Idle Issues With Ac Vacuum

[Arkfoxvert]

I have a tech question maybe someone can point me in the right direction. I picked up an 87 gt very the other day. Only mods are stainless shortys, o/r h pipe, Flowmaster dumps, smog delete.

It fires right up and drives great. The idle has always been at 1000-1100 rpm. I was trying to get the ac to work as it only blows defrost so I checked the vacuum block and sure enough the ac was unplugged. So I plugged it in and the blower works great. However the idle drops to 500-600 rpm and surges like crazy then dies. It will run fine just can’t hold idle. The car is still speed density. Any ideas? I’m thinking there must be a leak.


The second question, some times, when the car is just the right temp it really halls ass. Typically it’s quick for what it is, but 3 or 4 times it’s shocked me when I got on it, sideways in 2 second. I’d love it to run like that all the time lol. Could this be a vacuum issue as well?

Last The oil pressure gauge will fall really low on idle about half the time, the rest of the time it’s in normal levels. I’m guessing this is just a bad gauge. It doesn’t smoke at all other than a quick puff on a big down shift.


I really want to get this sorted before I start the mods. Thanks for the help!

 

[jrichker]

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

 

[mikestang63]

do a base idle reset- the car should be idling around 650 not 1000. chances are the prior owner has it that high to mask something wrong
The AC on puts more of a load on the engine and also draws vacuum
Stock gauges are notorious for being inaccurate. Your sending unit could be bad, you could have a failing oil pump, or the engine could be tired

 

[Blown88GT]

I have a tech question maybe someone can point me in the right direction. I picked up an 87 gt very the other day. Only mods are stainless shortys, o/r h pipe, Flowmaster dumps, smog delete.

It fires right up and drives great. The idle has always been at 1000-1100 rpm. I was trying to get the ac to work as it only blows defrost so I checked the vacuum block and sure enough the ac was unplugged. So I plugged it in and the blower works great. However the idle drops to 500-600 rpm and surges like crazy then dies. It will run fine just can’t hold idle. The car is still speed density. Any ideas? I’m thinking there must be a leak.


The second question, some times, when the car is just the right temp it really halls ass. Typically it’s quick for what it is, but 3 or 4 times it’s shocked me when I got on it, sideways in 2 second. I’d love it to run like that all the time lol. Could this be a vacuum issue as well?

Last The oil pressure gauge will fall really low on idle about half the time, the rest of the time it’s in normal levels. I’m guessing this is just a bad gauge. It doesn’t smoke at all other than a quick puff on a big down shift.


I really want to get this sorted before I start the mods. Thanks for the help!

All typical issues for the era.
There is no CEL, but see if any codes are stored.
Could be a vacuum leak, there are a lot of vacuum lines.
Stock 75A 2G alternator doesn't put out enough amps at idle, at least upgrade to a 130A 3G. It will solve a lot of electrical problems.