93' civic vx. Federally.
So I've had a perplexing thing happen. This has happened before, but I didn't have a gauge to prove my theory.
After driving for about 24 miles, (it was -3at the start of my drive) I was traveling on a flat plane at 40 mph and holding 65mpg on the gauge. Suddenly, an increase in low end torque (I'd estimate about 1/4-1/3 more) and big increase in mpg's. The gauge shot up to 104mpg and stayed there. The car started to accelerate. I hadn't changed throttle position the whole time.
The rest of the drive home, I could hold much higher mpgs at higher speeds and with much less effort up hills. The remaining 10 miles raised my trip mpg by 7mpg's. (total about a 34 mile trip or so)
I have noticed this happen several times on long drives. After 15-20 miles all of a sudden I start getting much better torque without losing lean burn. I couldn't prove it was reality (and not my head) until I was staring at the gauge when it happened.
So, honda experts, what happened?!?!?
I'm thinking vtec. I'm going to head out to the garage now to test the electronics side of it and to pull the valve cover to inspect the components. I've been reading that if the solenoid fails, it fails in the "higher power" mode. That is to say, it fails in the more aggressive, 16valve mode as opposed to the less aggressive, better swirl, almost 12 valve mode. The ecu turns off the solenoid to achieve "higher power" mode. Is this true?
If so, it may explain the problem. However, I have not found any codes so that makes me confused. The book says it will show a code if it fails.
This brings up another question: If I unplug the vtec solenoid, would the car run all the time in the "lower power economy" mode?
So the big question is, is the problem mechanical or electrical or both?
Any other possible scenarios? Could it be something other than vtec? Could it be the ECU? I have a spare one of those I can swap in.
Thanks for the help!
Okay, three other things:
I was in 5th gear at the time of the incident.
I checked the electrical side of things from the haynes manual. The only thing that was supposedly wrong is that there is continuity between ground and the ECU side of the plug to the solenoid under the hood. (not the pressure switch) This connector has one single wire. In the key off position, ground is present. Is this right or sign of a short?
I have never felt what people call VTEC "engaging." That is to say, no sudden boost of power at 2k-redline. Even under full throttle in first gear all the way to redline (just got done seafoaming it) no sudden boost in power. Perhaps another symptom of a not working vtec system? Or a symptom of bad ecu? I haven't tested the ECU side of things yet.
I tried that when I got home. No beans. Just a check engine light.
Here's my current theory:
The mechanical end of my vtec is stuck in 16v mode all the time. THis would explain the very low torque usually available at low rpms. I would explain the less than awesome FE. It would explain the rough idle.
So, to test this theory, all I need to do is pull the valve cover and spin the crank. If all 4 valves operate completely on all cylinders, I'll know the problem. However, it could be the oil pressure switch keeping it locked in 16v perhaps. If that was the case, it would only be evident if the car was running with the valve cover off. That would be messy but it would only take 5 seconds to determine if it were true.
Assuming it is true, then a long drive getting to full operating temp and keeping it there a while would somehow disengage back to 12v mode until the vtec system needed to be activated again (for instance if I needed to accelerate from a stop to 55mph going up a hill).
What could cause to stick? I'm going to start pullng it apart I guess. More input welcomed!
I wish I could be more help but I have never owned a VX. A friend of mine owned one about five years ago and was having issues with the VTEC solenoid. But as I remember it was stuck in 12v mode and had no power from 3000rpm on.
From what you describe it sounds like your diagnoses is right. Do you have a spare solenoid that you could block off the VTEC gasket oil filter screen to simulate a plug screen to see if you would have 12v mode all the time? For that matter even a spare VTEC gasket with the built in screen that you could block off.
Then if it went and stayed in 12v mode all the time you could rule out the mech. valve-train as the cause.
EDIT after reading your other post if the car change its power out put and fuel mileaged then the mech side of the valve train is working.
I like the idea of doing a visual inspection with valve cover off/engine off.
If you are lucky- maybe your oil filler hole is on rear (intake side) of the head... That might allow you to peek through the hole and see the two adjacent intake rocker arms of one cylinder.
It looks like vtec-e uses oil pressure to open the 2nd intake valve (http://www.b18c5eg.com/VTEC/sohcvtece.html) - so keeping it from doing so would keep you in the economy high swirl mode as long as you didn't ever need the power.
The factory service manual says that the vtec spool valve should have a resistance between 14-30 ohms. So, you could go to radio shack and find a such a resistor and attach one end to ground and the other end to the wire from the ecu. This would not trigger a CEL.
About 10 years ago, I used a pair of radio shack resistors to keep the CEL off on my Acura Integra which had a bad idle air control valve (the engine would idle way too fast unless I unplugged the connector). I wasn't knowledgeable enough at the time to know I could remove it and clean the valve.
I pulled the valve cover and spun the crank manually. All cylinders are operating in 3 valve (12 valve) mode with the engine off. I put the cover back on (more work than it sounds like; one of the head bolt threads I helicoiled broke in half at the casting. I had to drill it all the way down to the end of the hole and out the bottom. I then re-tapped it with a slightly smaller 1/4"x20 tap. I used a longer 1/4"x20 bolt and re-used the gasket/washer assembly (which I had to cut the bolt to get off intact) to secure it. Good news is, the valve cover doesn't leak anymore because I could finally put factory torque back on the bolt. Bad news is I have an sae bolt head on my valve cover now...
Anyway, through the oil fill hole I observe the number one cylinder's intake valves. Sure enough, only 1 is opening at idle. (12v mode) At least during revving with no load, it does not switch to 16v mode.
I went further and popped on a spare vtec solenoid I have. No change in driving characteristics that I can see. A curious thing though; that extra torque I got last night was not noticeable at idle. It was really only in 4th or 5th gear around 40 to 55 mph. I can't tell today if it's now got that better torque all the time because I haven't taken a highway drive (thus trying to maintain lean burn while ascending a small incline). Although, it does feel like it has that torque at that speed/gear now. I can't tell. I have a long drive tonight and will report back findings on that front.
I went further to install a 12v test light in the cabin of the car for the vtec solenoid. Sure enough, at 2500 to 3500 under heavy throttle, it lights up and seems to work. That implies that the electrical side of things is working just ducky.
I also took apart the rest of the spool valve. There is a spring in the bottom, a sliding pin that either lets oil past or not, and a small round fat washer looking thing with a hole in the middle of it that is not centered. I did just do a seafoam treatment including crankcase seafoam just prior to an oil change. Perhaps the sliding pin was stuck in the open position and is now free to move as it should? That's just speculation at best.
Another theory is that the spring may not be powerful enough (worn out) to keep the valve closed after it has been opened once by the solenoid. It seems to have enough energy to hold the valve closed, but I can't be certain. The parts motor will have a spool vale I can swap on to test this hypothesis.
Good idea on the resistor front. I'm thinking now that the electrical side of things is functional, but perhaps the mechanical side was up until recently not functional or that under high load (and thus high oil pressure) the solenoid may have opened on it's own without 12v thus activating 16valve operation.
Blocking off the orifice with something solid (like a fabricated alloy plate I could make and gasket material) would prove or disprove the oil pressure theory. Hmmm.
I'll post again after tonight's drive and give a synopsis.
Thanks for all the input!
Okay, so plot thickens.
On the drive tonight (40 miles each way) it was snowing heavily on the way there. It was also very windy (gusty, not one direction). The extra torque was there, no doubt. I used the block heater to bring the car up to operating temp beforehand. The traffic was heavy and traveling slowly then fast, etc. This trip was 63.3mpg.
On the way home, the car had to warm up on it's own. There was zero traffic. The speeds were about 5 mph faster overall. No snow coming down, the roads were clear as well. The new found torque, however, was absent until the last 3 miles of the drive. This pushed the FE down to 55mpg for the same trip in the opposite direction. No, there was no significant change in elevation from point at to point b.
So, I don't know what is different, but something sure is.
It's possible it has nothing to do with vtec but I really don't know. I have not yet looked in the valve cover and found two valves opening at idle. I've been checking consistently. I'll check it tomorrow at stone cold when oil pressures are likely highest.
What the heck else could cause this?
Maybe a cylinder not firing (or getting adequate fuel) during lean burn?
I'm totally out of ideas. Whatever it is, it is the cause of my less than stellar FE after all these mods and driving changes.