Miata rough idle in drive smooth idle in neutral?

scottmandue · 08-05-2015, 10:09 AM

Yeah I know, an automatic Miata!?!

Now that we all had a good laugh (at my expense).
Been trying to hunt down a rough idle on my 1995 Miata.
All new tune up, checked all hoses manifolds, everything is clean and tight.
Then on a whim I'm sitting at a stop light with her burbling as usual and I get the idea to pop it in neutral... she smoothes out...
Any ideas?

onewhippedpuppy · 08-05-2015, 10:27 AM

Typically that shift includes an RPM change. Compare the RPM in drive and in neutral then rev to that RPM in drive with your foot on the brake, I bet it smooths out. Assuming you've checked all the usual tune up stuff, what about motor mounts? Did you just visually inspect the vac lines? It's really hard to see cracks and they will cause a tough idle. On an older car I always just buy bulk rubber vac lines and replace them.

scottmandue · 08-05-2015, 10:38 AM

Motor mounts have been mentioned before... AFAIK they are OEM...

wayner · 08-05-2015, 10:44 AM

Every spring when my wife's miata comes out of storage it has a rough idle.

I thought it was related to the throttle position sensor. It goes away at idle with slight pressure on the accelerator but returns when foot is off the gas and idling

It was not that. Instead I used a Q tip dipped in alcohol to clean the hot wire airflow meter in the air intake.

Cures it every spring.

scottmandue · 08-05-2015, 12:33 PM

Quote:

Originally Posted by wayner

Every spring when my wife's miata comes out of storage it has a rough idle.

I thought it was related to the throttle position sensor. It goes away at idle with slight pressure on the accelerator but returns when foot is off the gas and idling

It was not that. Instead I used a Q tip dipped in alcohol to clean the hot wire airflow meter in the air intake.

Cures it every spring.

I presume that is the thing just before the airbox/air filter? (MAF?) I disassembled it and sprayed it liberally with cleaner.

asphaltgambler · 08-05-2015, 01:49 PM

Yes, but what kind of cleaner?

fastfredracing · 08-05-2015, 01:55 PM

Maybe try cleaning up the throttle body, and making sure base idle settings are close to where they need to be. I cant remember if you can adjust or not on these...

Por_sha911 · Por_sha911's Garage

Quote:

Originally Posted by onewhippedpuppy

Did you just visually inspect the vac lines? It's really hard to see cracks and they will cause a tough idle. On an older car I always just buy bulk rubber vac lines and replace them.

This. Vacuum test all lines.

Nostril Cheese · 08-05-2015, 02:09 PM

Vacuum leak or throttle body would be my first guesses.

scottmandue · 08-05-2015, 02:31 PM

Quote:

Originally Posted by fastfredracing

Maybe try cleaning up the throttle body, and making sure base idle settings are close to where they need to be. I cant remember if you can adjust or not on these...

Actually, everything I have read strongly recommends not to touch the idle settings. Keep in mind it idles fine in neutral.
Even more bizarre it idles better when cold Vs warm.

Quote:

Originally Posted by asphaltgambler

Yes, but what kind of cleaner?

Yes, I know a vacuum leak is the obvious cuprite... but everything looks tight and clean (no cracks on any rubber). I have pulled everything from the MAF to the throttle body apart and everything looks tight.

Por_sha911 · Por_sha911's Garage

Have you sprayed carb cleaner on the hoses while the car is idling to see if the idle goes up? As someone already stated, looks of hoses can be deceiving. Ask me how I know.

scottmandue · 08-05-2015, 03:27 PM

Quote:

Originally Posted by Por_sha911

Have you sprayed carb cleaner on the hoses while the car is idling to see if the idle goes up? As someone already stated, looks of hoses can be deceiving. Ask me how I know.

Do you think the MAF cleaner would work for that?

Por_sha911 · Por_sha911's Garage

Don't know. You can use plain old starting fluid but be careful not to use too much at once and torch the vehicle.

masraum · 08-05-2015, 04:37 PM

Well, when you put the car in drive (vs N) that puts a slight load on the engine which usually also makes the idle drop. I think there's supposed to be a solenoid that bumps the idle up a bit (or is that just when the AC is on?). As asked previously, what are the rpms in N and what are they in D? Do you run the AC? If so, test in N and D with the AC on and then again with the AC off and see what the results are. If you never run the AC, you still may want to test.

It does seem odd that it idles better cold than warm, unless it is a vacuum leak, in which case it kind of makes sense that it's better cold than warm. I know these things don't have a choke like a carb, but I'm sure the computers make the mixture a little rich when the car is cold to simulate a choke. If you have a vacuum leak, that may bring the mixture back to something approaching normal due to the extra air that's getting in due to the vacuum leak. At least, that sounds reasonable to me.

onewhippedpuppy · 08-05-2015, 04:49 PM

You can use an unlit propane soldering torch to check for vac leaks. If there's a leak it will suck in the propane and idle up.

Arizona_928 · 08-05-2015, 05:48 PM

Quote:

Originally Posted by onewhippedpuppy

You can use an unlit propane soldering torch to check for vac leaks. If there's a leak it will suck in the propane and idle up.

Carb cleaner works too.

Definitely air leaking somewhere.

VincentVega · 08-05-2015, 07:05 PM

I replaced engine mounts in 1 with a rough idle, easy enough to check. Does it just run rough or is their a rpm drop?

onewhippedpuppy · 08-06-2015, 03:36 AM

Quote:

Originally Posted by AZ_porschekid

Carb cleaner works too.

Definitely air leaking somewhere.

Yup. Or starting fluid, or anything else flammable. The only problem is that some of that stuff isn't very friendly to the hoses and seals that you are spraying it on.

wayner · 08-06-2015, 05:35 AM

[QUOTE=masraum;8741338]Well, when you put the car in drive (vs N) that puts a slight load on the engine which usually also makes the idle drop. I think there's supposed to be a solenoid that bumps the idle up a bit

Depending on the year, these things are all computer now not solenoids. At idles it uses one program. When it senses the throttle position movement (TP sensor), it switches to another program.THe idle program tries to keep it at a set RPM regardless of load. As long as the TPS does not detect movement, the idle program balances airflow, fuel, O2 exhaust etc. to keep it stable.

If there is an air leak, a dirty MAf sensor, a bad 02 sensor etc, it will hunt.

scottmandue · 08-06-2015, 06:20 AM

Experimenting last night, there is about 100 RPM drop (estimating between 900RPM in neutral/park and 800RPM in Drive) and yes the A/C makes it worse... so much that I tend to turn the A/C off when stopped in traffic.

If it is a vacuum leak wouldn't the rough idle be persistent and continue even when in park/neutral?