ben_tech

Update: Fixed

Hi everyone!

So I have an up until now faithful 1994 Accord EX 4-door Automatic with VTEC engine. All of a sudden about a month ago it started surging intermittently and it's gotten worse. I thought it was an idle issue at first (CEL code 14 persists to this day) but now it's doing it all the time even when the throttle is open. The RPM goes up and down, RRR - RARR -- RRR --- RARR -- you get the idea. It's bad enough that the engine moves back and forth like its nodding it's head.

I've tried everything I found online as a remedy. Here is what I've done so far, some of which seemed to fix it but after a day or two it disappointed:

New spark plugs NGK iridium

New spark plug wires NGK

Royal Purple injector cleaner added to premium gas

Cleaned out original Idle Air Control Valve

Adjusted Fast Idle Thermo Valve

New distributor cap and rotor (did this twice, Duralast then YEC)

New air filter

New pcv valve (oem)

New antifreeze/coolant (oem) -- bled system too, after every part change involving coolant lines

Replaced Idle Air Control Valve with New part

Replaced Fast Idle Thermo Valve with New part

New synthetic oil and filter

Disassembled throttle body and cleaned it out very very well including both sides and edges of butterfly and IACV passage, replaced TB gasket

New Throttle Position Sensor calibrated to .5-4.5 (original one tested fine too)

Tried removing vac line (plugged it) and wiring connector from EGR, no change in symptoms

Tried removing wiring connector from IACV, no change in symptoms at speeds above idle

Tried adjusting idle speed screw (paid a mechanic for diagnostic and that's all he came up with unfortunately) which seemed like a fix but wasn't.

Tried placing hand in front of open throttle body and engine stalled completely and quickly after hand was sucked up to the opening, my understanding is that pretty much eliminates a vacuum leak as the culprit?

Little to no suction on lower port in TB (FITV)

Lots of suction on upper port in TB (IACV)

ben_tech

Forgot to mention I did try spraying carb cleaner around on all vacuum interfaces I could see with the engine running. The back of the manifold gasket was out of reach. Not 100% sure but don't think it's a vac leak. As mentioned anything else I can try (test procedures) would be appreciated.

MAD_MIKE

If you have an IACV issue(setting the code) then check the wiring to the IACV. Check the engine harness ground(on thermostat housing) is not corroded, broken or loose. Backprobe with a voltmeter the IACV and see if the ECM is commanding the IACV to open/close you will see a change in voltage that should corrospond with the engine revving.
Verify all accessories, AC, heater, fans, radio, lights, rear defrost, etc is off when testing.
With the engine warm, disconnect the IACV and see if that stops the revving. If so then there is an issue with the IACV circuit that needs to be addressed. Either the ECM is commanding the rpm changes or there is a problem with the wiring.
If no changes occur with the IACV disconnected, verify that it is receiving voltage at the connector.

If the MAP sensor is faulty it may be changing the voltage slightly, the ECM see's this voltage change as an added load and tries to compensate with the IACV.

IACV can affect engine rpm in the lower rpms as well. Until the throttle is opened to let in more air than the IACV can bypass, it will still have some control in the lower rpm range.

ben_tech

Thanks so much Mike, I'll get into that as soon as possible, sadly I came down with a bad cold but getting this car back on the road is a high priority.

All I did so far last week is make sure the wiring harness to the IACV is receiving battery voltage (with connector disconnected) - it is, nice and steady 12v. I'll get into the back probe and ground and so on and reply to you in a lot more detail on everything you suggested as soon as possible.

Thanks again for your help.

Scott Little

I pretty much did all of those things on my wife's Civic as well and it finally stopped once I replaced the O2 Sensor. I cant speak to that being a sound troubleshooting step but it worked for my B20.


After I changed to the Intake manifold to the F23 one in my f22b1 accord I was battling the issue again. The thing that finally resolved it was capping one of the FIA lines and capping the Intake Manifold side.

ben_tech

Mike, here are some test results from the IACV backprobe I did today:

engine cold:

both idle and open throttle surging/rough
13.4v constant when acting up
7-8v when behaving for a moment

engine warm:

4.4v constant regardless of throttle position (behaving now)
idle smooth most of the time
surging problem arises intermittently while driving or idling

disconnect IACV: virtually no change, idle drops maybe 100 rpm

I also noticed when the car is started (engine warm) that it bounces up and down between 1500-1600 RPM (sounds like the surging issue) then up to 2500 RPM then down to normal idle speed. When the car was healthy it didn't do that.

I looked at thermostat housing but didn't find the ground. Used a digital camera with a flash to take a few pics under it but don't see a ground in those pics either, probably my fault wrong angle. It's dark now, will try again to find it tomorrow. Will check service manual to see if this ground is illustrated somewhere. I did loosen and tighten a ground on the side of Manifold B a while back, that one looked fine.

As an aside, this car is totally stock, no mods.

Senorsquez23

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ben_tech

Spent all afternoon on this today.

Tested IACV harness yellow/black wire to ground got battery voltage constant - PASS

Looked at engine grounds G1 G2 and G101, all look tight and no loose wire strands or any obvious corrosion

Tested MAP sensor. Results:
5v - pass
gnd - pass
signal - 2.91v decrease gradually to 2.79v with suction by mouth then back to 2.91 - pass

Cleaned EGR passages in manifold - one passage was blocked completely with carbon - big job

Cleaned EGR valve

End of the day, no change, she's still bucking like a mule (CEL code 14 as always).

Sprayed carb cleaner all around Manifold B gasket as best I could with poor access to back of it. Felt all around it with bare hand also, hoping to feel a breeze. Didn't notice anything.

Worth repeating (or in case I didn't mention it yet) that the engine bucking still happens with the IACV harness disconnected. Idle is lower as expected and smooth but opening throttle brings back the bucking. With IACV connected the bucking happens at idle when the engine is cold but when it warms up it seems to happen more with the throttle open, but sometimes at idle too (intermittent). Today it was really bad (weather: cold, rainy).

Any and all testing/troubleshooting tips would be most welcome.

ben_tech

Someone told me it could be the coolant temp sensor. I've been looking around at all of the sensors in there and I'm not quite sure which one.

I took a picture under my distributor cap and circled the sensor I think is the coolant sensor specific to the ECU:

Is that the right one?

Can this be tested in place? I'd test the harness voltage and ground with the key in the accessory position and I assume ohms on the sensor while the car is off. Can I backprobe the electrical plug while the car is running and see the sensor vary the ohms reading as the coolant heats or does it have to be measured without the electrical connection attached? I saw the hot water technique but you have to remove the sensor to do that and I'm a little tired of draining and bleeding coolant at this point.

Also today I did clamp a piece of cardboard in between my Fast Idle Thermo Valve and the throttle body as I read that's a way to eliminate that part as a suspect. No change.

I'm also looking at doing some continuity tests between the Idle Air Control Valve wiring connector and wherever it connects to the ECU/ECM. I have the Honda shop manual but quite frankly I can't find anything in that thing. So if anyone can point me to where the other end of those wires are that would be great.

Finally, does anyone think the o2 sensor could be the problem even though the only code I'm getting from the check engine light is 14 (Idle Air Control Valve). There are codes specific to the o2 sensor if it is bad....

MAD_MIKE

Running out the door so this will be a short post.
Engine vacuum at idle will be ~20-25inHg with the throttle closed, your lungs cannot pull that kind of negative pressure.

Beg, borrow, steal a hand held vacuum pump. And put a proper vacuum on the sensor. You want to test it at operating values. Your gonna want to see ~ .5V change per 5inHg change.
If the sensor is faulty it may have an air leak and not hold vacuum or it fluctutates voltage at higher vacuum.
Properly, fully test a component before calling it 'good'.

And yes, the larger sensor is the ECT for the ECM. Smaller one is for the gauge.

poorman212

Sorry, I admit I did not read all of this but if you have a code for the IACV (14)....solve that.

ben_tech

Today I accessed the ECU and backprobed pins A9 and A23 as the code 14 troubleshooting flowchart indicates.

Apparently on the OBD1 ECU's the numbers are odd numbers on top and even on bottom. This picture helped me:


Every time I connected the two pins the IACV clicked. The chart says to then substitute a known-good ECU. As I don't have one handy I opened up the ECU and studied the components and traces. I noticed several capacitors had leaked a bit of electrolyte - not unusual for old electrolytic caps. Upon closer inspection I noticed that capacitor C11 actually had one leg broken or charred all the way through. Here's a pic:

Could perhaps be an intermittent contact and getting worse which would explain the intermittent and increasingly worse symptoms.

Leaking cap (same value), part C17:


These are both 33uf 35v caps. There is another 33uf 35v on the board that is not doing too great either.

So I'll order some of these caps and desolder the old and solder the new and see if it's a miracle or not. If not there would of course still be some doubt as to whether the traces or other components were affected which still would point toward the ECU possibly being at fault. We shall see. If not, I'll pursue the sensors.

ben_tech

Here's what I did today on this:

- Replaced the compromised old Nichicon 33uf capacitors on the ECU circuit board with new Nichicons of same value. No immediate change in symptoms.

- Tested with transmission in Neutral, no change in symptoms. Presumably since it's still acting up in Neutral and I haven't seen any flashing from D4 (although I haven't jumped the transmission service connector) it's not the transmission.

- Back probed the engine coolant temperature sensor connector. There was no way I could get multimeter probes in there by hand with sufficient accuracy to maintain contact with the pins inside the sensor. Instead, I used needles to backprobe the connector and attached alligator clips to those. The measurements were taken with the electrical connector still plugged into the sensor. The reading was 800 ohms or so with the engine off and stone cold. I then ran the engine hard to reach max operating temp (fans on several times). Turned engine off. 140 ohms. Increased smoothly up to 200 ohms or so within a few minutes as the engine cooled just a bit (still hot).

- Unplugged several vacuum lines from the throttle body one at a time, plugging the holes on the throttle body to see if the engine smoothed out. No changes.

- Did the full idle speed adjustment routine again:
1) Unplug service connector and apply jumper
2) Unplug IACV electrical connector
3) Turn car on
4) Wait about 30 minutes for engine to heat up fully, fans on a few times
5) Set idle screw so that RPM tach hits just below the second notch on the gauge, about 700 rpm
6) Turn engine off
7) Pull two fuses (backup and ECM) for a couple minutes to reset ECU
8) plug in service connector
9) Plug in IACV
10) Turn on car and let idle for 20 minutes with everything off and not touching throttle for ECU to relearn

The symptoms are always better when the engine is thoroughly heated up but very bouncy between 1.25K and 1.5K RPM.

There's the fuel pressure, pump, filter to look at I suppose. Also I did notice some liquid maybe water (no odor) on the ground under the tailpipe and around the engine valve cover gasket, may be just condensation. Could try water remover in fuel just in case.

There's also that 220uf cap in the ECU that looked like it had leaked a little bit, didn't have a replacement for that. Leaking a little only causes a cap to be slightly off value though not sure that would cause the problem.

Could get a replacement used ECU off of ebay perhaps for testing.

Sucking on MAP sensor may have been inadequate test, could get vacuum pump to test it more thoroughly.

ben_tech

Today I decided to test the Engine Coolant Temperature (ECT) sensor more thoroughly.

I drained the coolant (will re-use it later).

Then I took the ECT off (tricky to reach but loosening up the wiring harness in front of it made it possible to get a deep socket on the ECT then jam a ratchet in there.

Once I had it out, I first put the ECT sensor under a cold tap -

Cold tap: 3700 ohms (maybe a little low but I didn't have ice)

I then let it warm up to room temp, the reading ranged smoothly up to:

Room temp 21 degrees C : 2000 ohms (good)

I then heated some water -

Hot stove water at exactly 100 degrees C : 154 ohms

154 ohms is out of tolerance. This part is supposed to be between 200 and 400 ohms, about 300 ohms at 100 degrees C.

Whether that would be the problem here I can't yet say after so many dead-end leads on this project.

I ordered Honda part number 37870-PK2-015 since the knockoffs had such bad reviews. Will report back if this fixed the issues on Thursday.

Also ordered a cheap manual vacuum pump to try out Mike's MAP sensor idea in case the new ECT is not a cure.

holmesnmanny

That is incorrect. To set base idle, you need to set it to 500-600. Anywhere in there will be fine. Setting it to 700 would cause it to not be able to idle correctly.

Everything else that you're doing appears to be fine, yes it's really important to make sure it's warmed up.

Keep in mind that the very first white line after 0 is 500 not 200 so when you have it at the first white line after that it will be 600 which is what you want. In between 500-600 the car will act like it's going to die, this is what you want.

If you still have a code 14 then it's possible the connector for the iacv has broken pins. Look inside at the two metal pins and make sure they are not worn or bent or even broken.

ben_tech

I did the click test from the service manual troubleshooting flowchart for the IACV so I think the wiring is fine. The test involves going right to the ECU connector and activating the IACV by creating a circuit there. A9 (blue/black) to ground. I suspect a bad ECU unfortunately. I'm trying a new ECT sensor tomorrow which was a bit out of tolerance then I'll try to get my hands on a different ECU. On the idle I believe the screw was turned all the way in before and it didn't make any difference but I'll be sure to set it to 500 - 600 next time as you recommended.

holmesnmanny

So I guess disconnecting the iacv connector and looking inside is too much work for ya.

If you're not able to get the base idle down to 600 then you have a vacuum leak somewhere.

Your problem isn't hard to figure out, you just need to start with the basics. The more work you make it the more work it's going to be.

I'm still not entirely sure if your issue is engine surging or idle surging.

ben_tech

I assure you it's not too much work, I've actually disconnected and looked at that connector 50 times at least already, sorry for not mentioning that. The IACV is brand new but didn't change symptoms vs. original part. I have tested for ground and battery voltage at the connector both on top and backprobe. I've tested the wiring at the other end, at the ECU.

Today I replaced the ECT sensor with brand new OEM part, no change. As a part of that job I did drain/fill/bleed the coolant system and squeezed the hoses and got the engine heated up with a no-spill funnel set attached to the radiator with the front end jacked up 2 feet for optimal air removal from the system.

I redid the idle setting procedure and this time set the screw as far in as it would go which looks like maybe 650 rpm if I had to make a guess by looking at the tach. No difference (been here before also).

The issue is engine surging pretty sure. When the car is acting up it surges / bucks when idling while cold in particular but always when the throttle is opened a bit. When the engine is cold and the throttle is opened more (say 2K RPM but I'm not sure), the engine goes to what sounds like redline immediately, there is no smooth transition. Now when it revs that high I can't tell if it's still bucking or not. I don't think it is, just loud as heck. When the car is not acting up, as mentioned this is an intermittent problem, the throttle has the normal smooth transition.

Worth repeating (or in case I didn't mention it yet) that the engine bucking still happens with the IACV harness disconnected. Idle is lower as expected and smooth when the engine is hot but opening throttle brings back the bucking. With IACV connected the bucking happens at idle when the engine is cold but when it warms up it seems to happen more with the throttle open, but sometimes at idle too (intermittent).

The symptoms are always better when the engine is thoroughly heated up but very bouncy between 1.25K and 1.5K RPM if I hold the throttle around there.

I'm about to re-test the MAP sensor with a vacuum pump. If that tests good, I'll be looking at a replacement ECU as that's what the Honda flowchart says to do for code 14 at this point.

Thanks for trying to help me, I do appreciate it. Any other ideas are welcome.

ben_tech

Hi Mike,

Tested the MAP sensor properly today with a vacuum pump.

Results:

2.9v on the third pin (backprobed) to battery negative.
5 inHg: 2.41v
10 inHg: 1.93v
15 inHg: 1.47v
20 inHg: 1v
25 inHg: .6v

Pressure released: 2.9v

No leaks, pressure held as long as I wanted.

I think we can call the MAP sensor good now.

Thanks for the idea though!

Got to be the ECU.... or maybe a fuel pressure issue but less likely that due to constant CEL code 14.... at least the ECU is part of Honda's troubleshooting procedure for CEL 14.

ben_tech

I'm happy to report that replacing the ECU / ECM module finally fixed the problem as far as I can tell. I mean I'm not 100% sure because it's an intermittent issue and it's only been one day but it's very promising as it was acting up more consistently these days. So I tried dozens of 'conventional wisdom' fixes found on the Internet for the engine surging / idle issues but in the final analysis, "read the service manual" (the code 14 troubleshooting chart) was the solution when followed through all the way to the end.

I'll never know if replacing all of the capacitors in the original ECM would have solved the problem. I replaced about half of them but didn't have the other values. May or may not have been an avenue to pursue further for a low cost fix.

All of the other things I did (replacing all of the valves and sensors, full tune up and more) I'll chalk up to preventative maintenance and a learning experience. All told it probably cost less than the dealership would have charged to replace just the ECM. That's one way of looking at it.

Thanks again for the consulting.

Will update if the issue crops up again.

I'm including below a copy of the troubleshooting chart for any poor souls dealing with this issue in the future. This is for your 94-97 Accords.

ts12easyas

Congrats on this fix! Did it indeed end up being the ECU/ECM? I admire your tenacity! You really did a lot. How's the car doing now?