Miss at idle

[hardtopguy]

I'm wondering if this has been a problem for anyone else. My car is a 92 with 28,000 miles and completely stock.
When the car is running at an idle (with all accessories off (a/c, radio)), it sounds like I have a subwoofer in the trunk of the car.
I've pulled the plugs and put in new OEM plugs.
I've checked the fuel injectors and ran some BG 44k fuel injector cleaner through the car.
I've replaced the air cleaner.
I've Ohm'd the coils and replaced them with known good ones. No difference so I put the old coils back in.
When I turn the A/C on the miss seems to lesson.
The car runs fine and get great gas mileage.
I've check for vacuum leaks and the car has good vacuum and no leaks.
Any suggestions?

 

[Briank]

Hardtopguy

Can't believe you have not gotten a response here yet? Normally there is lots of opinions here on prime.

I have some questions for you?

1) Is the miss regular or intermittent

2) Do have a check engine light on

3) Is your idle speed normal

Briank

 

[hardtopguy]

Briank Thanks for responding. I too thought I would get a lot of help on this.
To answer your questions:
No check engine light.
Normal idle speed
It does sound rythmic like the same cylinder is misfiring but its not misifiring every time.
If I could hook up the OBD scanner here at work (Ganley Honda) the problem would be solved. But the car doesn't have the capability.

 

[Briank]

Hardtopguy

My 91 has some unevenness at idle but I would not call it a dead miss. Have you done a recent emission test, is it even required in your area? When you pulled the plugs did they all look the same? When was the last time the valves were adjusted? Has the timming belt been changed? There is a copy of the service manual here on Prime that you can look up and find how to get the codes from the computer but with no check engine light you may not have any.

Briank

 

[Jin1976]

How do you adjust the idle RPM? Mine is at 600RPM @ idle.

 

[Briank]

Jin1976

http://www.nsxprime.com/FAQ/Reference/1991_svcman/1991_svcman.htm

(page 260)

Briank

 

[Jin1976]

Jin1976

http://www.nsxprime.com/FAQ/Reference/1991_svcman/1991_svcman.htm

(page 260)

Briank

I've checked. Mine is automatic and idle should be 600 +/- 50RPM therefore mine is okay. Manual should be 650 +/- 50RPM at idle.

Briank, thank you.

 

[Briank]

Jin1976

If you go to page 261 you will find that your idle speed for your automatic should be 750, the 600 is the basic setting without the computer in control.

BrianK

 

[hardtopguy]

When I pulled the plugs they all looked fine. I couldn't say after looking at the plugs which cylinder was causing the problem. The car was e-checked in May and passed with flying colors (even with the miss). I haven't changed the timing belt or adjusted the valves yet. That was going to be the next step unless someone on the board had a better idea.
It just seems odd that it is only at an idle. Once I crack the throttle its gone.

 

[Jin1976]

Jin1976

If you go to page 261 you will find that your idle speed for your automatic should be 750, the 600 is the basic setting without the computer in control.

BrianK

Is it okay if the RPM is a bit low than normal after spirited drive?

 

[Lud]

For a problem like this only at idle, I would really double check very carefully for vacuum leak. Then double-check plugs, fuel injectors and coils.

If it's none of the above, you have to consider the possibility of a compression problem.

Timing belt has nothing to do with it. A valve adjust will not fix the current problem, but being way out of adjutment could have caused a burnt valve and thus a compression problem.

 

[hardtopguy]

Found the problem. After checking everything over for the fifth time I decided to pull the plugs out and make sure they were OK. All looked OK and I started to put them back in. When I went to put the fourth one in I noticed a hairline crack in the ceramic. I swapped that plug out and she's running fine.
Thanks for all the help

 

[JPS Europa]

You are sooooo lucky. I had a piece of the ceramic fall off inside an engine once...happily not my NSX. Wrecked the piston, cylinder liner and head. It was not a good day.

 

[Lud]

Glad it's fixed, but I'm confused... In your first message you said as part of troubleshooting you had already replaced all plugs with new OEM plugs... ?? Was one of the new plugs cracked? If so, was the plug you replaced also damaged?