86 Accord - Irregular minor miss only at idle

[Gunter]

Hello,

My son's 1986 Honda Accord 4 door sedan (with a carburetor-ed A20A1 engine) has an irregular minor miss only at idle. While idling, the car always has a minor miss - but the timing between misses is irregular. Under load it runs like a champ and I don't detect any miss. It has a 151,000 miles and a 5 speed transmission.

I had no idea how old the plugs were. So I pulled the plugs and discovered plug #4 was heavily carbon fouled. The other three looked surprising good. I suspected the miss was from either:
- plug #4
- the secondary ignition cable #4, or
- a compression problem on cylinder #4

I replaced all 4 plugs with new NGK plugs. The miss did not disappear.

I then completed a compression test and the results were impressive:
#1 = 180 psi
#2 = 180 psi
#3 = 180 psi
#4 = 175 psi
The specs per AllDtata is 135 to 178 psi

I then checked the secondary ignition cables for resistance and #4 fluctuated wildly while I manipulated the cable. So I replaced all the secondary ignition cables. The miss did not disappear.

I checked the ignition coil's resistance and it was within spec per AllData.

I checked the distributor cap and there was some corrosion. So I replaced the distributor cap and rotor. The miss did not disappear.

I checked the ignition timing and it was set at 20 degrees BTDC. Per the emissions decal in the engine bay, the timing is set correctly.

So i have eliminated:
- compression
- spark (the only thing left to check is the ignition igniter - which is highly unlikely to be the cause)
- spark timing



That only leaves air/fuel as possible causes of the miss.

The air filter is like new.

In the recent past, I checked for vacuum leaks with a smoke machine and could not locate any.

About 1.5 years ago, the fuel pump died. So I replaced the fuel pump and both fuel filters (in the engine bay and next to the fuel tank). At the time, I tested the fuel pump for both pressure and volume. Both were within spec. I believe I had the minor irregular miss prior to the fuel pump being replaced, but I am not 100% sure of that.

The vehicle has been in our family since brand new. It is completely stock and it has never been modified.

Any suggestions on what to check next?

Any and all help is greatly appreciated.

Thank you!

[ShiRen]

So it runs fine while driving and it doesn't do it all the time, how can you actually tell it's a miss? I know that might sound stupid, but I don't know if my car has ever idled without a little burble, it's cammed now so of course it idled all over the place. Idk, if it's not falling completely on its face, you have good spark, it runs good, etc, I might not look too far into it. Obviously keep an eye on it.

[Gunter]

Good question: How do I know I have a miss?

Based upon the heavily carbon fouled #4 spark plug, I believe I had a miss. The other 3 plus were fine.

Listening at the tailpipe, it still sounds like the engine is missing.

I recently purchased a single channel digital oscilloscope (AESWAVE uScope) and tried to obtain a screen capture of the miss, but was not successful. I am just learning how to use the scope, so it could be user error. I will try a few more times.

I will examine the plugs in a few weeks and see if any of them have fouled up again. If all are good, then I will move on.

Thank you!

[Gunter]

Besides examining the plugs in a few weeks to see if any of them have fouled up again, any other suggestions?

Any and all help is greatly appreciated.

Thank you.

[Oldblueaccord]

Besides examining the plugs in a few weeks to see if any of them have fouled up again, any other suggestions?

Any and all help is greatly appreciated.

Thank you.

I would pull the valve cover off and look at the cam lobe. Adjust the valves while your there.

Very cool you have had that car since new!!

[ShiRen]

Set the valve lash while you're in there lol

[Gunter]

Set the valve lash while you're in there lol

When my father gifted the car to my son 2.5 years ago, he was not sure if the timing belt was EVER replaced. So I made that a priority and when I had the valve cover off I checked and adjusted the valve lash clearance. Only one of the valves needed adjusting. So I believe they will still be within spec.

Thank you for the suggestion!

I am stuck. Any other ideas???

[Oldblueaccord]

When my father gifted the car to my son 2.5 years ago, he was not sure if the timing belt was EVER replaced. So I made that a priority and when I had the valve cover off I checked and adjusted the valve lash clearance. Only one of the valves needed adjusting. So I believe they will still be within spec.

Thank you for the suggestion!

I am stuck. Any other ideas???

Was the cylinder that you adjusted #4...?

[ShiRen]

Hard for me to believe only one needed adjustment, unless he'd been keeping up on it. I find them to be pretty finicky, so make sure they're right

[Gunter]

Was the cylinder that you adjusted #4...?

It was more than 2 years ago, but I am certain it was either #1 or #2 cylinder that needed adjusting. All valves clearances were measured with a feeler gauge and all were within spec - except one valve on either #1 or #2 cylinder.

[ShiRen]

I find that mine like to get tight, so if they are at all near that end I reset them

[Gunter]

I find that mine like to get tight, so if they are at all near that end I reset them

I will pull the valve cover and check the valve clearances again.

I also did a bit of research and learned that a faulty EGR valve/system could cause the same symptoms i.e. rough idle/miss at idle but no problems under load. I need to pick up a vacuum pump to test the EGR for leaks. Assuming the EGR valve is the problem, I searched Rockauto for an EGR valve and they have only one listing for a DX/LX model: a Wells WVE 4F1369. The only problem is it is out of stock. Based upon the age of the car, I wonder if they will ever get any more stock. I checked Ebay Motors and found nothing. I called my local dealership and they say the EGR valve is discontinued and they can't get me one.

They provided me with two Honda part #s 18710-PH4-661 and 18710-PH4-734. Denso is the OEM supplier of that part and I can't find it on the Denso website.

Any suggestions (besides my local auto recyclers) where I could get an EGR valve? I would prefer new over recycled, but I can't be too choosy.

Thanks again for any and all suggestions!

[Oldblueaccord]

No suggestions really but you might get lucky and another Honda type car has one that works.

Im not sure EGR is associated with #4 cylinder...ill have to think on that. I guess your could take it off and try and clean it.

I have mine hooked up but the hot air pipe going to it is plugged. Its always worked for me pretty well.