96 LX manual. Since replacing my TPS, I've been having serious issues with my IACV. Initially, when I received a bad replacement TPS, the IACV tended to stick open on upshifts, holding revs indefinitely (up to around 3.5K!), making it hard to get into the next gear, and extending braking distance in gear. I left the IACV unplugged till I got a properly working TPS.

Problem is, idle never went back to normal once the TPS was fixed. I tried the idle tuning procedure in the manual, and even replaced the IACV with no luck. No CELs showed up or stored in memory. The car runs perfectly besides startup (revs 1K too high for a split second) and idle (500RPM when warm, since IACV isn't assisting). If I up the throttle with the adjustment screw back to 700RPM, the car will idle normally until AC/accessories are used, and then will drop to around 550. The problem there is, revs will still be held a bit on upshifts, and will be delayed a bit from a stop, so that's not an option.

Here's the weird part: if I take off the intake hose and cover the top port inside the throttle body, the one before the throttle plate, the idle will start to increase. There's definitely vacuum there. When I let off of the port, the idle will shoot up, then settle back at 500RPM. The bottom port is somewhat similar, but doesn't have as big of an effect.

Any idea what's going on? Note that even before the TPS/IACV issue, the IACV never idled up when I used AC or accessories (fan, defog, vent, headlights, etc.), so this probably isn't a new issue, but it's at the point where it affects driving now.

 

Have you taken the battery off and reset the computer. The car could have "learned" a bad habit and is slowly relearning. But if you reset it, it might get better.

Just a thought.

 

A million times, at this point. I actually unplugged the IACV again last night, along with resetting the ECU. First time after the TPS was repaired, and the car freaked out. The car bucked like crazy until I got up to around 2500RPM. Absolutely undrivable, and I barely made it out of my driveway. Though I kept going to see if the ECU would learn, but it didn't. The throttle also had a massive dead spot once I was up to speed, and had a sharp cutoff once it finally accelerated/decelerated, whipping the engine back and forth. I might yank the ECU this week to see if everything's OK inside, as that's really the only other thing I can think of that might be broken.

 

It's a long shot, but a bad, or failing, Oxygen Sensor can cause the IACV to act like that because the intake might not be getting enough, or too much, fuel, for the load on the engine. In the case of a bad O2 sensor, the bucking and jerking is actually choking/starving going on in the throttle body.

How did you diagnose the bad Throttle Position Sensor?

 

My car started getting a dead spot right off throttle, and over the period of a year or so, it felt more like a slipping auto tranny off the line, it hit so hard. If I raised the idle a bit, the problem went away. Threw in a new TPS, and things were super smooth again. It's just the IACV that's acting up now.

About the O2 sensor, I replaced it 2 years ago with a Denso sensor. Would unplugging it for a test drive confirm if it's connected to the IACV issue? Also, I remember at one point in tuning the new TPS (before I got a scan tool), I got a CEL that contained an O2 sensor code (along with a couple of TPS codes). Didn't bother to remember it, since it never came up before or after that.

 

Took a quick drive with the O2 sensor disconnected, and the idle didn't change. The car was a bit slower and threw an O2 sensor CEL from the start, but that's it. Drove a bit with the scanner reattached, and below is a graph of the O2 sensor voltage. Flat high spots are closed-throttle deceleration and idle. Note that the scanner doesn't have a fast refresh rate (only $50).

 

Yeah, unless you really racked up the miles, that Denso O2 sensor should last longer than 2 years. So if nothing changed with it disconnected, something else is wrong.

The O2 code you got is because of unplugging the unit. I sometimes think Onboard Diagnostics only checks for the presence of the O2, but not its health.

You seem pretty handy with diagnosing things, so perhaps hook your scanning device up to the unplugged IACV to make sure its Ohms (resistance) are in spec. They can go out of spec without showing a code.
Maybe try these tests. Leave the key in the Off position.

Test 1. Setup to measure Ohms, Probe the IACV terminals (not the plug) and look for between 8 and 15 Ohms. Those numbers are for my 4th generation; your 5th generation may differ.

Test 2. Set up to measure Continuity, and check for continuity between each terminal of the IACV (not the plug) and a ground. There should be NO continuity. If continuity is found, the IACV is bad.

 

I’m having the exact same problem!!! Did you ever find out what the problem is or was? I could really use an answer to this my car is inoperable.

 

Always verify that the main engine harness ground(end of the intake plenum near the BMC '94-97) is attached and in one piece.
Verify the main engine bonding jumper(engine mount cable).
And the main ground from the battery cable are all in good condition, clean and tight connections.

A faulty main engine harness ground can cause a fauly like a TPS, MAP, IAT or any other sensor is going bad, replace and the problem 'jumps' to the next component.

Always back probe and look for any errant or weird voltage changes as well.
A loose connection can cause similar issues, check those connectors and pins.

 

On a chance, I found a super cheap used ECU on eBay ($22!) and decided to give it a try, since nothing else was making sense. It didn't solve the problem when I installed it, but the idle did try to twitch a bit here and there and raised up consistently by around 50 RPM from the 550 RPM default idle, which it never did before. The car felt a bit peppier at low RPM, too. After that I decided to throw the old IAC back in for giggles, and viola! Somehow everything worked perfectly! So I'm guessing it could have just been a bad ECU and a faulty aftermarket IAC (never actually worked). And the car runs smooth as butter right off idle to redline, with no delays in throttle input anywhere. Even between shifts, where it was most noticeable (jerky unless I slipped the clutch a lot).

Something interesting I noted on the used ECU: my scanner was able to access more internal testing modes on the new one than on the old one. Not sure if it's because it might be a newer ECU (last model # is 1 higher), or because the old ECU was pretty far gone.

 

I feel like I am seeing more and more ECU -related issues coming from these cars. I would not have guessed that. Very interesting.

 

Mike, is BMC = Brake Master Cylinder?

 

Yup. Description is for LHD vehicles(USA).

 

It's really not surprising as many of the cars over 25 years old now. The real problem in any electronic device especially in harsh conditions like car electronics is components called
"electrolytic capacitors" they are usually small (usually less than 1/2 in in dia 1/2 height) round cans with two leads coming out of the bottom, they are gel filled, when they become very old usually more than 20 years old they will usually either dry out or they will leak...this changes the value of the part and will cause all manner of strange problems..These can be easily replaced if you have basic soldering skills and the replacement parts are cheap.

 

It's just so interesting that this cannot be symptomatic of Honda vehicles, it makes me think that as any car gets to this age, it will be a new age of cars getting trashed and discarded, not because the mechanics have failed or are unsafe, but because there will be a long line of electronics failures happening which runs everything, from fuel injection to timing to air conditioning vents. And while we are dealing with this with one of the most sold vehicles and so people have enough of a group to look at and determine the issue, any of the lesser selling vehicles will just disappear. I find it so sad because these vehicles are well-made enough that they could go another 20-30 years mechanically since it's easy to rebuild and they will run for hundreds of thousands of miles between these big repairs, but it's the electronics that will be the Achilles heel and cut these out of daily driving. I suppose entire businesses will be setup around either rebuilding these OEM ECU's or there will be an aftermarket flourishing here, such as Megasquirt devices to bolt back into place. And unfortunately, unless laws change, you can't take a car and switch it back to a non-ecu setup without failing emissions or inspections in many places.

But maybe that's for the best as we move to autonomous driving vehicles, there will come a time where we won't be able to drive on a street any longer, only in race parks.