What is 'normal' idle RPMs for a stock MM?

[IwantmyMMnow!]

When I was at Russ's (03mmmonroe) place the weekend before last, he noticed that my RPMs at idle seemed to be low; in fact, when he pulled it out of the garage, it stalled out on him...I've never had it happen to me. We hooked up his laptop and took her out for some spirited driving to check numbers and everything looked good. The RPMs do take a little 'nose-dive' at stops that are not gradual ones...down to around 400, then comes back up.

When I'm at idle, my RPMs are usually around 550-600; with the A/C on, it sits around 700. Yesterday, I noticed that at idle the needle on the tach was dead-on the 500 mark (A/C was off). Is there a 'standard' for RPMs at idle for a stock MM? I'm beginning to think that I might be gradually loosing RPMs at idle...what would cause this?

I don't want to make any 'computerized' adjustments to my idle RPMs since this may possibly 'cover up' an existing issue that needs to be addressed. I will be getting an oil change this week and will replace the air filter as well. I'm also thinking a fuel filter change is in order; hasn't been done since I bought it and I've put almost 40K the clock. Anything else I should consider? TIA...

[babbage]

Fuel filter for sure! Should do that every 10-15K miles.
Seafoam would be a good idea too, clean the Tbody with it. Clean your IAC (Idle air control) valve - it's on the side of the tbody.

Clean - Clean - Clean = Vrooom!

[SC Cheesehead]

IAC?

http://www.mercurymarauder.net/forums/showthread.php?t=68153

[Blackened300a]

Sounds like the IAC needs a cleaning or check for a vacuum leak.

[Mike M]

Also the throttle body must be cleaned. With engine off throttle gets opened wide and the lip of black crud gets removed where the throttle plates normally comes to rest. Use your finger to clean along with either WD-40 or throttle body spray.

Fuel filter is always a good maintenance item but won't make any difference to your idle speed.

[RF Overlord]

Agree with the others ^^^ about the IAC for the idle speed.

Also, when cleaning the throttle body, DON'T use plain carb cleaner or brake cleaner...use a cleaner that says it's safe for coated throttle bodies.

[capt512]

Just my .02, I have the same issue... I have replaced my IAC (with motorcraft and new motorcraft gasket, cleaned TB (and new motorcraft gasket), cleaned MAF, replaced fuel filter, replaced plugs, replaced PCV, and still the same. 4-500 in drive at idle stopped, sometimes dips a tiny bit lower when first going in reverse from stop.

[Bluerauder]

Is there a 'standard' for RPMs at idle for a stock MM? I'm beginning to think that I might be gradually loosing RPMs at idle...what would cause this?

My idle is about 650-700 RPMs after she warms up. Stock with a tune.

[IwantmyMMnow!]

Thanks for all the info guys.

Last Friday I had oil change and new filter, new air filter, trans fluid change (had full flush done last summer), and new fuel filter done. Car still idles dead-on the 500 tick-mark...I seem to recall it used to idle around 600 (pure speculation; the needle would be 2-3 widths above the 500 mark), and was at 550 according to the laptop after Russ installed new plugs.

All of this got me to thinking...could the following be a possible reason for my RPMs to be lower? With new plugs and fuel filter, could the engine be running more efficiently now, thus it not needing to work as hard (making RPMs lower) as it was before? It doesn't appear that I'm in any danger with my RPM situation, but will keep an eye on the situation.

I do plan on cleaning my IAC and throttle body. Also wondering if the alternator could be a player as well...any warning signs of when the alternator is getting ready to crap out, or is it just one of those deals where it just dies w/o any warning?

[Zim Hosein]

Sounds like the IAC needs a cleaning or check for a vacuum leak.

Would a vacuum leak related to the IAC cause A/C issues, or are they completely unrelated when dealing w/ vacuum leaks under the hood vs. behind the dash? :confused:

[JoeBoomz]

Is there a 'standard' for RPMs at idle for a stock MM?

Straight from SCT Advantage III Stock Values from BMD0:


Idle Speed in Drive = 544
Max Allowed Idle Speed Drive = 1000
Idle Speed in Neutral = 640
Idle Speed Adder with A/C On In Drive = 16 (add 16RPM when AC Clutch engaged)
Idle Speed Adder with A/C On In Neutral = 0
Idle Speed Adder with A/C On = 1426.7 (this is poorly labeled, it is actually the max idle speed allowed when the A/C clutch is engaged)


There are some other modifiers not listed here, idle adder for power steering, coolant temp, etc (i.e., at 110F coolant temp it adds 96 RPM to help warm up, more RPM at lower coolant temperatures).

But a stock Marauder when warmed up with AC off and the steering wheel still should be idling at 640 in neutral and 544 in drive.

[IwantmyMMnow!]

Straight from SCT Advantage III Stock Values from BMD0:


There are some other modifiers not listed here, idle adder for power steering, coolant temp, etc (i.e., at 110F coolant temp it adds 96 RPM to help warm up, more RPM at lower coolant temperatures).

But a stock Marauder when warmed up with AC off and the steering wheel still should be idling at 640 in neutral and 544 in drive.

Thanks, Joe. The RPMs do kick up ~ 100 when I have the car in neutral, but hard to tell the 'exact' number by looking at the tach. Also, I realize that there may be a +/- % error with the tach as well, but I think it's pretty close...when it was hooked up to the laptop and it read 550, the needle was one width above the 500 tick mark...several days later is when I noticed it was sitting dead-on that mark, which threw up a yellow flag in my mind.

She's running smooth, so I'm not overly concerned at this point. Will see where she stands after I get to cleaning the IAC and TB valve...if she's still at 500, do you think there would be any benefit to adjusting the idle up a smidge to get it closer to the numbers you provided?

Your mention of coolant temps reminded me that I need to have a coolant flush done before I make the trek for MVX...should've already had it done, but you know how that goes...

Thanks again Joe...you've got the best sig block on the site!

[JoeBoomz]

if she's still at 500, do you think there would be any benefit to adjusting the idle up a smidge to get it closer to the numbers you provided?

Thanks again Joe...you've got the best sig block on the site!

Kind words appreciated - thanks ;)

Bringing up the idle won't hurt anything, you use a bit more fuel at idle at a higher RPM but it's a miniscule difference. Increasing the idle solved stumbling and stalling issues in my SC Marauder (the Vortech puts load on the engine and increased idle RPM is required to compensate).

My current idle values are at 700 for both drive and neutral. And the car runs great.