I have a hesitation/miss or something in my truck under certain conditions. Specifically, beginning a climb on a steep hill. Its fairly violent. Googling indicates that a vacuum leak may be the issue and, in fact, I sometimes hear a whistling sound that is probably just that. It comes and goes and I can’t reproduce it at will. My question is: what role does the vacuum play in engine performance? Why is doing this under load but not while cruising? The truck is going in a few days to get checked for leaks and a new fuel filter and I just cleaned the MAF sensor. Fight my ignorance.
It’s not the “Vacuum System” that is the issue - it’s the excess un-metered air that is being sucked into the engine, messing up the air/fuel ratio.
Engine Vacuum is a side effect of the pumping action of the engine. It is useful as a control system, for vacuum motors (like, heat blend doors), but the system needs to be pretty leak-free.
If there is a leak, the air will be sucked into the engine without being metered by the Mass Air Flow sensor. That means that the computer will calculate the wrong amount of fuel to inject, leading to a very lean condition. It can do some amount of compensation by looking at the readings from the oxygen sensors in the exhaust, but that’s not ideal.
Is your Check Engine light on?
Yes, the CE light is on. A code reader tells me its an evap leak. Its had this for years. The mechanic said that tracking these leaks down can be expensive and they have no impact on performance. I know, its bad for the environment but I rarely use the truck. Its 20 years old and I don’t want to dump a bunch of money into it. The catch is that if the light is always on, I don’t know when a new code is being thrown. I recently went to clear the code prior to state inspection and saw a new code - a misfire on the #2 cylinder. I cleared both codes, passed inspection and the misfire code hasn’t returned. The evap code did after several days of driving, as usual. So, where is this low pressure that causes the vacuum being created? It seems this leak would have to be between the MAF sensor and the intake valve, right? Where would an air leak likely be in that case? I guess with corona-related free time on my hands I should read up on this. Its not that I need to know, I just want to. i’ll let you know what the mechanic finds.
Wiki does a pretty good job of explaining it. I should have Googled first!
If you can hear it whistle, you should be able to find the leak - use a piece of hose as a stethoscope to isolate where the sound is coming from.
What make / model truck?
2000 Explorer V8. Its in the shop right now. I asked for a new fuel filter while its there. The problem is that I won’t know if the problem is fixed until I drive up some fairly steep hills and there none for about 60 miles. The whistling sound comes and goes. It normally stops after a few seconds of being stopped at a traffic light and is barely audible over the sound of the engine or road noise. In any case, its not constant and I can’t “make” it happen.
It can be, but a bottle of brake cleaner or a butane blow torch can often find it in a matter of minutes.
Mechanics use a smoke machine that fills all the various cavities with smoke (or rather, vapor, much like an e-cigarette). Then you can watch to see where the smoke pours out of.
The one the mechanic has is expensive, you can make your own at home for a few dollars. The only expensive thing you’d need is a source of compressed air.
Scotty Kilmer can show you how to do it with a cigar/cigarette, but I wouldn’t trust my lungs to be able to push the smoke far enough into the system to find a leak reliably.
If it was me, I’d start by seeing if I could track it down with some brake cleaner or a blow torch. It’s quick and easy (and mostly, sorta safe) and you might find a broken hose or a cracked fitting somewhere that you can replace easily.
You can, with mixed results, do the same test with a light misting of water as well.
It’s much harder to use brake cleaner to find leaks these days.
Before computer-controlled engines, adding extra fuel would make the engine race, but these days, the computer reacts so fast that it’s hard to notice the change in the idle speed. I had a rather enormous leak in the PCV elbow, and I “thought” I could detect a change in the engine RPM when I sprayed solvent near it, but was never sure. I finally found it with a stethoscope.
Agreed. I just tossed it out because it’s something quick you can try that might get you a result, and a lot of people have something in the garage they can use (brake cleaner, blow torch etc).
brake cleaner isn’t combustible. you’re thinking of carb cleaner. it can be used to check for external vacuum leaks and the engine should respond to it (some vacuum leaks are internal and can’t be found with this method).
your mechanic was correct that the evaporative emissions typically don’t cause the type of problem that you’re experiencing.
wild ass guess but your (OP) problem sounds more ignition related to me.
it’s in the shop now it sounds like. they shouldn’t need our help diagnosing it (and won’t even be impressed if we guess what it is correctly).
let us know what they figure out.
The mechanic says that there is no vacuum leak. I’m not sure how he determined this, yet. The wife made a call to see if he was done yet and this is what he told her. I had also asked for a new fuel filter to be installed and he is waiting for the part. Cmore - if it is ignition-related what could account for the symptom (under a load climbing a hill)? My next move may be to replace the coils. I have a vague memory of replacing one or both of them years ago due to a crack after similar/identical symptoms. I forgot all about that until just the other day. I’ll have to see if I have a record of this somewhere.
Don’t just throw parts at the problem - that’s just a waste of money.
Use diagnostics, like the long-term fuel tables, and any pending or current error codes to track down the issue.
Sure it is, you just have to get the kind that says ‘extremely flammable’ instead of the one that says non-flammable. They usually sell them right next to each other in the store.
I always get the flammable kind just so I can use it like starting fluid if (when) I need to.
If I were you, my next move would be to go rent a fuel pressure gauge and make sure it’s getting exactly what is required.
On one my trucks, it should have been getting, IIRC 55psi. The pressure gauge said 51psi so I figured it was fine. I eventually replaced the pump, the PSI was back up to 55, and all was fine. But I spent a long time trying to figure the problem out. Now, this was for a Vortec engine with a spider injector which is considerably more sensitive to low fuel pressure than other types of injectors.
The problem I was having was that the truck would start lugging and shaking and sounding like it was going to die and the speed would drop any time I punched the gas pedal. It made for a real nightmare trying to make a left across a busy road. See a spot, punch the gas, roll through traffic at 2mph.
I’ve only ever seen two codes on this Explorer - evap and a miss on #2 cylinder. The miss code hasn’t returned after being cleared. The real nuisance is that I can’t get the hesitation or whatever it is to repeat on demand (without driving for 90 minutes or more to get to a steep enough hill) so the mechanic can get a sense for what is going on. I could just live with it but when I do drive in the mountains I’m often pulling a trailer (the reason I have this vehicle) and I fear breaking down. That would be a huge pain in the ass. To be clear, I don’t need to be pulling the trailer to have this happen. As a matter of fact, I don’t recall it happening while pulling the trailer but its been almost two years. I turn the OD off while pulling so maybe its tranny-related? On the other hand, when it does happen, its not when OD would be kicking in. Coils are cheap enough and I can do that myself. After that and with nothing else to do because of COVID, maybe I’ll head to hills for a test ride.
I’m assuming the mechanic pulled the freeze frame data. That tells him what’s going on when the MIL turned on.
Before you throw good money after bad, maybe pick up a scanner that has live data. Ask the mechanic what data PIDs he’d be interested in and record them for him. Either get a screen recorder or be ready to take some screenshots. Next time you’re out in the mountains, get it up and running and you’ll be able to show him exactly what was going on when it happened.
Also, before you buy new plugs and coils, just swap the number 2 coil and plug with another cylinder and see if the misfire follows the plug/coil or stays with the cylinder. If it follows it, then you have to figure out if it’s the plug or the coil. If it stays with #2, it’s likely either something to do with the fuel going to that cylinder, or something to do with the electrical signal to the coil.
Also, google your problems and codes with the your make/model/year and see if anything interesting pops up. For example, on my dad’s truck, he’s having a lot of problems with the #6 cylinder. Turns out the way the valve cover is designed, it splashes oil directly at one of the valve stems for that cylinder and it fouls the plug after a few thousand miles.
You might find something you never would have thought of, but turns out to be a common problem.
I believe that the OP will find that the problem is with one or more of the coils.
Different car, different engine, but when I had a coil get blown by a bad spark plug wire it acted the same way. Ran fine but would shudder under acceleration.
A caution on brake cleaner.
We have a vehicle repair department and electronics repair department where I work.
Mechanics have used brake cleaner on various items besides brakes. It can melt a lot of plastic items. Be careful.
I’ve been on the Explorer forum and got a bunch of, “It could be x, y, z” etc. but no “This is a known/common problem” kind of replies. I have been Googling and YouTubing and had no idea the amount of data available with the “freeze frame” feature. Whoda thunk? By clearing the codes before I took it to the shop, would I have deleted the freeze frame data? The code reader I have is the most basic model. Maybe I’ll invest in a better one like Joey P is talking about. Any suggestions for a non-mechanic?