware that O'Reilly's has a very short return period.

You can do a smoke test into a vac line (like the brake booster) to see if you have an intake leak.

If your O2 sensors are original, yes I'd replace them but I doubt that's the issue w/o a code indicating O2 sensor fault. The upstream sensor(s) would be the one to try first.
 




Juanhmi

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'99 XLS V6 OHV
Well, ran to Harbor Freight for a cheap fuel pressure gauge and I'm at 60-65 psi which I assume is right around where it ought to be. So while I will probably do O2 sensors, my suspicion is that I'm still chasing a pesky vacuum leak. Never smoke-tested an engine before, does anyone have a method (or link to a thread) on how to do it without an expensive smoke machine?
 




J_C

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Yes 60-65 PSI is in the right range.

There are youtube videos showing several methods, but one of the cheaper without having to build something elaborate is just getting a basic hand pump, sticking a lit cigar in a short sacrificial piece of hose going to the intake port (so you don't melt the intake nipple), and using an adapter on the output (or quick 'n dirty would be to use duct or electrical tape if the size difference isn't much) to seal onto the hose you're blowing the smoke into.

Example pump: Amazon product
or cheaper at Harbor Freight but i don't know if the quality is the same: Multi-Use Transfer Pump


I forgot to mention that if you're not pumping it into your intake snorkel, you need to block that off too, a plastic bag and rubber band will work. Do that between the filter box and MAF sensor so any leaks after it will be included in the test. I happened to find a video of someone using the HF pump linked above:


 




Juanhmi

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'99 XLS V6 OHV
What a hilarious method and a good excuse to smoke a cigar! I'll definitely be trying this. Hoping it's something stupid and I don't have to do intake gaskets, but the intake really isn't that tough to pull on the OHV motor.
 




beepinX

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Fuel pressure is the next thing I'd check. You can get a gauge to do that as a free loaner tool from some of the auto parts stores but beware that O'Reilly's has a very short return period.

You can do a smoke test into a vac line (like the brake booster) to see if you have an intake leak.

If your O2 sensors are original, yes I'd replace them but I doubt that's the issue w/o a code indicating O2 sensor fault. The upstream sensor(s) would be the one to try first.
We will include that on the things we check for the incoming 1999 Explorer project. The owner mentioned it for fuel leak before being sat under the tree. Rust seems not a major issue but we will see. Gonna take a closer look next week after we finished installing the shocks and winch from 4Wheelonline onto the Wrangler and clear the garage.
 




Juanhmi

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'99 XLS V6 OHV
Actually did this DIY cigar smoke test and sure enough, smoke immediately blew out of the surface between the idle air control motor and the intake. I removed it, and it looks like the previous owner had no gasket whatsoever on. The bolts were not very tight. I just replaced the IAC and gasket, that's a 45-second job.

Drove to get gas (for the sake of checking fuel mileage over the next few days) with the scan tool giving live data, and still had fairly high long term fuel trim values. Will see what my mpgs are the next week, but I suspect the IAC leak was just the closest leak to the brake booster inlet and there may be more leaks to worry about. Or, those O2 sensors, which I'm resisting because they're just more expensive. Still, optimistic I won't have to do all the intake gaskets.

I will say, noticeable difference in steady idle with a properly gasketed new Duralast IAC.

Most of the reason I'm even worrying about all this is I have a pretty big road trip ahead and am checking things out for the sake of safety. Part of me wonders if I've just been doing way too much in-town low-speed driving since covid started and I just need to blow the cobwebs out on the highway.

Still no codes!
 




J_C

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If you haven't yet, I might try disconnecting the battery to get it to relearn parameters, maybe this will change fuel trims? That's also something I'd do after replacing O2 sensors.
 




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