Bad fuel mileage/slightly high fuel trim

[Juanhmi]

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).

 

[J_C]

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]

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]

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 ASIN B0181EDCZ6
or cheaper at Harbor Freight but i don't know if the quality is the same: Multi-Use Transfer Pump

https://www.youtube.com/results?search_query=DIY+smoke+tester

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]

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]

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 onto the Wrangler and clear the garage.

 

[Juanhmi]

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]

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.

 

[Juanhmi]

Well, punting this one back up as I'm back at it. The original problem in this thread wasn't bad enough to really cause any issues, but it does seem like it's become steadily worse and finally has thrown lean running codes. Since then, I've done upper and lower intake gaskets on this engine, valve cover gaskets, checked all vacuum lines a million times over, done the EGR solenoid, and a handful of other minor jobs. Took fuel pressure today and it's a lousy 48 psi. I'm suspecting most everything is healthy here except for possible bad injectors, pressure regulator, fuel pump or filter. Seems to me like if a pump were gonna quit it would just quit instead of getting weak, but I'm not too experienced with how they fail. Going to start with a new K&N fuel filter as it's been about three years and 30-40k miles since the last one.

 

[Fix4Dirt]

Well, punting this one back up as I'm back at it. The original problem in this thread wasn't bad enough to really cause any issues, but it does seem like it's become steadily worse and finally has thrown lean running codes. Since then, I've done upper and lower intake gaskets on this engine, valve cover gaskets, checked all vacuum lines a million times over, done the EGR solenoid, and a handful of other minor jobs. Took fuel pressure today and it's a lousy 48 psi. I'm suspecting most everything is healthy here except for possible bad injectors, pressure regulator, fuel pump or filter. Seems to me like if a pump were gonna quit it would just quit instead of getting weak, but I'm not too experienced with how they fail. Going to start with a new K&N fuel filter as it's been about three years and 30-40k miles since the last one.

usually the pumps start to lower pressure when they get hot. they usually progressively fail, not just drop... usually.. how long since the lst pump? im on my 3rd i think at 320k (this last change was preemptative)

 

[Juanhmi]

Good question. I'm at almost 250k miles but I'm not the original owner, got it around 190k. For all I know, it could be the original pump. I'll still change the filter and see if my pressure goes back up, but I'll be prepared to do a fuel pump. Never done one before. Am I looking at a real bear of a job?

 

[Fix4Dirt]

pump aint hard. i can do it in a day with lunch taking time, as long as nothing comes up. remove the lines, and put a jack up against the tank (use a board in between to prevent dent. lower it slightly, and then disconnect the lines as its at an angle, they can be a pain to remove, use a metal tools not the plastic ones. then lower it to the ground. then its as simple as unbolting the lock ring for the pump and remving it and replacing. install is reverse of removal... im probably omitting steps, but should get you close. it really aint bad. if its original, my money is on the pump

 

[Turdle]

Try cleaning the maf sensor, and try a normal paper filter. The oil on in filters can get hot and contaminate the maf.

This might be the whole problem.

 

[Juanhmi]

I do actually have a new clean MAF and while I like K&N for oil and fuel filters, I agree that their oily air filters are not great for sensors so I use normal paper Motorcraft filters in this vehicle. Been monitoring readings on this MAF sensor via OBD2 live data and it's been pretty normal. 48psi on fuel pressure is definitely my biggest lead so far...

 

[Fix4Dirt]

has it ever died when hot? usually, thts one of the first obvious sign (as in, something everyone woudl notice, not just someone who watches their fuel consumptions and trims... an average joe)

 

[Juanhmi]

Once I really warm it up on the highway, the idle is a little rough once I come to a stop and fuel trims will max out at 25% on both banks, throwing those lean codes. Guess that would point to the pump getting hot and not just a clogged filter.

Did a 900 mile road trip the past few days and at a rest stop it was getting so weird it had a few misfires. But it's never completely shut off. Does seem like the symptoms (when hot) have progressively gotten worse. Now that I know my fuel pressure is a little low on a cool day I'd be interested to measure it right after getting off the highway on a hot day.

 

[C420sailor]

Since you do road trips with it, and you’ve got 250k on the clock, do the pump.

If it’s the pump, great.

If it’s not the pump, it’s one less thing to leave you stranded far from home. Doing a pump on the side of the highway with a full tank of fuel is a really ****ty time.

BUY A QUALITY PUMP. Bosch is a favorite here.

 

[Fix4Dirt]

Since you do road trips with it, and you’ve got 250k on the clock, do the pump.

If it’s the pump, great.

If it’s not the pump, it’s one less thing to leave you stranded far from home. Doing a pump on the side of the highway with a full tank of fuel is a really ****ty time.

BUY A QUALITY PUMP. Bosch is a favorite here.

this, oh so much this!!! may i add. many when they do the pump, also cut a hole in the floorboard to access the pump easier next time, simple as just removing your access panel and changing it then and there. im not a fan of holes (heh...) so i didnt do one, but i know of many people who have. it makes it easier in the future, but thats up to you if you wanna do. personally, i usd a delphi pump and have no complains, but cant go wrong w bosch either... just no bosch spark plugs

 

[Juanhmi]

Understood. As a guy who carries a starter and the tools to replace it just in case, I'm all about preventative maintenance. Better to monitor fuel pressure and trims before a thing quits than to be surprised by a catastrophic failure. Ownership of this thing has always been about doing small jobs that probably need to be done so they don't leave me on the side of the road. So far, I've never been stranded by this rig.

Anybody got the best link to a Bosch fuel pump?

 

[Fix4Dirt]

i got my delphi from rockauto, maybe check there for the bosch one! fuel pump is great reventative maintainence at this point and time, as well as miles. no fuel=no start=stranded. and if it aint the pump, use your old one as a spare just in case never know... stay vigilant