Need help from "car people" (Honda Civic)

I have an '06 Civic with 175000 miles, auto tranny. Till lately, it got about 40 mpg highway… Then seemingly overnight, the mileage dropped to 25 mpg. Oddly enough, the car still was running fine, no misses or hesitation in the engine, plenty of power, same acceleration and smooth operation.

After months of investigating possible causes (I move slow), I replaced the obvious: plugs, wires, both O2 sensors… to no effect. Eventually I took it to a pro for a scan diagnosis that revealed no error codes or abnormalities EXCEPT he thought the converter was running “a little cold, could be plugging off”. Compression was “excellent”. SOOO… when I gathered the necessary cash (weeks later) I replaced the converter along with a new muffler (it was time). Still no change… good operation, terrible mileage.

Flash forward a couple more months to this week. Now I notice a definite “wanky-ness” in performance. I’m experiencing a definite loss in power, the engine is reving higher than it used to before shifting, and when crusing @ steady speeds the tranny seems to range around, upshifting and downshifting more than it used to do.
Casual diagnosis would imply that the tranny is shot, and perhaps the cause of my grief. But if so, how would one explain all the previous months of flawless operation?

How are ya!?

I know its in obvious one but when has the transmission fluid been changed? How’s the color/smell?

Air filter changed? Check to see if throttle body is clean, then check IAT/MAF sensors or throttle position sensor. Sometimes these can start to fail without throwing a code either from wear in the the case of the TPS or the tip of the IAT/MAF being dirty. They can all seriously affect mileage and performance. If the converter isn’t throwing a P0420 code, I would not worry about that being the cause.

Doing well, thank you. You?
Fluid changed a year ago. Appearance: normal
New(er) air filter, air body: clean.

I have not however, changed any sensors except for the two O2s on the converter.

Have you looked at the brakes? I had a 90s Civic that had a stuck-on rear disc caliper and I think it took a pretty long time for me to figure it out. There was a smell from the overheated pads but it doesn’t always blow into the cabin. I noticed the terrible smell and a small amount of smoke by chance in a parking lot after a short drive. You and a friend could push the car slowly around in neutral with the engine off and listen to each wheel.

Tire pressures ok?

Bad fuel?

Pull one of the battery cables off for 10 minutes to reset the computer? (this is the go-to IT solution, turning it off and on again) Maybe it hasn’t learned that the brand new O2 sensors are reading slightly differently from the original ones?

Has the fuel filter been changed recently? If not, there’s one other thing to check.

Good to hear doing well! Doing great as well.
Those sensors may be the culprit. There should also be another O2 sensor on the exhaust manifold, but if no P0420 codes stored in the OBD/Computer then thats not likely it.

engineer_comp_geek may be right, if there is a removable fuel filter it should be changed if it hasn’t, or you have a leaky injector/one that dribbles. This is of course after the other sensors have been checked (you can get all 3 on RockAuto for way under $50). Its best to start with the cheapest fixes first. You can dump a few ounces of Marvel Mystery Oil* in the tank and see if anything improves at all. It will at least narrow down if its in fuel delivery or air flow/temp sensors.

To rule out the transmission, sit in the car and get it up to temperature then shift through each gear and notice the result. If there is significant delay, slamming into gear or any difference above of +500RPM on the tachometer when doing so it may just be the transmission. If these changes happen only at hot/operating temperature it could be the fluid is expanding and then foaming which causes symptoms of a failing transmission but not necessarily are happening (yet), if it happens at cold, too little fluid.

You can always add some Lucas Transmission Fix* to see if there are any changes, in a high mileage transmission like that it can only help and if it makes you uneasy you can always change the fluid out again (not necessary).

*I typically only use these additive products for diagnosing stuff, since the two mentioned actually are ones that work and not snake oil

During the “runs fine but fuel economy down” phase was it shifting into fourth gear, or only up to third?

I have a 2007 civic automatic and had almost the same symptoms. In my case, it turned out to be a vacuum leak - a very small one at the brake booster. I got no codes on the OBDII. I had no symptoms of deteriorating acceleration or engine rpm problems at low speed / idle.

It’s hard to find the vacuum leak without the smoke test, so I’d recommend going to a competent shop or the dealer.

Even if you don’t smoke you can do this at home. Unplug radiator fan if it comes on too quickly and light a cigar. Follow where the smoke gets sucked in.

That’s a great idea anomalous1, except it doesn’t work for this specific car because the brake booster is behind the air filter and very hard to access or even see the vacuum lines.

In answer to the previous questions/suggestions.
Brakes: OK. New all around.
Tire pressure: OK
Fuel: Good. Have put in three additives: Marvel Mystery, Sea Foam, Sta-Bil
Fuel filter: N/A (no external filter)
Tranny lockup & shifting: Smooth… no slop, slips, jerks, bumps, or grinds. It is utilizing all it’s “gears”. Just excessive up/down shifting at abnormal RPMs.

Before resorting to randomly replacing potentially numerous (and expensive) sensors I think I’ll look into…
Vacuum leaks!!!:smack: There’s an idea! I had that problem years ago on another vehicle. Not with identical symptoms, but somewhat similar.

:smack: here too. I forgot all about that. I never see it that often so I am always inclined to forget about vacuum leaks. Keep us posted?

am77494 Good call on the vaccuum leak idea, not intimately familiar with that car myself.

I agree that the excessive shifting does sound like it might be a vacuum leak.

The non-smoking method of finding a vacuum leak is to use some WD-40 or carb cleaner (if it burns easy and sprays, it will work) with a long applicator straw. Spray it where you suspect the leak to be, if the engine speeds up, you found a leak. Try to do it on a cool engine, and generally be careful over the exhaust.

Damn… I had such high hopes for a vacuum leak, but couldn’t find any. I used the WD-40 method with no indications of a leak. Perhaps I’ll try propane today just to be sure.
At least now the engine compartment is nice and shiny.

…and you haven’t written back. Hope their wasn’t an incident with propane :stuck_out_tongue:

If nothing else, try those sensors. MAF/IAT or applicable. If that isn’t it then I would also be inclined to say it was a problem with the transmission, slipping band or the torque converter. Way too costly.

Are you sure the car is coming up to proper operating temperature?

No problems with the propane late today, but same end result…nada.
I ordered MAF and MAP sensors this evening with a new PCV to boot.

Within reason.The factory gauge responds as it always has, and the radiator fans cycle on and off. The engine compartment and components seem normally hot to the touch.

Hmmm, yeah, if it’s an auto transmission issue, the bills can be costly. If it does end up being the transmission, you’re looking at a question of how much you really like this car to consider a rebuild or a low-mileage donor vs. just spending the extra money in gas until it dies and leaves you stranded.

Since you mentioned the excessive shifting, I don’t think the MAF/IAT is going to make any difference, but I could be wrong and the cost is minimal compared to a transmission rebuild. Let us know what happens.

For my car, 2008 Civic, I couldn’t find the vacuum leak either using the WD40 or Propane either. This is because the booster is well hidden and it’s hard to spray or take propane near it or the associated hoses.

When you take it to the dealer, they are going to pressurize the vacuum system with smoke and the leak will be visible then. Good luck.