Idle changed after spark plug replacement

[M_3_X]

Hi all,
91 325ic, yesterday I went ahead and replaced the old Autolite's the previous owner had in there and put some fresh NGK ZGR5A per forum recommendation.As soon as I started the car, I noticed it started idling faster (around 2200 RPM), engine was cold, but it is too high anyway.
Once it warmed up, it decreased to ~1400, it also seems to take longer to decelerate when shifting gears.
I guess this engine didn't like NGK's so I'll get a new set of the Autolite copper it had before and put those in.

Anybody have an idea why this would happen....

[packratbimmer]

hmmm...NGK should be fine

maybe do a stomp test, reset the ecm and try it again

usually idle relates to vacuum leaks on the M20

[reelizmpro]

The forum recommends NGK ZGR5A? That's news to me. Looks like a generic plug that fits many models. The fact that they fit is not enough. Make sure the NGK electrode is as far out as the Bosch or Autolite. I've had issue with NGK's that were too short. Also make sure to remove the screw caps on the end of the plugs as the M20 plug wires do not fit over them. I never had issues with good old Bosch W8LCR coppers or whatever it is called these days.

[digger]

The forum recommends NGK ZGR5A? That's news to me. Looks like a generic plug that fits many models. The fact that they fit is not enough. Make sure the NGK electrode is as far out as the Bosch or Autolite. I've had issue with NGK's that were too short. Also make sure to remove the screw caps on the end of the plugs as the M20 plug wires do not fit over them. I never had issues with good old Bosch W8LCR coppers or whatever it is called these days.

that is the correct NGK plug and it works fine. its the proper head range, projection and fitment etc etc i always recommend NGK over bosch. for the same heat range they seemed to have less deposits (no tune change) the quality looks better to IMO. bosch will also be ok as they are basically equivalent but most m20 guys i know also say they prefer NGK. Seems to be preffered by UK guys to.

seems unliekly a plug change causes that high of an idle unless some other problem was uncovered, arisen during the change out

[Northern]

probably bumped something or moved something and created a vacuum leak

[M_3_X]

I went ahead and got a new set of the same Autolite that were in there (#68 I think...) and the idle came back to normal. I further inspected the NGK's and noticed that they have this new thing (to me) that the call "V" technology, where the electrode has a cut in the middle in a "V" shape, no idea how that could affect...

In any case, the idle came back to the way it was, (around 950 RPM at operating temp.) now the issue I'm having is it takes too long to drop during shifts, only with the A/C ON.[EDIT]

I have ordered new intake and ICV hoses, and I'll clean the ICV to see if that takes care of the issue. What other rubber hoses should I be looking to replace that directly affect the idle?
This car has 125,000 miles and the PO gave me the records it has been well-mantained but it does not show record of any vacuum hoses replaced, even they look good visually. (except the main intake boot which looks a bit tired).

Thanks all for your input.

[dnguyen1963]

Have a shop perform a smoke test to find any vacuum leaks.

[5Toes]

thats WEIRD. the zgr5a plugs are great and every racer recommends them. nobody really runs autolite plugs.

reset the DME (digital motor electronics) by unplugging the battery over night and try the NGK plugs again.

[digger]

the slow rate which revs come down is probably a vacuum or icv issue something along that nature that can also cause a fast idle. Also make sure the throttle butterfly is adjusted properly and the tps working

[packratbimmer]

950 is still a high idle for a stock engine. I would keep digging.

[Northern]

As far as what I read yesterday when buying colder plugs for my car, if the part number has an E at the end, it will have that V groove electrode (ex. BKR7E-E vs BKR7E for the non-grooved)

[digger]

the stock NGK plugs have the V (comes from the A in ZGR5A it uses a different coding system to say BPR6ES where the S dictates the firing end )

[M_3_X]

I cleaned ICV and replaced the small hose that goes to the intake, also replaced the big elbow boot and air filter, no major changes.

I discover a decent size crack on the catalytic converter, big enough to make some noise and leak fumes. Not sure how big of an impact this would have on the idle behavior, but I will have that fixed before I continue to put more efforts in diagnosing this.
Thanks for all of your feedback!